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Cascading g-C3N4 and Peroxygenases for Selective Oxyfunctionalization Reactions

Cite this: ACS Catal. 2019, 9, 8, 7409–7417
Publication Date (Web):July 9, 2019
https://doi.org/10.1021/acscatal.9b01341

Copyright © 2022 American Chemical Society. This publication is licensed under CC-BY-NC-ND.

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Abstract

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Peroxygenases are very interesting catalysts for specific oxyfunctionalization chemistry. Instead of relying on complicated electron transport chains, they rely on simple hydrogen peroxide as the stoichiometric oxidant. Their poor robustness against H2O2 can be addressed via in situ generation of H2O2. Here we report that simple graphitic carbon nitride (g-C3N4) is a promising photocatalyst to drive peroxygenase-catalyzed hydroxylation reactions. The system has been characterized by outlining not only its scope but also its current limitations. In particular, spatial separation of the photocatalyst from the enzyme is shown as a solution to circumvent the undesired inactivation of the biocatalyst. Overall, very promising turnover numbers of the biocatalyst of more than 60.000 have been achieved.

KEYWORDS:
Selective oxyfunctionalization of nonactivated sp3 carbon–hydrogen bonds is a challenge in organic synthesis. Catalysts exhibiting both high oxidation potential and high selectivity are rarely found. (1) In this respect, so-called unspecific peroxygenases (UPOs), next to the established P450 monooxygenases and some other nonheme-monooxygenases, (2) have attracted considerable interest. (3−5) Monooxygenases generate their catalytically active species through reductive activation of molecular oxygen. For this, monooxygenases rely on complex electron transport chains delivering the reducing equivalents from NAD(P)H to the monooxygenases’ active sites. This complicates their practical use. Furthermore, quite frequently, a significant portion of the reducing equivalents is uncoupled into a futile reaction with molecular oxygen, thereby wasting valuable reducing equivalents and generating hazardous reactive oxygen species. (6)
UPOs do not rely on the aforementioned electron transport chains but rather use simple H2O2 as the oxidant. This makes UPOs very attractive from a preparative point-of-view. H2O2, however, is also a potent inhibitor of peroxygenases as already small excesses oxidatively inactivate the prosthetic heme group. (7) Generating H2O2 in situ through catalytic reduction of O2 is the most common approach to alleviate the inactivation issue. (8) In essence, these systems provide H2O2 at rates that enable efficient peroxygenase activity while minimizing the H2O2-related inactivation. These methods comprise a range of chemical, (9−15) electrochemical, (16−21) enzymatic, (22−26) and photocatalytic (27−34) approaches.
The latter is particularly interesting as it, in principle, enables using light energy to access simple sacrificial electron donors to drive the reduction of O2 to H2O2. To make light energy available for this reaction, a photosensitizer (or photocatalyst) is necessary. Both homogeneously dissolved molecular and semiconductor-based solid material photocatalysts have been evaluated. While the first often suffer from issues of photobleaching and inactivation, the latter excel by their high robustness and reusability. So far, mainly TiO2-based semiconductor photocatalysts have been evaluated to promote peroxygenase-catalyzed oxyfunctionalization reactions.
Therefore, we set out to investigate a broader set of (in)organic photocatalyst systems (35) to promote peroxygenase-catalyzed reactions (Scheme 1).

Scheme 1

Scheme 1. Photoenzymatic Hydroxylation of Ethyl Benzene Combining Heterogeneous Photocatalysts for the Reductive Activation of O2 to H2O2 with a Peroxygenase-Catalyzed Oxyfunctionalization Reaction
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We chose the selective hydroxylation of ethylbenzene to (R)-1-phenyl ethanol catalyzed by the peroxygenase from Agrocybe aegerita (rAaeUPO) as the model reaction. (36−39)
In a first set of experiments, we evaluated several reported and/or commercially available heterogeneous photocatalysts (Figure 1) with respect to their ability to form H2O2; particularly, we investigated their performance in the envisioned photoenzymatic cascade transforming ethylbenzene to (R)-1-phenyl ethanol (Figure 1). It is worth mentioning that in the absence of the photocatalysts or light, no product formation was observed. In the absence of rAaeUPO, upon prolonged reaction times, traces of racemic product and the overoxidation product were observed in some cases.

Figure 1

Figure 1. Performance of several heterogeneous photocatalysts to promote rAaeUPO-catalyzed oxyfunctionalization, forming phenyl ethanol (blue) and the overoxidation product acetophenone (red), in absence (left) or presence (right) of methanol. Conditions: 5 mg mL–1 heterogeneous catalyst, 50 mM ethylbenzene, 0 or 250 mM methanol, and 100 nM rAaeUPO in a 100 mM phosphate buffer at pH 7, 30 °C and stirring at 300 rpm. Illumination by an Osram 200W light bulb for 30 min. Reactions were performed in independent duplicates. 1: Au-BiVO4; (40)2: Co3O4 (quantum dots); (41)3: Co4(H2O)2(W9O34)2; (42)4: Pt-TiO2 (Rutile); (43)5: MnO (on Faujasite); (44)6: Co-TiO2; (45)7: MnO (nanowires); (46)8: Ir@SiO2; 9: Fe2O3; (47)10: g-C3N4; 11: ZnO (nanoclusters). (48)

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Almost all heterogeneous photocatalysts tested enabled the desired reaction both in the presence and absence of an extra electron donor (methanol). However, reaction rates tended to be significantly lower in experiments where methanol was absent. In the absence of methanol, it may be assumed that water was the primary reductant. (49) As water oxidation is thermodynamically more challenging than methanol oxidation, this explains the generally lower product formation rates in the absence of methanol. In some cases, significant amounts of acetophenone were found along with the desired (R)-1-phenyl ethanol. We attribute this to the oxidation of the desired product by the photocatalysts. (50) Platinum loaded titanium oxide (Pt-TiO2) and graphitic carbon nitride (g-C3N4) stood out in terms of product formation rate. For reasons of ease of preparation, we further focused on g-C3N4 as photocatalyst. Furthermore, g-C3N4 has previously been reported to be highly selective for H2O2 over other (partially reduced) reactive oxygen species. (51)
Figure 2 shows the overall product formation of g-C3N4 using either water (blue), methanol (red), or formate (green) as sacrificial electron donor to promote the photoenzymatic hydroxylation of ethylbenzene. Assumptions made to obtain the bar-graphs can be found in the experimental section.

Figure 2

Figure 2. g-C3N4 as photocatalyst to promote rAaeUPO-catalyzed hydroxylation of ethylbenzene in the absence of external electron donors (blue), or 250 mM methanol (red) or 250 mM formate (green). (A) Time course of (R)-1-phenyl ethanol formation and (B) time course of acetophenone formation. From these time courses, parameters such as reaction rate (C), rAaeUPO inactivation rate (D), maximal product concentration (E), and selectivity (F) were calculated. General conditions: [rAaeUPO] = 100 nM, [ethylbenzene] = 50 mM, [g-C3N4] = 5 mg mL–1, KPi buffer pH 7.0 (100 mM), 30 °C, magnetic stirring at 600 rpm, illumination by an Osram 200W light bulb. Reactions were performed in independent duplicates.

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From a conceptual and an environmental point-of-view, water would have been the most desirable source of reducing equivalents. However, this system fell back in terms of catalytic rate and selectivity compared with the use of methanol or formate as sacrificial electron donor. In comparative experiments, illuminating g-C3N4 in phosphate buffer gave negligible H2O2 formation rates (<1.0 μM h–1), which more than doubled to 2.0 (±0.02) μM min–1 upon addition of formate. Similar observations have been made before for TiO2. (52) Since g-C3N4 is not an efficient water oxidation catalyst, (52) other molecules are more likely to serve as reductants in the g-C3N4-catalyzed reduction of O2. Both ethylbenzene and phenyl ethanol can be oxidized by g-C3N4. (50) This was evident from the lower optical purity of phenyl ethanol and the increased overoxidation to acetophenone.
Another interesting effect of formate was that the robustness of the overall reaction was significantly higher than when using methanol or no sacrificial electron donor. We attribute this to the hydroxyl radical scavenging activity of formate (vide infra). Methanol oxidation proceeds via methoxy radicals, which again may be assumed to have a detrimental influence on the stability of the biocatalyst. (29)
Overall, formate proved to be a suitable sacrificial electron donor for the reaction. We therefore systematically investigated the factors influencing the activity, selectivity, and robustness of the formate-driven photoenzymatic reaction system.
Increasing the concentration of formate had a positive effect on the reaction rate, selectivity, and robustness of the overall system (Figure 3). Hence, we concluded that the photocatalytic oxidation of formate is the overall rate-limiting step in the reaction scheme.

Figure 3

Figure 3. Influence of the formate concentration on the performance of the photoenzymatic hydroxylation of ethylbenzene. (A) Time course of (R)-1-phenyl ethanol formation and (B) time course of acetophenone formation. From these time courses, parameters such as reaction rate (C), rAaeUPO inactivation rate (D), maximal product concentration (E), and selectivity (F) were calculated. [HCO2–] = 0 mM (black), 50 mM (green), 100 mM (red), 250 mM (blue) or 500 mM (purple). General conditions: [rAaeUPO] = 100 nM, [ethylbenzene] = 50 mM, [g-C3N4] = 5 mg mL–1, KPi buffer pH 7.0 (100 mM), 30 °C, magnetic stirring at 600 rpm, illumination by an Osram 200W light bulb. Reactions were performed in independent duplicates.

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The concentration of the biocatalyst also had a significant influence on the robustness of the overall reaction but hardly influenced the initial product formation rate (Figure 4). This supports our assumption that the photocatalytic H2O2 formation was rate-limiting and that undesired rAaeUPO-inactivation by the photocatalyst represented the major undesired side reaction. Incubation of g-C3N4 with rAaeUPO in the darkness did not result in a significant inactivation of the biocatalyst (Figure S10), suggesting that photocatalytically generated reactive oxygen species are responsible for the enzyme inactivation observed (vide infra).

Figure 4

Figure 4. Influence of the rAaeUPO concentration on the performance of the photoenzymatic hydroxylation of ethylbenzene. A: time course of (R)-1-phenyl ethanol formation and B: time course of acetophenone formation. From these time courses, parameters such as reaction rate (C), rAaeUPO inactivation rate (D), maximal product concentration (E) and selectivity (F) were calculated. [rAaeUPO] = 20 nM (black), 50 nM (red), 100 nM (blue), 200 nM (green) or 500 nM (purple). General conditions: [NaHCO2] = 250 mM, [ethylbenzene] = 50 mM, [g-C3N4] = 5 mg mL–1, KPi buffer pH 7.0 (100 mM), 30 °C, magnetic stirring at 600 rpm, illumination by an Osram 200W light bulb. Reactions were performed in independent duplicates.

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Next, we systematically varied the concentration of the photocatalyst (Figure 5). Very much to our surprise, the anticipated positive correlation between g-C3N4 concentration and H2O2 formation rate was less pronounced than expected. Moreover, rAaeUPO inactivation even decreased at higher g-C3N4 concentrations.

Figure 5

Figure 5. Influence of the g-C3N4 concentration on the performance of the photoenzymatic hydroxylation of ethylbenzene. (A) Time course of (R)-1-phenyl ethanol formation and (B) time course of acetophenone formation. From these time courses, parameters such as reaction rate (C), rAaeUPO inactivation rate (D), maximal product concentration (E) and selectivity (F) were calculated. [g-C3N4] = 1 mg mL–1 (black), 2.5 mg mL–1 (red), 5 mg mL–1 (blue), 10 mg mL–1 (green), or 15 mg mL–1 (purple). General conditions: [NaHCO2] = 250 mM, [ethylbenzene] = 50 mM, [rAaeUPO] = 100 nM, KPi buffer pH 7.0 (100 mM), 30 °C, magnetic stirring at 600 rpm, illumination by an Osram 200W light bulb. Reactions were performed in independent duplicates.

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The most likely explanation for this observation is a self-shading effect of g-C3N4 (already at a photocatalyst concentration of 1 mg mL–1, the reaction mixture was turbid). Hence, because of a decreased light penetration at elevated photocatalyst concentrations, the majority of the photocatalyst remains inactive neither contributing to H2O2 generation nor to inactivating the biocatalyst.
The overall rate of the reaction was highest at neutral to slightly alkaline pH values (Figure S6). Currently, we are lacking a plausible explanation for this observation as the H2O2 generation rate of g-C3N4 was reported to be largely pH-independent, (53) whereas the highest activity of rAaeUPO may be expected in slightly acidic media. (39) Similar trends have been observed before for TiO2, (54) suggesting that the pH-dependency of the complex O2-reduction mechanism may play an important role here. (54)
Also the morphology of the g-C3N4 catalyst had a significant influence on the overall reaction rate as well as the rAaeUPO inactivation rate. The form of g-C3N4 used so far are the so-called g-C3N4 sheets, obtained from calcination of urea. (55) Further thermal treatment process leads to so-called amorphous g-C3N4, exhibiting a higher surface area. (56) A preparation with lower specific surface area (bulk g-C3N4) was obtained by starting the synthesis from melamine instead of urea (Figure S1). In line with our previous observations that the photocatalytic H2O2 generation is overall rate-limiting, we also observed a correlation between surface area and overall reaction rate (Figure 6). The same, however, was also true for the inactivation rate of rAaeUPO, which increased with increasing surface area.

Figure 6

Figure 6. Influence of g-C3N4 morphology on the photoenzymatic hydroxylation of ethylbenzene. (A) Time course of (R)-1-phenyl ethanol formation and (B) time course of acetophenone formation. From these time courses, parameters such as reaction rate (C), rAaeUPO inactivation rate (D), maximal product concentration (E), and selectivity (F) were calculated. Amorphous g-C3N4(black), g-C3N4 sheets (red), or g-C3N4 bulk (green). Reaction conditions: [NaHCO2] = 250 mM, [ethylbenzene] = 50 mM, [g-C3N4] = 5 mg mL–1, [rAaeUPO] = 100 nM, KPi buffer pH 7.0 (100 mM), 30 °C, magnetic stirring at 600 rpm, illumination by an Osram 200W light bulb. Reactions were performed in independent duplicates.

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To evaluate the synthetic potential of the proposed photoenzymatic oxyfunctionalization system, we performed the reaction at semipreparative scale (Figure S7). To compensate for the high volatility of the starting material and the inactivation of the biocatalyst, both were replenished at intervals (see SI for details of the experimental procedure). In this experiment, 16.9 mM of essentially pure (97.8% ee) (R)-1-phenyl ethanol accumulated in the reaction mixture. From this, 42 mg of the desired product was isolated. Admittedly, this is not yet a practical protocol for the synthesis of (R)-1-phenyl ethanol. Especially, the high volatility of the reagents caused significant losses in the mass balance, which will have to be addressed in future, via improved experimental setups (circumventing the evaporation issue). Nevertheless, a respectable turnover number of more than 21 000 for the biocatalyst was obtained in this experiment.
The major bottleneck of the current system is its poor robustness; generally within 24 h, accumulation of (R)-1-phenyl ethanol ceased, which we attribute to the inactivation of the biocatalyst. As mentioned above, hydroxyl radicals may be assumed to be the primary products of g-C3N4-catalyzed water photo-oxidations. Indeed, using the spin trap method, (57) we could confirm the occurrence of hydroxyl radicals (Figure 7). The short lifetime of ·OH in aqueous media (58) suggests their predominant occurrence at the photocatalyst surface. We therefore also investigated whether g-C3N4 showed a tendency to absorb the biocatalyst. Indeed, we found that rAaeUPO and other proteins absorbed significantly to the polar surface of g-C3N4 (Figure 7). (59) Here they are exposed to locally high concentrations of hydroxyl radicals, which sufficiently explains the rather poor robustness of the photoenzymatic reactions so far. As mentioned above, the absorption per se did not result in inactivation of rAaeUPO (Figure S10).

Figure 7

Figure 7. Investigating the molecular reasons for the decreased rAaeUPO-stability under process conditions. (A) Detection of hydroxyl radicals formed by irradiated g-C3N4 using the spin-trap method. Signals marked with a star (★) are assigned to the oxidation product of DMPO, 5,5-dimethyl-2-oxopyrroline-1-oxyl (DMPOX). Signals marked with diamonds (◆) belong to the spin adduct DMPO–OH. (B) Protein in solution before (blue) or after (red) incubation with g-C3N4 in the dark, for bovine serum albumin (BSA) or rAaeUPO.

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We hypothesized that spatial separation of rAaeUPO from the photocatalyst may enhance the stability of the enzyme under reaction conditions. Therefore, we tested this hypothesis by placing rAaeUPO into a dialysis bag, thereby preventing its direct contact with the photocatalyst (Figure 8). Here, the UPO and g-C3N4 are contained, while both ethylbenzene as H2O2 could pass the membrane.

Figure 8

Figure 8. Time course of the photoenzymatic hydroxylation of ethylbenzene to (R)-1-phenyl ethanol (black) and overoxidation to acetophenone (red) using the dialysis bag approach. Conditions: 10 mL of reaction solution equally divided inside and outside the dialysis bag (20 kDa cutoff). Inside the bag: [NaHCO2] = 250 mM, [ethylbenzene] = 50 mM, [rAaeUPO] = 100 nM, KPi buffer pH 7.0 (100 mM). Outside the bag: [NaHCO2] = 250 mM, [ethylbenzene] = 50 mM, [g-C3N4] = 5 mg mL–1, KPi buffer pH 7.0 (100 mM). The reaction was performed once at room temperature while stirring at 600 rpm. The reaction solution was illuminated by a LIGHTNINGCURE spot light (Hamamatsu) at 50% intensity with an UV filter.

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To our delight, physically separating the biocatalyst from the photocatalyst had the desired effect of stable product accumulation for more than 4 days. Compared with the previous experiments, this corresponds to an improvement by more than 4 times. However, this improvement came at the expense of a significantly decreased reaction rate, which is most likely to be attributed to diffusion limitations for the substrates over the dialysis membrane (Figure S4).
Overall, in this contribution we have expanded the scope of photoenzymatic oxyfunctionalization reactions to a broader range of heterogeneous photocatalysts. g-C3N4 appeared to be a good alternative to the established TiO2-based photocatalysts. Total turnover numbers of over 60.000 have been achieved for the rAaeUPO. The major limitations identified in this study were (1) the relatively low specific H2O2-generation rate of the photocatalysts and (2) the undesired inactivation of the biocatalysts at the photocatalyst surface. The first limitation can be addressed by further optimizing the catalyst and light intensity. The catalyst can be improved by chemically modifying the g-C3N4 (e.g., doping with donor- or acceptor-type dopants may be successful). (60) Some preliminary results using KOH- (61) or Co2O3-modified (62) g-C3N4 indeed demonstrated that doping can significantly influence the H2O2-generation activity. Further systematic studies will validate this approach.
The second limitation (i.e., the oxidative inactivation of the biocatalyst by surface-borne reactive oxygen species) can be alleviated by physical separation. To circumvent the massive diffusion limitations observed in this double-heterogeneous reaction system, a linear plug-flow reactor concept may prove to be beneficial.

Experimental Section

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Catalysts Preparation

rAaeUPO was produced by heterologous expression in Pichia pastoris performed following a previously reported procedure. (38)
g-C3N4 was synthesized either from urea or melamine as starting compound by heating it up in a furnace to 550 °C (heat ramp: 5 °C min–1). In a typical procedure, from 10 g of urea, approximately 0.5 g of g-C3N4 sheets was obtained, whereas from 10 g of melamine, approximately 2 g of g-C3N4 bulk was obtained. Amorphous g-C3N4 was synthesized by further calcination of g-C3N4 sheets to 620 °C under an inert argon atmosphere. (63)
All other photocatalysts were either commercially available or prepared following literature procedures.

Reaction Setup

Unless stated differently, reactions were performed in 4 mL glass vials. Two milliliter reaction mixtures were stirred at 600 rpm using a small stirring magnet (6 mm). The vials were placed around an incandescent white light bulb (Osram, 205W Halolux Ceram) at a distance of approximately 1 cm. The water bath was continuously cooled at 30 °C. Reaction mixtures generally contained 5 mg mL–1 g-C3N4, 100 nM rAaeUPO, and 50 mM ethylbenzene in a 100 mM KPi buffer at pH 7.0. Sacrificial electron donors were added to a concentration of 250 mM. Before use, the g-C3N4 was dispersed via sonication in 1 mL of the phosphate buffer. Samples were taken using a syringe and needle, keeping the reactors closed and preventing evaporation of the ethylbenzene. The reaction mixture (200 μL) was taken and extracted with an aliquot of ethyl acetate containing 5 mM octanol as internal standard. The mixtures were intensively mixed for 10 s, centrifuged for 2 min, and the organic phase was dried over magnesium sulfate and subsequently analyzed via achiral CG chromatography (CP-WAX 52 CB). Optical purities were determined using chiral GC (CP-Chirasil-Dex CB).

Scale-Up Reaction

A 24 mL reaction, divided over 2 mL samples, was illuminated 80 h for increased product titers. The reaction was performed at 30 °C in a KPi buffer (100 mM, pH 7.0) containing ethylbenzene (50 mM), g-C3N4 (5 mg mL–1), rAaeUPO (100 nM), and sodium formate (250 mM). rAaeUPO (100 nM) was added to the reaction every 12 h, and the ethylbenzene was replenished by adding 50 mM extra substrate after 36 h. At the end of the reaction, the compound was extracted with ethyl acetate, dried with MgSO4, and purified under reduced pressure at room temperature, also evaporating remaining substrate. The purity of the product was determined by 1H NMR, while optical purity was determined using chiral GC (CP-Chirasil-Dex CB). NMR spectra were recorded on an Agilent 400 spectrometer in CDCl3. Chemical shifts are given in ppm with respect to tetramethylsilane. Coupling constants are reported as J-values in Hz (s: singlet. d: doublet. t: triplet. q: quartet. m: multiplet. br: broad). 1H NMR (400 MHz, CDCl3) δ 7.40–7.26 (m, 5H), 4.91 (q, J = 6.5 Hz, 1H), 1.82 (br s, 1H, −OH), 1.51 (d, J = 6.4 Hz, 3H). 13C NMR (100 MHz, CDCl3) δ 128.5, 127.5, 125.4, 70.4, 25.10.

Data Manipulation

As the reaction rate, the initial formation rate of phenyl ethanol was taken, considering at least three data points. To estimate the enzyme deactivation rate, the following formula was used:
With:
The reaction selectivity was calculated by dividing the maximum amount of phenyl ethanol reached by the total amount of product formed in that same point in time.

Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.9b01341.

  • Preparation of the catalysts, reaction setup, analytical data, and additional results (PDF)

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Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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  • Corresponding Authors
    • Morten M. C. H. van Schie - Department of Biotechnology, Delft University of Technology, van der Maasweg 9, 2629HZ Delft, The Netherlands;  Email: [email protected]
    • Frank Hollmann - Department of Biotechnology, Delft University of Technology, van der Maasweg 9, 2629HZ Delft, The Netherlands;  Orcidhttp://orcid.org/0000-0003-4821-756X;  Email: [email protected]
  • Authors
    • Wuyuan Zhang - Department of Biotechnology, Delft University of Technology, van der Maasweg 9, 2629HZ Delft, The Netherlands;  Orcidhttp://orcid.org/0000-0002-3182-5107
    • Florian Tieves - Department of Biotechnology, Delft University of Technology, van der Maasweg 9, 2629HZ Delft, The Netherlands
    • Da Som Choi - Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), 335 Science Road, Daejeon 305-701, Republic of Korea
    • Chan Beum Park - Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), 335 Science Road, Daejeon 305-701, Republic of Korea;  Orcidhttp://orcid.org/0000-0002-0767-8629
    • Bastien O. Burek - DECHEMA Forschungsinstitut, Theodor-Heuss-Allee 25, 60486 Frankfurt am Main, Germany;  Orcidhttp://orcid.org/0000-0002-2180-7458
    • Jonathan Z. Bloh - DECHEMA Forschungsinstitut, Theodor-Heuss-Allee 25, 60486 Frankfurt am Main, Germany
    • Isabel W. C. E. Arends - University of Utrecht, Faculty of Science, Budapestlaan 6, 3584 CD Utrecht, The Netherlands
    • Caroline E. Paul - Department of Biotechnology, Delft University of Technology, van der Maasweg 9, 2629HZ Delft, The Netherlands;  Orcidhttp://orcid.org/0000-0002-7889-9920
    • Miguel Alcalde - Department of Biocatalysis, Institute of Catalysis, CSIC, 28049 Madrid, Spain;  Orcidhttp://orcid.org/0000-0001-6780-7616
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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We thank The Netherlands Organization for Scientific Research for financial support through a VICI grant (No. 724.014.003) and by the European Union Project H2020-BBI-PPP-2015-2-720297-ENZOX2. CEP acknowledges a VENI grant (No. 722.015.011).

References

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    Wang, Y.; Lan, D.; Durrani, R.; Hollmann, F. Peroxygenases en route to becoming dream catalysts. What are the Opportunities and Challenges?. Curr. Opin. Chem. Biol. 2017, 37, 1– 9,  DOI: 10.1016/j.cbpa.2016.10.007
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    Peroxygenases en route to becoming dream catalysts. What are the opportunities and challenges?
    Wang, Yonghua; Lan, Dongming; Durrani, Rabia; Hollmann, Frank
    Current Opinion in Chemical Biology (2017), 37 (), 1-9CODEN: COCBF4; ISSN:1367-5931. (Elsevier B.V.)
    Peroxygenases are promising catalysts for preparative oxyfunctionalization chem. as they combine the versatility of P 450 monooxygenases with simplicity of cofactor-independent enzymes. Though many interesting applications have been reported, today 'we have only scratched the surface' and significant efforts are necessary to solve issues related to selectivity of the wild type enzymes and low product titers. For this, further elucidation of the vast natural diversity as well as protein and reaction engineering approaches are discussed.
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  4. 4
    Martínez, A. T.; Ruiz-Dueñas, F. J.; Camarero, S.; Serrano, A.; Linde, D.; Lund, H.; Vind, J.; Tovborg, M.; Herold-Majumdar, O. M.; Hofrichter, M.; Liers, C.; Ullrich, R.; Scheibner, K.; Sannia, G.; Piscitelli, A.; Pezzella, C.; Sener, M. E.; Kılıç, S.; van Berkel, W. J. H.; Guallar, V.; Lucas, M. F.; Zuhse, R.; Ludwig, R.; Hollmann, F.; Fernández-Fueyo, E.; Record, E.; Faulds, C. B.; Tortajada, M.; Winckelmann, I.; Rasmussen, J.-A.; Gelo-Pujic, M.; Gutiérrez, A.; del Río, J. C.; Rencoret, J.; Alcalde, M. Oxidoreductases on their way to Industrial Biotransformations. Biotechnol. Adv. 2017, 35, 815– 831,  DOI: 10.1016/j.biotechadv.2017.06.003
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    Oxidoreductases on their way to industrial biotransformations
    Martinez, Angel T.; Ruiz-Duenas, Francisco J.; Camarero, Susana; Serrano, Ana; Linde, Dolores; Lund, Henrik; Vind, Jesper; Tovborg, Morten; Herold-Majumdar, Owik M.; Hofrichter, Martin; Liers, Christiane; Ullrich, Rene; Scheibner, Katrin; Sannia, Giovanni; Piscitelli, Alessandra; Pezzella, Cinzia; Sener, Mehmet E.; Kilic, Sibel; van Berkel, Willem J. H.; Guallar, Victor; Lucas, Maria Fatima; Zuhse, Ralf; Ludwig, Roland; Hollmann, Frank; Fernandez-Fueyo, Elena; Record, Eric; Faulds, Craig B.; Tortajada, Marta; Winckelmann, Ib; Rasmussen, Jo-Anne; Gelo-Pujic, Mirjana; Gutierrez, Ana; del Rio, Jose C.; Rencoret, Jorge; Alcalde, Miguel
    Biotechnology Advances (2017), 35 (6), 815-831CODEN: BIADDD; ISSN:0734-9750. (Elsevier)
    Fungi produce heme-contg. peroxidases and peroxygenases, flavin-contg. oxidases and dehydrogenases, and different copper-contg. oxidoreductases involved in the biodegrdn. of lignin and other recalcitrant compds. Heme peroxidases comprise the classical ligninolytic peroxidases and the new dye-decolorizing peroxidases, while heme peroxygenases belong to a still largely unexplored superfamily of heme-thiolate proteins. Nevertheless, basidiomycete unspecific peroxygenases have the highest biotechnol. interest due to their ability to catalyze a variety of regio- and stereo-selective monooxygenation reactions with H2O2 as the source of oxygen and final electron acceptor. Flavo-oxidases are involved in both lignin and cellulose decay generating H2O2 that activates peroxidases and generates hydroxyl radical. The group of copper oxidoreductases also includes other H2O2 generating enzymes - copper-radical oxidases - together with classical laccases that are the oxidoreductases with the largest no. of reported applications to date. However, the recently described lytic polysaccharide monooxygenases have attracted the highest attention among copper oxidoreductases, since they are capable of oxidatively breaking down cryst. cellulose, the disintegration of which is still a major bottleneck in lignocellulose biorefineries, along with lignin degrdn. Interestingly, some flavin-contg. dehydrogenases also play a key role in cellulose breakdown by directly/indirectly "fueling" electrons for polysaccharide monooxygenase activation. Many of the above oxidoreductases have been engineered, combining rational and computational design with directed evolution, to attain the selectivity, catalytic efficiency and stability properties required for their industrial utilization. Indeed, using ad hoc software and current computational capabilities, it is now possible to predict substrate access to the active site in biophys. simulations, and electron transfer efficiency in biochem. simulations, reducing in orders of magnitude the time of exptl. work in oxidoreductase screening and engineering. What has been set out above is illustrated by a series of remarkable oxyfunctionalization and oxidn. reactions developed in the frame of an intersectorial and multidisciplinary European RTD project. The optimized reactions include enzymic synthesis of 1-naphthol, 25-hydroxyvitamin D3, drug metabolites, furandicarboxylic acid, indigo and other dyes, and conductive polyaniline, terminal oxygenation of alkanes, biomass delignification and lignin oxidn., among others. These successful case stories demonstrate the unexploited potential of oxidoreductases in medium and large-scale biotransformations.
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  5. 5
    Hofrichter, M.; Ullrich, R. Oxidations Catalyzed by Fungal Peroxygenases. Curr. Opin. Chem. Biol. 2014, 19, 116– 125,  DOI: 10.1016/j.cbpa.2014.01.015
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    5
    Oxidations catalyzed by fungal peroxygenases
    Hofrichter, Martin; Ullrich, Rene
    Current Opinion in Chemical Biology (2014), 19 (), 116-125CODEN: COCBF4; ISSN:1367-5931. (Elsevier B.V.)
    A review. The enzymic oxyfunctionalization of org. mols. under physiol. conditions has attracted keen interest from the chem. community. Unspecific peroxygenases (EC 1.11.2.1) secreted by fungi represent an intriguing enzyme type that selectively transfers peroxide-borne oxygen with high efficiency to diverse substrates including unactivated hydrocarbons. They are glycosylated heme-thiolate enzymes that form a sep. superfamily of hemoproteins. Among the catalyzed reactions are hydroxylations, epoxidns., dealkylations, oxidns. of org. hetero atoms, and inorg. halides, as well as one-electron oxidns. The substrate spectrum of fungal peroxygenases and the product patterns show similarities both to cytochrome P 450 monooxygenases and classic heme peroxidases. Given that selective oxyfunctionalizations are among the most difficult to realize chem. reactions and that resp. transformed mols. are of general importance in org. and pharmaceutical syntheses, it will be worth developing peroxygenase biocatalysts for industrial applications.
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  6. 6
    Holtmann, D.; Hollmann, F. The Oxygen Dilemma: A Severe Challenge for the Application of Monooxygenases?. ChemBioChem 2016, 17, 1391– 1398,  DOI: 10.1002/cbic.201600176
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    The Oxygen Dilemma: A Severe Challenge for the Application of Monooxygenases?
    Holtmann, Dirk; Hollmann, Frank
    ChemBioChem (2016), 17 (15), 1391-1398CODEN: CBCHFX; ISSN:1439-4227. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. Monooxygenases are promising catalysts because they in principle enable the org. chemist to perform highly selective oxyfunctionalisation reactions that are otherwise difficult to achieve. For this, monooxygenases require reducing equiv., to allow reductive activation of mol. oxygen at the enzymes' active sites. However, these reducing equiv. are often delivered to O2 either directly or via a reduced intermediate (uncoupling), yielding hazardous reactive oxygen species and wasting valuable reducing equiv. The oxygen dilemma arises from monooxygenases' dependency on O2 and the undesired uncoupling reaction. With this contribution we hope to generate a general awareness of the oxygen dilemma and to discuss its nature and some promising solns.
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  7. 7
    Valderrama, B.; Ayala, M.; Vazquez-Duhalt, R. Suicide Inactivation of Peroxidases and the Challenge of Engineering More Robust Enzymes. Chem. Biol. 2002, 9, 555– 565,  DOI: 10.1016/S1074-5521(02)00149-7
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    Suicide inactivation of peroxidases and the challenge of engineering more robust enzymes
    Valderrama, Brenda; Ayala, Marcela; Vazquez-Duhalt, Rafael
    Chemistry & Biology (2002), 9 (5), 555-565CODEN: CBOLE2; ISSN:1074-5521. (Cell Press)
    A review with 171 refs. As the no. of industrial applications for proteins continues to expand, the exploitation of protein engineering becomes crit. It is predicted that protein engineering can generate enzymes with new catalytic properties and create desirable, high-value, products at lower prodn. costs. Peroxidases are ubiquitous enzymes that catalyze a variety of O2-transfer reactions and are thus potentially useful for industrial and biomedical applications. However, peroxidases are unstable and are readily inactivated by their substrate, H2O2. Researchers rely on the powerful tools of mol. biol. to improve the stability of these enzymes, either by protecting residues sensitive to oxidn. or by devising more efficient intramol. pathways for free-radical allocation. Here, the authors discuss the catalytic cycle of peroxidases and the mechanism of the suicide inactivation process to establish a broad knowledge base for future rational protein engineering.
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    Burek, B. O. O.; Bormann, S.; Hollmann, F.; Bloh, J.; Holtmann, D. Hydrogen peroxide Driven Biocatalysis. Green Chem. 2019, 21, 3232– 3249,  DOI: 10.1039/C9GC00633H
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    Hydrogen peroxide driven biocatalysis
    Burek, B. O.; Bormann, S.; Hollmann, F.; Bloh, J. Z.; Holtmann, D.
    Green Chemistry (2019), 21 (12), 3232-3249CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
    A review. In general, hydrogen peroxide is a stable and relatively mild oxidant and it can be regarded as the ultimate "green" reagent because water and oxygen are the only byproducts. Besides the direct application of H2O2 in chem. processes more and more enzymic syntheses based on hydrogen peroxide are being developed. Different types of reactions can be addressed by using a hydrogen-peroxide driven biocatalysis (e.g. hydroxylations, epoxidns., sulfoxidns., halogenations, Baeyer-Villiger oxidns., decarboxylations). H2O2-driven reactions can often be used to substitute NAD(P)H dependent reactions. Therefore, laborious cofactor regeneration systems can be avoided by using H2O2-dependent enzymes. The tremendous increase in the no. of publications dealing with this type of reactions clearly demonstrates the progress in this area in recent years. The described innovations range from new enzymes and types of reaction to novel reaction engineering approaches. This review aims to give the scope of possible advantageous applications of peroxyzymes and a crit. discussion of their current limitations. The versatile reactions, the ecol. advantageous and the great progress in the discovery and engineering of novel enzymes make a tech. use feasible.
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  9. 9
    Paul, C. E.; Churakova, E.; Maurits, E.; Girhard, M.; Urlacher, V. B.; Hollmann, F. In situ formation of H2O2 for P450 Peroxygenases. Bioorg. Med. Chem. 2014, 22, 5692– 5696,  DOI: 10.1016/j.bmc.2014.05.074
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    In situ formation of H2O2 for P450 peroxygenases
    Paul, Caroline E.; Churakova, Ekaterina; Maurits, Elmer; Girhard, Marco; Urlacher, Vlada B.; Hollmann, Frank
    Bioorganic & Medicinal Chemistry (2014), 22 (20), 5692-5696CODEN: BMECEP; ISSN:0968-0896. (Elsevier B.V.)
    An in situ H2O2 generation approach to promote P 450 peroxygenases catalysis was developed through the use of the nicotinamide cofactor analog 1-benzyl-1,4-dihydronicotinamide (BNAH) and FMN. Final productivity could be enhanced due to higher enzyme stability at low H2O2 concns. The H2O2 generation represented the rate-limiting step, however it could be easily controlled by varying both FMN and BNAH concns. Further characterization can result in an optimized ratio of FMN/BNAH/O2/biocatalyst enabling high reaction rates while minimizing H2O2-related inactivation of the enzyme.
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  10. 10
    Hollmann, F.; Schmid, A. Towards [Cp*Rh(bpy)(H2O)]2+-promoted P450 Catalysis: Direct Regeneration of CytC. J. Inorg. Biochem. 2009, 103, 313– 315,  DOI: 10.1016/j.jinorgbio.2008.11.005
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    10
    Towards [Cp*Rh(bpy)(H2O)]2+-promoted P450 catalysis: Direct regeneration of CytC
    Hollmann, Frank; Schmid, Andreas
    Journal of Inorganic Biochemistry (2009), 103 (3), 313-315CODEN: JIBIDJ; ISSN:0162-0134. (Elsevier B.V.)
    The organometallic complex pentamethylcyclopentadienyl rhodium 2,2'-bipyridine ([Cp*Rh(bpy)(H2O)]2+) was applied as regeneration catalyst for cytochrome C (CytC). Direct redn. of CytC-bound FeIII was achieved in this model system pointing towards a potential usefulness of this concept to promote cell-free P 450 catalysis. In addn., controlled in situ provision with hydrogen peroxide was performed using [Cp*Rh(bpy)(H2O)]2+ resulting in improved CytC-catalyzed sulfoxidn. of thioanisol. This work represents the first step towards the direct-[Cp*Rh(bpy)(H2O)]2+ catalyzed regeneration of P 450 monooxygenases and peroxidases.
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    Karmee, S. K.; Roosen, C.; Kohlmann, C.; Lütz, S.; Greiner, L.; Leitner, W. Chemo-Enzymatic Cascade Oxidation in Supercritical Carbon Dioxide/Water Biphasic Media. Green Chem. 2009, 11, 1052– 1055,  DOI: 10.1039/b820606f
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    11
    Chemo-enzymatic cascade oxidation in supercritical carbon dioxide/water biphasic media
    Karmee, Sanjib Kumar; Roosen, Christoph; Kohlmann, Christina; Luetz, Stephan; Greiner, Lasse; Leitner, Walter
    Green Chemistry (2009), 11 (7), 1052-1055CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
    Enantioselective sulfoxidn. was carried out by cascade reaction of Pd(0)-catalyzed formation of H2O2 and enzymic oxidn. using chloroperoxidase from Caldariomyces fumago. Supercrit. CO2 (scCO2) was used as medium for in-situ generation of H2O2 directly from H2 and O2 using Pd-catalysts. Subsequently, H2O2 was utilized by the chloroperoxidase as an oxidant for the asym. sulfoxidn. in the aq. phase. This chemo-enzymic cascade transformation exemplifies the potential of compartmentalization of catalytic processes in multiphase systems.
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    Ranganathan, S.; Sieber, V. Recent Advances in the Direct Synthesis of Hydrogen Peroxide Using Chemical Catalysis—A Review. Catalysts 2018, 8, 379,  DOI: 10.3390/catal8090379
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    Recent advances in the direct synthesis of hydrogen peroxide using chemical Catalysis-A review
    Ranganathan, Sumanth; Sieber, Volker
    Catalysts (2018), 8 (9), 379/1-379/22CODEN: CATACJ; ISSN:2073-4344. (MDPI AG)
    Hydrogen peroxide is an important chem. of increasing demand in today's world. Currently, the anthraquinone autoxidn. process dominates the industrial prodn. of hydrogen peroxide. Herein, hydrogen and oxygen are reacted indirectly in the presence of quinones to yield hydrogen peroxide. Owing to the complexity and multi-step nature of the process, it is advantageous to replace the process with an easier and straightforward one. The direct synthesis of hydrogen peroxide from its constituent reagents is an effective and clean route to achieve this goal. Factors such as water formation due to thermodn., explosion risk, and the stability of the hydrogen peroxide produced hinder the applicability of this process at an industrial level. Currently, the catalysis for the direct synthesis reaction is palladium based and the research into finding an effective and active catalyst has been ongoing for more than a century now. Palladium in its pure form, or alloyed with certain metals, are some of the new generation of catalysts that are extensively researched. Addnl., to prevent the decompn. of hydrogen peroxide to water, the process is stabilized by adding certain promoters such as mineral acids and halides. A major part of today's research in this field focusses on the reactor and the mode of operation required for synthesizing hydrogen peroxide. The emergence of microreactor technol. has helped in setting up this synthesis in a continuous mode, which could possibly replace the anthraquinone process in the near future. This review will focus on the recent findings of the scientific community in terms of reaction engineering, catalyst and reactor design in the direct synthesis of hydrogen peroxide.
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    Ranganathan, S.; Zeitlhofer, S.; Sieber, V. Development of a Lipase-Mediated Epoxidation Process for Monoterpenes in Choline Chloride-based Deep Eutectic Solvents. Green Chem. 2017, 19, 2576– 2586,  DOI: 10.1039/C7GC01127J
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    13
    Development of a lipase-mediated epoxidation process for monoterpenes in choline chloride-based deep eutectic solvents
    Ranganathan, Sumanth; Zeitlhofer, Sandra; Sieber, Volker
    Green Chemistry (2017), 19 (11), 2576-2586CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
    Chem. syntheses in contemporary process industries today are predominantly conducted using org. solvents, which are potentially hazardous to humans and the environment alike. Green chem. was developed as a means to overcome this hazard and it also holds enormous potential for designing clean, safe and sustainable processes. The present work incorporates the concepts of green chem. in its design of a lipase-mediated epoxidn. process for monoterpenes; the process uses alternative reaction media, namely deep eutectic solvents (DESs), which have not been reported for such an application before. Choline chloride (ChCl), in combination with a variety of hydrogen bond donors (HBD) at certain molar ratios, was screened and tested for this purpose. The process was optimized through the design of expts. (DoE) using the Taguchi method for four controllable parameters (temp., enzyme amt., peroxide amt. and type of substrate) and one uncontrollable parameter (DES reaction media) in a crossed-array design. Two distinct DESs, namely glycerol : choline chloride (GlCh) and sorbitol : choline chloride (SoCh), were found to be the best systems and they resulted in a complete conversion of the substrates within 8 h. Impurities (esters) were found to form in both the DESs, which was a concern; as such, we developed a novel minimal DES system that incorporated a co-substrate into the DES so that this issue could be overcome. The minimal DES consisted of urea·H2O2 (U·H2O2) and ChCl and exhibited better results than both the GlCh and SoCh systems; complete conversions were achieved within 2 h for 3-carene and within 3 h for both limonene and α-pinene. Product isolation with a simple water/ethyl acetate based procedure gave isolated yields of 87.2 ± 2.4%, 77.0 ± 5.0% and 84.6 ± 3.7% for 3-carene, limonene and α-pinene resp.
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    Ranganathan, S.; Sieber, V. Development of Semi-Continuous Chemo-Enzymatic Terpene Epoxidation: Combination of Anthraquinone Autooxidation and the Lipase-Mediated Epoxidation Process. React. Chem. Eng. 2017, 2, 885– 895,  DOI: 10.1039/C7RE00112F
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    14
    Development of semi-continuous chemo-enzymatic terpene epoxidation: combination of anthraquinone autooxidation and the lipase-mediated epoxidation process
    Ranganathan, Sumanth; Sieber, Volker
    Reaction Chemistry & Engineering (2017), 2 (6), 885-895CODEN: RCEEBW; ISSN:2058-9883. (Royal Society of Chemistry)
    Lipase has been used for epoxidizing olefins such as monoterpenes for more than two decades. This epoxidn. is accomplished by adding hydrogen peroxide (H2O2) to a carboxylic acid in the presence of a lipase such as Candida antartica lipase B (CALB) to produce percarboxylic acid, which then epoxidized monoterpenes according to the Prilezhaev mechanism. One drawback of this process is the need for continuous addn. of hydrogen peroxide to maintain max. productivity. To overcome this hurdle, the industrial anthraquinone autoxidn. process for hydrogen peroxide prodn. was scaled down and coupled with lipase-mediated epoxidn. in a semi-continuous manner. Palladium on alumina pellets (5% loading) was used as the catalyst for obtaining high yields of high-concn. hydrogen peroxide (50% wt. by vol.), followed by epoxidn. of 3-carene, (+) limonene, and α-pinene. A total reaction time of 5 h was used for hydrogen peroxide prodn. and 2-3 h for the epoxidn. reactions. Pure 3-carene epoxide and α-pinene epoxide were obtained in isolated yields of 88.8 ± 2.8% and 83.8 ± 2.6%, resp. Limonene epoxide was obtained as a mixt. of mono- and di-epoxides in a ratio of 70% and 30%, resp., with an isolated yield of 71.5 ± 3.1%.
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    Ranganathan, S.; Tebbe, J.; Wiemann, L.; Sieber, V. Optimization of the Lipase Mediated Epoxidation of Monoterpenes using the Design of Experiments-Taguchi Method. Process Biochem. 2016, 51, 1479– 1485,  DOI: 10.1016/j.procbio.2016.07.005
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    15
    Optimization of the lipase mediated epoxidation of monoterpenes using the design of experiments-Taguchi method
    Ranganathan, Sumanth; Tebbe, Johannes; Wiemann, Lars O.; Sieber, Volker
    Process Biochemistry (Oxford, United Kingdom) (2016), 51 (10), 1479-1485CODEN: PBCHE5; ISSN:1359-5113. (Elsevier Ltd.)
    This work deals with the optimization of the Candida antarctica lipase B (CALB) mediated epoxidn. of monoterpenes by using the design of expts. (DoE) working with the Taguchi Method. Epoxides are essential org. intermediates that find various industrial applications making epoxidn. one of the most investigated processes in chem. industry. As many as 8 parameters such as the reaction medium, carboxylic acid type, carboxylic acid concn., temp., monoterpene type, monoterpene concn., hydrogen peroxide concn. and amt. of lipase were optimized using as little as 18 runs in triplicates (54 runs). As a result, the hydrogen peroxide concn. used was found to be the most influential parameter of this process while the type of monoterpene was least influential. Scaling up of the reaction conditions according to the findings of the optimization achieved full conversion in less than 6 h. In addn., a purifn. process for the epoxides was developed leading to an isolated yield of ca. 72.3%, 88.8% and 62.5% for α-pinene, 3-carene and limonene, resp.
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    Kohlmann, C.; Lütz, S. Electroenzymatic Synthesis of Chiral Sulfoxides. Eng. Life Sci. 2006, 6, 170– 174,  DOI: 10.1002/elsc.200620907
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    Electroenzymatic synthesis of chiral sulfoxides
    Kohlmann, C.; Luetz, S.
    Engineering in Life Sciences (2006), 6 (2), 170-174CODEN: ELSNAE; ISSN:1618-0240. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Chloroperoxidase (CPO) from Caldariomyces fumago (E.C.1.11.1.10) is able to enantioselectively oxidize various sulfides to the corresponding (R)-enantiomer of the sulfoxides. For these oxidns. the enzyme requires an oxidant. Most commonly, tert-Bu hydroperoxide (TBHP) and hydrogen peroxide are used. As it is known that these oxidants inactivate the enzyme, the enzymic reaction was combined with the electrochem. in situ generation of hydrogen peroxide. As substrates for this combination of an enzymic and an electrochem. reaction Me p-tolyl sulfide, 1-methoxy-4-(methylthio)benzene and N-MOC-L-methionine Me ester were used to carry out batch expts.
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    Lutz, S.; Steckhan, E.; Liese, A. First Asymmetric Electroenzymatic Oxidation Catalyzed by a Peroxidase. Electrochem. Commun. 2004, 6, 583– 587,  DOI: 10.1016/j.elecom.2004.04.009
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    First asymmetric electroenzymatic oxidation catalyzed by a peroxidase
    Lutz, Stephan; Steckhan, Eberhard; Liese, Andreas
    Electrochemistry Communications (2004), 6 (6), 583-587CODEN: ECCMF9; ISSN:1388-2481. (Elsevier Science B.V.)
    Thioanisole is selectively oxidized to (R)-methylphenylsulfoxide (ee > 98.5%) with electrochem. generated hydrogen peroxide catalyzed by chloroperoxidase (E.C. 1.11.1.10) from Caldariomyces fumago. Hydrogen peroxide is generated in situ by cathodic redn. of oxygen. This is the first example of an asym. electroenzymic synthesis with a peroxidase. The reaction was carried out on 300 mL scale with a productivity of 30 g L-1 d-1.
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    Horst, A. E. W.; Bormann, S.; Meyer, J.; Steinhagen, M.; Ludwig, R.; Drews, A.; Ansorge-Schumacher, M.; Holtmann, D. Electro-Enzymatic Hydroxylation of Ethylbenzene by the Evolved Unspecific Peroxygenase of Agrocybeaegerita. J. Mol. Catal. B: Enzym. 2016, 133, S137– S142,  DOI: 10.1016/j.molcatb.2016.12.008
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    Holtmann, D.; Krieg, T.; Getrey, L.; Schrader, J. Electroenzymatic Process to Overcome Enzyme Instabilities. Catal. Commun. 2014, 51, 82– 85,  DOI: 10.1016/j.catcom.2014.03.033
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    Electroenzymatic process to overcome enzyme instabilities
    Holtmann, Dirk; Krieg, Thomas; Getrey, Laura; Schrader, Jens
    Catalysis Communications (2014), 51 (), 82-85CODEN: CCAOAC; ISSN:1566-7367. (Elsevier B.V.)
    The versatile enzyme chloroperoxidase was used in a reaction system, based on a gas diffusion electrode, for enzymic chlorinations. Due to an adjusted and continuous electro-generation of the co-substrate hydrogen peroxide a ttn up to 1,150,000 for the CPO was achieved. Space time yields were dependent on the electrochem. produced H2O2 and reached up to 52 g L- 1 d- 1. The ratio of hydrogen peroxide prodn. per added enzyme unit can be used as a dimensionless parameter for process characterization and a knowledge based process design.
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    Getrey, L.; Krieg, T.; Hollmann, F.; Schrader, J.; Holtmann, D. Enzymatic Halogenation of the Phenolic Monoterpenes Thymol and Carvacrol with Chloroperoxidase. Green Chem. 2014, 16, 1104– 1108,  DOI: 10.1039/C3GC42269K
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    Enzymatic halogenation of the phenolic monoterpenes thymol and carvacrol with chloroperoxidase
    Getrey, Laura; Krieg, Thomas; Hollmann, Frank; Schrader, Jens; Holtmann, Dirk
    Green Chemistry (2014), 16 (3), 1104-1108CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
    The conversion of the phenolic monoterpenes thymol and carvacrol into antimicrobials by (electro)-chemoenzymic halogenation was investigated using a chloroperoxidase (CPO) catalyzed process. The CPO catalyzed process enables for the first time the biotechnol. prodn. of chlorothymol, chlorocarvacrol and bromothymol as well as a dichlorothymol with high conversion rates, total turnover nos. and space time yields of up to 90%, 164 000 and 4.6 mM h-1, resp.
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    Krieg, T.; Huttmann, S.; Mangold, K.-M.; Schrader, J.; Holtmann, D. Gas Diffusion Electrode as Novel Reaction System for an Electro-Enzymatic Process with Chloroperoxidase. Green Chem. 2011, 13, 2686– 2689,  DOI: 10.1039/c1gc15391a
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    Gas diffusion electrode as novel reaction system for an electro-enzymatic process with chloroperoxidase
    Krieg, Thomas; Huettmann, Sonja; Mangold, Klaus-Michael; Schrader, Jens; Holtmann, Dirk
    Green Chemistry (2011), 13 (10), 2686-2689CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
    The versatile enzyme chloroperoxidase was used in a new reaction system, based on a gas diffusion electrode, for enzymic chlorinations, sulfoxidns. and oxidns. This is the first report on the combination of hydrogen peroxide prodn. at a GDE with an enzymic reaction.
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    Pereira, P. C.; Arends, I.; Sheldon, R. A. Optimizing the Chloroperoxidase-Glucose Oxidase System: The Effect of Glucose Oxidase on Activity and Enantioselectivity. Process Biochem. 2015, 50, 746– 751,  DOI: 10.1016/j.procbio.2015.02.006
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    Optimizing the chloroperoxidase-glucose oxidase system: The effect of glucose oxidase on activity and enantioselectivity
    Pereira, Pedro C.; Arends, Isabel W. C. E.; Sheldon, Roger A.
    Process Biochemistry (Oxford, United Kingdom) (2015), 50 (5), 746-751CODEN: PBCHE5; ISSN:1359-5113. (Elsevier Ltd.)
    The optimum application of chloroperoxidase from Caldariomyces fumago in oxidns. with hydrogen peroxide depends on the mode of addn. of the oxidant. The use of the previously reported combination of chloroperoxidase and glucose oxidase, for in situ generation of hydrogen peroxide, was studied in more detail using thioanisole as a model substrate. Maximum yields and enantiopurities were obsd. at high chloroperoxidase reaction rates and not at low hydrogen peroxide formation rates, as would be expected considering the instability of CPO at high hydrogen peroxide concns. Glucose oxidase catalyzed aerobic sulfoxidn., affording racemic sulfoxide, was obsd. as an unexpected and novel side-reaction. It was attributed to oxidn. by a flavin hydroperoxide formed by reaction of the free flavin cofactor assocd. with glucose oxidase with dioxygen. The rate of this side-reaction depended on the amt. of co-solvent in the system and the enantiopurity of the oxidn. product could thus be improved by lowering the co-solvent concn.
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    Tieves, F.; Willot, S. J.-P.; van Schie, M. M. C. H.; Rauch, M. C. R.; Younes, S. H. H.; Zhang, W.; Dong, J.; de Santos, P. G.; Robbins, J. M.; Bommarius, B.; Alcalde, M.; Bommarius, A.; Hollmann, F. Formate Oxidase (FOx) from Aspergillus oryzae: One Catalyst to Promote H2O2-Dependent Biocatalytic Oxidation Reactions. Angew. Chem., Int. Ed. 2019, 58, 7873– 7877,  DOI: 10.1002/anie.201902380
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    Formate oxidase (FOx) from Aspergillus oryzae: One catalyst enables diverse H2O2-dependent biocatalytic oxidation reactions
    Tieves, Florian; Willot, Sebastien Jean-Paul; van Schie, Morten Martinus Cornelis Harald; Rauch, Marine Charlene Renee; Younes, Sabry Hamdy Hamed; Zhang, Wuyuan; Dong, JiaJia; Gomez de Santos, Patricia; Robbins, John Mick; Bommarius, Bettina; Alcalde, Miguel; Bommarius, Andreas Sebastian; Hollmann, Frank
    Angewandte Chemie, International Edition (2019), 58 (23), 7873-7877CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    An increasing no. of biocatalytic oxidn. reactions rely on H2O2 as a clean oxidant. The poor robustness of most enzymes towards H2O2, however, necessitates more efficient systems for in situ H2O2 generation. In analogy to the well-known formate dehydrogenase to promote NADH-dependent reactions, we here propose employing formate oxidase (FOx) to promote H2O2-dependent enzymic oxidn. reactions. Even under non-optimized conditions, high turnover nos. for coupled FOx/peroxygenase catalysis were achieved.
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    Pesic, M.; Willot, S. J.-P.; Fernández-Fueyo, E.; Tieves, F.; Alcalde, M.; Hollmann, F. Multienzymatic in situ Hydrogen Peroxide Generation Cascade for Peroxygenase-Catalysed Oxyfunctionalisation Reactions. Z. f. Naturforsch. C 2019, 74, 101– 104,  DOI: 10.1515/znc-2018-0137
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    24
    Multienzymatic in situ hydrogen peroxide generation cascade for peroxygenase-catalysed oxyfunctionalisation reactions
    Pesic, Milja; Willot, Sebastien Jean-Paul; Fernandez-Fueyo, Elena; Tieves, Florian; Alcalde, Miguel; Hollmann, Frank
    Zeitschrift fuer Naturforschung, C: Journal of Biosciences (2019), 74 (3-4), 101-104CODEN: ZNCBDA; ISSN:1865-7125. (Walter de Gruyter GmbH)
    There is an increasing interest in the application of peroxygenases in biocatalysis, because of their ability to catalyze the oxyfunctionalisation reaction in a stereoselective fashion and with high catalytic efficiencies, while using hydrogen peroxide or org. peroxides as oxidant. However, enzymes belonging to this class exhibit a very low stability in the presence of peroxides. With the aim of bypassing this fast and irreversible inactivation, we study the use of a gradual supply of hydrogen peroxide to maintain its concn. at stoichiometric levels. In this contribution, we report a multienzymic cascade for in situ generation of hydrogen peroxide. In the first step, in the presence of NAD+ cofactor, formate dehydrogenase from Candida boidinii (FDH) catalyzed the oxidn. of formate yielding CO2. Reduced NADH was reoxidised by the redn. of the FMN cofactor bound to an old yellow enzyme homolog from Bacillus subtilis (YqjM), which subsequently reacts with mol. oxygen yielding hydrogen peroxide. Finally, this system was coupled to the hydroxylation of ethylbenzene reaction catalyzed by an evolved peroxygenase from Agrocybe aegerita (rAaeUPO). Addnl., we studied the influence of different reaction parameters on the performance of the cascade with the aim of improving the turnover of the hydroxylation reaction.
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  25. 25
    Ma, Y.; Li, P.; Li, Y.; Willot, S. J.-P.; Zhang, W.; Ribitsch, D.; Choi, Y. H.; Zhang, T.; Verpoorte, R.; Hollmann, F.; Wang, Y. Natural Deep Eutectic Solvents as Multifunctional Media for the Valorisation of Agricultural Wastes. ChemSusChem 2019, 12, 1310– 1315,  DOI: 10.1002/cssc.201900043
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    Natural Deep Eutectic Solvents as Multifunctional Media for the Valorization of Agricultural Wastes
    Ma, Yunjian; Li, Peilin; Li, Yongru; Willot, Sebastien J.-P.; Zhang, Wuyuan; Ribitsch, Doris; Choi, Young Hae; Verpoorte, Robert; Zhang, Tianyu; Hollmann, Frank; Wang, Yonghua
    ChemSusChem (2019), 12 (7), 1310-1315CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The use of natural deep eutectic solvents (NADES) as multifunctional solvents for limonene bioprocessing was reported. NADES were used for the extn. of limonene from orange peel wastes, as solvent for the chemoenzymic epoxidn. of limonene, and as sacrificial electron donor for the in situ generation of H2O2 to promote the epoxidn. reaction. The proof-of-concept for this multifunctional use was provided, and the scope and current limitations of the concept were outlined.
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    Ni, Y.; Fernández-Fueyo, E.; Baraibar, A. G.; Ullrich, R.; Hofrichter, M.; Yanase, H.; Alcalde, M.; van Berkel, W. J. H.; Hollmann, F. Peroxygenase-Catalyzed Oxyfunctionalization Reactions Promoted by the Complete Oxidation of Methanol. Angew. Chem., Int. Ed. 2016, 55, 798– 801,  DOI: 10.1002/anie.201507881
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    Peroxygenase-Catalyzed Oxyfunctionalization Reactions Promoted by the Complete Oxidation of Methanol
    Ni, Yan; Fernandez-Fueyo, Elena; Baraibar, Alvaro Gomez; Ullrich, Rene; Hofrichter, Martin; Yanase, Hideshi; Alcalde, Miguel; van Berkel, Willem J. H.; Hollmann, Frank
    Angewandte Chemie, International Edition (2016), 55 (2), 798-801CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Peroxygenases catalyze a broad range of (stereo)selective oxyfunctionalization reactions. However, to access their full catalytic potential, peroxygenases need a balanced provision of hydrogen peroxide to achieve high catalytic activity while minimizing oxidative inactivation. Herein, we report an enzymic cascade process that employs methanol as a sacrificial electron donor for the reductive activation of mol. oxygen. Full oxidn. of methanol is achieved, generating three equiv. of hydrogen peroxide that can be used completely for the stereoselective hydroxylation of ethylbenzene as a model reaction. Overall we propose and demonstrate an atom-efficient and easily applicable alternative to established hydrogen peroxide generation methods, which enables the efficient use of peroxygenases for oxyfunctionalization reactions.
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    Willot, S. J. P.; Fernández-Fueyo, E.; Tieves, F.; Pesic, M.; Alcalde, M.; Arends, I. W. C. E.; Park, C. B.; Hollmann, F. Expanding the Spectrum of Light-Driven Peroxygenase Reactions. ACS Catal. 2019, 9, 890– 894,  DOI: 10.1021/acscatal.8b03752
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    Expanding the spectrum of light-driven peroxygenase reactions
    Willot, Sebastien J.-P.; Fernandez-Fueyo, Elena; Tieves, Florian; Pesic, Milja; Alcalde, Miguel; Arends, Isabel W. C. E.; Park, Chan Beum; Hollmann, Frank
    ACS Catalysis (2019), 9 (2), 890-894CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Peroxygenases require a controlled supply of H2O2 to operate efficiently. Here, we propose a photocatalytic system for the reductive activation of ambient O2 to produce H2O2 which uses the energy provided by visible light more efficiently based on the combination of wavelength-complementary photosensitizers. This approach was coupled to an enzymic system to make formate available as a sacrificial electron donor. The scope and current limitations of this approach are reported and discussed.
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    Zhang, W.; Fernández-Fueyo, E.; Ni, Y.; van Schie, M.; Gacs, J.; Renirie, R.; Wever, R.; Mutti, F. G.; Rother, D.; Alcalde, M.; Hollmann, F. Selective Aerobic Oxidation Reactions using a Combination of Photocatalytic Water Oxidation and Enzymatic Oxyfunctionalizations. Nat. Catal. 2018, 1, 55– 62,  DOI: 10.1038/s41929-017-0001-5
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    Selective aerobic oxidation reactions using a combination of photocatalytic water oxidation and enzymatic oxyfunctionalizations
    Zhang, Wuyuan; Fernandez-Fueyo, Elena; Ni, Yan; van Schie, Morten; Gacs, Jenoe; Renirie, Rokus; Wever, Ron; Mutti, Francesco G.; Rother, Doerte; Alcalde, Miguel; Hollmann, Frank
    Nature Catalysis (2018), 1 (1), 55-62CODEN: NCAACP; ISSN:2520-1158. (Nature Research)
    Peroxygenases offer an attractive means to address challenges in selective oxyfunctionalization chem. Despite this, their application in synthetic chem. remains challenging due to their facile inactivation by the stoichiometric oxidant H2O2. Often atom-inefficient peroxide generation systems are required, which show little potential for large-scale implementation. Here, we show that visible-light-driven, catalytic water oxidn. can be used for in situ generation of H2O2 from water, rendering the peroxygenase catalytically active. In this way, the stereoselective oxyfunctionalization of hydrocarbons can be achieved by simply using the catalytic system, water and visible light.
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    Zhang, W.; Burek, B. O.; Fernández-Fueyo, E.; Alcalde, M.; Bloh, J. Z.; Hollmann, F. Selective Activation of C-H Bonds by Cascading Photochemistry with Biocatalysis. Angew. Chem., Int. Ed. 2017, 56, 15451– 15455,  DOI: 10.1002/anie.201708668
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    29
    Selective Activation of C-H Bonds in a Cascade Process Combining Photochemistry and Biocatalysis
    Zhang, Wuyuan; Burek, Bastien O.; Fernandez-Fueyo, Elena; Alcalde, Miguel; Bloh, Jonathan Z.; Hollmann, Frank
    Angewandte Chemie, International Edition (2017), 56 (48), 15451-15455CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Selective oxyfunctionalizations of inert C-H bonds can be achieved under mild conditions by using peroxygenases. This approach, however, suffers from the poor robustness of these enzymes in the presence of hydrogen peroxide as the stoichiometric oxidant. Herein, we demonstrate that inorg. photocatalysts such as gold-titanium dioxide efficiently provide H2O2 through the methanol-driven reductive activation of ambient oxygen in amts. that ensure that the enzyme remains highly active and stable. Using this approach, the stereoselective hydroxylation of ethylbenzene to (R)-1-phenylethanol was achieved with high enantioselectivity (>98 % ee) and excellent turnover nos. for the biocatalyst (>71 000).
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    Choi, D. S.; Ni, Y.; Fernández-Fueyo, E.; Lee, M.; Hollmann, F.; Park, C. B. Photoelectroenzymatic Oxyfunctionalization on Flavin-Hybridized Carbon Nanotube Electrode Platform. ACS Catal. 2017, 7, 1563– 1567,  DOI: 10.1021/acscatal.6b03453
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    Photoelectroenzymatic Oxyfunctionalization on Flavin-Hybridized Carbon Nanotube Electrode Platform
    Choi, Da Som; Ni, Yan; Fernandez-Fueyo, Elena; Lee, Minah; Hollmann, Frank; Park, Chan Beum
    ACS Catalysis (2017), 7 (3), 1563-1567CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Peroxygenases are very promising catalysts for oxyfunctionalization reactions. Their practical applicability, however, is hampered by their sensitivity against the oxidant (H2O2), therefore necessitating in situ generation of H2O2. Here, we report a photoelectrochem. approach to provide peroxygenases with suitable amts. of H2O2 while reducing the electrochem. overpotential needed for the redn. of mol. oxygen to H2O2. When tethered on single-walled carbon nanotubes (SWNTs) under illumination, flavins allowed for a marked anodic shift of the oxygen redn. potential in comparison to pristine SWNT and/or nonilluminated electrodes. This flavin-SWNT-based photoelectrochem. platform enabled peroxygenases-catalyzed, selective hydroxylation reactions.
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    Churakova, E.; Kluge, M.; Ullrich, R.; Arends, I.; Hofrichter, M.; Hollmann, F. Specific Photobiocatalytic Oxyfunctionalization Reactions. Angew. Chem., Int. Ed. 2011, 50, 10716– 10719,  DOI: 10.1002/anie.201105308
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    Specific Photobiocatalytic Oxyfunctionalization Reactions
    Churakova, Ekaterina; Kluge, Martin; Ullrich, Rene; Arends, Isabel; Hofrichter, Martin; Hollmann, Frank
    Angewandte Chemie, International Edition (2011), 50 (45), 10716-10719, S10716/1-S10716/11CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The present work demonstrates that the arom. peroxygenase from Agrocybe aegerita (AaeAPO) is an active and versatile catalyst for enantiospecific hydroxylation and epoxidn. reactions. In situ generation of H2O2 via photochem. redn. of O2 permits AaeAPO to sustain robust oxyfunctionalization activity for periods of up to several hours. Under nonoptimized reaction conditions the enzyme turnover nos. for AaeAPO exceed those of comparable systems like chloroperoxidase (CPO) from Caldariomyces fumago and cytochrome P 450 enzymes. Furthermore, unlike CPO, AaeAPO is able to catalyze the hydroxylation of nonactivated C-H bonds.
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    Gulder, T.; Seel, C. J. Biocatalysis Fueled by Light: On the Versatile Combination of Photocatalysis and Enzymes. ChemBioChem 2019,  DOI: 10.1002/cbic.201800806 .
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    Seel, C. J.; Králík, A.; Hacker, M.; Frank, A.; König, B.; Gulder, T. Atom-Economic Electron Donors for Photobiocatalytic Halogenations. ChemCatChem 2018, 10, 3960– 3963,  DOI: 10.1002/cctc.201800886
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    Atom-Economic Electron Donors for Photobiocatalytic Halogenations
    Seel, Catharina Julia; Kralik, Antonin; Hacker, Melanie; Frank, Annika; Koenig, Burkhard; Gulder, Tanja
    ChemCatChem (2018), 10 (18), 3960-3963CODEN: CHEMK3; ISSN:1867-3880. (Wiley-VCH Verlag GmbH & Co. KGaA)
    In vitro cofactor supply and regeneration have been a major obstacle for biocatalytic processes, in particular on a large scale. Peroxidases often suffer from inactivation by their oxidative co-factor. Combining photocatalysis and biocatalysis offers an innovative soln. to this problem, but lacks atom economy due to the sacrificial electron donors needed. Herein, we show that redox-active buffers or even water alone can serve as efficient, biocompatible electron sources, when combined with photocatalysis. Mechanistic investigations revealed first insights into the possibilities and limitations of this approach and allowed adjusting the reaction conditions to the specific needs of biocatalytic transformations. Proof-of-concept for the applicability of this photobiocatalytic reaction setup was given by enzymic halogenations.
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    Schmermund, L.; Jurkaš, V.; Özgen, F. F.; Barone, G. D.; Büchsenschütz, H. C.; Winkler, C. K.; Schmidt, S.; Kourist, R.; Kroutil, W. Photo-Biocatalysis: Biotransformations in the Presence of Light. ACS Catal. 2019, 9, 4115– 4144,  DOI: 10.1021/acscatal.9b00656
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    Photo-Biocatalysis: Biotransformations in the Presence of Light
    Schmermund, Luca; Jurkas, Valentina; Oezgen, F. Feyza; Barone, Giovanni D.; Buechsenschuetz, Hanna C.; Winkler, Christoph K.; Schmidt, Sandy; Kourist, Robert; Kroutil, Wolfgang
    ACS Catalysis (2019), 9 (5), 4115-4144CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    A review. Light has received increased attention for various chem. reactions but also in combination with biocatalytic reactions. Because currently only a few enzymic reactions are known, which per se require light, most transformations involving light and a biocatalyst exploit light either for providing the cosubstrate or cofactor in an appropriate redox state for the biotransformation. In selected cases, a promiscuous activity of known enzymes in the presence of light could be induced. In other approaches, light-induced chem. reactions have been combined with a biocatalytic step, or light-induced biocatalytic reactions were combined with chem. reactions in a linear cascade. Finally, enzymes with a light switchable moiety have been investigated to turn off/on or tune the actual reaction. This Review gives an overview of the various approaches for using light in biocatalysis.
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    Goldstein, S.; Aschengrau, D.; Diamant, Y.; Rabani, J. Photolysis of Aqueous H2O2: Quantum Yield and Applications for Polychromatic UV Actinometry in Photoreactors. Environ. Sci. Technol. 2007, 41, 7486– 7490,  DOI: 10.1021/es071379t
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    Photolysis of Aqueous H2O2: Quantum Yield and Applications for Polychromatic UV Actinometry in Photoreactors
    Goldstein, Sara; Aschengrau, Dorit; Diamant, Yishay; Rabani, Joseph
    Environmental Science & Technology (2007), 41 (21), 7486-7490CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Methanol is used to measure the yield of •OH radicals produced in the photolysis of H2O2 in aq. solns. The UV photolysis of H2O2 generates •OH radicals, which in the presence of methanol, oxygen, and phosphate buffer form formaldehyde, namely, Φ(HCHO) = Φ(•OH). The quantum yield of •OH has been redetd. in view of literature inconsistencies resulting in Φ(•OH) = 1.11 ± 0.07 in the excitation range of 205-280 nm. The constancy of Φ(•OH) and the ease and sensitivity of the formaldehyde product anal. makes the H2O2/CH3OH system suitable for polychromatic UV actinometry. In addn., the relatively low cost of the main components and the possibility of destroying the methanol before disposal qualify the system for both monochromatic and polychromatic actinometry in a large vol. of water. The H2O2/CH3OH system was applied in different com. UV photoreactors.
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  36. 36
    Ullrich, R.; Hofrichter, M. The Haloperoxidase of the Agaric Fungus AgrocybeaegeritaHydroxylates Toluene and Naphthalene. FEBS Lett. 2005, 579, 6247– 6250,  DOI: 10.1016/j.febslet.2005.10.014
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    36
    The haloperoxidase of the agaric fungus Agrocybe aegerita hydroxylates toluene and naphthalene
    Ullrich, Rene; Hofrichter, Martin
    FEBS Letters (2005), 579 (27), 6247-6250CODEN: FEBLAL; ISSN:0014-5793. (Elsevier B.V.)
    The mushroom Agrocybe aegerita secretes a peroxidase (AaP) that catalyzes halogenations and hydroxylations. Phenol was brominated to 2- and 4-bromophenol (ratio 1:4) and chlorinated to a lesser extent to 2-chlorophenol. The purified enzyme was found to oxidize toluene via benzyl alc. and benzaldehyde into benzoic acid. A second fraction of toluene was hydroxylated to give p-cresol as well as o-cresol and methyl-p-benzoquinone. The UV-Vis absorption spectrum of purified AaP showed high similarity to a resting state cytochrome P 450 with the Soret band at 420 nm and addnl. maxima at 278, 358, 541 and 571 nm; the AaP CO-complex had a distinct absorption max. at 445 nm that is characteristic for heme-thiolate proteins. AaP regioselectively hydroxylated naphthalene to 1-naphthol and traces of 2-naphthol (ratio 36:1). H2O2 was necessarily required for AaP function and hence the hydroxylations catalyzed by AaP can be designated as peroxygenation and the enzyme as an extracellular peroxygenase.
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  37. 37
    Ullrich, R.; Nüske, J.; Scheibner, K.; Spantzel, J.; Hofrichter, M. Novel Haloperoxidase from the Agaric Basidiomycete AgrocybeaegeritaOxidizes Aryl Alcohols and Aldehydes. Appl. Environ. Microbiol. 2004, 70, 4575– 4581,  DOI: 10.1128/AEM.70.8.4575-4581.2004
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    37
    Novel haloperoxidase from the agaric basidiomycete Agrocybe aegerita oxidizes aryl alcohols and aldehydes
    Ullrich, Rene; Nueske, Joerg; Scheibner, Katrin; Spantzel, Joerg; Hofrichter, Martin
    Applied and Environmental Microbiology (2004), 70 (8), 4575-4581CODEN: AEMIDF; ISSN:0099-2240. (American Society for Microbiology)
    Agrocybe aegerita, a bark mulch- and wood-colonizing basidiomycete, was found to produce a peroxidase (AaP) that oxidizes aryl alcs., such as veratryl and benzyl alcs., into the corresponding aldehydes and then into benzoic acids. The enzyme also catalyzed the oxidn. of typical peroxidase substrates, such as 2,6-dimethoxyphenol (DMP) or 2,2'-azinobis-(3-ethylbenzothiazoline-6-sulfonate) (ABTS). A. aegerita peroxidase prodn. depended on the concn. of org. nitrogen in the medium, and highest enzyme levels were detected in the presence of soybean meal. Two fractions of the enzyme, AaP I and AaP II, which had identical mol. masses (46 kDa) and isoelec. points of 4.6 to 5.4 and 4.9 to 5.6, resp. (corresponding to six different isoforms), were identified after several steps of purifn., including anion- and cation-exchange chromatog. The optimum pH for the oxidn. of aryl alcs. was found to be around 7, and the enzyme required relatively high concns. of H2O2 (2 mM) for optimum activity. The apparent Km values for ABTS, DMP, benzyl alc., veratryl alc., and H2O2 were 37, 298, 1001, 2367 and 1313 μM, resp. The N-terminal amino acid sequences of the main AaP II spots blotted after two-dimensional gel electrophoresis were almost identical and exhibited almost no homol. to the sequences of other peroxidases from basidiomycetes, but they shared the first three amino acids, as well as two addnl. amino acids, with the heme chloroperoxidase (CPO) from the ascomycete Caldariomyces fumago. This finding is consistent with the fact that AaP halogenates monochlorodimedone, the specific substrate of CPO. The existence of haloperoxidases in basidiomycetous fungi may be of general significance for the natural formation of chlorinated org. compds. in forest soils.
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    Molina-Espeja, P.; Ma, S.; Mate, D. M.; Ludwig, R.; Alcalde, M. Tandem-Yeast Expression System for Engineering and Producing Unspecific Peroxygenase. Enzyme Microb. Technol. 2015, 73–74, 29– 33,  DOI: 10.1016/j.enzmictec.2015.03.004
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    Tandem-yeast expression system for engineering and producing unspecific peroxygenase
    Molina-Espeja, Patricia; Ma, Su; Mate, Diana M.; Ludwig, Roland; Alcalde, Miguel
    Enzyme and Microbial Technology (2015), 73-74 (), 29-33CODEN: EMTED2; ISSN:0141-0229. (Elsevier)
    Unspecific peroxygenase (UPO) is a highly efficient biocatalyst with a peroxide dependent monooxygenase activity and many biotechnol. applications, but the absence of suitable heterologous expression systems has precluded its use in different industrial settings. Recently, the UPO from Agrocybe aegerita was evolved for secretion and activity in Saccharomyces cerevisiae [8]. In the current work, we describe a tandem-yeast expression system for UPO engineering and large scale prodn. By harnessing the directed evolution process in S. cerevisiae, the beneficial mutations for secretion enabled Pichia pastoris to express the evolved UPO under the control of the methanol inducible alc. oxidase 1 promoter. While secretion levels were found similar for both yeasts in flask fermn. (∼8 mg/L), the recombinant UPO from P. pastoris showed a 27-fold enhanced prodn. in fed-batch fermn. (217 mg/L). The P. pastoris UPO variant maintained similar biochem. properties of the S. cerevisiae counterpart in terms of catalytic consts., pH activity profiles and thermostability. Thus, this tandem-yeast expression system ensures the engineering of UPOs to use them in future industrial applications as well as large scale prodn.
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  39. 39
    Molina-Espeja, P.; Garcia-Ruiz, E.; Gonzalez-Perez, D.; Ullrich, R.; Hofrichter, M.; Alcalde, M. Directed Evolution of Unspecific Peroxygenase from Agrocybeaegerita. Appl. Environ. Microbiol. 2014, 80, 3496– 3507,  DOI: 10.1128/AEM.00490-14
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    Directed evolution of unspecific peroxygenase from Agrocybe aegerita
    Molina-Espeja, Patricia; Garcia-Ruiz, Eva; Gonzalez-Perez, David; Ullrich, Rene; Hofrichter, Martin; Alcalde, Miguel
    Applied and Environmental Microbiology (2014), 80 (11), 3496-3507, 13 pp.CODEN: AEMIDF; ISSN:1098-5336. (American Society for Microbiology)
    Unspecific peroxygenase (UPO) represents a new type of heme-thiolate enzyme with self-sufficient mono(per)oxygenase activity and many potential applications in org. synthesis. With a view to taking advantage of these properties, we subjected the Agrocybe aegerita UPO1-encoding gene to directed evolution in Saccharomyces cerevisiae. To promote functional expression, several different signal peptides were fused to the mature protein, and the resulting products were tested. Over 9,000 clones were screened using an ad hoc dual-colorimetric assay that assessed both peroxidative and oxygen transfer activities. After 5 generations of directed evolution combined with hybrid approaches, 9 mutations were introduced that resulted in a 3,250-fold total activity improvement with no alteration in protein stability. A breakdown between secretion and catalytic activity was performed by replacing the native signal peptide of the original parental type with that of the evolved mutant; the evolved leader increased functional expression 27-fold, whereas an 18-fold improvement in the kcat/Km value for oxygen transfer activity was obtained. The evolved UPO1 was active and highly stable in the presence of org. cosolvents. Mutations in the hydrophobic core of the signal peptide contributed to enhance functional expression up to 8 mg/L, while catalytic efficiencies for peroxidative and oxygen transfer reactions were increased by several mutations in the vicinity of the heme access channel. Overall, the directed-evolution platform described is a valuable point of departure for the development of customized UPOs with improved features and for the study of structure-function relationships.
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  40. 40
    Hirakawa, H.; Shiota, S.; Shiraishi, Y.; Sakamoto, H.; Ichikawa, S.; Hirai, T. Au Nanoparticles Supported on BiVO4: Effective Inorganic Photocatalysts for H2O2Production from Water and O2 under Visible Light. ACS Catal. 2016, 6, 4976– 4982,  DOI: 10.1021/acscatal.6b01187
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    Au nanoparticles supported on BiVO4: Effective inorganic photocatalysts for H2O2 production from water and O2 under visible light
    Hirakawa, Hiroaki; Shiota, Shingo; Shiraishi, Yasuhiro; Sakamoto, Hirokatsu; Ichikawa, Satoshi; Hirai, Takayuki
    ACS Catalysis (2016), 6 (8), 4976-4982CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    The design of a safe and sustainable process for the synthesis of hydrogen peroxide (H2O2) is a very important subject from the viewpoint of green chem. Photocatalytic H2O2 prodn. with earth-abundant water and mol. oxygen (O2) as resources is an ideal process. A successful system based on an org. semiconductor has been proposed; however, it suffers from poor photostability. Here we report an inorg. photocatalyst for H2O2 synthesis. Visible light irradn. (λ >420 nm) of the semiconductor BiVO4 loaded with Au nanoparticles (Au/BiVO4) in pure water with O2 successfully produces H2O2. The bottom of the BiVO4 conduction band (0.02 V vs NHE, pH 0) is more pos. than the one-electron redn. potential of O2 (-0.13 V) while more neg. than the two-electron redn. potential of O2 (0.68 V). This thus suppresses one-electron redn. of O2 and selectively promotes two-electron redn. of O2, resulting in efficient H2O2 formation.
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    Shi, N.; Cheng, W.; Zhou, H.; Fan, T.; Niederberger, M. Facile Synthesis of Monodisperse Co3O4Quantum Dots with Efficient Oxygen Evolution Activity. Chem. Commun. 2015, 51, 1338– 1340,  DOI: 10.1039/C4CC08179J
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    Facile synthesis of monodisperse Co3O4 quantum dots with efficient oxygen evolution activity
    Shi, Nan; Cheng, Wei; Zhou, Han; Fan, Tongxiang; Niederberger, Markus
    Chemical Communications (Cambridge, United Kingdom) (2015), 51 (7), 1338-1340CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    Monodisperse, water-dispersible Co3O4 quantum dots with sizes of around 4.5 nm are prepd. through a simple soln. method. The resultant cobalt oxide quantum dots exhibit excellent visible-light-driven oxygen evolution activities in the [Ru(bpy)3]2+-persulfate system under mild pH conditions.
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  42. 42
    Yin, Q. S.; Tan, J. M.; Besson, C.; Geletii, Y. V.; Musaev, D. G.; Kuznetsov, A. E.; Luo, Z.; Hardcastle, K. I.; Hill, C. L. A Fast Soluble Carbon-Free Molecular Water Oxidation Catalyst Based on Abundant Metals. Science 2010, 328, 342– 345,  DOI: 10.1126/science.1185372
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    A Fast Soluble Carbon-Free Molecular Water Oxidation Catalyst Based on Abundant Metals
    Yin, Qiushi; Tan, Jeffrey Miles; Besson, Claire; Geletii, Yurii V.; Musaev, Djamaladdin G.; Kuznetsov, Aleksey E.; Luo, Zhen; Hardcastle, Ken I.; Hill, Craig L.
    Science (Washington, DC, United States) (2010), 328 (5976), 342-345CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Traditional homogeneous water oxidn. catalysts are plagued by instability under the reaction conditions. We report that the complex [Co4(H2O)2(PW9O34)2]10-, comprising a Co4O4 core stabilized by oxidatively resistant polytungstate ligands, is a hydrolytically and oxidatively stable homogeneous water oxidn. catalyst that self-assembles in water from salts of earth-abundant elements (Co, W, and P). With [Ru(bpy)3]3+ (bpy is 2,2'-bipyridine) as the oxidant, we observe catalytic turnover frequencies for O2 prodn. ≥5 s-1 at pH = 8. The rate's pH sensitivity reflects the pH dependence of the four-electron O2-H2O couple. Extensive spectroscopic, electrochem., and inhibition studies firmly indicate that [Co4(H2O)2(PW9O34)2]10- is stable under catalytic turnover conditions: Neither hydrated cobalt ions nor cobalt hydroxide/oxide particles form in situ.
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    Priebe, J. B.; Radnik, J.; Lennox, A. J. J.; Pohl, M. M.; Karnahl, M.; Hollmann, D.; Grabow, K.; Bentrup, U.; Junge, H.; Beller, M.; Bruckner, A. Solar Hydrogen Production by Plasmonic Au-TiO2Catalysts: Impact of Synthesis Protocol and TiO2Phase on Charge Transfer Efficiency and H2Evolution Rates. ACS Catal. 2015, 5, 2137– 2148,  DOI: 10.1021/cs5018375
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    Solar Hydrogen Production by Plasmonic Au-TiO2 Catalysts: Impact of Synthesis Protocol and TiO2 Phase on Charge Transfer Efficiency and H2 Evolution Rates
    Priebe, Jacqueline B.; Radnik, Joerg; Lennox, Alastair J. J.; Pohl, Marga-Martina; Karnahl, Michael; Hollmann, Dirk; Grabow, Kathleen; Bentrup, Ursula; Junge, Henrik; Beller, Matthias; Brueckner, Angelika
    ACS Catalysis (2015), 5 (4), 2137-2148CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    The activity of plasmonic Au-TiO2 catalysts for solar H prodn. from H2O/MeOH mixts. depends strongly on the support phase (anatase, rutile, brookite, or composites thereof) as well as on specific structural properties caused by the method of Au deposition (sol-immobilization, photodeposition, or deposition-pptn.). Structural and electronic rationale have been identified for this behavior. Using a combination of spectroscopic in situ techniques (EPR, XANES, and UV-visible spectroscopy), the formation of plasmonic Au particles from precursor species was monitored, and the charge-carrier sepn. and stabilization under photocatalytic conditions was explored in relation to H2 evolution rates. By in situ EPR spectroscopy, it was directly shown that abundant surface vacancies and surface OH groups enhance the stabilization of sepd. electrons and holes, whereas the enrichment of Ti3+ in the support lattice hampers an efficient electron transport. Under the given exptl. conditions, these properties were most efficiently generated by depositing Au particles on anatase/rutile composites using the deposition-pptn. technique.
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    Najafpour, M. M.; Pashaei, B. Nanoscale Manganese Oxide within Faujasite Zeolite as an Efficient and Biomimetic Water Oxidizing Catalyst. Dalton Trans. 2012, 41, 10156– 10160,  DOI: 10.1039/c2dt30891f
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    Nanoscale manganese oxide within faujasite zeolite as an efficient and biomimetic water oxidizing catalyst
    Najafpour, Mohammad Mahdi; Pashaei, Babak
    Dalton Transactions (2012), 41 (34), 10156-10160CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
    Nanoscale manganese oxides within Faujasite zeolite have been synthesized with a simple method and characterized by SEM, X-ray diffraction spectrometry, N2 adsorption-desorption isotherms, transmission electron microscopy, and at. absorption spectroscopy. These oxides showed efficient water oxidizing activity in the presence of cerium(IV) ammonium nitrate as a non-oxo transfer oxidant.
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    Maeda, K.; Ishimaki, K.; Okazaki, M.; Kanazawa, T.; Lu, D.; Nozawa, S.; Kato, H.; Kakihana, M. Cobalt Oxide Nanoclusters on Rutile Titania as Bifunctional Units for Water Oxidation Catalysis and Visible Light Absorption: Understanding the Structure-Activity Relationship. ACS Appl. Mater. Interfaces 2017, 9, 6114– 6122,  DOI: 10.1021/acsami.6b15804
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    45
    Cobalt Oxide Nanoclusters on Rutile Titania as Bifunctional Units for Water Oxidation Catalysis and Visible Light Absorption: Understanding the Structure-Activity Relationship
    Maeda, Kazuhiko; Ishimaki, Koki; Okazaki, Megumi; Kanazawa, Tomoki; Lu, Daling; Nozawa, Shunsuke; Kato, Hideki; Kakihana, Masato
    ACS Applied Materials & Interfaces (2017), 9 (7), 6114-6122CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    The structure of cobalt oxide (CoOx) nanoparticles dispersed on rutile TiO2 (R-TiO2) was characterized by X-ray diffraction, UV-vis-NIR diffuse reflectance spectroscopy, high-resoln. transmission electron microscopy, X-ray absorption fine-structure spectra, and XPS spectra. The CoOx nanoparticles were loaded onto R-TiO2 by an impregnation method from an aq. soln. contg. Co(NO3)2·6H2O followed by heating in air. Modification of the R-TiO2 with 2.0 wt. % Co followed by heating at 423 K for 1 h resulted in the highest photocatalytic activity with good reproducibility. Structural analyses revealed that the activity of this photocatalyst depended strongly on the generation of Co3O4 nanoclusters with an optimal distribution. These nanoclusters are thought to interact with the R-TiO2 surface, resulting in visible light absorption and active sites for water oxidn.
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    Boppana, V. B. R.; Jiao, F. Nanostructured MnO2: an Efficient and Robust Water Oxidation Catalyst. Chem. Commun. 2011, 47, 8973– 8975,  DOI: 10.1039/c1cc12258d
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    Nanostructured MnO2: an efficient and robust water oxidation catalyst
    Boppana, Venkata Bharat Ram; Jiao, Feng
    Chemical Communications (Cambridge, United Kingdom) (2011), 47 (31), 8973-8975CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    Nanostructured MnO2 exhibits a high turnover frequency for oxygen evolution under visible light and high stability in strong acidic conditions.
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    Idris, A.; Hassan, N.; Mohd Ismail, N. S.; Misran, E.; Yusof, N. M.; Ngomsik, A.-F.; Bee, A. Photocatalytic Magnetic Separable Beads for Chromium (VI) Reduction. Water Res. 2010, 44, 1683– 1688,  DOI: 10.1016/j.watres.2009.11.026
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    Photocatalytic magnetic separable beads for chromium (VI) reduction
    Idris, Ani; Hassan, Nursia; Ismail, Nur Suriani Mohd.; Misran, Effaliza; Yusof, Noordin Mohd.; Ngomsik, Audrey-Flore; Bee, Agnes
    Water Research (2010), 44 (6), 1683-1688CODEN: WATRAG; ISSN:0043-1354. (Elsevier B.V.)
    Magnetically separable photocatalyst beads contg. nano-sized iron oxide in alginate polymer were prepd. This magnetic photocatalyst beads are used in slurry-type reactors. The magnetism of the catalyst arises from the nanostructured particles γ-Fe2O3, by which the catalyst can be easily recovered by the application of an external magnetic field. These synthesized beads are sunlight-driven photocatalyst. In the system without magnetic photocatalyst beads, no chromium redn. was obsd. under sunlight irradn. due to the stability of the chromium (VI). Upon the addn. of magnetic photocatalyst beads, the photo-redn. of Cr(VI) was completed in just after only 50 min under sunlight irradn. due to the photocatalytic activity of the beads. However when placed away from sunlight, the redn. rate of the chromium is just about 10%. These observations were explained in terms of absorption occurrence of chromium (VI) onto the catalyst surface which took place in this reaction. In addn., photo-redn. rate of chromium (VI) was more significant at lower pH. The results suggest that the use of magnetic separable photocatalyst beads is a feasible strategy for eliminating Cr(VI).
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    Hamid, B. A. S.; Teh, S. J.; Lai, C. W. Photocatalytic Water Oxidation on ZnO: A Review. Catalysts 2017, 7, 93,  DOI: 10.3390/catal7030093
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    Kim, H. W.; Ross, M. B.; Kornienko, N.; Zhang, L.; Guo, J.; Yang, P.; McCloskey, B. D. Efficient Hydrogen Peroxide Generation using Reduced Graphene Oxide-Based Oxygen Reduction Electrocatalysts. Nature Catal. 2018, 1, 282– 290,  DOI: 10.1038/s41929-018-0044-2
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    Zhang, W.; Bariotaki, A.; Smonou, I.; Hollmann, F. Visible-Light-Driven Photooxidation of Alcohols using Surface-Doped Graphitic Carbon Nitride. Green Chem. 2017, 19, 2096– 2100,  DOI: 10.1039/C7GC00539C
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    Visible-light-driven photooxidation of alcohols using surface-doped graphitic carbon nitride
    Zhang, Wuyuan; Bariotaki, Anna; Smonou, Ioulia; Hollmann, Frank
    Green Chemistry (2017), 19 (9), 2096-2100CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
    Carbon-nanodot-doped g-C3N4 is used as a photocatalyst to promote the aerobic oxidn. of alcs. ROH (R = C6H5, c-C6H11, 1,2,3,4-tetrahydronaphthalen-1-yl, etc.) and oxyfunctionalization of activated hydrocarbons such as methylbenzene, 1-chloro-4-methylbenzene, cyclohexane, etc. A crit. E-factor anal. of the current reaction system reveals its limitations en route to environmentally acceptable oxidn. procedures.
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    Shiraishi, Y.; Kanazawa, S.; Sugano, Y.; Tsukamoto, D.; Sakamoto, H.; Ichikawa, S.; Hirai, T. Highly Selective Production of Hydrogen Peroxide on Graphitic Carbon Nitride (g-C3N4) Photocatalyst Activated by Visible Light. ACS Catal. 2014, 4, 774– 780,  DOI: 10.1021/cs401208c
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    51
    Highly Selective Production of Hydrogen Peroxide on Graphitic Carbon Nitride (g-C3N4) Photocatalyst Activated by Visible Light
    Shiraishi, Yasuhiro; Kanazawa, Shunsuke; Sugano, Yoshitsune; Tsukamoto, Daijiro; Sakamoto, Hirokatsu; Ichikawa, Satoshi; Hirai, Takayuki
    ACS Catalysis (2014), 4 (3), 774-780CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Photocatalytic prodn. of hydrogen peroxide (H2O2) on semiconductor catalysts with alc. as a hydrogen source and mol. oxygen (O2) as an oxygen source is a potential method for safe H2O2 synthesis because the reaction can be carried out without the use of explosive H2/O2 mixed gases. Early reported photocatalytic systems, however, produce H2O2 with significantly low selectivity (∼1%). We found that visible light irradn. (λ > 420 nm) of graphitic carbon nitride (g-C3N4), a polymeric semiconductor, in an alc./water mixt. with O2 efficiently produces H2O2 with very high selectivity (∼90%). Raman spectroscopy and ESR anal. revealed that the high H2O2 selectivity is due to the efficient formation of 1,4-endoperoxide species on the g-C3N4 surface. This suppresses one-electron redn. of O2 (superoxide radical formation), resulting in selective promotion of two-electron redn. of O2 (H2O2 formation).
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    Hydrogen peroxide (H2O2) is of great significance in biol. and environmental processes as well as in chem. industry. Even though anthraquinone autoxidn. (AO) process has been the major artificial way to produce H2O2, its energy cost and non-green nature have been motivating people to develop more efficient, economic and green technologies as alternatives. Here we demonstrated that photocatalytic H2O2 prodn. at g-C3N4 could be improved by as much as 14 times in the absence of org. scavenger through a carbon vacancy-based strategy. Both the exptl. and theor. calcn. results indicated that the creation of carbon vacancies could reduce the symmetry of g-C3N4 and produce the effect of electron delocalization. This will allow g-C3N4 to possess more excitable electrons and a narrower band gap. On the other hand, carbon vacancies provided more sites to adsorb mol. oxygen and thereby help electrons transfer from g-C3N4 to the surface adsorbed O2. More interestingly, the presence of carbon vacancies changed the H2O2 generation pathway from a two-step single-electron indirect redn. to an one-step two-electron direct redn. This study could not only develop a novel strategy to improve the H2O2 prodn. activity of semiconductors, but also shed light on the deep understanding of the role played by surface defect structure on photocatalytic activity of semiconductor photocatalysts.
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    Two precursors, melamine and urea, were used to prep. graphitic carbon nitride through a thermal decompn. (pyrolysis) method. The pyrolysis was carried out at different temps. in open air condition in a crucible with cover. The as-prepd. samples were characterized by SEM, TEM, BET, XRD, XPS, FTIR and DRS. The characterization results revealed that the samples synthesized from different precursors had different phys. and chem. properties. Specifically, it was found that the pyrolysis of urea yielded product with smaller cryst. domains but larger surface areas compared to that of melamine. To further qualify the as-prepd. samples, the adsorption and photocatalytic activities were measured by using Rhodamine B (RhB) as target pollutant. It was found out that the precursors as well as pyrolysis temps. had big influences on the adsorption and photocatalytic activities. Higher photocatalytic activities were achieved by samples synthesized from urea at higher temps. The mechanism of the degrdn. process was explored on the basis of the band structure and the roles of photo-generated radicals.
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    Chemical Engineering Journal (Amsterdam, Netherlands) (2015), 260 (), 469-477CODEN: CMEJAJ; ISSN:1385-8947. (Elsevier B.V.)
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    Moon, G.-H.; Fujitsuka, M.; Kim, S.; Majima, T.; Wang, X.; Choi, W. Eco-Friendly Photochemical Production of H2O2 through O2 Reduction over Carbon Nitride Frameworks Incorporated with Multiple Heteroelements. ACS Catal. 2017, 7, 2886– 2895,  DOI: 10.1021/acscatal.6b03334
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    Eco-Friendly Photochemical Production of H2O2 through O2 Reduction over Carbon Nitride Frameworks Incorporated with Multiple Heteroelements
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    ACS Catalysis (2017), 7 (4), 2886-2895CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    We report that in situ incorporation of both potassium and phosphate species into a polymeric carbon nitride (CN) framework highly enhanced the photoprodn. of hydrogen peroxide (H2O2) without the use of any noble-metal cocatalysts. The incorporation of earth-abundant heteroelements (K, P, and O) (i) introduced the neg. surface charge over the entire pH range through surface functionalization by phosphate species, (ii) increased the lifetime of the transient species to a picosecond time scale via the formation of charge sepn. states, (iii) facilitated the interfacial electron transfer to dioxygen, and (iv) inhibited the decompn. of in situ generated H2O2. As a result, the modified CN showed apparent quantum yields (Φ, for H2O2 prodn.) that are enhanced by about 25 and 17 times (Φ420 = 8.0%; Φ320 = 26.2%) from those of bare CN (Φ420 = 0.32%; Φ320 = 1.55%) under monochromatic irradn. of 420 and 320 nm, resp. This study clearly demonstrated a simple way to design multiple heteroelement-incorporated CN compds. that consist of earth-abundant elements only (C, N, K, P, O) for the development of practical and economical solar conversion photocatalytic materials.
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    Band gap-tunable potassium doped graphitic carbon nitride with enhanced mineralization ability
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    Dalton Transactions (2015), 44 (3), 1084-1092CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
    Band gap-tunable potassium doped graphitic carbon nitride with enhanced mineralization ability was prepd. using dicyandiamide monomer and potassium hydrate as precursors. X-ray diffraction (XRD), N2 adsorption, UV-Vis spectroscopy, Fourier transform IR (FT-IR) spectroscopy, SEM (SEM), photoluminescence (PL) and XPS were used to characterize the prepd. catalysts. The CB and VB potentials of graphitic carbon nitride could be tuned from -1.09 and +1.56 to -0.31 and +2.21 eV by controlling the K concn. Besides, the addn. of potassium inhibited the crystal growth of graphitic carbon nitride, enhanced the surface area and increased the sepn. rate for photogenerated electrons and holes. The visible-light-driven Rhodamine B (RhB) photodegrdn. and mineralization performances were significantly improved after potassium doping. A possible influence mechanism of the potassium concn. on the photocatalytic performance was proposed.
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    Zhang, H.; Tian, W.; Zhou, L.; Sun, H.; Tade, M.; Wang, S. Monodisperse Co3O4Quantum Dots on Porous Carbon Nitride Nanosheets for Enhanced Visible-Light-Driven Water Oxidation. Appl. Catal., B 2018, 223, 2– 9,  DOI: 10.1016/j.apcatb.2017.03.028
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    Monodisperse Co3O4 quantum dots on porous carbon nitride nanosheets for enhanced visible-light-driven water oxidation
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    Applied Catalysis, B: Environmental (2018), 223 (), 2-9CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
    Here we report a facile annealing process for homogeneous deposition of Co3O4 quantum dots (Co3O4 QDs) onto porous g-C3N4 nanosheets. It was discovered that pores were catalytically produced around Co3O4 QDs. In the synthesis, annealing temp. was found to be crucial for the textural property, optical absorption, and the corresponding photocatalytic water oxidn. as well as photochem. performances. The highest sp. surface area, pore vol. and optimal O2 prodn. rate as well as the highest photocurrent were obtained on 0.8 wt.% Co3O4 QDs decorated g-C3N4 nanosheets annealed at 300 °C (0.8% Co3O4-C3N4-300). These results underline the importance of surface heterojunction and afford us a feasible protocol for rational design of g-C3N4 based photocatalysts for water oxidn.
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    Son, E. J.; Lee, Y. W.; Ko, J. W.; Park, C. B. Amorphous Carbon Nitride as a Robust Photocatalyst for Biocatalytic Solar-to-Chemical Conversion. ACS Sustainable Chem. Eng. 2019, 7, 2545– 2552,  DOI: 10.1021/acssuschemeng.8b05487
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    Amorphous carbon nitride as a robust photocatalyst for biocatalytic solar-to-chemical conversion
    Son, Eun Jin; Lee, Yang Woo; Ko, Jong Wan; Park, Chan Beum
    ACS Sustainable Chemistry & Engineering (2019), 7 (2), 2545-2552CODEN: ASCECG; ISSN:2168-0485. (American Chemical Society)
    In situ regeneration of nicotinamide cofactor (NADH) is imperative because it is required as a reducing power for driving many redox enzymic cycles useful in industry. Here, we report that amorphous carbon nitride (ACN) is a promising and robust photocatalyst for solar-driven biotransformation via NADH regeneration. Under visible light (λ > 420 nm), NADH regeneration yields by ACN reached 62.3% within an hour, whereas partially cryst. polymeric carbon nitride (CCN) hardly reduced NAD+ to NADH. Subsequently, the regenerated cofactor was consumed by L-glutamate dehydrogenase, a NADH-dependent enzyme, achieving the conversion of α-ketoglutarate with a turnover frequency of 2640 h-1. ACN showed excellent catalytic activity and long-term stability for light-driven biocatalysis; NADH regeneration efficiency after eight cycles remained above 92% of the first cycle's efficiency, and the enzymic reaction proceeded for more than 12 h without significant loss of ACN's photoactivity. The remarkable photocatalytic activity of ACN originated from its unique microstructure that lacks hydrogen bonds that link polymeric melon units, leading to extended visible light absorption and less charge recombination. Our results suggest that ACN efficiently drives biocatalytic photosynthesis with exceptional catalytic sustainability.
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  • Abstract

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    Scheme 1

    Scheme 1. Photoenzymatic Hydroxylation of Ethyl Benzene Combining Heterogeneous Photocatalysts for the Reductive Activation of O2 to H2O2 with a Peroxygenase-Catalyzed Oxyfunctionalization Reaction
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    Figure 1

    Figure 1. Performance of several heterogeneous photocatalysts to promote rAaeUPO-catalyzed oxyfunctionalization, forming phenyl ethanol (blue) and the overoxidation product acetophenone (red), in absence (left) or presence (right) of methanol. Conditions: 5 mg mL–1 heterogeneous catalyst, 50 mM ethylbenzene, 0 or 250 mM methanol, and 100 nM rAaeUPO in a 100 mM phosphate buffer at pH 7, 30 °C and stirring at 300 rpm. Illumination by an Osram 200W light bulb for 30 min. Reactions were performed in independent duplicates. 1: Au-BiVO4; (40)2: Co3O4 (quantum dots); (41)3: Co4(H2O)2(W9O34)2; (42)4: Pt-TiO2 (Rutile); (43)5: MnO (on Faujasite); (44)6: Co-TiO2; (45)7: MnO (nanowires); (46)8: Ir@SiO2; 9: Fe2O3; (47)10: g-C3N4; 11: ZnO (nanoclusters). (48)

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    Figure 2

    Figure 2. g-C3N4 as photocatalyst to promote rAaeUPO-catalyzed hydroxylation of ethylbenzene in the absence of external electron donors (blue), or 250 mM methanol (red) or 250 mM formate (green). (A) Time course of (R)-1-phenyl ethanol formation and (B) time course of acetophenone formation. From these time courses, parameters such as reaction rate (C), rAaeUPO inactivation rate (D), maximal product concentration (E), and selectivity (F) were calculated. General conditions: [rAaeUPO] = 100 nM, [ethylbenzene] = 50 mM, [g-C3N4] = 5 mg mL–1, KPi buffer pH 7.0 (100 mM), 30 °C, magnetic stirring at 600 rpm, illumination by an Osram 200W light bulb. Reactions were performed in independent duplicates.

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    Figure 3

    Figure 3. Influence of the formate concentration on the performance of the photoenzymatic hydroxylation of ethylbenzene. (A) Time course of (R)-1-phenyl ethanol formation and (B) time course of acetophenone formation. From these time courses, parameters such as reaction rate (C), rAaeUPO inactivation rate (D), maximal product concentration (E), and selectivity (F) were calculated. [HCO2–] = 0 mM (black), 50 mM (green), 100 mM (red), 250 mM (blue) or 500 mM (purple). General conditions: [rAaeUPO] = 100 nM, [ethylbenzene] = 50 mM, [g-C3N4] = 5 mg mL–1, KPi buffer pH 7.0 (100 mM), 30 °C, magnetic stirring at 600 rpm, illumination by an Osram 200W light bulb. Reactions were performed in independent duplicates.

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    Figure 4

    Figure 4. Influence of the rAaeUPO concentration on the performance of the photoenzymatic hydroxylation of ethylbenzene. A: time course of (R)-1-phenyl ethanol formation and B: time course of acetophenone formation. From these time courses, parameters such as reaction rate (C), rAaeUPO inactivation rate (D), maximal product concentration (E) and selectivity (F) were calculated. [rAaeUPO] = 20 nM (black), 50 nM (red), 100 nM (blue), 200 nM (green) or 500 nM (purple). General conditions: [NaHCO2] = 250 mM, [ethylbenzene] = 50 mM, [g-C3N4] = 5 mg mL–1, KPi buffer pH 7.0 (100 mM), 30 °C, magnetic stirring at 600 rpm, illumination by an Osram 200W light bulb. Reactions were performed in independent duplicates.

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    Figure 5

    Figure 5. Influence of the g-C3N4 concentration on the performance of the photoenzymatic hydroxylation of ethylbenzene. (A) Time course of (R)-1-phenyl ethanol formation and (B) time course of acetophenone formation. From these time courses, parameters such as reaction rate (C), rAaeUPO inactivation rate (D), maximal product concentration (E) and selectivity (F) were calculated. [g-C3N4] = 1 mg mL–1 (black), 2.5 mg mL–1 (red), 5 mg mL–1 (blue), 10 mg mL–1 (green), or 15 mg mL–1 (purple). General conditions: [NaHCO2] = 250 mM, [ethylbenzene] = 50 mM, [rAaeUPO] = 100 nM, KPi buffer pH 7.0 (100 mM), 30 °C, magnetic stirring at 600 rpm, illumination by an Osram 200W light bulb. Reactions were performed in independent duplicates.

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    Figure 6

    Figure 6. Influence of g-C3N4 morphology on the photoenzymatic hydroxylation of ethylbenzene. (A) Time course of (R)-1-phenyl ethanol formation and (B) time course of acetophenone formation. From these time courses, parameters such as reaction rate (C), rAaeUPO inactivation rate (D), maximal product concentration (E), and selectivity (F) were calculated. Amorphous g-C3N4(black), g-C3N4 sheets (red), or g-C3N4 bulk (green). Reaction conditions: [NaHCO2] = 250 mM, [ethylbenzene] = 50 mM, [g-C3N4] = 5 mg mL–1, [rAaeUPO] = 100 nM, KPi buffer pH 7.0 (100 mM), 30 °C, magnetic stirring at 600 rpm, illumination by an Osram 200W light bulb. Reactions were performed in independent duplicates.

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    Figure 7

    Figure 7. Investigating the molecular reasons for the decreased rAaeUPO-stability under process conditions. (A) Detection of hydroxyl radicals formed by irradiated g-C3N4 using the spin-trap method. Signals marked with a star (★) are assigned to the oxidation product of DMPO, 5,5-dimethyl-2-oxopyrroline-1-oxyl (DMPOX). Signals marked with diamonds (◆) belong to the spin adduct DMPO–OH. (B) Protein in solution before (blue) or after (red) incubation with g-C3N4 in the dark, for bovine serum albumin (BSA) or rAaeUPO.

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    Figure 8

    Figure 8. Time course of the photoenzymatic hydroxylation of ethylbenzene to (R)-1-phenyl ethanol (black) and overoxidation to acetophenone (red) using the dialysis bag approach. Conditions: 10 mL of reaction solution equally divided inside and outside the dialysis bag (20 kDa cutoff). Inside the bag: [NaHCO2] = 250 mM, [ethylbenzene] = 50 mM, [rAaeUPO] = 100 nM, KPi buffer pH 7.0 (100 mM). Outside the bag: [NaHCO2] = 250 mM, [ethylbenzene] = 50 mM, [g-C3N4] = 5 mg mL–1, KPi buffer pH 7.0 (100 mM). The reaction was performed once at room temperature while stirring at 600 rpm. The reaction solution was illuminated by a LIGHTNINGCURE spot light (Hamamatsu) at 50% intensity with an UV filter.

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      Fungi produce heme-contg. peroxidases and peroxygenases, flavin-contg. oxidases and dehydrogenases, and different copper-contg. oxidoreductases involved in the biodegrdn. of lignin and other recalcitrant compds. Heme peroxidases comprise the classical ligninolytic peroxidases and the new dye-decolorizing peroxidases, while heme peroxygenases belong to a still largely unexplored superfamily of heme-thiolate proteins. Nevertheless, basidiomycete unspecific peroxygenases have the highest biotechnol. interest due to their ability to catalyze a variety of regio- and stereo-selective monooxygenation reactions with H2O2 as the source of oxygen and final electron acceptor. Flavo-oxidases are involved in both lignin and cellulose decay generating H2O2 that activates peroxidases and generates hydroxyl radical. The group of copper oxidoreductases also includes other H2O2 generating enzymes - copper-radical oxidases - together with classical laccases that are the oxidoreductases with the largest no. of reported applications to date. However, the recently described lytic polysaccharide monooxygenases have attracted the highest attention among copper oxidoreductases, since they are capable of oxidatively breaking down cryst. cellulose, the disintegration of which is still a major bottleneck in lignocellulose biorefineries, along with lignin degrdn. Interestingly, some flavin-contg. dehydrogenases also play a key role in cellulose breakdown by directly/indirectly "fueling" electrons for polysaccharide monooxygenase activation. Many of the above oxidoreductases have been engineered, combining rational and computational design with directed evolution, to attain the selectivity, catalytic efficiency and stability properties required for their industrial utilization. Indeed, using ad hoc software and current computational capabilities, it is now possible to predict substrate access to the active site in biophys. simulations, and electron transfer efficiency in biochem. simulations, reducing in orders of magnitude the time of exptl. work in oxidoreductase screening and engineering. What has been set out above is illustrated by a series of remarkable oxyfunctionalization and oxidn. reactions developed in the frame of an intersectorial and multidisciplinary European RTD project. The optimized reactions include enzymic synthesis of 1-naphthol, 25-hydroxyvitamin D3, drug metabolites, furandicarboxylic acid, indigo and other dyes, and conductive polyaniline, terminal oxygenation of alkanes, biomass delignification and lignin oxidn., among others. These successful case stories demonstrate the unexploited potential of oxidoreductases in medium and large-scale biotransformations.
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    5. 5
      Hofrichter, M.; Ullrich, R. Oxidations Catalyzed by Fungal Peroxygenases. Curr. Opin. Chem. Biol. 2014, 19, 116– 125,  DOI: 10.1016/j.cbpa.2014.01.015
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      Oxidations catalyzed by fungal peroxygenases
      Hofrichter, Martin; Ullrich, Rene
      Current Opinion in Chemical Biology (2014), 19 (), 116-125CODEN: COCBF4; ISSN:1367-5931. (Elsevier B.V.)
      A review. The enzymic oxyfunctionalization of org. mols. under physiol. conditions has attracted keen interest from the chem. community. Unspecific peroxygenases (EC 1.11.2.1) secreted by fungi represent an intriguing enzyme type that selectively transfers peroxide-borne oxygen with high efficiency to diverse substrates including unactivated hydrocarbons. They are glycosylated heme-thiolate enzymes that form a sep. superfamily of hemoproteins. Among the catalyzed reactions are hydroxylations, epoxidns., dealkylations, oxidns. of org. hetero atoms, and inorg. halides, as well as one-electron oxidns. The substrate spectrum of fungal peroxygenases and the product patterns show similarities both to cytochrome P 450 monooxygenases and classic heme peroxidases. Given that selective oxyfunctionalizations are among the most difficult to realize chem. reactions and that resp. transformed mols. are of general importance in org. and pharmaceutical syntheses, it will be worth developing peroxygenase biocatalysts for industrial applications.
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    6. 6
      Holtmann, D.; Hollmann, F. The Oxygen Dilemma: A Severe Challenge for the Application of Monooxygenases?. ChemBioChem 2016, 17, 1391– 1398,  DOI: 10.1002/cbic.201600176
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      The Oxygen Dilemma: A Severe Challenge for the Application of Monooxygenases?
      Holtmann, Dirk; Hollmann, Frank
      ChemBioChem (2016), 17 (15), 1391-1398CODEN: CBCHFX; ISSN:1439-4227. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. Monooxygenases are promising catalysts because they in principle enable the org. chemist to perform highly selective oxyfunctionalisation reactions that are otherwise difficult to achieve. For this, monooxygenases require reducing equiv., to allow reductive activation of mol. oxygen at the enzymes' active sites. However, these reducing equiv. are often delivered to O2 either directly or via a reduced intermediate (uncoupling), yielding hazardous reactive oxygen species and wasting valuable reducing equiv. The oxygen dilemma arises from monooxygenases' dependency on O2 and the undesired uncoupling reaction. With this contribution we hope to generate a general awareness of the oxygen dilemma and to discuss its nature and some promising solns.
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    7. 7
      Valderrama, B.; Ayala, M.; Vazquez-Duhalt, R. Suicide Inactivation of Peroxidases and the Challenge of Engineering More Robust Enzymes. Chem. Biol. 2002, 9, 555– 565,  DOI: 10.1016/S1074-5521(02)00149-7
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      Suicide inactivation of peroxidases and the challenge of engineering more robust enzymes
      Valderrama, Brenda; Ayala, Marcela; Vazquez-Duhalt, Rafael
      Chemistry & Biology (2002), 9 (5), 555-565CODEN: CBOLE2; ISSN:1074-5521. (Cell Press)
      A review with 171 refs. As the no. of industrial applications for proteins continues to expand, the exploitation of protein engineering becomes crit. It is predicted that protein engineering can generate enzymes with new catalytic properties and create desirable, high-value, products at lower prodn. costs. Peroxidases are ubiquitous enzymes that catalyze a variety of O2-transfer reactions and are thus potentially useful for industrial and biomedical applications. However, peroxidases are unstable and are readily inactivated by their substrate, H2O2. Researchers rely on the powerful tools of mol. biol. to improve the stability of these enzymes, either by protecting residues sensitive to oxidn. or by devising more efficient intramol. pathways for free-radical allocation. Here, the authors discuss the catalytic cycle of peroxidases and the mechanism of the suicide inactivation process to establish a broad knowledge base for future rational protein engineering.
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    8. 8
      Burek, B. O. O.; Bormann, S.; Hollmann, F.; Bloh, J.; Holtmann, D. Hydrogen peroxide Driven Biocatalysis. Green Chem. 2019, 21, 3232– 3249,  DOI: 10.1039/C9GC00633H
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      Hydrogen peroxide driven biocatalysis
      Burek, B. O.; Bormann, S.; Hollmann, F.; Bloh, J. Z.; Holtmann, D.
      Green Chemistry (2019), 21 (12), 3232-3249CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
      A review. In general, hydrogen peroxide is a stable and relatively mild oxidant and it can be regarded as the ultimate "green" reagent because water and oxygen are the only byproducts. Besides the direct application of H2O2 in chem. processes more and more enzymic syntheses based on hydrogen peroxide are being developed. Different types of reactions can be addressed by using a hydrogen-peroxide driven biocatalysis (e.g. hydroxylations, epoxidns., sulfoxidns., halogenations, Baeyer-Villiger oxidns., decarboxylations). H2O2-driven reactions can often be used to substitute NAD(P)H dependent reactions. Therefore, laborious cofactor regeneration systems can be avoided by using H2O2-dependent enzymes. The tremendous increase in the no. of publications dealing with this type of reactions clearly demonstrates the progress in this area in recent years. The described innovations range from new enzymes and types of reaction to novel reaction engineering approaches. This review aims to give the scope of possible advantageous applications of peroxyzymes and a crit. discussion of their current limitations. The versatile reactions, the ecol. advantageous and the great progress in the discovery and engineering of novel enzymes make a tech. use feasible.
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    9. 9
      Paul, C. E.; Churakova, E.; Maurits, E.; Girhard, M.; Urlacher, V. B.; Hollmann, F. In situ formation of H2O2 for P450 Peroxygenases. Bioorg. Med. Chem. 2014, 22, 5692– 5696,  DOI: 10.1016/j.bmc.2014.05.074
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      In situ formation of H2O2 for P450 peroxygenases
      Paul, Caroline E.; Churakova, Ekaterina; Maurits, Elmer; Girhard, Marco; Urlacher, Vlada B.; Hollmann, Frank
      Bioorganic & Medicinal Chemistry (2014), 22 (20), 5692-5696CODEN: BMECEP; ISSN:0968-0896. (Elsevier B.V.)
      An in situ H2O2 generation approach to promote P 450 peroxygenases catalysis was developed through the use of the nicotinamide cofactor analog 1-benzyl-1,4-dihydronicotinamide (BNAH) and FMN. Final productivity could be enhanced due to higher enzyme stability at low H2O2 concns. The H2O2 generation represented the rate-limiting step, however it could be easily controlled by varying both FMN and BNAH concns. Further characterization can result in an optimized ratio of FMN/BNAH/O2/biocatalyst enabling high reaction rates while minimizing H2O2-related inactivation of the enzyme.
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    10. 10
      Hollmann, F.; Schmid, A. Towards [Cp*Rh(bpy)(H2O)]2+-promoted P450 Catalysis: Direct Regeneration of CytC. J. Inorg. Biochem. 2009, 103, 313– 315,  DOI: 10.1016/j.jinorgbio.2008.11.005
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      Towards [Cp*Rh(bpy)(H2O)]2+-promoted P450 catalysis: Direct regeneration of CytC
      Hollmann, Frank; Schmid, Andreas
      Journal of Inorganic Biochemistry (2009), 103 (3), 313-315CODEN: JIBIDJ; ISSN:0162-0134. (Elsevier B.V.)
      The organometallic complex pentamethylcyclopentadienyl rhodium 2,2'-bipyridine ([Cp*Rh(bpy)(H2O)]2+) was applied as regeneration catalyst for cytochrome C (CytC). Direct redn. of CytC-bound FeIII was achieved in this model system pointing towards a potential usefulness of this concept to promote cell-free P 450 catalysis. In addn., controlled in situ provision with hydrogen peroxide was performed using [Cp*Rh(bpy)(H2O)]2+ resulting in improved CytC-catalyzed sulfoxidn. of thioanisol. This work represents the first step towards the direct-[Cp*Rh(bpy)(H2O)]2+ catalyzed regeneration of P 450 monooxygenases and peroxidases.
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    11. 11
      Karmee, S. K.; Roosen, C.; Kohlmann, C.; Lütz, S.; Greiner, L.; Leitner, W. Chemo-Enzymatic Cascade Oxidation in Supercritical Carbon Dioxide/Water Biphasic Media. Green Chem. 2009, 11, 1052– 1055,  DOI: 10.1039/b820606f
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      Chemo-enzymatic cascade oxidation in supercritical carbon dioxide/water biphasic media
      Karmee, Sanjib Kumar; Roosen, Christoph; Kohlmann, Christina; Luetz, Stephan; Greiner, Lasse; Leitner, Walter
      Green Chemistry (2009), 11 (7), 1052-1055CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
      Enantioselective sulfoxidn. was carried out by cascade reaction of Pd(0)-catalyzed formation of H2O2 and enzymic oxidn. using chloroperoxidase from Caldariomyces fumago. Supercrit. CO2 (scCO2) was used as medium for in-situ generation of H2O2 directly from H2 and O2 using Pd-catalysts. Subsequently, H2O2 was utilized by the chloroperoxidase as an oxidant for the asym. sulfoxidn. in the aq. phase. This chemo-enzymic cascade transformation exemplifies the potential of compartmentalization of catalytic processes in multiphase systems.
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    12. 12
      Ranganathan, S.; Sieber, V. Recent Advances in the Direct Synthesis of Hydrogen Peroxide Using Chemical Catalysis—A Review. Catalysts 2018, 8, 379,  DOI: 10.3390/catal8090379
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      Recent advances in the direct synthesis of hydrogen peroxide using chemical Catalysis-A review
      Ranganathan, Sumanth; Sieber, Volker
      Catalysts (2018), 8 (9), 379/1-379/22CODEN: CATACJ; ISSN:2073-4344. (MDPI AG)
      Hydrogen peroxide is an important chem. of increasing demand in today's world. Currently, the anthraquinone autoxidn. process dominates the industrial prodn. of hydrogen peroxide. Herein, hydrogen and oxygen are reacted indirectly in the presence of quinones to yield hydrogen peroxide. Owing to the complexity and multi-step nature of the process, it is advantageous to replace the process with an easier and straightforward one. The direct synthesis of hydrogen peroxide from its constituent reagents is an effective and clean route to achieve this goal. Factors such as water formation due to thermodn., explosion risk, and the stability of the hydrogen peroxide produced hinder the applicability of this process at an industrial level. Currently, the catalysis for the direct synthesis reaction is palladium based and the research into finding an effective and active catalyst has been ongoing for more than a century now. Palladium in its pure form, or alloyed with certain metals, are some of the new generation of catalysts that are extensively researched. Addnl., to prevent the decompn. of hydrogen peroxide to water, the process is stabilized by adding certain promoters such as mineral acids and halides. A major part of today's research in this field focusses on the reactor and the mode of operation required for synthesizing hydrogen peroxide. The emergence of microreactor technol. has helped in setting up this synthesis in a continuous mode, which could possibly replace the anthraquinone process in the near future. This review will focus on the recent findings of the scientific community in terms of reaction engineering, catalyst and reactor design in the direct synthesis of hydrogen peroxide.
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      Ranganathan, S.; Zeitlhofer, S.; Sieber, V. Development of a Lipase-Mediated Epoxidation Process for Monoterpenes in Choline Chloride-based Deep Eutectic Solvents. Green Chem. 2017, 19, 2576– 2586,  DOI: 10.1039/C7GC01127J
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      Development of a lipase-mediated epoxidation process for monoterpenes in choline chloride-based deep eutectic solvents
      Ranganathan, Sumanth; Zeitlhofer, Sandra; Sieber, Volker
      Green Chemistry (2017), 19 (11), 2576-2586CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
      Chem. syntheses in contemporary process industries today are predominantly conducted using org. solvents, which are potentially hazardous to humans and the environment alike. Green chem. was developed as a means to overcome this hazard and it also holds enormous potential for designing clean, safe and sustainable processes. The present work incorporates the concepts of green chem. in its design of a lipase-mediated epoxidn. process for monoterpenes; the process uses alternative reaction media, namely deep eutectic solvents (DESs), which have not been reported for such an application before. Choline chloride (ChCl), in combination with a variety of hydrogen bond donors (HBD) at certain molar ratios, was screened and tested for this purpose. The process was optimized through the design of expts. (DoE) using the Taguchi method for four controllable parameters (temp., enzyme amt., peroxide amt. and type of substrate) and one uncontrollable parameter (DES reaction media) in a crossed-array design. Two distinct DESs, namely glycerol : choline chloride (GlCh) and sorbitol : choline chloride (SoCh), were found to be the best systems and they resulted in a complete conversion of the substrates within 8 h. Impurities (esters) were found to form in both the DESs, which was a concern; as such, we developed a novel minimal DES system that incorporated a co-substrate into the DES so that this issue could be overcome. The minimal DES consisted of urea·H2O2 (U·H2O2) and ChCl and exhibited better results than both the GlCh and SoCh systems; complete conversions were achieved within 2 h for 3-carene and within 3 h for both limonene and α-pinene. Product isolation with a simple water/ethyl acetate based procedure gave isolated yields of 87.2 ± 2.4%, 77.0 ± 5.0% and 84.6 ± 3.7% for 3-carene, limonene and α-pinene resp.
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      Ranganathan, S.; Sieber, V. Development of Semi-Continuous Chemo-Enzymatic Terpene Epoxidation: Combination of Anthraquinone Autooxidation and the Lipase-Mediated Epoxidation Process. React. Chem. Eng. 2017, 2, 885– 895,  DOI: 10.1039/C7RE00112F
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      14
      Development of semi-continuous chemo-enzymatic terpene epoxidation: combination of anthraquinone autooxidation and the lipase-mediated epoxidation process
      Ranganathan, Sumanth; Sieber, Volker
      Reaction Chemistry & Engineering (2017), 2 (6), 885-895CODEN: RCEEBW; ISSN:2058-9883. (Royal Society of Chemistry)
      Lipase has been used for epoxidizing olefins such as monoterpenes for more than two decades. This epoxidn. is accomplished by adding hydrogen peroxide (H2O2) to a carboxylic acid in the presence of a lipase such as Candida antartica lipase B (CALB) to produce percarboxylic acid, which then epoxidized monoterpenes according to the Prilezhaev mechanism. One drawback of this process is the need for continuous addn. of hydrogen peroxide to maintain max. productivity. To overcome this hurdle, the industrial anthraquinone autoxidn. process for hydrogen peroxide prodn. was scaled down and coupled with lipase-mediated epoxidn. in a semi-continuous manner. Palladium on alumina pellets (5% loading) was used as the catalyst for obtaining high yields of high-concn. hydrogen peroxide (50% wt. by vol.), followed by epoxidn. of 3-carene, (+) limonene, and α-pinene. A total reaction time of 5 h was used for hydrogen peroxide prodn. and 2-3 h for the epoxidn. reactions. Pure 3-carene epoxide and α-pinene epoxide were obtained in isolated yields of 88.8 ± 2.8% and 83.8 ± 2.6%, resp. Limonene epoxide was obtained as a mixt. of mono- and di-epoxides in a ratio of 70% and 30%, resp., with an isolated yield of 71.5 ± 3.1%.
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      Ranganathan, S.; Tebbe, J.; Wiemann, L.; Sieber, V. Optimization of the Lipase Mediated Epoxidation of Monoterpenes using the Design of Experiments-Taguchi Method. Process Biochem. 2016, 51, 1479– 1485,  DOI: 10.1016/j.procbio.2016.07.005
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      Optimization of the lipase mediated epoxidation of monoterpenes using the design of experiments-Taguchi method
      Ranganathan, Sumanth; Tebbe, Johannes; Wiemann, Lars O.; Sieber, Volker
      Process Biochemistry (Oxford, United Kingdom) (2016), 51 (10), 1479-1485CODEN: PBCHE5; ISSN:1359-5113. (Elsevier Ltd.)
      This work deals with the optimization of the Candida antarctica lipase B (CALB) mediated epoxidn. of monoterpenes by using the design of expts. (DoE) working with the Taguchi Method. Epoxides are essential org. intermediates that find various industrial applications making epoxidn. one of the most investigated processes in chem. industry. As many as 8 parameters such as the reaction medium, carboxylic acid type, carboxylic acid concn., temp., monoterpene type, monoterpene concn., hydrogen peroxide concn. and amt. of lipase were optimized using as little as 18 runs in triplicates (54 runs). As a result, the hydrogen peroxide concn. used was found to be the most influential parameter of this process while the type of monoterpene was least influential. Scaling up of the reaction conditions according to the findings of the optimization achieved full conversion in less than 6 h. In addn., a purifn. process for the epoxides was developed leading to an isolated yield of ca. 72.3%, 88.8% and 62.5% for α-pinene, 3-carene and limonene, resp.
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      Kohlmann, C.; Lütz, S. Electroenzymatic Synthesis of Chiral Sulfoxides. Eng. Life Sci. 2006, 6, 170– 174,  DOI: 10.1002/elsc.200620907
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      Electroenzymatic synthesis of chiral sulfoxides
      Kohlmann, C.; Luetz, S.
      Engineering in Life Sciences (2006), 6 (2), 170-174CODEN: ELSNAE; ISSN:1618-0240. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Chloroperoxidase (CPO) from Caldariomyces fumago (E.C.1.11.1.10) is able to enantioselectively oxidize various sulfides to the corresponding (R)-enantiomer of the sulfoxides. For these oxidns. the enzyme requires an oxidant. Most commonly, tert-Bu hydroperoxide (TBHP) and hydrogen peroxide are used. As it is known that these oxidants inactivate the enzyme, the enzymic reaction was combined with the electrochem. in situ generation of hydrogen peroxide. As substrates for this combination of an enzymic and an electrochem. reaction Me p-tolyl sulfide, 1-methoxy-4-(methylthio)benzene and N-MOC-L-methionine Me ester were used to carry out batch expts.
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      Lutz, S.; Steckhan, E.; Liese, A. First Asymmetric Electroenzymatic Oxidation Catalyzed by a Peroxidase. Electrochem. Commun. 2004, 6, 583– 587,  DOI: 10.1016/j.elecom.2004.04.009
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      First asymmetric electroenzymatic oxidation catalyzed by a peroxidase
      Lutz, Stephan; Steckhan, Eberhard; Liese, Andreas
      Electrochemistry Communications (2004), 6 (6), 583-587CODEN: ECCMF9; ISSN:1388-2481. (Elsevier Science B.V.)
      Thioanisole is selectively oxidized to (R)-methylphenylsulfoxide (ee > 98.5%) with electrochem. generated hydrogen peroxide catalyzed by chloroperoxidase (E.C. 1.11.1.10) from Caldariomyces fumago. Hydrogen peroxide is generated in situ by cathodic redn. of oxygen. This is the first example of an asym. electroenzymic synthesis with a peroxidase. The reaction was carried out on 300 mL scale with a productivity of 30 g L-1 d-1.
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      Horst, A. E. W.; Bormann, S.; Meyer, J.; Steinhagen, M.; Ludwig, R.; Drews, A.; Ansorge-Schumacher, M.; Holtmann, D. Electro-Enzymatic Hydroxylation of Ethylbenzene by the Evolved Unspecific Peroxygenase of Agrocybeaegerita. J. Mol. Catal. B: Enzym. 2016, 133, S137– S142,  DOI: 10.1016/j.molcatb.2016.12.008
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      Holtmann, D.; Krieg, T.; Getrey, L.; Schrader, J. Electroenzymatic Process to Overcome Enzyme Instabilities. Catal. Commun. 2014, 51, 82– 85,  DOI: 10.1016/j.catcom.2014.03.033
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      Electroenzymatic process to overcome enzyme instabilities
      Holtmann, Dirk; Krieg, Thomas; Getrey, Laura; Schrader, Jens
      Catalysis Communications (2014), 51 (), 82-85CODEN: CCAOAC; ISSN:1566-7367. (Elsevier B.V.)
      The versatile enzyme chloroperoxidase was used in a reaction system, based on a gas diffusion electrode, for enzymic chlorinations. Due to an adjusted and continuous electro-generation of the co-substrate hydrogen peroxide a ttn up to 1,150,000 for the CPO was achieved. Space time yields were dependent on the electrochem. produced H2O2 and reached up to 52 g L- 1 d- 1. The ratio of hydrogen peroxide prodn. per added enzyme unit can be used as a dimensionless parameter for process characterization and a knowledge based process design.
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      Getrey, L.; Krieg, T.; Hollmann, F.; Schrader, J.; Holtmann, D. Enzymatic Halogenation of the Phenolic Monoterpenes Thymol and Carvacrol with Chloroperoxidase. Green Chem. 2014, 16, 1104– 1108,  DOI: 10.1039/C3GC42269K
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      Enzymatic halogenation of the phenolic monoterpenes thymol and carvacrol with chloroperoxidase
      Getrey, Laura; Krieg, Thomas; Hollmann, Frank; Schrader, Jens; Holtmann, Dirk
      Green Chemistry (2014), 16 (3), 1104-1108CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
      The conversion of the phenolic monoterpenes thymol and carvacrol into antimicrobials by (electro)-chemoenzymic halogenation was investigated using a chloroperoxidase (CPO) catalyzed process. The CPO catalyzed process enables for the first time the biotechnol. prodn. of chlorothymol, chlorocarvacrol and bromothymol as well as a dichlorothymol with high conversion rates, total turnover nos. and space time yields of up to 90%, 164 000 and 4.6 mM h-1, resp.
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      Krieg, T.; Huttmann, S.; Mangold, K.-M.; Schrader, J.; Holtmann, D. Gas Diffusion Electrode as Novel Reaction System for an Electro-Enzymatic Process with Chloroperoxidase. Green Chem. 2011, 13, 2686– 2689,  DOI: 10.1039/c1gc15391a
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      21
      Gas diffusion electrode as novel reaction system for an electro-enzymatic process with chloroperoxidase
      Krieg, Thomas; Huettmann, Sonja; Mangold, Klaus-Michael; Schrader, Jens; Holtmann, Dirk
      Green Chemistry (2011), 13 (10), 2686-2689CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
      The versatile enzyme chloroperoxidase was used in a new reaction system, based on a gas diffusion electrode, for enzymic chlorinations, sulfoxidns. and oxidns. This is the first report on the combination of hydrogen peroxide prodn. at a GDE with an enzymic reaction.
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      Pereira, P. C.; Arends, I.; Sheldon, R. A. Optimizing the Chloroperoxidase-Glucose Oxidase System: The Effect of Glucose Oxidase on Activity and Enantioselectivity. Process Biochem. 2015, 50, 746– 751,  DOI: 10.1016/j.procbio.2015.02.006
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      Optimizing the chloroperoxidase-glucose oxidase system: The effect of glucose oxidase on activity and enantioselectivity
      Pereira, Pedro C.; Arends, Isabel W. C. E.; Sheldon, Roger A.
      Process Biochemistry (Oxford, United Kingdom) (2015), 50 (5), 746-751CODEN: PBCHE5; ISSN:1359-5113. (Elsevier Ltd.)
      The optimum application of chloroperoxidase from Caldariomyces fumago in oxidns. with hydrogen peroxide depends on the mode of addn. of the oxidant. The use of the previously reported combination of chloroperoxidase and glucose oxidase, for in situ generation of hydrogen peroxide, was studied in more detail using thioanisole as a model substrate. Maximum yields and enantiopurities were obsd. at high chloroperoxidase reaction rates and not at low hydrogen peroxide formation rates, as would be expected considering the instability of CPO at high hydrogen peroxide concns. Glucose oxidase catalyzed aerobic sulfoxidn., affording racemic sulfoxide, was obsd. as an unexpected and novel side-reaction. It was attributed to oxidn. by a flavin hydroperoxide formed by reaction of the free flavin cofactor assocd. with glucose oxidase with dioxygen. The rate of this side-reaction depended on the amt. of co-solvent in the system and the enantiopurity of the oxidn. product could thus be improved by lowering the co-solvent concn.
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    23. 23
      Tieves, F.; Willot, S. J.-P.; van Schie, M. M. C. H.; Rauch, M. C. R.; Younes, S. H. H.; Zhang, W.; Dong, J.; de Santos, P. G.; Robbins, J. M.; Bommarius, B.; Alcalde, M.; Bommarius, A.; Hollmann, F. Formate Oxidase (FOx) from Aspergillus oryzae: One Catalyst to Promote H2O2-Dependent Biocatalytic Oxidation Reactions. Angew. Chem., Int. Ed. 2019, 58, 7873– 7877,  DOI: 10.1002/anie.201902380
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      Formate oxidase (FOx) from Aspergillus oryzae: One catalyst enables diverse H2O2-dependent biocatalytic oxidation reactions
      Tieves, Florian; Willot, Sebastien Jean-Paul; van Schie, Morten Martinus Cornelis Harald; Rauch, Marine Charlene Renee; Younes, Sabry Hamdy Hamed; Zhang, Wuyuan; Dong, JiaJia; Gomez de Santos, Patricia; Robbins, John Mick; Bommarius, Bettina; Alcalde, Miguel; Bommarius, Andreas Sebastian; Hollmann, Frank
      Angewandte Chemie, International Edition (2019), 58 (23), 7873-7877CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      An increasing no. of biocatalytic oxidn. reactions rely on H2O2 as a clean oxidant. The poor robustness of most enzymes towards H2O2, however, necessitates more efficient systems for in situ H2O2 generation. In analogy to the well-known formate dehydrogenase to promote NADH-dependent reactions, we here propose employing formate oxidase (FOx) to promote H2O2-dependent enzymic oxidn. reactions. Even under non-optimized conditions, high turnover nos. for coupled FOx/peroxygenase catalysis were achieved.
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      Pesic, M.; Willot, S. J.-P.; Fernández-Fueyo, E.; Tieves, F.; Alcalde, M.; Hollmann, F. Multienzymatic in situ Hydrogen Peroxide Generation Cascade for Peroxygenase-Catalysed Oxyfunctionalisation Reactions. Z. f. Naturforsch. C 2019, 74, 101– 104,  DOI: 10.1515/znc-2018-0137
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      Multienzymatic in situ hydrogen peroxide generation cascade for peroxygenase-catalysed oxyfunctionalisation reactions
      Pesic, Milja; Willot, Sebastien Jean-Paul; Fernandez-Fueyo, Elena; Tieves, Florian; Alcalde, Miguel; Hollmann, Frank
      Zeitschrift fuer Naturforschung, C: Journal of Biosciences (2019), 74 (3-4), 101-104CODEN: ZNCBDA; ISSN:1865-7125. (Walter de Gruyter GmbH)
      There is an increasing interest in the application of peroxygenases in biocatalysis, because of their ability to catalyze the oxyfunctionalisation reaction in a stereoselective fashion and with high catalytic efficiencies, while using hydrogen peroxide or org. peroxides as oxidant. However, enzymes belonging to this class exhibit a very low stability in the presence of peroxides. With the aim of bypassing this fast and irreversible inactivation, we study the use of a gradual supply of hydrogen peroxide to maintain its concn. at stoichiometric levels. In this contribution, we report a multienzymic cascade for in situ generation of hydrogen peroxide. In the first step, in the presence of NAD+ cofactor, formate dehydrogenase from Candida boidinii (FDH) catalyzed the oxidn. of formate yielding CO2. Reduced NADH was reoxidised by the redn. of the FMN cofactor bound to an old yellow enzyme homolog from Bacillus subtilis (YqjM), which subsequently reacts with mol. oxygen yielding hydrogen peroxide. Finally, this system was coupled to the hydroxylation of ethylbenzene reaction catalyzed by an evolved peroxygenase from Agrocybe aegerita (rAaeUPO). Addnl., we studied the influence of different reaction parameters on the performance of the cascade with the aim of improving the turnover of the hydroxylation reaction.
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    25. 25
      Ma, Y.; Li, P.; Li, Y.; Willot, S. J.-P.; Zhang, W.; Ribitsch, D.; Choi, Y. H.; Zhang, T.; Verpoorte, R.; Hollmann, F.; Wang, Y. Natural Deep Eutectic Solvents as Multifunctional Media for the Valorisation of Agricultural Wastes. ChemSusChem 2019, 12, 1310– 1315,  DOI: 10.1002/cssc.201900043
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      Natural Deep Eutectic Solvents as Multifunctional Media for the Valorization of Agricultural Wastes
      Ma, Yunjian; Li, Peilin; Li, Yongru; Willot, Sebastien J.-P.; Zhang, Wuyuan; Ribitsch, Doris; Choi, Young Hae; Verpoorte, Robert; Zhang, Tianyu; Hollmann, Frank; Wang, Yonghua
      ChemSusChem (2019), 12 (7), 1310-1315CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The use of natural deep eutectic solvents (NADES) as multifunctional solvents for limonene bioprocessing was reported. NADES were used for the extn. of limonene from orange peel wastes, as solvent for the chemoenzymic epoxidn. of limonene, and as sacrificial electron donor for the in situ generation of H2O2 to promote the epoxidn. reaction. The proof-of-concept for this multifunctional use was provided, and the scope and current limitations of the concept were outlined.
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    26. 26
      Ni, Y.; Fernández-Fueyo, E.; Baraibar, A. G.; Ullrich, R.; Hofrichter, M.; Yanase, H.; Alcalde, M.; van Berkel, W. J. H.; Hollmann, F. Peroxygenase-Catalyzed Oxyfunctionalization Reactions Promoted by the Complete Oxidation of Methanol. Angew. Chem., Int. Ed. 2016, 55, 798– 801,  DOI: 10.1002/anie.201507881
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      Peroxygenase-Catalyzed Oxyfunctionalization Reactions Promoted by the Complete Oxidation of Methanol
      Ni, Yan; Fernandez-Fueyo, Elena; Baraibar, Alvaro Gomez; Ullrich, Rene; Hofrichter, Martin; Yanase, Hideshi; Alcalde, Miguel; van Berkel, Willem J. H.; Hollmann, Frank
      Angewandte Chemie, International Edition (2016), 55 (2), 798-801CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Peroxygenases catalyze a broad range of (stereo)selective oxyfunctionalization reactions. However, to access their full catalytic potential, peroxygenases need a balanced provision of hydrogen peroxide to achieve high catalytic activity while minimizing oxidative inactivation. Herein, we report an enzymic cascade process that employs methanol as a sacrificial electron donor for the reductive activation of mol. oxygen. Full oxidn. of methanol is achieved, generating three equiv. of hydrogen peroxide that can be used completely for the stereoselective hydroxylation of ethylbenzene as a model reaction. Overall we propose and demonstrate an atom-efficient and easily applicable alternative to established hydrogen peroxide generation methods, which enables the efficient use of peroxygenases for oxyfunctionalization reactions.
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      Willot, S. J. P.; Fernández-Fueyo, E.; Tieves, F.; Pesic, M.; Alcalde, M.; Arends, I. W. C. E.; Park, C. B.; Hollmann, F. Expanding the Spectrum of Light-Driven Peroxygenase Reactions. ACS Catal. 2019, 9, 890– 894,  DOI: 10.1021/acscatal.8b03752
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      Expanding the spectrum of light-driven peroxygenase reactions
      Willot, Sebastien J.-P.; Fernandez-Fueyo, Elena; Tieves, Florian; Pesic, Milja; Alcalde, Miguel; Arends, Isabel W. C. E.; Park, Chan Beum; Hollmann, Frank
      ACS Catalysis (2019), 9 (2), 890-894CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      Peroxygenases require a controlled supply of H2O2 to operate efficiently. Here, we propose a photocatalytic system for the reductive activation of ambient O2 to produce H2O2 which uses the energy provided by visible light more efficiently based on the combination of wavelength-complementary photosensitizers. This approach was coupled to an enzymic system to make formate available as a sacrificial electron donor. The scope and current limitations of this approach are reported and discussed.
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    28. 28
      Zhang, W.; Fernández-Fueyo, E.; Ni, Y.; van Schie, M.; Gacs, J.; Renirie, R.; Wever, R.; Mutti, F. G.; Rother, D.; Alcalde, M.; Hollmann, F. Selective Aerobic Oxidation Reactions using a Combination of Photocatalytic Water Oxidation and Enzymatic Oxyfunctionalizations. Nat. Catal. 2018, 1, 55– 62,  DOI: 10.1038/s41929-017-0001-5
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      Selective aerobic oxidation reactions using a combination of photocatalytic water oxidation and enzymatic oxyfunctionalizations
      Zhang, Wuyuan; Fernandez-Fueyo, Elena; Ni, Yan; van Schie, Morten; Gacs, Jenoe; Renirie, Rokus; Wever, Ron; Mutti, Francesco G.; Rother, Doerte; Alcalde, Miguel; Hollmann, Frank
      Nature Catalysis (2018), 1 (1), 55-62CODEN: NCAACP; ISSN:2520-1158. (Nature Research)
      Peroxygenases offer an attractive means to address challenges in selective oxyfunctionalization chem. Despite this, their application in synthetic chem. remains challenging due to their facile inactivation by the stoichiometric oxidant H2O2. Often atom-inefficient peroxide generation systems are required, which show little potential for large-scale implementation. Here, we show that visible-light-driven, catalytic water oxidn. can be used for in situ generation of H2O2 from water, rendering the peroxygenase catalytically active. In this way, the stereoselective oxyfunctionalization of hydrocarbons can be achieved by simply using the catalytic system, water and visible light.
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      Zhang, W.; Burek, B. O.; Fernández-Fueyo, E.; Alcalde, M.; Bloh, J. Z.; Hollmann, F. Selective Activation of C-H Bonds by Cascading Photochemistry with Biocatalysis. Angew. Chem., Int. Ed. 2017, 56, 15451– 15455,  DOI: 10.1002/anie.201708668
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      Selective Activation of C-H Bonds in a Cascade Process Combining Photochemistry and Biocatalysis
      Zhang, Wuyuan; Burek, Bastien O.; Fernandez-Fueyo, Elena; Alcalde, Miguel; Bloh, Jonathan Z.; Hollmann, Frank
      Angewandte Chemie, International Edition (2017), 56 (48), 15451-15455CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Selective oxyfunctionalizations of inert C-H bonds can be achieved under mild conditions by using peroxygenases. This approach, however, suffers from the poor robustness of these enzymes in the presence of hydrogen peroxide as the stoichiometric oxidant. Herein, we demonstrate that inorg. photocatalysts such as gold-titanium dioxide efficiently provide H2O2 through the methanol-driven reductive activation of ambient oxygen in amts. that ensure that the enzyme remains highly active and stable. Using this approach, the stereoselective hydroxylation of ethylbenzene to (R)-1-phenylethanol was achieved with high enantioselectivity (>98 % ee) and excellent turnover nos. for the biocatalyst (>71 000).
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      Choi, D. S.; Ni, Y.; Fernández-Fueyo, E.; Lee, M.; Hollmann, F.; Park, C. B. Photoelectroenzymatic Oxyfunctionalization on Flavin-Hybridized Carbon Nanotube Electrode Platform. ACS Catal. 2017, 7, 1563– 1567,  DOI: 10.1021/acscatal.6b03453
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      30
      Photoelectroenzymatic Oxyfunctionalization on Flavin-Hybridized Carbon Nanotube Electrode Platform
      Choi, Da Som; Ni, Yan; Fernandez-Fueyo, Elena; Lee, Minah; Hollmann, Frank; Park, Chan Beum
      ACS Catalysis (2017), 7 (3), 1563-1567CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      Peroxygenases are very promising catalysts for oxyfunctionalization reactions. Their practical applicability, however, is hampered by their sensitivity against the oxidant (H2O2), therefore necessitating in situ generation of H2O2. Here, we report a photoelectrochem. approach to provide peroxygenases with suitable amts. of H2O2 while reducing the electrochem. overpotential needed for the redn. of mol. oxygen to H2O2. When tethered on single-walled carbon nanotubes (SWNTs) under illumination, flavins allowed for a marked anodic shift of the oxygen redn. potential in comparison to pristine SWNT and/or nonilluminated electrodes. This flavin-SWNT-based photoelectrochem. platform enabled peroxygenases-catalyzed, selective hydroxylation reactions.
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      Churakova, E.; Kluge, M.; Ullrich, R.; Arends, I.; Hofrichter, M.; Hollmann, F. Specific Photobiocatalytic Oxyfunctionalization Reactions. Angew. Chem., Int. Ed. 2011, 50, 10716– 10719,  DOI: 10.1002/anie.201105308
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      Specific Photobiocatalytic Oxyfunctionalization Reactions
      Churakova, Ekaterina; Kluge, Martin; Ullrich, Rene; Arends, Isabel; Hofrichter, Martin; Hollmann, Frank
      Angewandte Chemie, International Edition (2011), 50 (45), 10716-10719, S10716/1-S10716/11CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The present work demonstrates that the arom. peroxygenase from Agrocybe aegerita (AaeAPO) is an active and versatile catalyst for enantiospecific hydroxylation and epoxidn. reactions. In situ generation of H2O2 via photochem. redn. of O2 permits AaeAPO to sustain robust oxyfunctionalization activity for periods of up to several hours. Under nonoptimized reaction conditions the enzyme turnover nos. for AaeAPO exceed those of comparable systems like chloroperoxidase (CPO) from Caldariomyces fumago and cytochrome P 450 enzymes. Furthermore, unlike CPO, AaeAPO is able to catalyze the hydroxylation of nonactivated C-H bonds.
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      Gulder, T.; Seel, C. J. Biocatalysis Fueled by Light: On the Versatile Combination of Photocatalysis and Enzymes. ChemBioChem 2019,  DOI: 10.1002/cbic.201800806 .
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      Seel, C. J.; Králík, A.; Hacker, M.; Frank, A.; König, B.; Gulder, T. Atom-Economic Electron Donors for Photobiocatalytic Halogenations. ChemCatChem 2018, 10, 3960– 3963,  DOI: 10.1002/cctc.201800886
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      Atom-Economic Electron Donors for Photobiocatalytic Halogenations
      Seel, Catharina Julia; Kralik, Antonin; Hacker, Melanie; Frank, Annika; Koenig, Burkhard; Gulder, Tanja
      ChemCatChem (2018), 10 (18), 3960-3963CODEN: CHEMK3; ISSN:1867-3880. (Wiley-VCH Verlag GmbH & Co. KGaA)
      In vitro cofactor supply and regeneration have been a major obstacle for biocatalytic processes, in particular on a large scale. Peroxidases often suffer from inactivation by their oxidative co-factor. Combining photocatalysis and biocatalysis offers an innovative soln. to this problem, but lacks atom economy due to the sacrificial electron donors needed. Herein, we show that redox-active buffers or even water alone can serve as efficient, biocompatible electron sources, when combined with photocatalysis. Mechanistic investigations revealed first insights into the possibilities and limitations of this approach and allowed adjusting the reaction conditions to the specific needs of biocatalytic transformations. Proof-of-concept for the applicability of this photobiocatalytic reaction setup was given by enzymic halogenations.
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      Schmermund, L.; Jurkaš, V.; Özgen, F. F.; Barone, G. D.; Büchsenschütz, H. C.; Winkler, C. K.; Schmidt, S.; Kourist, R.; Kroutil, W. Photo-Biocatalysis: Biotransformations in the Presence of Light. ACS Catal. 2019, 9, 4115– 4144,  DOI: 10.1021/acscatal.9b00656
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      Photo-Biocatalysis: Biotransformations in the Presence of Light
      Schmermund, Luca; Jurkas, Valentina; Oezgen, F. Feyza; Barone, Giovanni D.; Buechsenschuetz, Hanna C.; Winkler, Christoph K.; Schmidt, Sandy; Kourist, Robert; Kroutil, Wolfgang
      ACS Catalysis (2019), 9 (5), 4115-4144CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      A review. Light has received increased attention for various chem. reactions but also in combination with biocatalytic reactions. Because currently only a few enzymic reactions are known, which per se require light, most transformations involving light and a biocatalyst exploit light either for providing the cosubstrate or cofactor in an appropriate redox state for the biotransformation. In selected cases, a promiscuous activity of known enzymes in the presence of light could be induced. In other approaches, light-induced chem. reactions have been combined with a biocatalytic step, or light-induced biocatalytic reactions were combined with chem. reactions in a linear cascade. Finally, enzymes with a light switchable moiety have been investigated to turn off/on or tune the actual reaction. This Review gives an overview of the various approaches for using light in biocatalysis.
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      Goldstein, S.; Aschengrau, D.; Diamant, Y.; Rabani, J. Photolysis of Aqueous H2O2: Quantum Yield and Applications for Polychromatic UV Actinometry in Photoreactors. Environ. Sci. Technol. 2007, 41, 7486– 7490,  DOI: 10.1021/es071379t
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      Photolysis of Aqueous H2O2: Quantum Yield and Applications for Polychromatic UV Actinometry in Photoreactors
      Goldstein, Sara; Aschengrau, Dorit; Diamant, Yishay; Rabani, Joseph
      Environmental Science & Technology (2007), 41 (21), 7486-7490CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      Methanol is used to measure the yield of •OH radicals produced in the photolysis of H2O2 in aq. solns. The UV photolysis of H2O2 generates •OH radicals, which in the presence of methanol, oxygen, and phosphate buffer form formaldehyde, namely, Φ(HCHO) = Φ(•OH). The quantum yield of •OH has been redetd. in view of literature inconsistencies resulting in Φ(•OH) = 1.11 ± 0.07 in the excitation range of 205-280 nm. The constancy of Φ(•OH) and the ease and sensitivity of the formaldehyde product anal. makes the H2O2/CH3OH system suitable for polychromatic UV actinometry. In addn., the relatively low cost of the main components and the possibility of destroying the methanol before disposal qualify the system for both monochromatic and polychromatic actinometry in a large vol. of water. The H2O2/CH3OH system was applied in different com. UV photoreactors.
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      Ullrich, R.; Hofrichter, M. The Haloperoxidase of the Agaric Fungus AgrocybeaegeritaHydroxylates Toluene and Naphthalene. FEBS Lett. 2005, 579, 6247– 6250,  DOI: 10.1016/j.febslet.2005.10.014
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      The haloperoxidase of the agaric fungus Agrocybe aegerita hydroxylates toluene and naphthalene
      Ullrich, Rene; Hofrichter, Martin
      FEBS Letters (2005), 579 (27), 6247-6250CODEN: FEBLAL; ISSN:0014-5793. (Elsevier B.V.)
      The mushroom Agrocybe aegerita secretes a peroxidase (AaP) that catalyzes halogenations and hydroxylations. Phenol was brominated to 2- and 4-bromophenol (ratio 1:4) and chlorinated to a lesser extent to 2-chlorophenol. The purified enzyme was found to oxidize toluene via benzyl alc. and benzaldehyde into benzoic acid. A second fraction of toluene was hydroxylated to give p-cresol as well as o-cresol and methyl-p-benzoquinone. The UV-Vis absorption spectrum of purified AaP showed high similarity to a resting state cytochrome P 450 with the Soret band at 420 nm and addnl. maxima at 278, 358, 541 and 571 nm; the AaP CO-complex had a distinct absorption max. at 445 nm that is characteristic for heme-thiolate proteins. AaP regioselectively hydroxylated naphthalene to 1-naphthol and traces of 2-naphthol (ratio 36:1). H2O2 was necessarily required for AaP function and hence the hydroxylations catalyzed by AaP can be designated as peroxygenation and the enzyme as an extracellular peroxygenase.
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      Ullrich, R.; Nüske, J.; Scheibner, K.; Spantzel, J.; Hofrichter, M. Novel Haloperoxidase from the Agaric Basidiomycete AgrocybeaegeritaOxidizes Aryl Alcohols and Aldehydes. Appl. Environ. Microbiol. 2004, 70, 4575– 4581,  DOI: 10.1128/AEM.70.8.4575-4581.2004
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      37
      Novel haloperoxidase from the agaric basidiomycete Agrocybe aegerita oxidizes aryl alcohols and aldehydes
      Ullrich, Rene; Nueske, Joerg; Scheibner, Katrin; Spantzel, Joerg; Hofrichter, Martin
      Applied and Environmental Microbiology (2004), 70 (8), 4575-4581CODEN: AEMIDF; ISSN:0099-2240. (American Society for Microbiology)
      Agrocybe aegerita, a bark mulch- and wood-colonizing basidiomycete, was found to produce a peroxidase (AaP) that oxidizes aryl alcs., such as veratryl and benzyl alcs., into the corresponding aldehydes and then into benzoic acids. The enzyme also catalyzed the oxidn. of typical peroxidase substrates, such as 2,6-dimethoxyphenol (DMP) or 2,2'-azinobis-(3-ethylbenzothiazoline-6-sulfonate) (ABTS). A. aegerita peroxidase prodn. depended on the concn. of org. nitrogen in the medium, and highest enzyme levels were detected in the presence of soybean meal. Two fractions of the enzyme, AaP I and AaP II, which had identical mol. masses (46 kDa) and isoelec. points of 4.6 to 5.4 and 4.9 to 5.6, resp. (corresponding to six different isoforms), were identified after several steps of purifn., including anion- and cation-exchange chromatog. The optimum pH for the oxidn. of aryl alcs. was found to be around 7, and the enzyme required relatively high concns. of H2O2 (2 mM) for optimum activity. The apparent Km values for ABTS, DMP, benzyl alc., veratryl alc., and H2O2 were 37, 298, 1001, 2367 and 1313 μM, resp. The N-terminal amino acid sequences of the main AaP II spots blotted after two-dimensional gel electrophoresis were almost identical and exhibited almost no homol. to the sequences of other peroxidases from basidiomycetes, but they shared the first three amino acids, as well as two addnl. amino acids, with the heme chloroperoxidase (CPO) from the ascomycete Caldariomyces fumago. This finding is consistent with the fact that AaP halogenates monochlorodimedone, the specific substrate of CPO. The existence of haloperoxidases in basidiomycetous fungi may be of general significance for the natural formation of chlorinated org. compds. in forest soils.
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    38. 38
      Molina-Espeja, P.; Ma, S.; Mate, D. M.; Ludwig, R.; Alcalde, M. Tandem-Yeast Expression System for Engineering and Producing Unspecific Peroxygenase. Enzyme Microb. Technol. 2015, 73–74, 29– 33,  DOI: 10.1016/j.enzmictec.2015.03.004
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      38
      Tandem-yeast expression system for engineering and producing unspecific peroxygenase
      Molina-Espeja, Patricia; Ma, Su; Mate, Diana M.; Ludwig, Roland; Alcalde, Miguel
      Enzyme and Microbial Technology (2015), 73-74 (), 29-33CODEN: EMTED2; ISSN:0141-0229. (Elsevier)
      Unspecific peroxygenase (UPO) is a highly efficient biocatalyst with a peroxide dependent monooxygenase activity and many biotechnol. applications, but the absence of suitable heterologous expression systems has precluded its use in different industrial settings. Recently, the UPO from Agrocybe aegerita was evolved for secretion and activity in Saccharomyces cerevisiae [8]. In the current work, we describe a tandem-yeast expression system for UPO engineering and large scale prodn. By harnessing the directed evolution process in S. cerevisiae, the beneficial mutations for secretion enabled Pichia pastoris to express the evolved UPO under the control of the methanol inducible alc. oxidase 1 promoter. While secretion levels were found similar for both yeasts in flask fermn. (∼8 mg/L), the recombinant UPO from P. pastoris showed a 27-fold enhanced prodn. in fed-batch fermn. (217 mg/L). The P. pastoris UPO variant maintained similar biochem. properties of the S. cerevisiae counterpart in terms of catalytic consts., pH activity profiles and thermostability. Thus, this tandem-yeast expression system ensures the engineering of UPOs to use them in future industrial applications as well as large scale prodn.
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    39. 39
      Molina-Espeja, P.; Garcia-Ruiz, E.; Gonzalez-Perez, D.; Ullrich, R.; Hofrichter, M.; Alcalde, M. Directed Evolution of Unspecific Peroxygenase from Agrocybeaegerita. Appl. Environ. Microbiol. 2014, 80, 3496– 3507,  DOI: 10.1128/AEM.00490-14
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      39
      Directed evolution of unspecific peroxygenase from Agrocybe aegerita
      Molina-Espeja, Patricia; Garcia-Ruiz, Eva; Gonzalez-Perez, David; Ullrich, Rene; Hofrichter, Martin; Alcalde, Miguel
      Applied and Environmental Microbiology (2014), 80 (11), 3496-3507, 13 pp.CODEN: AEMIDF; ISSN:1098-5336. (American Society for Microbiology)
      Unspecific peroxygenase (UPO) represents a new type of heme-thiolate enzyme with self-sufficient mono(per)oxygenase activity and many potential applications in org. synthesis. With a view to taking advantage of these properties, we subjected the Agrocybe aegerita UPO1-encoding gene to directed evolution in Saccharomyces cerevisiae. To promote functional expression, several different signal peptides were fused to the mature protein, and the resulting products were tested. Over 9,000 clones were screened using an ad hoc dual-colorimetric assay that assessed both peroxidative and oxygen transfer activities. After 5 generations of directed evolution combined with hybrid approaches, 9 mutations were introduced that resulted in a 3,250-fold total activity improvement with no alteration in protein stability. A breakdown between secretion and catalytic activity was performed by replacing the native signal peptide of the original parental type with that of the evolved mutant; the evolved leader increased functional expression 27-fold, whereas an 18-fold improvement in the kcat/Km value for oxygen transfer activity was obtained. The evolved UPO1 was active and highly stable in the presence of org. cosolvents. Mutations in the hydrophobic core of the signal peptide contributed to enhance functional expression up to 8 mg/L, while catalytic efficiencies for peroxidative and oxygen transfer reactions were increased by several mutations in the vicinity of the heme access channel. Overall, the directed-evolution platform described is a valuable point of departure for the development of customized UPOs with improved features and for the study of structure-function relationships.
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    40. 40
      Hirakawa, H.; Shiota, S.; Shiraishi, Y.; Sakamoto, H.; Ichikawa, S.; Hirai, T. Au Nanoparticles Supported on BiVO4: Effective Inorganic Photocatalysts for H2O2Production from Water and O2 under Visible Light. ACS Catal. 2016, 6, 4976– 4982,  DOI: 10.1021/acscatal.6b01187
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      40
      Au nanoparticles supported on BiVO4: Effective inorganic photocatalysts for H2O2 production from water and O2 under visible light
      Hirakawa, Hiroaki; Shiota, Shingo; Shiraishi, Yasuhiro; Sakamoto, Hirokatsu; Ichikawa, Satoshi; Hirai, Takayuki
      ACS Catalysis (2016), 6 (8), 4976-4982CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      The design of a safe and sustainable process for the synthesis of hydrogen peroxide (H2O2) is a very important subject from the viewpoint of green chem. Photocatalytic H2O2 prodn. with earth-abundant water and mol. oxygen (O2) as resources is an ideal process. A successful system based on an org. semiconductor has been proposed; however, it suffers from poor photostability. Here we report an inorg. photocatalyst for H2O2 synthesis. Visible light irradn. (λ >420 nm) of the semiconductor BiVO4 loaded with Au nanoparticles (Au/BiVO4) in pure water with O2 successfully produces H2O2. The bottom of the BiVO4 conduction band (0.02 V vs NHE, pH 0) is more pos. than the one-electron redn. potential of O2 (-0.13 V) while more neg. than the two-electron redn. potential of O2 (0.68 V). This thus suppresses one-electron redn. of O2 and selectively promotes two-electron redn. of O2, resulting in efficient H2O2 formation.
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    41. 41
      Shi, N.; Cheng, W.; Zhou, H.; Fan, T.; Niederberger, M. Facile Synthesis of Monodisperse Co3O4Quantum Dots with Efficient Oxygen Evolution Activity. Chem. Commun. 2015, 51, 1338– 1340,  DOI: 10.1039/C4CC08179J
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      Facile synthesis of monodisperse Co3O4 quantum dots with efficient oxygen evolution activity
      Shi, Nan; Cheng, Wei; Zhou, Han; Fan, Tongxiang; Niederberger, Markus
      Chemical Communications (Cambridge, United Kingdom) (2015), 51 (7), 1338-1340CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      Monodisperse, water-dispersible Co3O4 quantum dots with sizes of around 4.5 nm are prepd. through a simple soln. method. The resultant cobalt oxide quantum dots exhibit excellent visible-light-driven oxygen evolution activities in the [Ru(bpy)3]2+-persulfate system under mild pH conditions.
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    42. 42
      Yin, Q. S.; Tan, J. M.; Besson, C.; Geletii, Y. V.; Musaev, D. G.; Kuznetsov, A. E.; Luo, Z.; Hardcastle, K. I.; Hill, C. L. A Fast Soluble Carbon-Free Molecular Water Oxidation Catalyst Based on Abundant Metals. Science 2010, 328, 342– 345,  DOI: 10.1126/science.1185372
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      42
      A Fast Soluble Carbon-Free Molecular Water Oxidation Catalyst Based on Abundant Metals
      Yin, Qiushi; Tan, Jeffrey Miles; Besson, Claire; Geletii, Yurii V.; Musaev, Djamaladdin G.; Kuznetsov, Aleksey E.; Luo, Zhen; Hardcastle, Ken I.; Hill, Craig L.
      Science (Washington, DC, United States) (2010), 328 (5976), 342-345CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Traditional homogeneous water oxidn. catalysts are plagued by instability under the reaction conditions. We report that the complex [Co4(H2O)2(PW9O34)2]10-, comprising a Co4O4 core stabilized by oxidatively resistant polytungstate ligands, is a hydrolytically and oxidatively stable homogeneous water oxidn. catalyst that self-assembles in water from salts of earth-abundant elements (Co, W, and P). With [Ru(bpy)3]3+ (bpy is 2,2'-bipyridine) as the oxidant, we observe catalytic turnover frequencies for O2 prodn. ≥5 s-1 at pH = 8. The rate's pH sensitivity reflects the pH dependence of the four-electron O2-H2O couple. Extensive spectroscopic, electrochem., and inhibition studies firmly indicate that [Co4(H2O)2(PW9O34)2]10- is stable under catalytic turnover conditions: Neither hydrated cobalt ions nor cobalt hydroxide/oxide particles form in situ.
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    43. 43
      Priebe, J. B.; Radnik, J.; Lennox, A. J. J.; Pohl, M. M.; Karnahl, M.; Hollmann, D.; Grabow, K.; Bentrup, U.; Junge, H.; Beller, M.; Bruckner, A. Solar Hydrogen Production by Plasmonic Au-TiO2Catalysts: Impact of Synthesis Protocol and TiO2Phase on Charge Transfer Efficiency and H2Evolution Rates. ACS Catal. 2015, 5, 2137– 2148,  DOI: 10.1021/cs5018375
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      43
      Solar Hydrogen Production by Plasmonic Au-TiO2 Catalysts: Impact of Synthesis Protocol and TiO2 Phase on Charge Transfer Efficiency and H2 Evolution Rates
      Priebe, Jacqueline B.; Radnik, Joerg; Lennox, Alastair J. J.; Pohl, Marga-Martina; Karnahl, Michael; Hollmann, Dirk; Grabow, Kathleen; Bentrup, Ursula; Junge, Henrik; Beller, Matthias; Brueckner, Angelika
      ACS Catalysis (2015), 5 (4), 2137-2148CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      The activity of plasmonic Au-TiO2 catalysts for solar H prodn. from H2O/MeOH mixts. depends strongly on the support phase (anatase, rutile, brookite, or composites thereof) as well as on specific structural properties caused by the method of Au deposition (sol-immobilization, photodeposition, or deposition-pptn.). Structural and electronic rationale have been identified for this behavior. Using a combination of spectroscopic in situ techniques (EPR, XANES, and UV-visible spectroscopy), the formation of plasmonic Au particles from precursor species was monitored, and the charge-carrier sepn. and stabilization under photocatalytic conditions was explored in relation to H2 evolution rates. By in situ EPR spectroscopy, it was directly shown that abundant surface vacancies and surface OH groups enhance the stabilization of sepd. electrons and holes, whereas the enrichment of Ti3+ in the support lattice hampers an efficient electron transport. Under the given exptl. conditions, these properties were most efficiently generated by depositing Au particles on anatase/rutile composites using the deposition-pptn. technique.
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    44. 44
      Najafpour, M. M.; Pashaei, B. Nanoscale Manganese Oxide within Faujasite Zeolite as an Efficient and Biomimetic Water Oxidizing Catalyst. Dalton Trans. 2012, 41, 10156– 10160,  DOI: 10.1039/c2dt30891f
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      44
      Nanoscale manganese oxide within faujasite zeolite as an efficient and biomimetic water oxidizing catalyst
      Najafpour, Mohammad Mahdi; Pashaei, Babak
      Dalton Transactions (2012), 41 (34), 10156-10160CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
      Nanoscale manganese oxides within Faujasite zeolite have been synthesized with a simple method and characterized by SEM, X-ray diffraction spectrometry, N2 adsorption-desorption isotherms, transmission electron microscopy, and at. absorption spectroscopy. These oxides showed efficient water oxidizing activity in the presence of cerium(IV) ammonium nitrate as a non-oxo transfer oxidant.
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    45. 45
      Maeda, K.; Ishimaki, K.; Okazaki, M.; Kanazawa, T.; Lu, D.; Nozawa, S.; Kato, H.; Kakihana, M. Cobalt Oxide Nanoclusters on Rutile Titania as Bifunctional Units for Water Oxidation Catalysis and Visible Light Absorption: Understanding the Structure-Activity Relationship. ACS Appl. Mater. Interfaces 2017, 9, 6114– 6122,  DOI: 10.1021/acsami.6b15804
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      45
      Cobalt Oxide Nanoclusters on Rutile Titania as Bifunctional Units for Water Oxidation Catalysis and Visible Light Absorption: Understanding the Structure-Activity Relationship
      Maeda, Kazuhiko; Ishimaki, Koki; Okazaki, Megumi; Kanazawa, Tomoki; Lu, Daling; Nozawa, Shunsuke; Kato, Hideki; Kakihana, Masato
      ACS Applied Materials & Interfaces (2017), 9 (7), 6114-6122CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      The structure of cobalt oxide (CoOx) nanoparticles dispersed on rutile TiO2 (R-TiO2) was characterized by X-ray diffraction, UV-vis-NIR diffuse reflectance spectroscopy, high-resoln. transmission electron microscopy, X-ray absorption fine-structure spectra, and XPS spectra. The CoOx nanoparticles were loaded onto R-TiO2 by an impregnation method from an aq. soln. contg. Co(NO3)2·6H2O followed by heating in air. Modification of the R-TiO2 with 2.0 wt. % Co followed by heating at 423 K for 1 h resulted in the highest photocatalytic activity with good reproducibility. Structural analyses revealed that the activity of this photocatalyst depended strongly on the generation of Co3O4 nanoclusters with an optimal distribution. These nanoclusters are thought to interact with the R-TiO2 surface, resulting in visible light absorption and active sites for water oxidn.
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    46. 46
      Boppana, V. B. R.; Jiao, F. Nanostructured MnO2: an Efficient and Robust Water Oxidation Catalyst. Chem. Commun. 2011, 47, 8973– 8975,  DOI: 10.1039/c1cc12258d
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      46
      Nanostructured MnO2: an efficient and robust water oxidation catalyst
      Boppana, Venkata Bharat Ram; Jiao, Feng
      Chemical Communications (Cambridge, United Kingdom) (2011), 47 (31), 8973-8975CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      Nanostructured MnO2 exhibits a high turnover frequency for oxygen evolution under visible light and high stability in strong acidic conditions.
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    47. 47
      Idris, A.; Hassan, N.; Mohd Ismail, N. S.; Misran, E.; Yusof, N. M.; Ngomsik, A.-F.; Bee, A. Photocatalytic Magnetic Separable Beads for Chromium (VI) Reduction. Water Res. 2010, 44, 1683– 1688,  DOI: 10.1016/j.watres.2009.11.026
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      Photocatalytic magnetic separable beads for chromium (VI) reduction
      Idris, Ani; Hassan, Nursia; Ismail, Nur Suriani Mohd.; Misran, Effaliza; Yusof, Noordin Mohd.; Ngomsik, Audrey-Flore; Bee, Agnes
      Water Research (2010), 44 (6), 1683-1688CODEN: WATRAG; ISSN:0043-1354. (Elsevier B.V.)
      Magnetically separable photocatalyst beads contg. nano-sized iron oxide in alginate polymer were prepd. This magnetic photocatalyst beads are used in slurry-type reactors. The magnetism of the catalyst arises from the nanostructured particles γ-Fe2O3, by which the catalyst can be easily recovered by the application of an external magnetic field. These synthesized beads are sunlight-driven photocatalyst. In the system without magnetic photocatalyst beads, no chromium redn. was obsd. under sunlight irradn. due to the stability of the chromium (VI). Upon the addn. of magnetic photocatalyst beads, the photo-redn. of Cr(VI) was completed in just after only 50 min under sunlight irradn. due to the photocatalytic activity of the beads. However when placed away from sunlight, the redn. rate of the chromium is just about 10%. These observations were explained in terms of absorption occurrence of chromium (VI) onto the catalyst surface which took place in this reaction. In addn., photo-redn. rate of chromium (VI) was more significant at lower pH. The results suggest that the use of magnetic separable photocatalyst beads is a feasible strategy for eliminating Cr(VI).
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    48. 48
      Hamid, B. A. S.; Teh, S. J.; Lai, C. W. Photocatalytic Water Oxidation on ZnO: A Review. Catalysts 2017, 7, 93,  DOI: 10.3390/catal7030093
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      Kim, H. W.; Ross, M. B.; Kornienko, N.; Zhang, L.; Guo, J.; Yang, P.; McCloskey, B. D. Efficient Hydrogen Peroxide Generation using Reduced Graphene Oxide-Based Oxygen Reduction Electrocatalysts. Nature Catal. 2018, 1, 282– 290,  DOI: 10.1038/s41929-018-0044-2
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      Zhang, W.; Bariotaki, A.; Smonou, I.; Hollmann, F. Visible-Light-Driven Photooxidation of Alcohols using Surface-Doped Graphitic Carbon Nitride. Green Chem. 2017, 19, 2096– 2100,  DOI: 10.1039/C7GC00539C
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      50
      Visible-light-driven photooxidation of alcohols using surface-doped graphitic carbon nitride
      Zhang, Wuyuan; Bariotaki, Anna; Smonou, Ioulia; Hollmann, Frank
      Green Chemistry (2017), 19 (9), 2096-2100CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
      Carbon-nanodot-doped g-C3N4 is used as a photocatalyst to promote the aerobic oxidn. of alcs. ROH (R = C6H5, c-C6H11, 1,2,3,4-tetrahydronaphthalen-1-yl, etc.) and oxyfunctionalization of activated hydrocarbons such as methylbenzene, 1-chloro-4-methylbenzene, cyclohexane, etc. A crit. E-factor anal. of the current reaction system reveals its limitations en route to environmentally acceptable oxidn. procedures.
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    51. 51
      Shiraishi, Y.; Kanazawa, S.; Sugano, Y.; Tsukamoto, D.; Sakamoto, H.; Ichikawa, S.; Hirai, T. Highly Selective Production of Hydrogen Peroxide on Graphitic Carbon Nitride (g-C3N4) Photocatalyst Activated by Visible Light. ACS Catal. 2014, 4, 774– 780,  DOI: 10.1021/cs401208c
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      51
      Highly Selective Production of Hydrogen Peroxide on Graphitic Carbon Nitride (g-C3N4) Photocatalyst Activated by Visible Light
      Shiraishi, Yasuhiro; Kanazawa, Shunsuke; Sugano, Yoshitsune; Tsukamoto, Daijiro; Sakamoto, Hirokatsu; Ichikawa, Satoshi; Hirai, Takayuki
      ACS Catalysis (2014), 4 (3), 774-780CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      Photocatalytic prodn. of hydrogen peroxide (H2O2) on semiconductor catalysts with alc. as a hydrogen source and mol. oxygen (O2) as an oxygen source is a potential method for safe H2O2 synthesis because the reaction can be carried out without the use of explosive H2/O2 mixed gases. Early reported photocatalytic systems, however, produce H2O2 with significantly low selectivity (∼1%). We found that visible light irradn. (λ > 420 nm) of graphitic carbon nitride (g-C3N4), a polymeric semiconductor, in an alc./water mixt. with O2 efficiently produces H2O2 with very high selectivity (∼90%). Raman spectroscopy and ESR anal. revealed that the high H2O2 selectivity is due to the efficient formation of 1,4-endoperoxide species on the g-C3N4 surface. This suppresses one-electron redn. of O2 (superoxide radical formation), resulting in selective promotion of two-electron redn. of O2 (H2O2 formation).
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    52. 52
      Burek, B. O.; Timm, J.; Bahnemann, D. W.; Bloh, J. Z. Kinetic Effects and Oxidation Pathways of Sacrificial Electron Donors on the Example of the Photocatalytic Reduction of Molecular Oxygen to Hydrogen Peroxide over Illuminated Titanium Dioxide. Catal. Today 2019, 335, 354– 364,  DOI: 10.1016/j.cattod.2018.12.044
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      52
      Kinetic effects and oxidation pathways of sacrificial electron donors on the example of the photocatalytic reduction of molecular oxygen to hydrogen peroxide over illuminated titanium dioxide
      Burek, Bastien O.; Timm, Jana; Bahnemann, Detlef W.; Bloh, Jonathan Z.
      Catalysis Today (2019), 335 (), 354-364CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
      Sacrificial electron donors are frequently used in photocatalytic reactions to enhance the performance of the reaction, typically short-chain alcs. as well as their resp. aldehydes and acids are used. This study focuses on the differences between the individual electron donors regarding their oxidn. rates, mechanistic pathways, the influence of the intermediates and their direct impact on the H2O2 generation. The individual H2O2 formation rates of 16 different electron donors, photonic and faradaic efficiencies for H2O2 prodn. are carefully discussed. Furthermore, a new multi-reaction pathway for t-butanol oxidn. is postulated and critically examd.
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    53. 53
      Li, S.; Dong, G.; Hailili, R.; Yang, L. L.; Li, Y.; Wang, F.; Zeng, Y.; Wang, C. Effective Photocatalytic H2O2Production under Visible Light Irradiation at g-C3N4Modulated by Carbon Vacancies. Appl. Catal., B 2016, 190, 26– 35,  DOI: 10.1016/j.apcatb.2016.03.004
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      53
      Effective photocatalytic H2O2 production under visible light irradiation at g-C3N4 modulated by carbon vacancies
      Li, Shuna; Dong, Guohui; Hailili, Reshalaiti; Yang, Liping; Li, Yingxuan; Wang, Fu; Zeng, Yubin; Wang, Chuanyi
      Applied Catalysis, B: Environmental (2016), 190 (), 26-35CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
      Hydrogen peroxide (H2O2) is of great significance in biol. and environmental processes as well as in chem. industry. Even though anthraquinone autoxidn. (AO) process has been the major artificial way to produce H2O2, its energy cost and non-green nature have been motivating people to develop more efficient, economic and green technologies as alternatives. Here we demonstrated that photocatalytic H2O2 prodn. at g-C3N4 could be improved by as much as 14 times in the absence of org. scavenger through a carbon vacancy-based strategy. Both the exptl. and theor. calcn. results indicated that the creation of carbon vacancies could reduce the symmetry of g-C3N4 and produce the effect of electron delocalization. This will allow g-C3N4 to possess more excitable electrons and a narrower band gap. On the other hand, carbon vacancies provided more sites to adsorb mol. oxygen and thereby help electrons transfer from g-C3N4 to the surface adsorbed O2. More interestingly, the presence of carbon vacancies changed the H2O2 generation pathway from a two-step single-electron indirect redn. to an one-step two-electron direct redn. This study could not only develop a novel strategy to improve the H2O2 prodn. activity of semiconductors, but also shed light on the deep understanding of the role played by surface defect structure on photocatalytic activity of semiconductor photocatalysts.
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      Burek, B. O.; de Boer, S. R.; Tieves, F.; Zhang, W.; van Schie, M.; Bormann, S.; Alcalde, M.; Holtmann, D.; Hollmann, F.; Bahnemann, D. W.; Bloh, J. Z., Photoenzymatic Hydroxylation of Ethylbenzene Catalyzed by Unspecific Peroxygenase: Origin of Enzyme Inactivation and the Impact of Light Intensity and Temperature. ChemCatChem 2019,  DOI: 10.1002/cctc.201900610 .
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      Zheng, Y.; Zhang, Z.; Li, C. A Comparison of Graphitic Carbon Nitrides Synthesized from Different Precursors through Pyrolysis. J. Photochem. Photobiol., A 2017, 332, 32– 44,  DOI: 10.1016/j.jphotochem.2016.08.005
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      A comparison of graphitic carbon nitrides synthesized from different precursors through pyrolysis
      Zheng, Yu; Zhang, Zisheng; Li, Chunhu
      Journal of Photochemistry and Photobiology, A: Chemistry (2017), 332 (), 32-44CODEN: JPPCEJ; ISSN:1010-6030. (Elsevier B.V.)
      Two precursors, melamine and urea, were used to prep. graphitic carbon nitride through a thermal decompn. (pyrolysis) method. The pyrolysis was carried out at different temps. in open air condition in a crucible with cover. The as-prepd. samples were characterized by SEM, TEM, BET, XRD, XPS, FTIR and DRS. The characterization results revealed that the samples synthesized from different precursors had different phys. and chem. properties. Specifically, it was found that the pyrolysis of urea yielded product with smaller cryst. domains but larger surface areas compared to that of melamine. To further qualify the as-prepd. samples, the adsorption and photocatalytic activities were measured by using Rhodamine B (RhB) as target pollutant. It was found out that the precursors as well as pyrolysis temps. had big influences on the adsorption and photocatalytic activities. Higher photocatalytic activities were achieved by samples synthesized from urea at higher temps. The mechanism of the degrdn. process was explored on the basis of the band structure and the roles of photo-generated radicals.
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      Kang, Y.; Yang, Y.; Yin, L.-C.; Kang, X.; Liu, G.; Cheng, H.-M. An Amorphous Carbon Nitride Photocatalyst with Greatly Extended Visible-Light-Responsive Range for Photocatalytic Hydrogen Generation. Adv. Mater. 2015, 27, 4572– 4577,  DOI: 10.1002/adma.201501939
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      An Amorphous Carbon Nitride Photocatalyst with Greatly Extended Visible-Light-Responsive Range for Photocatalytic Hydrogen Generation
      Kang, Yuyang; Yang, Yongqiang; Yin, Li-Chang; Kang, Xiangdong; Liu, Gang; Cheng, Hui-Ming
      Advanced Materials (Weinheim, Germany) (2015), 27 (31), 4572-4577CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      We show that amorphous carbon nitride (ACN) can be used as an effective visible light photocatalyst. ACN with a bandgap of 1.90 eV was obtained by simply heating partially cryst. graphitic carbon nitride (GCN) with a bandgap of 2.82 eV. ACN shows an order of magnitude higher photocatalytic activity in hydrogen evolution under visible light than the partially cryst. GCN counterpart. ACN is detd. to be active in hydrogen generation under visible light with wavelength beyond 600 nm. These findings may help to develop a class of amorphous photocatalysts for solar energy conversion.
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      Dvoranová, D.; Barbieriková, Z.; Brezová, V. Radical Intermediates in Photoinduced Reactions on TiO2 (an EPR Spin Trapping Study). Molecules 2014, 19, 17279,  DOI: 10.3390/molecules191117279
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      Radical intermediates in photoinduced reactions on TiO2 (an EPR spin trapping study)
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      The radical intermediates formed upon UVA irradn. of titanium dioxide suspensions in aq. and non-aq. environments were investigated applying the EPR spin trapping technique. The results showed that the generation of reactive species and their consecutive reactions are influenced by the solvent properties (e.g., polarity, soly. of mol. oxygen, rate const. for the reaction of hydroxyl radicals with the solvent). The formation of hydroxyl radicals, evidenced as the corresponding spin-adducts, dominated in the irradiated TiO2 aq. suspensions. The addn. of 17O-enriched water caused changes in the EPR spectra reflecting the interaction of an unpaired electron with the 17O nucleus. The photoexcitation of TiO2 in non-aq. solvents (dimethylsulfoxide, acetonitrile, methanol and ethanol) in the presence of 5,5-dimethyl-1-pyrroline N-oxide spin trap displayed a stabilization of the superoxide radical anions generated via electron transfer reaction to mol. oxygen, and various oxygen- and carbon-centered radicals from the solvents were generated. The character and origin of the carbon-centered spin-adducts was confirmed using nitroso spin trapping agents.
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      Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. Critical Review of Rate Constants for Reactions of Hydrated Electrons, Hydrogen Atoms and Hydroxyl Radicals (· OH/· O- in Aqueous Solution. J. Phys. Chem. Ref. Data 1988, 17, 513– 886,  DOI: 10.1063/1.555805
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      Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (·OH/·O-) in aqueous solution
      Buxton, George V.; Greenstock, Clive L.; Helman, W. Phillip; Ross, Alberta B.
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      Kinetic data for the radicals H and OH in aq. soln., and the corresponding radical anions, O- and eaq-, are critically reviewed with many refs. Reactions of the radicals in aq. soln. have been studied by pulse radiolysis, flash photolysis, and other methods. Rate consts. for >3,500 reactions are tabulated, including reactions with mols., ions, and other radicals derived from inorg. and org. solutes.
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      Hu, R.; Wang, X.; Dai, S.; Shao, D.; Hayat, T.; Alsaedi, A. Application of Graphitic Carbon Nitride for the Removal of Pb(II) and Aniline from Aqueous Solutions. Chem. Eng. J. 2015, 260, 469– 477,  DOI: 10.1016/j.cej.2014.09.013
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      Application of graphitic carbon nitride for the removal of Pb(II) and aniline from aqueous solutions
      Hu, Rui; Wang, Xiangke; Dai, Songyuan; Shao, Dadong; Hayat, Tasawar; Alsaedi, Ahmed
      Chemical Engineering Journal (Amsterdam, Netherlands) (2015), 260 (), 469-477CODEN: CMEJAJ; ISSN:1385-8947. (Elsevier B.V.)
      Graphitic carbon nitride (g-C3N4) was synthesized from urea with a facile approach and was characterized by SEM (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transformed IR (FT-IR) spectroscopy and XPS. The as-synthesized g-C3N4 was applied as sorbent to remove Pb(II) and aniline from aq. solns. as a function of contact time, solid content, pH, ionic strength, temp., and initial concns. of Pb(II) and aniline. The results indicated that the sorption of Pb(II) was mainly dominated by outer-sphere surface complexation or ion exchange at pH < 7.0, but by inner-sphere surface complexation at pH > 7.0. The sorption of aniline was mainly attributed to electrostatic interaction at pH < 5.0, whereas the π-π electron donor-acceptor (EDA) interaction was the predominant sorption mechanism at pH > 5.0. The sorption isotherms of Pb(II) and aniline on g-C3N4 were well described by the Langmuir model. The thermodn. parameters calcd. from the temp.-dependent sorption isotherms indicated that the sorption of Pb(II) and aniline on g-C3N4 was endothermic and spontaneous processes. Moreover, g-C3N4 could be regenerated through the desorption of Pb(II) and aniline by using 1.0 M HCl soln. and alc., resp., and no obvious decline of sorption capacity was found for the recycling results. All these results indicated that g-C3N4 was a promising material for the preconcn. of Pb(II) and aniline from aq. solns. in real pollution management.
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      Moon, G.-H.; Fujitsuka, M.; Kim, S.; Majima, T.; Wang, X.; Choi, W. Eco-Friendly Photochemical Production of H2O2 through O2 Reduction over Carbon Nitride Frameworks Incorporated with Multiple Heteroelements. ACS Catal. 2017, 7, 2886– 2895,  DOI: 10.1021/acscatal.6b03334
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      Eco-Friendly Photochemical Production of H2O2 through O2 Reduction over Carbon Nitride Frameworks Incorporated with Multiple Heteroelements
      Moon, Gun-hee; Fujitsuka, Mamoru; Kim, Sooyeon; Majima, Tetsuro; Wang, Xinchen; Choi, Wonyong
      ACS Catalysis (2017), 7 (4), 2886-2895CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      We report that in situ incorporation of both potassium and phosphate species into a polymeric carbon nitride (CN) framework highly enhanced the photoprodn. of hydrogen peroxide (H2O2) without the use of any noble-metal cocatalysts. The incorporation of earth-abundant heteroelements (K, P, and O) (i) introduced the neg. surface charge over the entire pH range through surface functionalization by phosphate species, (ii) increased the lifetime of the transient species to a picosecond time scale via the formation of charge sepn. states, (iii) facilitated the interfacial electron transfer to dioxygen, and (iv) inhibited the decompn. of in situ generated H2O2. As a result, the modified CN showed apparent quantum yields (Φ, for H2O2 prodn.) that are enhanced by about 25 and 17 times (Φ420 = 8.0%; Φ320 = 26.2%) from those of bare CN (Φ420 = 0.32%; Φ320 = 1.55%) under monochromatic irradn. of 420 and 320 nm, resp. This study clearly demonstrated a simple way to design multiple heteroelement-incorporated CN compds. that consist of earth-abundant elements only (C, N, K, P, O) for the development of practical and economical solar conversion photocatalytic materials.
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      Hu, S.; Li, F.; Fan, Z.; Wang, F.; Zhao, Y.; Lv, Z. Band gap-tunable potassium doped graphitic carbon nitride with enhanced mineralization ability. Dalton Trans. 2015, 44, 1084– 1092,  DOI: 10.1039/C4DT02658F
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      Band gap-tunable potassium doped graphitic carbon nitride with enhanced mineralization ability
      Hu, Shaozheng; Li, Fayun; Fan, Zhiping; Wang, Fei; Zhao, Yanfeng; Lv, Zhenbo
      Dalton Transactions (2015), 44 (3), 1084-1092CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
      Band gap-tunable potassium doped graphitic carbon nitride with enhanced mineralization ability was prepd. using dicyandiamide monomer and potassium hydrate as precursors. X-ray diffraction (XRD), N2 adsorption, UV-Vis spectroscopy, Fourier transform IR (FT-IR) spectroscopy, SEM (SEM), photoluminescence (PL) and XPS were used to characterize the prepd. catalysts. The CB and VB potentials of graphitic carbon nitride could be tuned from -1.09 and +1.56 to -0.31 and +2.21 eV by controlling the K concn. Besides, the addn. of potassium inhibited the crystal growth of graphitic carbon nitride, enhanced the surface area and increased the sepn. rate for photogenerated electrons and holes. The visible-light-driven Rhodamine B (RhB) photodegrdn. and mineralization performances were significantly improved after potassium doping. A possible influence mechanism of the potassium concn. on the photocatalytic performance was proposed.
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      Zhang, H.; Tian, W.; Zhou, L.; Sun, H.; Tade, M.; Wang, S. Monodisperse Co3O4Quantum Dots on Porous Carbon Nitride Nanosheets for Enhanced Visible-Light-Driven Water Oxidation. Appl. Catal., B 2018, 223, 2– 9,  DOI: 10.1016/j.apcatb.2017.03.028
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      62
      Monodisperse Co3O4 quantum dots on porous carbon nitride nanosheets for enhanced visible-light-driven water oxidation
      Zhang, Huayang; Tian, Wenjie; Zhou, Li; Sun, Hongqi; Tade, Moses; Wang, Shaobin
      Applied Catalysis, B: Environmental (2018), 223 (), 2-9CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
      Here we report a facile annealing process for homogeneous deposition of Co3O4 quantum dots (Co3O4 QDs) onto porous g-C3N4 nanosheets. It was discovered that pores were catalytically produced around Co3O4 QDs. In the synthesis, annealing temp. was found to be crucial for the textural property, optical absorption, and the corresponding photocatalytic water oxidn. as well as photochem. performances. The highest sp. surface area, pore vol. and optimal O2 prodn. rate as well as the highest photocurrent were obtained on 0.8 wt.% Co3O4 QDs decorated g-C3N4 nanosheets annealed at 300 °C (0.8% Co3O4-C3N4-300). These results underline the importance of surface heterojunction and afford us a feasible protocol for rational design of g-C3N4 based photocatalysts for water oxidn.
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    63. 63
      Son, E. J.; Lee, Y. W.; Ko, J. W.; Park, C. B. Amorphous Carbon Nitride as a Robust Photocatalyst for Biocatalytic Solar-to-Chemical Conversion. ACS Sustainable Chem. Eng. 2019, 7, 2545– 2552,  DOI: 10.1021/acssuschemeng.8b05487
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      63
      Amorphous carbon nitride as a robust photocatalyst for biocatalytic solar-to-chemical conversion
      Son, Eun Jin; Lee, Yang Woo; Ko, Jong Wan; Park, Chan Beum
      ACS Sustainable Chemistry & Engineering (2019), 7 (2), 2545-2552CODEN: ASCECG; ISSN:2168-0485. (American Chemical Society)
      In situ regeneration of nicotinamide cofactor (NADH) is imperative because it is required as a reducing power for driving many redox enzymic cycles useful in industry. Here, we report that amorphous carbon nitride (ACN) is a promising and robust photocatalyst for solar-driven biotransformation via NADH regeneration. Under visible light (λ > 420 nm), NADH regeneration yields by ACN reached 62.3% within an hour, whereas partially cryst. polymeric carbon nitride (CCN) hardly reduced NAD+ to NADH. Subsequently, the regenerated cofactor was consumed by L-glutamate dehydrogenase, a NADH-dependent enzyme, achieving the conversion of α-ketoglutarate with a turnover frequency of 2640 h-1. ACN showed excellent catalytic activity and long-term stability for light-driven biocatalysis; NADH regeneration efficiency after eight cycles remained above 92% of the first cycle's efficiency, and the enzymic reaction proceeded for more than 12 h without significant loss of ACN's photoactivity. The remarkable photocatalytic activity of ACN originated from its unique microstructure that lacks hydrogen bonds that link polymeric melon units, leading to extended visible light absorption and less charge recombination. Our results suggest that ACN efficiently drives biocatalytic photosynthesis with exceptional catalytic sustainability.
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