The first π-allyl complexes of Cu^III have been prepared and characterized by using rapid injection nuclear magnetic resonance spectroscopy (RI-NMR). The prototype, (η³-allyl)dimethylcopper(III), was prepared by injection of allyl chloride into a THF-d₈ solution of iodo-Gilman reagent, Me₂CuLi·LiI (A), spinning in the probe of an NMR spectrometer at −100 °C. A σ-allyl ate complex, lithium (η¹-allyl)trimethylcuprate(III), was prepared in high yield by including 1 equiv of tributylphosphine in the reaction mixture or by using allyl acetate as the substrate. Cyano ate complex, lithium cis-(η¹-allyl)cyanodimethylcuprate(III) was obtained in high yield by injecting allyl chloride or allyl acetate into the cyano-Gilman reagent, Me₂CuLi·LiCN (B), in THF-d₈ at −100 °C. Reactions of A with allylic substrates show a definite dependence on leaving group (chloride vs acetate), whereas those of B do not. Moreover, these reagents have different regioselectivities, which in the case of A vary with temperature. Finally, the exclusive formation of cis-cyano σ-allyl Cu^III intermediates in both the 1,4-addition of B to α-enones and its S_N2α reaction with allylic substrates now makes sense in terms of π-allyl intermediates in both cases, thus unifying the mechanisms of these two kinds of conjugate addition.

The present study utilizes rapid mixing techniques together with fast optical spectroscopy to probe the assembly of [Cu₂(5-OH-bdc)₂L₂]₁₂ formation in methanol. The data reveal five distinct kinetic steps involved in the formation of the Cu-nanoball in methanol with lifetimes of <15 ms, 26 ms, 310 ms, 4 s, and 45 s. From these results two possible mechanistic pathways are proposed based upon fragment isomerization followed by condensation. Both mechanisms involve fast building unit assembly (<15 ms) followed by either concerted or sequential fragment isomerization (activation controlled) coupled with condensation.

Rhodium-catalyzed C−H insertion of hydroxylamine-derived sulfamate esters makes possible the synthesis of unique oxathiadiazinane heterocycles, which upon mild reduction furnish differentially substituted 1,2-diamine products. This highly chemo- and diastereoselective transformation underscores the power of catalytic C−H functionalization as a general approach to C−N bond construction.

Experimental results supported by density functional theory calculations show carbonate formation and reaction on atomic oxygen precovered Au(111). Oxygen mixing is observed in temperature-programmed desorption measurements when a Au(111) precovered with ¹⁶O is exposed to isotopically labeled CO₂ (C¹⁸O₂). The presence of ¹⁶O¹⁸O is attributed to surface carbonate formation and decomposition at surface temperatures ranging from 77−400 K and initial oxygen coverages ranging from 0.18−2.1 ML. A reaction probability on the order of 10⁻⁴ and an activation energy of −0.15 ± 0.08 eV are estimated for this reaction.

Area-selective atomic layer deposition (ALD) allows the growth of highly uniform thin inorganic films on certain parts of the substrate while preventing the film growth on other parts. Although the selective ALD growth is working well at the micron and submicron scale, it has failed at the nanoscale, especially near the interface where there is growth on one side and no-growth on the other side. The reason is that methods so far solely rely on the chemical modification of the substrate, while neglecting the occurrence of lateral ALD growth at the nanoscale. Here we present a proof-of-concept for blocking the lateral ALD growth also at the nanoscale by combining the chemical surface modification with topographical features. We demonstrate that area-selective ALD of ZnO occurs by applying the diethylzinc/water ALD process on cicada wings that contain a dense array of nanoscopic pillars. The sizes of the features in the inorganic film are down to 25 nm which is, to the best of our knowledge, the smallest obtained by area-selective ALD. Importantly, our concept allows the synthesis of such small features even though the film is multiple times thicker.

Self-sorting organization of two AB-type heteroditopic monomers led to the formation of linear supramolecular alternating copolymers driven by host−guest noncovalent interactions based on the bis(p-phenylene)-34-crown-10/paraquat derivative and dibenzo-24-crown-8/dibenzylammonium salt recognition motifs as confirmed by ¹H NMR, cyclic voltammetry, dynamic light scattering, viscosity measurements, and scanning electron microscopy.

We report a large-scale self-etching approach for the synthesis of monodispersed mesoporous F−TiO₂ hollow microspheres. The self-etching derived from HF was elucidated by the morphology, chemical composition, and crystal size evolutions from solid to hollow microspheres with the increase in the concentration of H₂SO₄. The resulting TiO₂ hollow microspheres exhibited ease for the concurrent membrane filtration and photocatalysis, providing high potential for engineering application in advanced water treatment, for not only increasing water production but also improving water quality.

New rare-earth silicide oxides, La₁₀Si₈O₃ (1) and Ce₁₀Si₈O₃ (2), were synthesized through high-temperature reactions of the pure elements under controlled oxygen atmosphere conditions. The remarkable silicides crystallize in a unique crystal structure (space group P6/mmm; a = 10.975(3) Å (La) and 10.844(1) Å (Ce); c = 4.680(1) Å (La) and 4.561(1) Å (Ce)) that features a 3-D framework of corner-shared O-centered (La/Ce)₆ octahedra, reminiscent of hexagonal tungsten bronzes, with planar Si₆ rings enclosed within its hexagonal channels. Band structure calculations indicate the compounds are metallic, with optimized La−Si bonds, and a benzene-like [Si₆]⁶⁻ anion. Compound 1 exhibits temperature independent paramagnetism. Compound 2 exhibits Curie−Weiss paramagnetism, and an antiferromagnetic ordering below 7 K.

Dynamic mixtures of Rh-dye complexes can be used to determine the history of chemical events such as the addition of ATP and ADP by UV−vis spectroscopy.

The reaction of carbon monoxide with a β-diketiminato dimethyl niobium complex (BDI)Me₂Nb(N^tBu) is shown to lead to a variety of products whose distribution displays a remarkable dependence on the reaction conditions. Among these, the products of metal reduction, enediolate formation, and intramolecular C−H activation have been fully characterized. An investigation into the individual steps leading to these products points to a transient initial monoacyl complex, whose fate may be perturbed via reaction conditions to allow for control over the product distribution. Furthermore, the reaction of (BDI)Me₂Nb(NCMe₃) with XylNC (Xyl = 2,6-Me₂C₆H₃) yields the η²-ketimine complex (BDI)(Me₂C═NXyl)Nb(NCMe₃), whose characterization and reactivity enhance our understanding of the sequences involving CO.

Controlling selectivity in heterogeneous catalysis is critical for the design of environmentally friendly catalytic processes that minimize the production of undesired byproducts and operate with high energy efficiency. We show that the Ag nanowire catalysts exhibit higher selectivity in the ethylene epoxidation reaction than conventional spherical particle catalysts. The higher selectivity of the nanowire catalysts was attributed to a higher concentration of the Ag(100) surface facets in the nanowire catalysts compared to the particle catalysts. Density functional theory calculations show that the transformation of the surface oxametallacycle intermediate to form the selective product, EO, is more favorable on the Ag(100) than on Ag(111). The studies show that recent advances in the controlled synthesis of uniform nanostructures with well-defined surface facets might provide an important platform for the design of highly selective heterogeneous catalysts.

The presence of dipolar coupling waves within helical elements of proteins implies an effective tilt of the main axis of the helical element relative to the magnetic field. Here, we investigate the origin of dipolar waves observed in helical elements of partially folded proteins. We find that the dipolar waves result from an effective tilt of the helix relative to the alignment axis that is determined by the directionality of the unfolded chains projected from the helix caps. The amplitude and phase of the dipolar wave depend in a predictable way on helix length, providing direct insight into helix stability, nucleation, and fraying in partially folded proteins.

A novel aptamer-based molecular probe design employing intramolecular signal transduction is demonstrated. The probe is composed of three elements: an aptamer, a short, partially cDNA sequence, and a PEG linker conjugating the aptamer with the short DNA strand. We have termed this aptamer probe an “aptamer switch probe”, or ASP. The ASP design utilizes both a fluorophore and a quencher which are respectively modified at the two termini of the ASP. In the absence of the target molecule, the short DNA will hybridize with the aptamer, keeping the fluorophore and quencher in close proximity, thus switching off the fluorescence. However, when the ASP meets its target, the binding between the aptamer and the target molecule will disturb the intramolecular DNA hybridization, move the quencher away from the fluorophore, and, in effect, switch on the fluorescence. Both ATP and human α-thrombin aptamers were engineered to demonstrate this design, and both showed that fluorescence enhancement could be quantitatively mediated by the addition of various amounts of target molecules. Both of these ASPs presented excellent selectivity and prompt response toward their targets. With intrinsic advantages resulting from its intramolecular signal transduction architecture, the ASP design holds promising potential for future applications, such as biochip and in situ imaging, which require reusability, excellent stability, prompt response, and high sensitivity.

A novel chelate-controlled intermolecular oxidative Heck reaction is reported that proceeds with a wide range of nonresonance stabilized α-olefin substrates and organoboron reagents to afford internal olefin products in good yields and outstanding regio- and E:Z stereoselectivities. Pd−H isomerization, common in many Heck reactions, is not observed under these mild, oxidative conditions. This is evidenced by outstanding E:Z selectivities (>20:1 in all cases examined), no erosion in optical purity for proximal stereogenic centers, and a tolerance for unprotected alcohols. Remarkably, a single metal/ligand combination, Pd/bis-sulfoxide complex 1, catalyzes this reaction over a broad range of coupling partners. Given the high selectivities and broad scope, we anticipate this intermolecular Heck reaction will find heightened use in complex molecule synthesis.

Photolysis of a mixture of NaNO₂ with NaCl with an adsorbed coating of 1-oleoyl-2-palmitoyl-sn-glycero-3-phosphocholine (OPPC) is shown to lead to oxidation of the OPPC. This oxidation “from the bottom up” is due to the generation of the OH free radical from nitrite ion photochemistry in the presence of water and its addition to the OPPC double bond. Such chemistry may be important in the lower atmosphere.

Rice α-oxygenase (RαO) catalyzes the insertion of O₂ into the C_α−H bond of various fatty acids. The mechanism is thought to involve a tyrosyl radical as the oxidant on the basis of comparisons to the structurally homologous cyclooxygenase enzymes. Kinetic and spectroscopic results presented here for the wild-type RαO and the Tyr379Phe mutant indicate an irreversible H• abstraction mechanism and support the involvement of the proposed catalytic Tyr•. In addition, very large, weakly temperature dependent deuterium kinetic isotope effects (∼50) are observed, consistent with extensive nuclear tunneling. RαO, thus, presents a novel example where such quantum effects are associated with an amino acid radical-utilizing enzyme.

Cobalt-catalyzed reactions of haloalkanes with dimethylphenylsilylmethylmagnesium chloride result in highly regioselective dehydrohalogenation. The reaction does not follow the conventional E2 elimination mechanism but includes β-hydride elimination from the corresponding alkylcobalt intermediate. The interesting reaction mechanism of the cobalt-catalyzed dehydrohalogenation offered unique transformations that are otherwise difficult to attain.

We report a new strategy to minimize the hydrodynamic size of quantum dots (QDs) and to overcome their colloidal stability and photobleaching problems based on the use of multifunctional and multidentate polymer ligands. A novel finding is that a balanced composition of thiol (-SH) and amine (-NH₂) coordinating groups grafted to a linear polymer chain leads to highly compact nanocrystals with exceptional colloidal stability, a strong resistance to photobleaching, and high fluorescence quantum yields. In contrast to the standing brushlike conformation of PEGylated dihydrolipoic acid molecules, mutlidentate polymer ligands can wrap around the QDs in a closed “loops-and-trains” conformation. This structure is highly stable thermodynamically and is responsible for the excellent colloidal and optical properties. We have optimized this process for the preparation of ultrastable CdTe nanocrystals and have found the strategy to be broadly applicable to a wide range of nanocrystalline materials and heterostructures. This work has led to a new generation of bright and stable QDs with small hydrodynamic diameters between 5.6 and 9.7 nm with tunable fluorescence emission from the visible (515 nm) to the near-infrared (720 nm). These QDs are well suited for molecular and cellular imaging applications in which the nanoparticle hydrodynamic size must be minimized.

Electron transfer activity from excited single CdSe/ZnS core/shell quantum dots (QDs) to adsorbed Fluorescein 27 was studied by single QD fluorescence spectroscopy. In comparison with QDs, the QD-acceptor complexes showed a shorter average and broader distribution of QD emission lifetimes due to electron transfer to adsorbates. Large fluctuation of lifetimes in single QD/dye complexes was observed, indicating intermittent electron transfer activity from QDs.

Mismatched Hartmann−Hahn conditions between the protons and dilute spins (such as ¹⁵N) are found to cause intermolecular magnetization transfer between the low-gamma nuclei over long distances. This transfer is purely proton mediated and occurs even in the absence of direct ¹⁵N−¹⁵N couplings. This has been demonstrated experimentally using a static single crystal of n-acetyl Leucine with intermolecular distances between the ¹⁵N nuclei exceeding 6.5 Å. A quantum-mechanical explanation of this phenomenon is given based on the average-Hamiltonian theory which was confirmed by detailed numerical many-spin simulations. The theory and experiment presented in the present paper may help in the development of solid-state NMR methods for studying interhelical contacts in membrane proteins, as well as for their spectral assignment.

ssDNA exhibits much different adsorption behaviors on enantiomer modified surfaces, which can be explained by the stereoselective H-bond interaction between DNA and the chiral surfaces. This effect not only may help to understand the stereospecific cell/substrate interaction and the origin of the chiral preference in nature but also brings novel insights to the study of DNA properties and the application in biochemical devices.

Pyrimidinium betaïnes (1), readily accessible via a straightforward modular synthesis from a formamidine and a monosubstituted malonic acid, are readily deprotonated by nBuLi (or KHMDS) to give the stable carbene species [2]Li⁺ (abbreviated as maloNHC). The latter represents the archetype of a subgroup of N-heterocyclic carbenes incorporating a malonate as remote anionic functional group within their heterocyclic backbone. While playing the dual role of monodentate 2 e⁻ L type donor and noncoordinating charge carrier X, such ligands are seen to provide a rational route to zwitterionic complexes, as illustrated here by three examples (Rh, Fe, Ag). In particular, the reaction of [2]Li⁺ with [RhCl(1,5-COD)]₂ produces the neutral 14 e⁻ complex Rh(maloNHC)(COD) (3) in which coordinative unsaturation at the metal is relieved in the solid state by an uncommon labile bonding interaction between the C_ipso of one of the mesityl arms and the Rh center.

A facile route to well-defined “smart” polymer−protein conjugates with tunable bioactivity is reported. Protein modification with a reversible addition−fragmentation chain transfer (RAFT) agent and subsequent room temperature polymerization in aqueous media led to conjugates of poly(N-isopropylacrylamide) and a model protein. Representing the first example of polymer−protein conjugation with RAFT agent immobilization via the “R-group” approach, high molecular weight and reductively stable conjugates were accessible without extensive purification or adverse effects on the protein structure. An increase in molecular weight with conversion was observed for the chains grafted from the protein surface, confirming the controlled nature of the polymerization. The responsive behavior of the immobilized polymer facilitated conjugate isolation and also allowed environmental modulation of bioactivity.

Conditions for a site-selective ceric ammonium nitrate oxidation have been developed. The reactions proceed nicely on both 1K- and 12K-microelectrode arrays. The procedure for developing the reactions was very simple and demonstrated that the same reagents used for a solution-phase reaction can be used for a related site-selective reaction on a microelectrode array.

A short and efficient synthesis of enone 3, a key intermediate in the total synthesis of platencin (2), based on an intramolecular Diels−Alder reaction is described.

A bistable [2]rotaxane comprising an α-cyclodextrin (α-CD) ring and a dumbbell component containing a redox-active tetrathiafulvalene (TTF) ring system within its rod section has been synthesized using the Cu(I)-catalyzed azide−alkyne cycloaddition, and the redox-driven movements of the α-CD ring between the TTF and newly formed triazole ring systems have been elucidated. Microcalorimetric titrations on model complexes suggested that the α-CD ring prefers to reside on the TTF rather than on the triazole ring system by at least an order of magnitude. The fact that this situation does pertain in the bistable [2]rotaxane has not only been established quantitatively by electrochemical experiments and backed up by spectroscopic and chiroptical measurements but also been confirmed semiquantitatively by the recording of numerous cyclic voltammograms which point, along with the use of redox-active chemical reagents, to a mechanism of switching that involves the oxidation of the neutral TTF ring system to either its radical cationic (TTF^•+) or dicationic (TTF²⁺) counterparts, whereupon the α-CD ring, moves along the dumbbell to encircle the triazole ring system. Since redox control by both chemical and electrochemical means is reversible, the switching by the bistable [2]rotaxane can be reversed on reduction of the TTF^•+ or TTF²⁺ back to being a neutral TTF.

Total syntheses of (+)-nankakurine A (2) and (+)-nankakurine B (3) were accomplished by a sequence that employs an intramolecular dipolar cycloaddition of an azomethine imine intermediate to form the azatricyclic moiety and establish the relative configuration of the spiropiperidine ring. These syntheses, together with the synthesis of the originally purported structure 1 of nankakurine A, rigorously establish the relative and absolute configuration of these structurally unusual Lycopodium alkaloids. The syntheses are sufficiently concise that gram quantities of (+)-nankakurine A (2) and (+)-nankakurine B (3) will be available for further biological studies.

The first direct arylation via C−OH bond activation of tautomerizable heterocycles has been achieved using phosphonium salts, on the basis of a combination of the phosphonium coupling and Suzuki−Miyaura cross-coupling conditions. Optimal reaction condition is obtained through screening of phosphonium salts, Pd catalysts, and bases. The direct arylation via C−OH bond activation tolerates a variety of tautomerizable heterocycles and aryl boronic acids. The mechanism of the Pd-catalyzed phosphonium coupling is proposed to proceed via a domino seven-step process including the unprecedented heterocycle-Pd(II)−phosphonium species. Application of the Pd-catalyzed direct arylation via C−OH bond activation using PyBroP leads to the most efficient synthesis of the biologically important 6-arylpurine ribonucleoside in a single step from unactivated and unprotected inosine.

A series of six open-chain tetrapyrroles has been synthesized and used as chromophores for the plant photoreceptor protein phytochrome. The novel chromophores vary in the size of substituents 17 and 18 at ring D. This ring undergoes maximal conformational change upon light excitation (Z → E photoisomerization of the 15,16-double bond). Instead of methyl and vinyl substituents (positions 17, 18) as present in the native chromophore phytochromobilin, dimethyl, methyl and isopropyl, methyl and tert-butyl, ethyl and methyl, vinyl and methyl, and isopropyl and methyl substituents have been generated. All novel chromophores assemble with the apoprotein. The obtained chromoproteins show hypsochromic shifts of the absorbance maxima by 10 nm maximally, compared to the native pigment, except for the 17-isopropyl-18-methyl-substituted compound which showed a 100 nm hypsochromic shift of selectively the P_r form. The assembly kinetics were slowed down in correlation to the increasing size of the substituents, with stronger effects for modified substituents at position 17. The thermal stability of the photoinduced P_fr form for the 18-isopropyl and the 18-tert butyl substituents was even greater than that of the native pigments. Those chromophores carrying substituents at position 17 larger than the methyl group (ethyl and isopropyl) showed a very low stability of the respective P_fr forms. Time-resolved detection of the P_r to P_fr conversion (laser-induced flash photolysis) revealed a slower formation of the P_fr form for those chromophores carrying larger substituents at position 18, whereas the rise and decay kinetics of the early intermediates are only moderately changed. Introduction of larger substituents at position 17 (ethyl, vinyl, and isopropyl) causes drastic changes in the kinetics; in particular the formation of the first thermally stable intermediate, I₇₀₀, is significantly slowed, making a detection of its rise possible.

The paper describes the electronic charging and conducting properties of vertically oriented TiO₂ nanotube arrays formed by anodization of Ti foil samples. The resulting films, composed of vertically oriented nanotubes approximately 10 μm long, wall thickness 22 nm, and pore diameter 56 nm, are analyzed using impedance spectroscopy and cyclic voltammetry. Depending on the electrochemical conditions two rather different electronic behaviors are observed. Nanotube array samples in basic medium show behavior analogous to that of nanoparticulate TiO₂ films used in dye-sensitized solar cells: a chemical capacitance and electronic conductivity that increase exponentially with bias potential indicating a displacement of the Fermi level. Nanotube array samples in acidic medium, or samples in a basic medium submitted to a strong negative bias, exhibit a large increase in capacitance and conductivity indicating Fermi level pinning. The contrasting behaviors are ascribed to proton intercalation of the TiO₂. Our results suggest a route for controlling the electronic properties of the ordered metal-oxide nanostructures for their use in applications including supercapacitors, dye-sensitized solar cells, and gas sensing.

The reductive elimination of carbon−carbon bonds is one of the most fundamentally and synthetically important reaction steps in organometallic chemistry, yet relatively little is understood about the factors that govern the kinetics of this reaction. C−C elimination from complexes with the common d⁶ six-coordinate configuration generally proceeds via prior ligand loss, which greatly complicates any attempt to directly measure the rates of the specific elimination step. We report the synthesis of a series of five-coordinate d⁶ iridium complexes, (^tBuPCP)Ir(R)(R′), where R and R′ are Me, Ph, and (phenyl-substituted) vinyl and alkynyl groups. For several of these complexes (R/R′ = Ph/Vi, Me/Me, Me/Vi, Me/CCPh, and Vi/CCPh, where Vi = trans-CH═CHPh) we have measured the absolute rate of C−C elimination. For R/R′ = Ph/Ph, Ph/Me, and Ph/CCPh, we obtain upper limits to the elimination rate; and for R/R′ = CCPh/CCPh, a lower limit. In general, the rates decrease (activation barriers increase) according to the following order: acetylide < vinyl ∼ Me < Ph. Density functional theory (DFT) calculations offer significant insight into the factors behind this order, in particular the slow rates for elimination of the vinyl and, especially, phenyl complexes. The transition states are calculated to involve rotation of the aryl or vinyl group around the Ir−C bond, prior to C−C elimination, such that the group to which it couples can add to the face of the aryl or vinyl group. This rotation is severely hindered by the presence of the phosphino-t-butyl groups that lie above and below the plane of the aryl/vinyl group in the ground state. Accordingly, calculations predict dramatically different relative rates of elimination from the much less sterically hindered complexes (^HPCP)Ir(R)(R′). For example, the barrier to elimination from (^HPCP)Ir(Me)₂ is 20 kcal/mol, which is 2 kcal/mol greater than from the (^tBuPCP)Ir analogue. In contrast, the activation enthalpies calculated for vinyl−vinyl and phenyl−phenyl elimination from (^HPCP)Ir are remarkably low, only 2 and 9 kcal/mol, respectively; these values are 16 and 22 kcal/mol less than those of the corresponding (^tBuPCP)Ir complexes. Moreover, since these eliminations are very nearly thermoneutral, the barriers are calculated to be equally low for the reverse reactions [C−C oxidative addition to (^HPCP)Ir]. The absence of differences in intraligand C═C bond lengths in the transition states relative to the ground states, combined with a comparison of calculated “face-on” and “planar” transition states for C−C coupling, suggests that the critical importance of the aryl/vinyl rotation is based on geometric or steric factors rather than electronic ones. Thus there is no evidence for participation of the π or π* orbitals of the aryl or vinyl groups in the formation of the C−C bond, although a small π effect cannot be rigorously excluded. Likewise, the results do not support the hypothesis that the degree of directionality of the carbon-based orbital used for bonding to iridium (sp³ > sp² > sp) plays an important role in this system in determining the barrier to reductive elimination.

UVB irradiation of DNA produces photodimers in adjacent DNA bases and on rare occasions in nonadjacent bases. UVB irradiation (312 nm) of d(GTATCATGAGGTGC) gave rise to an unknown DNA photoproduct in approximately 40% yield at acidic pH of about 5. This product has a much shorter retention time in reverse phase HPLC compared to known dipyrimidine photoproducts of this sequence. A large upfield shift of two thymine H6 NMR signals and photoreversion to the parent ODN upon irradiation with 254 nm light indicates that the photoproduct is a cyclobutane thymine dimer. Exonuclease-coupled MS assay establishes that the photodimer forms between T2 and T7, which was confirmed by tandem mass spectrometric MS/MS identification of the endonuclease P1 digestion product pd(T2[A3])=pd(T7[G8]). Acidic hydrolysis of the photoproduct gave a product with the same retention time on reverse phase HPLC and the same MS/MS fragmentation pattern as authentic Thy[c,a]Thy. 2D NOE NMR data are consistent with a cis-anti cyclobutane dimer between the 3′-sides of T2 and T7 in anti glycosyl conformations that had to have arisen from an interstand type reaction. In addition to pH dependency, the photoproduct yield is highly sequence specific and concentration dependent, indicating that it results from a higher order folded structure. The efficient formation of this interstrand-type photoproduct suggests the existence of a new type of folding motif and the possibility that this type of photoproduct might also form in other folded structures, such as G-quadruplexes and i-motif structures which can be now studied by the methods described.

A convenient, sensitive, and label-free method to determine the DNA methylation status of CpG sites of plasmid and human colon cancer cell has been developed. The system relies on highly selective single base extension reaction and significant optical amplification of cationic conjugated polyelectrolytes (CCP-1). The higher fluorescence resonance energy transfer efficiency between CCP-1 and fluorescein-labeled dGTP (dGTP-Fl) is correlated to the incorporation of dGTP-Fl into the probe DNA by single base extension reaction when the target/probe pair is complementary at the methylation site. As low as 1% methylation status can be determined by this new assay method. Because of the optical amplification property of CCP-1, the method exhibited high sensitivity with a concentration of analyte DNA at the picomolar level. The CCP-1 can form a complex with negatively charged DNA through electrostatic interactions, avoiding labeling the DNA target and probe by covalent linking. The isolation steps employed in other typical assays were avoided to simplify operations and increase repeatability. These features make the system promising for future use for early cancer diagnosis.

Over the past two decades, the photolytic reactions of dibenzyl ketones sorbed on zeolites have been investigated. The reported results are consistent with a supramolecular model that takes into account the physical and chemical nature of the structure of the zeolites and their effect on the reactive radical intermediates produced by photolysis of adsorbed molecules. The model incorporates various phenomena such as surface coverage, external and internal sorption, surface diffusion, radical sieving, and the resulting product distributions. This account reports direct evidence for the validation of the model through FT-IR spectroscopy and through a new method for “titrating” the binding sites via EPR spectroscopy. It is shown that it is possible to adjust and modulate the photolytic product distribution by varying the parameters of the system. The effects of co-adsorbed spectator molecules with different polarities, namely water, pyridine, and benzene, on the photolysis of o-methyldibenzyl ketone and dibenzyl ketone sorbed on MFI zeolites is examined. This study provides insights into a displacement mechanism caused by spectator molecules and further demonstrates how the product distribution of photolysis of sorbed ketones can be controlled. The kinetics of persistent radicals formed by photolysis of ketones sorbed on zeolites is directly monitored over time by EPR, providing a measure of the lifetime of these reactive organic intermediates. Finally, measurement of Langmuir isotherms was employed to provide classical evidence for the model.

The compound (bpy)₂Mn^III(μ-O)₂Mn^IV(bpy)₂, a structural model relevant for the photosynthetic water oxidation complex, was coupled to single Cr^VI charge-transfer chromophores in the channels of the nanoporous oxide AlMCM-41. Mn K-edge EXAFS spectroscopy confirmed that the di-μ-oxo dinuclear Mn core of the complex is unaffected when loaded into the nanoscale pores. Observation of the 16-line EPR signal characteristic of Mn^III(μ-O)₂Mn^IV demonstrates that the majority of the loaded complexes retained their nascent oxidation state in the presence or absence of Cr^VI centers. The FT-Raman spectrum upon visible light excitation of the Cr^VI−O^II → Cr^V−O^I ligand-to-metal charge transfer reveals electron transfer from Mn^III(μ-O)₂Mn^IV (Mn−O stretch at 700 cm⁻¹) to Cr^VI, resulting in the formation of Cr^V and Mn^IV(μ-O)₂Mn^IV (Mn−O stretch at 645 cm⁻¹). All initial and final states are directly observed by FT-Raman or EPR spectroscopy, and the assignments are corroborated by X-ray absorption spectroscopy measurements. The endoergic charge separation products (ΔE _o = −0.6 V) remain after several minutes, which points to spatial separation of Cr^V and Mn^IV(μ-O)₂Mn^IV as a consequence of hole (O^I) hopping as a major contributing mechanism. This is the first observation of visible light-induced oxidation of a potential water oxidation complex by a metal charge-transfer pump in a nanoporous environment. These findings will allow for the assembly and photochemical characterization of well-defined transition metal molecular units, with the ultimate goal of performing endothermic, multielectron transformations that are coupled to visible light electron pumps in nanostructured scaffolds.

A nonaqueous liquid-phase route involving the reaction of vanadium oxychloride with benzyl alcohol leads to the formation of single-crystalline and semiconducting VO_1.52(OH)_0.77 nanorods with an ellipsoidal morphology, up to 500 nm in length and typically about 100 nm in diameter. Composition, structure, and morphology were thoroughly analyzed by neutron and synchrotron powder X-ray diffraction as well as by different electron microscopy techniques (SEM, (HR)TEM, EDX, and SAED). The data obtained point to a hollandite-type structure which, unlike other vanadates, contains oxide ions in the channels along the c-axis, with hydrogen atoms attached to the edge-sharing oxygen atoms, forming OH groups. According to structural probes and magnetic measurements (1.94 μ_B/V), the formal valence of vanadium is +3.81 (V⁴⁺/V³⁺ atomic ratio ≈ 4). The experimentally determined density of 3.53(5) g/cm³ is in good agreement with the proposed structure and nonstoichiometry. The temperature-dependent DC electrical conductivity exhibits Arrhenius-type behavior with a band gap of 0.64 eV. The semiconducting behavior is interpreted in terms of electron hopping between vanadium cations of different valence states (small polaron model). Ab initio density-functional calculations with a local spin density approximation including orbital potential (LSDA + U with an effective U value of 4 eV) have been employed to extract the electronic structure. These calculations propose, on the one hand, that the electronic conductivity is based on electron hopping between neighboring V³⁺ and V⁴⁺ sites, and, on the other hand, that the oxide ions in the channels act as electron donors, increasing the fraction of V³⁺ cations, and thus leading to self-doping. Experimental and simulated electron energy-loss spectroscopy data confirm both the presence of V⁴⁺ and the validity of the density-of-states calculation. Temperature-dependent magnetic susceptibility measurements indicate strongly frustrated antiferromagnetic interactions between the vanadium ions. A model involving the charge order of the V³⁺ sites is proposed to account for the observed formation of the magnetic moment below 25 K.

An optimized procedure for the selective dimetalation of [V(η⁶-C₆H₆)₂] by BuLi/tmeda allowed for the isolation and characterization of [V(η⁶-C₆H₅Li)₂]·tmeda. X-ray diffraction of its thf solvate [V(η⁶-C₆H₅Li)₂]·(thf)₇ revealed an unsymmetrical, dimeric composition in the solid state, in which both subunits are connected by three bridging lithium atoms. Treatment with several element dihalides facilitated the isolation of [n]vanadoarenophanes (n = 1, 2) with boron and silicon in the bridging positions. In agreement with the number and covalent radii of the bridging elements, these derivatives exhibit molecular ring strain to a greater or lesser extent. The B−B bond of the [2]bora species [V(η⁶-C₆H₅)₂B₂(NMe₂)₂] was readily cleaved by [Pt(PEt₃)₃] to afford the corresponding oxidative addition product. Subsequently, [V(η⁶-C₆H₅)₂B₂(NMe₂)₂] was employed as a diborane(4) precursor in the diboration of 2-butyne under stoichiometric, homogeneous, and heterogeneous catalysis conditions. This transformation is facilitated by the reduction of molecular ring strain, which was confirmed by a decrease of the tilt angle α observed in the corresponding solid-state structures. EPR spectroscopy was used to probe the electronic structure of strained [n]vanadoarenophanes and revealed an obvious correlation between the degree of molecular distortion and the observed hyperfine coupling constant a_iso. State-of-the-art DFT calculations were able to reproduce the measured isotropic vanadium hyperfine couplings and the coupling anisotropies. The calculations confirmed the decrease of the absolute isotropic hyperfine couplings with increasing tilt angle. Closer analysis showed that this is mainly due to increased positive contributions to the spin density at the vanadium nucleus from the spin polarization of doubly occupied valence orbitals of vanadium-ligand σ-antibonding character. The latter are destabilized and thus made more polarizable in the bent structures.

Thiamin thiazole biosynthesis in eukaryotes is still not completely understood. In this report, a late intermediate, tightly bound to the active site of the Saccharomyces cerevisiae thiazole synthase, was identified as an adenylated thiazole tautomer. The reactivity of this unusual compound was evaluated. Its identification provides an additional molecular snapshot of the complex reaction sequence catalyzed by the eukaryotic thiazole synthase and identifies the final step of the thiamin-thiazole biosynthesis.

Human carbonic anhydrase II (HCA II), among the fastest enzymes known, catalyzes the reversible hydration of CO₂ to HCO₃^-. The rate-limiting step of this reaction is believed to be the formation of an intramolecular water wire and transfer of a proton across the active site cavity from a zinc-bound solvent to a proton shuttling residue (His64). X-ray crystallographic studies have shown this intramolecular water wire to be directly stabilized through hydrogen bonds via a small well-defined set of amino acids, namely, Tyr7, Asn62, Asn67, Thr199, and Thr200. Furthermore, X-ray crystallographic and kinetic studies have shown that the mutation of tyrosine 7 to phenylalanine, Y7F HCA II, has the effect of increasing the proton transfer rate by 7-fold in the dehydration direction of the enzyme reaction compared to wild-type (WT). This increase in the proton transfer rate is postulated to be linked to the formation of a more directional, less branched, water wire. To evaluate this proposal, molecular dynamics simulations have been employed to study water wire formation in both the WT and Y7F HCA II mutant. These studies reveal that the Y7F mutant enhances the probability of forming small water wires and significantly extends the water wire lifetime, which may account for the elevated proton transfer seen in the Y7F mutant. Correlation analysis of the enzyme and intramolecular water wire indicates that the Y7F mutant significantly alters the interaction of the active site waters with the enzyme while occupancy data of the water oxygens reveals that the Y7F mutant stabilizes the intramolecular water wire in a manner that maximizes smaller water wire formation. This increase in the number of smaller water wires is likely to elevate the catalytic turnover of an already very efficient enzyme.

In this report, for the first time, a planar chiral photoresponsive compound has been employed in commercially available nematic liquid crystals to achieve phototunable reflection colors. We designed an azobenzenophane compound having conformational restriction on the free rotation of naphthalene moiety to impose an element of planar chirality and the corresponding enantiomers were resolved by HPLC on chiral column. We have determined the absolute configuration by comparison of density functional theory (DFT) calculations of its electronic circular dichroism (ECD) spectrum and specific rotation [α]_D to experimental ECD and [α]_D data. Enantiomers exhibit photochemically reversible isomerization in solution without undergoing thermal or photoinduced racemization. As chiroptic switches in different host nematic liquid crystals, they exhibit good solubility, moderately high helical twisting power, as well as a large change in helical twisting power due to photoisomerization. A unique feature of these chiral photochromic compounds is that no other auxiliary chiral agents is required to achieve a fast photon mode reversible full-range color control in induced cholesterics, that is, both the hypsochromic and bathochromic shift can be obtained from a single LC formulation by reversible photoisomerization of the single chiral compound.

We demonstrate that the key step for the reaction of CO₂ with hydrogen on Ni(110) is a change of the activated molecule coordination to the metal surface. At 90 K, CO₂ is negatively charged and chemically bonded via the carbon atom. When the temperature is increased and H approaches, the H−CO₂ complex flips and binds to the surface through the two oxygen atoms, while H binds to the carbon atom, thus yielding formate. We provide the atomic-level description of this process by means of conventional ultrahigh vacuum surface science techniques combined with density functional theory calculations and corroborated by high pressure reactivity tests. Knowledge about the details of the mechanisms involved in this reaction can yield a deeper comprehension of heterogeneous catalytic organic synthesis processes involving carbon dioxide as a reactant. We show why on Ni the CO₂ hydrogenation barrier is remarkably smaller than that on the common Cu metal-based catalyst. Our results provide a possible interpretation of the observed high catalytic activity of NiCu alloys.

A water-soluble self-assembled supramolecular host molecule catalyzes the hydrolysis of orthoformates in basic solution. Comparison of the rate constants of the catalyzed and uncatalyzed reactions for hydrolysis displays rate accelerations of up to 3900 for tri-n-propyl orthoformate. Kinetic analysis shows that the mechanism of hydrolysis with the supramolecular host obeys the Michaelis−Menten model. Mechanistic studies, including ¹³C-labeling experiments, revealed that the resting state of the catalytic system is the neutral substrate encapsulated in the host. Activation parameters for the k_cat step of the reaction revealed that upon substrate encapsulation in the assembly, the entropy of activation becomes more negative in contrast to the uncatalyzed reaction. Furthermore, solvent isotope effects reveal a normal k(H₂O)/k(D₂O) = 1.6, confirming an A-S_E2 mechanism in which proton transfer occurs in the rate-limiting step. This is in contrast with the A1 mechanism of the uncatalyzed reaction in which decomposition of the protonated substrate is rate-limiting.

We report the surfactant-directed assembly of mesoporous metal/germanium-based semiconducting materials from coupling of anionic (Ge₉)⁴⁻ clusters with various linking metal ions. The resulting materials feature a metal/Ge₉ framework perforated by regular arrays of mesoporous channels. The permanent mesoporosity of the materials NU-MGe-2 (M = Sb, In, Sn, Pb, Cd), determined by N₂ physisorption measurements, corresponds to high internal BET surface areas from 127 to 277 m²/g and total pore volumes from 0.15 to 0.26 cm³/g. The mesoporous structures exhibit energy gaps in the range of 1.48−1.70 eV as well as strong photoluminescence at room temperature with emission energies varying from 740 to 845 nm. The emission depends on pore wall thickness and framework composition. The photoemission intensity in the mesoporous intermetallic germanium-based frameworks can be selectively suppressed by adsorbing electron-acceptor species such as tetracyanoethylene molecules but remains unchanged when exposed to electron-donor species such as tetrathiafulvalene molecules.

A series of diacetylene monomers with benzoyl, 4-hexylbenzoyl, 4-dodecylbenzoyl, and perfluorobenzoyl substituents were synthesized and investigated with respect to their crystal structures and polymerizability. In the absence of perfluorophenyl−phenyl interactions, the crystal structures of related alkylated and nonalkylated derivatives were substantially different and dominated by the phase segregation between the alkylated side chains and the diaryl-substituted diacetylene cores. By contrast, the perfluorophenyl−phenyl interactions served as a reliable supramolecular synthon in that they persisted in the crystal structures of different alkylated and nonalkylated derivatives. The packing of the diacetylene functions was appropriate for a topochemical polymerization in these cases, and the perfluorophenyl−phenyl interaction determined the polymerization direction. As a result, soluble alternating diacetylene copolymers were obtained which were further characterized with solution phase methods.

Lasso peptides are a structurally unique class of bioactive peptides characterized by a knotted arrangement, where the C-terminus threads through an N-terminal macrolactam ring. Although ribosomally synthesized, only the gene cluster for the best studied lasso peptide MccJ25 from Escherichia coli consisting of the precursor protein McjA and the processing and immunity proteins McjB, McjC, and McjD is known. Through genome mining studies, we have identified homologues of all four proteins in Burkholderia thailandensis E264 and predicted this strain to produce a lasso peptide. Here we report the successful isolation of the predicted peptide, named capistruin. Upon optimization of the fermentation conditions, mass spectrometric and NMR structural studies proved capistruin to adopt a novel lasso fold. Heterologous production of the lasso peptide in Escherichia coli showed that the identified genes are sufficient for the biosynthesis of capistruin, which exhibits antimicrobial activity against closely related Burkholderia and Pseudomonas strains. In general, our rational approach should be widely applicable for the isolation of new lasso peptides to explore their high structural stability and diverse biological activity.

The facile synthesis of the metallaheteroborane [8,8-(PPh₃)₂-nido-8,7-RhSB₉H₁₀] (1) makes possible the systematic study of its reactivity. Addition of pyridine to 1 gives in high yield the 11-vertex nido-hydridorhodathiaborane [8,8,8-(PPh₃)₂H-9-(NC₅H₅)-nido-8,7-RhSB₉H₉] (2). 2 reacts with C₂H₄ or CO to form [1,1-(PPh₃)(L)-3-(NC₅H₅)-closo-RhSB₉H₈] [L = C₂H₄ (3), CO (4)]. In CH₂Cl₂ at reflux temperature 2 undergoes a nido to closo transformation to afford [1,1-(PPh₃)₂-3-(NC₅H₅)-closo-1,2-RhSB₉H₈] (5). Reaction of 2 with alkenes leads to hydrogenation and isomerization of the olefins. NMR spectroscopy indicates the presence of a labile phosphine ligand in 2, and DFT calculations have been used to determine which of the two phosphine groups is labile. Rationalization of the hydrogenation mechanism and the part played by the 2 → 3 nido to closo cluster change during the reaction cycle is suggested. In the proposed mechanism the classical hydrogen transfer from hydride metal complexes to olefins occurs twice: first upon coordination of the alkene to the rhodium centre in 2, and second concomitant with formation of a closo-hydridorhodathiaborane intermediate by migration of a BHB-bridging hydrogen atom to the metal. Reaction of H₂ with 3 or 5 regenerates 2, closing a reaction cycle that under catalytic conditions is capable of hydrogenating alkenes. Single-site versus cluster-bifunctional mechanisms are discussed as possible routes for H₂ activation.

Most low-molecular-weight platinum anticancer drugs have short blood circulation times that are reflected in their reduced tumor uptake and intracellular DNA binding. A platinum(IV) complex of the formula c,c,t-[Pt(NH₃)₂Cl₂(O₂CCH₂CH₂CO₂H)(O₂CCH₂CH₂CONH-PEG-FA)] (1), containing a folate derivative (FA) at an axial position, was prepared and characterized. Folic acid offers a means of targeting human cells that highly overexpress the folate receptor (FR). Compound 1 was attached to the surface of an amine-functionalized single-walled carbon nanotube (SWNT-PL-PEG-NH₂) through multiple amide linkages to use the SWNTs as a “longboat delivery system” for the platinum warhead, carrying it to the tumor cell and releasing cisplatin upon intracellular reduction of Pt(IV) to Pt(II). The ability of SWNT tethered 1 to destroy selectively FR(+) vs FR(−) cells demonstrated its ability to target tumor cells that overexpress the FR on their surface. That the SWNTs deliver the folate-bearing Pt(IV) cargos into FR(+) cancer cells by endocytosis was demonstrated by the localization of fluorophore-labeled SWNTs using fluorescence microscopy. Once inside the cell, cisplatin, formed upon reductive release from the longboat oars, enters the nucleus and reacts with its target nuclear DNA, as determined by platinum atomic absorption spectroscopy of cell extracts. Formation of the major cisplatin 1,2-intrastrand d(GpG) cross-links on the nuclear DNA was demonstrated by use of a monoclonal antibody specific for this adduct. The SWNT-tethered compound 1 is the first construct in which both the targeting and delivery moieties have been incorporated into the same molecule; it is also the first demonstration that intracellular reduction of a Pt(IV) prodrug leads to the cis-{Pt((NH₃)₂} 1,2-intrastrand d(GpG) cross-link in nuclear DNA.

The 68 residue peptide IA₃ is an intrinsically unstructured protein that serves as an endogenous inhibitor of the yeast aspartic proteinase A (YPrA). Although unstructured in free solution, IA₃ forms an N-terminal α helix as it binds to YPrA, leading to subnanomolar inhibition of the protease. Equilibrium structural and inhibition studies provide little insight into the mechanism and kinetics of the coupled folding and binding interaction. We have used laser temperature jump spectroscopy to study the kinetics of folding of free IA₃ and of the interaction between IA₃ and YPrA. Inducing folding with trifluoroethanol cosolvent allows us to determine the folding rate (k_f ≈ 0.3 (μs)⁻¹) and the unfolding rate (k_u ≈ 3 (μs)⁻¹) for free IA₃ in water at 25 °C. A substantially faster relaxation process is observed in the presence of the proteinase; this process appears to be the kinetic signature of an intermediate binding step in the coupled folding and binding interaction of IA₃ and YPrA.

The 1,3-dipolar cycloaddition of azides and activated alkynes has been used for site-selective labeling of biomolecules in vitro and in vivo. While copper catalysis has been widely employed to activate terminal alkynes for [3 + 2] cycloaddition, this method, often termed “click chemistry”, is currently incompatible with living systems because of the toxicity of the metal. We recently reported a difluorinated cyclooctyne (DIFO) reagent that rapidly reacts with azides in living cells without the need for copper catalysis. Here we report a novel class of DIFO reagents for copper-free click chemistry that are considerably more synthetically tractable. The new analogues maintained the same elevated rates of [3 + 2] cycloaddition as the parent compound and were used for imaging glycans on live cells. These second-generation DIFO reagents should expand the use of copper-free click chemistry in the hands of biologists.

The reactions of the arachno-4,6-C₂B₇H₁₃ carborane with the secondary and primary amines, Me₂NHBH₃ and ^tBuNH₂BH₃, in ionic liquid media result in both boron-insertion into the cage at a position across the two cage-carbons and additional hydrogen-elimination via the reaction of a hydridic B−H with a protonic amine N−H hydrogen to produce the 6-(RR′N)-nido-5,7-C₂B₈H₁₁ carboranes. Computational characterizations of these compounds and the previously reported 6-ClC₆H₄-9-(RR′N)-nido-6-NB₉H₁₀ azaboranes indicate that the amine-nitrogens form unique exopolyhedral dative B═N double bonds with a cage-boron.

We describe the functionalization of single-wall carbon nanotubes (SWNTs) with 4-(2-trimethylsilyl)ethynylaniline and the subsequent attachment of a zinc-phthalocyanine (ZnPc) derivative using the reliable Huisgen 1,3-dipolar cycloaddition. The motivation of this study was the preparation of a nanotube-based platform which allows the facile fabrication of more complex functional nanometer-scale structures, such as a SWNT−ZnPc hybrid. The nanotube derivatives described here were fully characterized by a combination of analytical techniques such as Raman, absorption and emission spectroscopy, atomic force and scanning electron microscopy (AFM and SEM), and thermogravimetric analysis (TGA). The SWNT−ZnPc nanoconjugate was also investigated with a series of steady-state and time-resolved spectroscopy experiments, and a photoinduced communication between the two photoactive components (i.e., SWNT and ZnPc) was identified. Such beneficial features lead to monochromatic internal photoconversion efficiencies of 17.3% when the SWNT−ZnPc hybrid material was tested as photoactive material in an ITO photoanode.

We report here a detailed study on the surface topology of well-known ordered mesoporous silica (SBA-15, MCM-41, and KIT-6) and a series of nanocast Co₃O₄, Co₃O₄/CoFe₂O₄ composites by high resolution scanning electron microscopy (HR-SEM). Images of the MCM-41 structure were obtained at a resolution of the pore size, as well as a real space image of the gyroid silica surface of KIT-6 for two different aging temperatures, clearly revealing the differences of the aging procedures. By using the low voltage HR-SEM technique with extremely high resolution, we could very clearly show the influence of the template properties on the structure of the nanocast metal oxides.

Using scanning tunneling microscope (STM) at 300 K, we studied the growth of one-dimensional molecular assemblies (molecular lines) on the Si(100)-(2 × 1)-H surface through the chain reaction of small ketone (CH₃COCH₃, PhCOPh, and PhCOCH₃) molecules with dangling bond (DB) sites of the substrate. Acetone and benzophenone show the growth of molecular lines exclusively parallel to the dimer row direction. In contrast, acetophenone molecules show some molecular lines perpendicular, in addition to parallel, to the dimer row direction. Most of the molecular lines perpendicular to the dimer row direction were grown by self-turning the propagation direction of a chain reaction from parallel to perpendicular directions relative to the dimer row. A chiral center created upon adsorption of an acetophenone molecule allows the adsorbed molecules to align with identical as well as alternate enantiomeric forms along the dimer row direction, whereas such variations in molecular arrangement are not observed in the case of acetone and benzophenone molecules. The observed molecular lines growth both parallel and perpendicular to dimer row directions appears to be unique to acetophenone among all the molecules studied to date. Hence, the present study opens new possibility for fabricating one-dimensional molecular assemblies of various compositions in both high-symmetry directions on the Si(100)-(2 × 1)-H surface.

The siderophore enterobactin (Ent) is produced by enteric bacteria to mediate iron uptake. Ent scavenges iron and is taken up by the bacteria as the highly stable ferric complex [Fe^III(Ent)]³⁻. This complex is also a specific target of the mammalian innate immune system protein, Siderocalin (Scn), which acts as an antibacterial agent by specifically sequestering siderophores and their ferric complexes during infection. Recent literature suggesting that Scn may also be involved in cellular iron transport has increased the importance of understanding the mechanism of siderophore interception and clearance by Scn; Scn is observed to release iron in acidic endosomes and [Fe^III(Ent)]³⁻ is known to undergo a change from catecholate to salicylate coordination in acidic conditions, which is predicted to be sterically incompatible with the Scn binding pocket (also referred to as the calyx). To investigate the interactions between the ferric Ent complex and Scn at different pH values, two recombinant forms of Scn with mutations in three residues lining the calyx were prepared: Scn-W79A/R81A and Scn-Y106F. Binding studies and crystal structures of the Scn-W79A/R81A:[Fe^III(Ent)]³⁻ and Scn-Y106F:[Fe^III(Ent)]³⁻ complexes confirm that such mutations do not affect the overall conformation of the protein but do weaken significantly its affinity for [Fe^III(Ent)]³⁻. Fluorescence, UV−vis, and EXAFS spectroscopies were used to determine Scn/siderophore dissociation constants and to characterize the coordination mode of iron over a wide pH range, in the presence of both mutant proteins and synthetic salicylate analogues of Ent. While Scn binding hinders salicylate coordination transformation, strong acidification results in the release of iron and degraded siderophore. Iron release may therefore result from a combination of Ent degradation and coordination change.

Oligomeric o-aryleneethynylenes with three triple bonds undergo cascade radical transformations in reaction with a Bu₃SnH/AIBN system. These cascades involve three consecutive cycle closures with the formation of substituted benzo[a]indeno[2,1-c]fluorene or benzo[1,2]fluoreno[4,3-b]silole derivatives. The success of this sequence depends on regioselectivity of the initial attack of the Bu₃Sn radical at the central triple bond of the o-aryleneethynylene moiety. The cascade is propagated through the sequence of 5-exo-dig and 6-exo-dig cyclizations which is followed by either a radical attack at the terminal Ar substituent or radical transposition which involves H-abstraction from the terminal TMS group and 5-endo-trig cyclization. Overall, the transformation has potential to be developed into an approach to a new type of graphite ribbons.

This full account presents the background on, discovery of, and extensive insight that has been gained into the oxidative intermolecular coupling of two different carbonyl species. Optimization of this process has culminated in reliable and scalable protocols for the union of amides, imides, ketones, and oxindoles using soluble copper(II) or iron(III) salts as oxidants. Extensive mechanistic studies point to a metal-chelated single-electron-transfer process in the case of copper(II), while iron(III)-based couplings appear to proceed through a non-templated heterodimerization. This work presents the most in-depth findings on the mechanism of oxidative enolate coupling to date. The scope of oxidative enolate heterocoupling is extensive (40 examples) and has been shown to be efficient even on a large scale (gram-scale or greater). Finally, the method has been applied to the total synthesis of the unsymmetrical lignan lactone (−)-bursehernin and a medicinally important 2,3-disubstituted succinate derivative.