Kinetically Controlled Coassembly of Multichromophoric Peptide Hydrogelators and the Impacts on Energy Transport
- Herdeline Ann M. Ardoña
- ,
- Emily R. Draper
- ,
- Francesca Citossi
- ,
- Matthew Wallace
- ,
- Louise C. Serpell
- ,
- Dave J. Adams
- , and
- John D. Tovar
View Author Information
† ‡ # †Department of Chemistry, Krieger School of Arts and Sciences, ‡Institute for NanoBioTechnology, and #Department of Materials Science and Engineering, Whiting School of Engineering, Johns Hopkins University, 3400 N. Charles Street, Baltimore, Maryland 21218, United States
§ School of Chemistry, WESTChem, University of Glasgow, Glasgow, G12 8QQ, United Kingdom
∥ School of Life Sciences, University of Sussex, Falmer, Brighton BN1 9QG, United Kingdom
⊥ Department of Chemistry, University of Liverpool, Liverpool L69 7ZD, United Kingdom
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Abstract
We report a peptide-based multichromophoric hydrogelator system, wherein π-electron units with different inherent spectral energies are spatially controlled within peptidic 1-D nanostructures to create localized energy gradients in aqueous environments. This is accomplished by mixing different π-conjugated peptides prior to initiating self-assembly through solution acidification. We can vary the kinetics of the assembly and the degree of self-sorting through the choice of the assembly trigger, which changes the kinetics of acidification. The hydrolysis of glucono-δ-lactone (GdL) provides a slow pH drop that allows for stepwise triggering of peptide components into essentially self-sorted nanostructures based on subtle pKa differences, whereas HCl addition leads to a rapid formation of mixed components within a nanostructure. Using 1H NMR spectroscopy and fiber X-ray diffraction, we determine the conditions and peptide mixtures that favor self-sorting or intimate comixing. Photophysical investigations in the solution phase provide insight into the correlation of energy-transport processes occurring within the assemblies to the structural organization of the π-systems.
Introduction
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Supramolecular systems can be engineered with dynamic properties as a result of the noncovalent nature of the intermolecular interactions that drive their assembly, so they offer promising materials for mimicking the dynamic nature of physiological environments. (1-3) Multicomponent supramolecular assemblies have been used toward creating artificial light-harvesting complexes, whereby the energy migration from an excited donor to an acceptor occurs within an array of donor–acceptor units. (4-8) There are several examples of multichromophoric assemblies aimed toward applications not only for photosynthetic mimicry but also for excitonic solar cells, (9) light-emitting assemblies, (10-13) and as biophysical molecular rulers. (14) The synthesis details and characterization of energy-transfer events are both equally important for the development of multichromophoric arrays in such applications. For example, Friend, Rowan, and Nolte and co-workers have developed Pt-porphyrin and perylene-bis(dicarboximide) units in polyisocyanopeptides and investigated the sequential energy and electron transfer between the donor and acceptor units. (15) Hodgkiss and Thordarson and co-workers reported a highly efficient multichromophore array based on perylene systems, whereby efficient incoherent energy transfer was reported. (16) Basché and Müllen and co-workers reported terrylene and perylene diimide dyad systems and quantified singlet–singlet annihilation. (17, 18) Ajayaghosh and co-workers extensively investigated a library of oligo(p-phenylenevinylene) organogelators that are capable of excitation energy transfer between specific donor–acceptor systems. (19-24) A system reported on C3-symmetric self-sorted units of oligo(p-phenylenevinylene) and perylene bisimide was even utilized as a sensing platform for volatile aromatic compounds. (25)
Among the biologically relevant supramolecular structures, peptide-based assemblies appended with π-electron systems, both in solution phase and in xerogels, hold promise for potential applications toward energy-harvesting materials under aqueous conditions (26) as well as solution-processable active layers for device applications such as in field-effect transistors (27-29) and biointerfacing (30-34) applications. Tovar and co-workers previously presented a two-component peptide scaffold bearing oligo(p-phenylenevinylene) and quaterthiophene π-electron units that demonstrated excitonic energy transfer. (35) They also demonstrated that quaterthiophene and naphthalenediimide chromophores embedded in the same peptide supported photoinduced electron transfer also under aqueous conditions. (36) Adams and co-workers reported self-sorting among perylene bisimide and stilbene gelators, creating heterojunction arrays with self-sorted fiber networks that led to a photoconductive material. (37) Side-chain functionalization of lysine units with tetraphenylporphyrin and naphthalene diimide was also shown to demonstrate a switch between the H- or J-type of aggregation depending on the protonation state of the porphyrin. (38) In a study that utilized chiral cholesterol moieties, the formation of p-n heterojunctions among self-sorted quaterthiophene- and perylene diimide organogelators was achieved via a heat–cool method, whereby the sequential dissociation and reassembly was governed by the different dissociation constants of each component. (39)
In this report, we combine three distinct π-electron units (oligo(p-phenylenevinylene), quaterthiophene, and naphthalenediimide) within peptidic moieties that form assemblies in aqueous environments under acidic conditions. The combination of photonic donor–acceptor pairs and electron donor–acceptor pairs within one nanostructure enables potentially simultaneous and/or sequential energy- and electron-transfer events and allows for the creation of localized electric fields between the donor and acceptor units that result from charge separation. Moreover, by utilizing different acidification triggers and controlling the kinetics of self-assembly, we aim to control the spatial ordering of the chromophores within the peptidic nanostructure and systematically impact the unique energy-transport processes fostered within these nanostructures.
Results and Discussion
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Design Considerations
We compare the photophysical responses of self-sorted and randomly coassembled binary mixtures of peptides bearing different π-electron systems (Figure 1). To achieve such control over the spatial ordering of the binary peptide mixtures, we modulated the rate of acidification. Although there are several studies conducted on controlling the supramolecular polymerization of multicomponent systems, (40) accurate control over the assembly dynamics and spatial ordering is rarely discussed in the context of engineering the optoelectronic function of a material. A previous report on the nonequilibrium hydrogelation of a quaterthiophene (41) demonstrated that the nanoscale morphologies vary depending on the rate of acid diffusion into the peptide solution: the rapid assembly leads to random but dense matted fiber networks, whereas the relatively slower assembly results in a more homogeneous and anisotropic fiber network.
Figure 1
Figure 1. (a) Molecular structures of the peptides studied herein and (b),(c) diagrams of the potential energy (e.g., resonance-energy transfer (RET)) and electron-transfer events occurring within a two-component peptidic nanostructure with three π-electron units for (b) self-sorted and (c) randomly coassembled systems.
Here, we use different assembly triggers that lower the pH of the peptide solutions at different rates in order to have control over the kinetics of assembly and the molecular ordering of binary peptide mixtures. Self-sorting was induced via chemical programming, utilizing the difference in the pKa of the two components as the pH is gradually decreased upon the slow hydrolysis of glucono-δ-lactone (GdL). (42, 43) Thus, as the pH slowly decreases, we expect a greater extent of protonation/charge screening for the higher pKa peptide, triggering it to preferentially assemble first. This should favor the formation of essentially self-sorted structures (Figure 1b). To form the randomly coassembled structures, acid was rapidly added to solutions of essentially dissolved peptide units, whereby global protonation/charge screening of all peptides occurs regardless of the pKa (Figure 1c). The Asp-Val-Val and Lys-Ala-Ala peptide sequences were chosen for the oligo(p-phenylenevinylene)- and quaterthiophene-appended peptides (OPV3 and OT4-NDI), respectively, to establish a difference in the pKa of each component (see below). The 1,4-distyrylbenzene, an oligo(p-phenylenevinylene), serves as an energy donor that upon photoexcitation can funnel the energy to a quaterthiophene unit acceptor. Similarly, the quaterthiophene serves as an excited-state electron donor to the naphthalenediimide in OT4-NDI. A control molecule was also synthesized in which the side-chain amines were acylated instead of bearing naphthalenediimide groups (OT4-Ac). This control molecule allows us to observe photonic energy-transfer events without the associated electron transfer. We investigate the occurrence of energy-transport processes under aqueous environments in the solution phase and whether the components are self-sorted because of the slow GdL hydrolysis or kinetically coassembled by rapid acidification.
Characterization of Assembly Behavior
The apparent pKas of the OPV3donor, the OT4-NDIacceptor, and the OT4-Accontrol acceptor were observed as pH 6.2, 6.5, and 5.4, respectively (Figure S10). The OT4-NDI-containing samples showed buffering regions around pH 12. The gradual titration with aqueous HCl for 1:1 binary mixtures of OPV3/ OT4-NDI and OPV3/ OT4-Ac only showed one apparent pKa rather than a distinct pKa for each component. To initiate the self-sorting behavior within the two-component peptide hydrogels, 30 mg/mL of GdL was added to the peptide solutions that contained 5 mg/mL of each component. In previous studies with related peptides appended with π-systems, only 5 to 10 mg/mL of GdL was necessary to form hydrogels from 5 mg/mL one-component or binary-peptide gelator solutions. (42, 43) The necessary higher GdL concentration for the hydrogelation of the systems reported herein can be possibly attributed to the multiple carboxylates (total of four, two from the backbone termini and two from the aspartic acid side chains, for the OPV3 donor units) that need to be protonated to favor the self-assembly process. To further monitor the self-assembly process with GdL, we followed the disappearance of 1H NMR peaks corresponding to the assembly of each gelator component together with the in situ measurement of pH. (44) The assembly is correlated to the disappearance of unique NMR peaks of each component over time, and the results showed a sequential assembly for the 1:1 OPV3 and OT4-Ac mixture (Figure 2a), whereby OPV3 assembles first. Alternatively, the 1:1 OPV3 and OT4-NDI mixture (Figure S11) did not show any peaks for OT4-NDI even at the beginning of the measurement (before the addition of GdL), suggesting that the assembly might have already started for this component even at higher pH conditions (ca. pH 8). This observation of assembly formation at higher pH was analogous to a previously reported hydrophobic oligo(p-phenylenevinylene) dipeptide, which favors the formation of wormlike micelles. (45) All homoassemblies (one-component samples) and 1:1 mixed assemblies successfully formed self-supporting gels, as confirmed by the rheology measurements (Figure 2b and S14; the storage modulus G′ for any given composition was an order of magnitude larger than the loss modulus respective G″ at the end of the time-sweep measurements). The storage moduli values for these hydrogels spanned from 0.02 to 2.5 kPa, with the 1:1 OPV3 and OT4-NDI mixed assemblies having the lowest G′ values and the 1:1 OPV3 and OT4 mixed assemblies having the highest G′ values. For the 1:1 OPV3 and OT4-NDI mixed assembled hydrogel (with a total of 10 mg/mL peptide content) the G′ is even lower than its individual components at 5 mg/mL, which demonstrates the complex and nonlinear nature of the mechanical properties of two-component supramolecular systems. These mechanical properties were also observed in another related binary system composed of dipeptide components appended with naphthalene units. (46) Considering the apparent pKa values from the titration experiments and the spectral information from time-resolved NMR measurements, OT4-NDI assembles first, followed by OPV3 and then OT4-Ac. The rheology plots show that the OT4-Ac forms the hydrogel the fastest, followed by OPV3 and then OT4-NDI (Figure S14). Correlating the 1H NMR results, which probe molecular aggregation events, to the time frame of gelation observed from rheological measurements, it is observed that the aggregation time does not coincide with the amount of time required to observe the inflection points for G′ and G″ plots for this peptidic system. These plots suggest that for this material and the conditions imposed upon assembly the kinetics of aggregation of the individual components do not directly correlate to the rate at which the fibrillar network reaches the critical fiber-network density to form a hydrogel. Furthermore, because the fiber network density is linked to the bulk mechanical property of hydrogels, there is also no direct correlation that can be made between the kinetics of assembly formation and hydrogel stiffness for these systems.
Figure 2
Figure 2. Monitoring of peptide assembly and hydrogel formation for 1:1 OPV3 and OT4-Ac solutions prepared with 30 mg/mL GdL via (a) 1H NMR and (b) rheology (G′ = solid circles; G″ = smaller circles). Each peptide component is at 5 mg/mL (total of 10 mg/mL peptides for the 1:1 mixed sample).
Further monitoring of the GdL hydrolysis-triggered gelation process via fiber X-ray diffraction supports the occurrence of self-sorting based on the presence of distinct signals from the individual components (Figure 3). No diffraction pattern was observed for OT4-NDI as a result of the difficulty in aligning the weaker gels (G′ < 100 Pa) between the capillaries used for the measurements. The persistence of diffraction signals from pure OPV3 (9.3 Å and 4.61/4.69 Å, Figure 3a) and pure OT4-Ac (5.6 and 4.61 Å, Figure 3b) in the mixed-peptide assemblies prepared with GdL indicates the self-sorted nature of the nanostructures (Figure 3c,d). (42) From these characterization techniques, we determined the chronology of aggregation and the onset of peptide gelation, and we established that self-sorting occurs when hydrogels are prepared with GdL for both OPV3/OT4-Ac and OPV3/OT4-NDI systems.
Figure 3
Figure 3. Fiber X-ray diffraction data for hydrogels prepared with 30 mg/mL GdL, composed of one peptide component ((a) OPV3 only; (b) OT4-Ac only) and two peptide components ((c) OPV3/OT4-NDI; (d) OPV3/OT4-Ac; both 1:1). Each peptide component is at 5 mg/mL (total of 10 mg/mL peptides for the 1:1 mixed samples).
TEM was used to visualize the nanostructures resulting from GdL or HCl addition (ca. 3 μM peptide concentration for each component). Under such dilute conditions, 10 mg/mL GdL was enough to observe nanostructure formation. For the OPV3 and OT4-Ac homoassemblies (Figure S15), both methods of triggering assembly resulted in the same type of 1-D morphologies. For the OT4-NDI peptides, rapid acidification with HCl shows some evidence of micellar-shaped random aggregates, whereas the addition of GdL results in structures with 1-D morphology coexisting with these micellar structures. The predominantly micellar nature of nanostructures formed by OT4-NDI with GdL and HCl further supports the formation of relatively weaker gels than for the other peptides because such low-aspect-ratio structures generally have a lower propensity to form cross-links among the nanostructures. For the OPV3 and OT4-NDI mixed assemblies, both the rapid acidification with HCl and the addition of GdL result in the formation of micellar structures along with 1-D nanostructures (Figure 4a,b). In contrast, the mixed assemblies of OPV3 and OT4-Ac (Figure 4c,d) both show 1-D nanostructures with high aspect ratios.
Figure 4
Figure 4. TEM images of 1:1 mixed assemblies (3 μM peptide concentration for each component) of OPV3 with (a),(b) OT4-NDI or (c),(d) OT4-Ac (1:1) assembled via (a),(c) HCl addition and (b),(d) GdL (10 mg/mL) hydrolysis.
Photophysical Characterization
We utilized the π-electron units embedded in the peptidic moieties, each with unique spectroscopic signatures, as reporters to obtain more information about the spatial organization occurring within the nanostructures under HCl or GdL assembly conditions. The steady-state photophysical spectra of the homoassemblies and mixed assemblies triggered to assemble via HCl and GdL in the aqueous solution phase (Figures 5, 6, and S16) consistently showed trends for H-like aggregation. Namely, this included blue-shifted, quenched absorption and red-shifted, quenched photoluminescence (PL) of the assembled samples as compared to their molecularly dissolved states. The recorded spectra from these aqueous solutions should be treated as ensemble averages of the response from polydisperse solutions of peptide assemblies. The spectra for HCl-triggered solutions were recorded immediately after acidification (i.e., within 1 h) whereas the GdL-triggered samples were measured at 20 h to allow enough time for the lactone to hydrolyze and produce enough acid to achieve the same pH as the HCl-triggered solutions. The HCl-assembled samples were then aged for the same amount of time that the GdL was allowed to hydrolyze, after which the absorption and PL spectra were then remeasured. For this multichromophoric system, the spectral overlap between the emission of the oligo(p-phenylenevinylene) donor with the absorption of quaterthiophene or naphthalenediimide acceptors encourages energy transfer via exciton migration or resonance-energy transfer (RET) from the donor to these acceptors, with a better spectral overlap with quaterthiophene. For quaterthiophene and naphthalenediimide, their relative HOMO and LUMO levels encourage excited-state electron transfer and inhibit energy transfer upon photoexcitation as previously reported. (36) As a point of reference, the spectral properties of the homoassemblies prepared via GdL- and HCl-mediated assembly showed no apparent differences in the maximum absorption peaks except for some slight intensity variations (Figure 5a–c). The aged HCl-assembled samples more closely resemble the absorption spectra of the GdL-assembled samples, suggesting the presence of kinetically trapped structures formed by rapid acidification. They also suggest the dynamic nature of these assemblies, which allow some extent of reorganization over time.
Figure 5
Figure 5. Solution-phase UV–vis absorption (a–c) and steady-state emission (d–f) spectra for GdL- (10 mg/mL) or HCl-assembled peptide homoassemblies of (a) OPV3 (blue), (b) OT4-Ac (red), and (c) OT4-NDI (black); solutions measured under different conditions: molecularly dissolved samples (---), (a),(d) with GdL (empty circles), (b),(e) with HCl (—), and (c),(f) with HCl, aged (...); [peptide] = 3 μM; X= Raman scatter of water.
Figure 6
Figure 6. Solution-phase steady-state PL spectra for mixed peptide structures of OPV3 with OT4-Ac (red) and with OT4-NDI (black) prepared with GdL (10 mg/mL) or HCl, measured under different conditions: (a–c) λexc= 350 nm; (d–f) λexc= 430 nm; (a) with GdL (empty circles), (b) with HCl (—), and (c) with HCl, aged (...); [OPV3] = 3 μM.
The PL spectra for the homoassemblies of OPV3, OT4-Ac, and OT4-NDI all showed different features when assembled via GdL hydrolysis or via rapid HCl addition (Figure 5d–f). The HCl-assembled OPV3 shows a broad emission with λmax = 470 nm, whereas the aged HCl- and GdL-triggered assemblies show blue-shifted peaks with more resolved vibronic bands at 420, 440, and 470 nm, suggesting a different packing pattern than for the initially trapped HCl assemblies. These bands can be assigned to strong exciton coupling and are consistent with those that we have observed for the thermally annealed structures of other OPV3 tetrapeptides. (35) These results support that the slower hydrolysis of GdL could be achieving a more thermodynamically favored structure that is accessed after a dynamic rearrangement due to heating or aging of the kinetically trapped structures formed after rapid acidification. (47-49) For OT4-Ac and OT4-NDI excited at 370 and 350 nm (maximum absorption for assembled OT4-Ac and OPV3, respectively; naphthalenediimide has minimal absorption at 350 nm), the PL peak of the aged HCl-assembled solutions does not vary from the early time-point measurements. The OT4-NDI peptide excited at 350 nm shows an emission profile that corresponds to minimal PL contributions from naphthalenediimide. However, for excitation at 430 nm, which corresponds to the quaterthiophene component of OT4-NDI, the GdL-assembled and aged HCl samples both show a very weak 510 nm peak similar to the maximum PL peak for a monomeric quaterthiophene peptide. The aged HCl-assembled sample shows a completely quenched spectrum (430 nm excitation), with the Raman peak from water at 500 nm as the only apparent feature. Figure 5e,f show signals with very weak intensities and apparent Raman scattering peaks for water. The weak signals are expected from the weakly fluorescent quaterthiophene assemblies but can also be a consequence of the electron transfer from quaterthiophene to naphthalenediimide units.
For the mixed assemblies, analogous to the observations in our previous study for an oligo(p-phenylenevinylene)-based donor–acceptor system, (50) it is expected that the photophysical behavior will reflect the spatial organization of the π-electron units, whether it is self-sorted via GdL-triggered assembly or randomly coassembled via HCl-triggered assembly. The absorption profiles are dominated by the OPV3 signals, which have the highest extinction coefficient among the three chromophores studied, even at 50 mol % of the acceptor units (Figure S16). At 10 and 50 mol %, higher-intensity shoulders at 450 nm are more evident for the HCl-assembled than for the GdL-assembled comixtures. For the PL spectra of mixed assemblies, the samples were excited at 350 nm (where oligo(p-phenylenevinylene) has the strongest absorption among the three chromophores) and at 430 nm (where only the quaterthiophene component absorbs). The 10 and 50 mol % OT4-Ac in OPV3 and OT4-NDI in OPV3 are compared under different assembly conditions. In all cases, the PL peak intensities of these mixed-peptide solutions under acidic conditions are lower than that of the OPV3 homoassemblies as shown in Figure 5d. The 10 and 50 mol % OT4-Ac in OPV3 GdL self-sorted assemblies excited at 350 nm (Figure 6a) both show quenched emission peaks in the OPV3 region with bimodal peaks around 490 and 520 nm resembling assembled oligothiophene emission in more hydrophobic environments, (51-54) demonstrating some extent of energy transfer. The self-sorted OT4-NDI and OPV3 GdL-assembled samples showed less OPV3 peak quenching than did the OT4-Ac and OPV3 self-sorted assemblies but maintained the OPV3 vibronic progression. The HCl-assembled samples (Figure 6b) showed a similar trend for quaterthiophene PL signatures but with less-resolved spectral features. Compared to the GdL-assembled samples, the OPV3 emission region was completely quenched, and the peak was further red-shifted to 540 nm at 50 mol %, showing a higher efficiency of energy transfer, which is expected from a more intimate comixing for HCl-assembled samples. Upon aging (Figure 6c), the HCl-assembled samples show more vibronic features in the OPV3 region but are still more quenched than the self-sorted assemblies prepared with GdL. Furthermore, comparing the OT4-Ac and OT4-NDI coassemblies with OPV3 prepared with HCl, the intensities (i.e., relative quantum yields) of the OT4-NDI coassemblies are much lower than those of the OT4-Ac coassemblies with OPV3, verifying the complete energy funneling from oligo(p-phenylenevinylene) to quaterthiophene, to relatively spectrally dark naphthalenediimide in the OT4-NDI/OPV3 coassemblies. This is indicative that energy transfer is still more efficient in the aged HCl-assembled samples than in the GdL-assembled samples, showing that the reorganization that occurs in the kinetically trapped structures formed upon rapid acidification does not achieve the complete self-sorting that is induced with GdL hydrolysis.
For the mixed assemblies excited at 430 nm (Figure 6d–f), the emission spectra provide information on any energy-transfer events that occur from quaterthiophene. The PL signals for self-sorted OT4-Ac and OPV3 mixed assemblies prepared with GdL coincide with the emission of assembled OT4-Ac homoassemblies, which is consistent with self-sorting. For the HCl-assemblies at 50 mol % OT4-Ac, the PL spectrum has a maximum emission peak at ca. 540 nm, which coincides with what was previously reported for quaterthiophene peptides, (29, 51) whereas the maximum emission peak for 10 mol % OT4-Ac at 510 nm is reminiscent of molecularly dissolved samples prepared under basic conditions. At 10 mol %, where OT4-Ac is a minority component in the comixture, the similarity of the PL profile to molecularly dissolved samples suggests that most of the OT4-Ac units are isolated within 90 mol % OPV3 1-D nanostructures. Upon aging of the HCl-assembled samples, the 50 mol % OT4-Ac sample showed a similar bimodal peak to that observed in GdL-assembled samples, whereas the 10 mol % sample showed a broader quenched peak. For the OT4-NDI/ OPV3 HCl-triggered mixed assemblies, the quaterthiophene PL region showed a significant quenching as compared to the OT4-Ac and OPV3 samples, which are prepared with the same concentration of quaterthiophene units.
These results support the idea that the selective electron transfer from quaterthiophene to naphthalenediimide upon excitation at 430 nm efficiently occurs without preference as to the assembly method used. For the mixed assemblies excited at 350 nm, the steady-state emission behavior supports that the sequential and/or concerted energy/electron transfer events from OPV3 to OT4-Ac/OT4-NDI occur more efficiently when the components are randomly mixed (prepared with HCl) than self-sorted (prepared with GdL). These data demonstrate the dependence of the photophysical behavior on the choice of assembly trigger (GdL vs HCl) for solution-phase assembly. Hence, the spatial organization of the chromophores within the peptide nanostructures coincides with the structural information obtained for these samples in the gel phase via 1H NMR and fiber XRD (Figures 2a and 3).
Circular dichroism (CD) measurements confirm that both the HCl and GdL-assembled peptide nanostructures maintain similar chirality in both pure and mixed assemblies, except for OT4-NDI (Figure S17). Low-intensity CD signals were recorded for OT4-NDI homoassemblies corresponding to the naphthalenediimide region of absorption but not for quaterthiophene. Bisignate signals within the region of chromophore absorption corresponding to an exciton-coupled Cotton band were observed for all other peptide nanostructures prepared under both assembly conditions, which is indicative of the local chiral environment of the π-systems undergoing exciton coupling. The CD signals within the high-energy region that distinguish the structural motifs for conventional peptides were not observed for GdL-assembled samples because of a high intensity signal from chiral GdL and gluconic acid as well as interference from sample scattering (Figure S18). For the HCl-assembled samples, the broad peak with minima at ∼250 nm corresponds to a red-shifted β-sheet signal, which is correlated to local twisting within individual assembly units. (55, 56)
Conclusions
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A two-component peptide hydrogelator system with multichromophoric units can be spatially engineered on the basis of the choice of the assembly trigger. The slow hydrolysis of GdL provides access to self-sorted structures if each of the components has a different pKa. However, rapid addition of HCl to the peptide solutions favors the random comixing of two components within the same nanostructure, which can reorganize upon aging. Both methods of assembly led to the formation of self-supporting hydrogels. 1H NMR spectroscopy and fiber XRD support that self-sorting behavior is induced when GdL is added to the binary peptide mixtures. Correlating the 1H NMR spectra and rheology profiles, aggregation occurs the fastest for OT4-NDI, but the fastest gelation was achieved with OT4-Ac. Thus, aggregation can start earlier in one component, but the propagation of the supramolecular polymerization leading to entangled networks that can support a hydrogel structure can occur at a different rate. Whether the two components were randomly coassembled or self-sorted, the chirality within the nanostructures was maintained as evidenced by the exciton-coupled CD bands within the region of the peptide-embedded chromophore absorptions. Both randomly coassembled and self-sorted peptide nanostructures showed evidence for energy and electron transfer. Selectively exciting OPV3 initiates exciton migration and resonance energy transfer, which both occur more efficiently if the chromophores are randomly coassembled within one nanostructure. However, the excited-state electron transfer from quaterthiophene to napthelenediimide does not require any specific comixing. This work provides new insights into the photophysical behavior of multichromophoric peptide assemblies whether the components are self-sorted, which is useful for establishing p-n heterojunctions, or randomly comixed, which is useful for creating photosynthetic mimics that require high energy-transfer efficiency. The observation of multiple energy-transport processes in completely aqueous environments, along with the ability to spatially control the distribution of π-electron units within peptidic nanostructures, is an important step in the continued engineering of functional bioelectronic materials.
Supporting Information
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b04006.
General synthesis procedures and experimental conditions; characterization data for peptides (1H NMR, ESI-MS, HPLC traces); pH titration curves; 1NMR stacked plots and data for monitoring OPV3 and OT4-NDI assembly with GdL; supplementary rheology data, TEM images, UV–vis absorption spectra, and CD spectra (PDF)
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Author Information
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- Dave J. Adams - School of Chemistry, WESTChem, University of Glasgow, Glasgow, G12 8QQ, United Kingdom;http://orcid.org/0000-0002-3176-1350;
- John D. Tovar - †Department of Chemistry, Krieger School of Arts and Sciences, ‡Institute for NanoBioTechnology, and #Department of Materials Science and Engineering, Whiting School of Engineering, Johns Hopkins University, 3400 N. Charles Street, Baltimore, Maryland 21218, United States; School of Chemistry, WESTChem, University of Glasgow, Glasgow, G12 8QQ, United Kingdom; School of Life Sciences, University of Sussex, Falmer, Brighton BN1 9QG, United Kingdom; Department of Chemistry, University of Liverpool, Liverpool L69 7ZD, United Kingdom;http://orcid.org/0000-0002-9650-2210;
- Herdeline Ann M. Ardoña - †Department of Chemistry, Krieger School of Arts and Sciences, ‡Institute for NanoBioTechnology, and #Department of Materials Science and Engineering, Whiting School of Engineering, Johns Hopkins University, 3400 N. Charles Street, Baltimore, Maryland 21218, United States; School of Chemistry, WESTChem, University of Glasgow, Glasgow, G12 8QQ, United Kingdom; School of Life Sciences, University of Sussex, Falmer, Brighton BN1 9QG, United Kingdom; Department of Chemistry, University of Liverpool, Liverpool L69 7ZD, United Kingdom;http://orcid.org/0000-0003-0640-1262
- Louise C. Serpell - School of Life Sciences, University of Sussex, Falmer, Brighton BN1 9QG, United Kingdom;http://orcid.org/0000-0001-9335-7751
Acknowledgment
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We thank Johns Hopkins University and the NSF DMR Biomaterials program (1407493). H.A.M.A. is thankful for the generous support from Howard Hughes Medical Institute (International Student Research Fellowship) and Schlumberger Foundation (Faculty for the Future Fellowship). We also thank the Center for Molecular Biophysics (JHU) where circular dichroism measurements were conducted. The NMR spectrometer used for the aggregation time measurements was funded by the EPSRC (EP/C005643/1 and EP/K039687/1). D.J.A. thanks the EPSRC for a Fellowship, which also funded E.R.D. (EP/L021978/1). M.W. thanks Unilever for a Case Award and the EPSRC for funding a DTA. We thank Dr. Laura L. E. Mears for the discussions regarding the structural characterizations of these peptide nanomaterials and for her insightful comments on the manuscript. We dedicate this paper to Professor Samuel I. Stupp (Northwestern University) in recognition of his 40 years of leadership in self-assembling materials.
References
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This article references 56 other publications.
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- 26Krieg, E.; Bastings, M. M. C.; Besenius, P.; Rybtchinski, B. Chem. Rev. 2016, 116 (4) 2414– 2477 DOI: 10.1021/acs.chemrev.5b00369[ACS Full Text], [CAS], Google Scholar26https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhtVCls7Y%253D&md5=f925f8858d9766bad8c978db3870c589Supramolecular Polymers in Aqueous MediaKrieg, Elisha; Bastings, Maartje M. C.; Besenius, Pol; Rybtchinski, BorisChemical Reviews (Washington, DC, United States) (2016), 116 (4), 2414-2477CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)This review discusses one-dimensional supramol. polymers that form in aq. media. First, naturally occurring supramol. polymers are described, in particular, amyloid fibrils, actin filaments, and microtubules. Their structural, thermodn., kinetic, and nanomech. properties are highlighted, as well as their importance for the advancement of biol. inspired supramol. polymer materials. Second, five classes of synthetic supramol. polymers are described: systems based on (1) hydrogen-bond motifs, (2) large π-conjugated surfaces, (3) host-guest interactions, (4) peptides, and (5) DNA. We focus on recent studies that address key challenges in the field, providing mechanistic understanding, rational polymer design, important functionality, robustness, or unusual thermodn. and kinetic properties.
- 27Sanders, A. M.; Dawidczyk, T. J.; Katz, H. E.; Tovar, J. D. ACS Macro Lett. 2012, 1 (11) 1326– 1329 DOI: 10.1021/mz3004665[ACS Full Text], [CAS], Google Scholar27https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhsFygt7rO&md5=61217ed255dc27c5e489890df36b81cdPeptide-based supramolecular semiconductor nanomaterials via Pd-catalyzed solid-phase "dimerizations"Sanders, Allix M.; Dawidczyk, Thomas J.; Katz, Howard E.; Tovar, John D.ACS Macro Letters (2012), 1 (11), 1326-1329CODEN: AMLCCD; ISSN:2161-1653. (American Chemical Society)We report a streamlined method for the synthesis of peptides embedded with complex and easily variable π-conjugated oligomeric subunits from com. available precursors. These modified peptides self-assemble under aq. conditions to form one-dimensional nanomaterials contg. networks of π-stacked conduits, despite the inclusion of π-conjugated oligomers with quadrupoles extended over larger areas. The procedure has circumvented soly. and other synthetic issues to allow for the facile formation of a diverse library of bioelectronic nanomaterials, including a complex sexithiophene-contg. peptide whose nanostructures display gate-induced cond. within field effect transistors.
- 28Besar, K.; Ardoña, H. A. M.; Tovar, J. D.; Katz, H. E. ACS Nano 2015, 9 (12) 12401– 12409 DOI: 10.1021/acsnano.5b05752
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- 30Liyanage, W.; Ardoña, H. A. M.; Mao, H. Q.; Tovar, J. D. Bioconjugate Chem. 2017, 28 (3) 751– 759 DOI: 10.1021/acs.bioconjchem.6b00593[ACS Full Text], [CAS], Google Scholar30https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhvVenu7fO&md5=2a8594256b856fa77d9f034b3abae8c3Cross-Linking Approaches to Tuning the Mechanical Properties of Peptide π-Electron HydrogelsLiyanage, Wathsala; Ardona, Herdeline Ann M.; Mao, Hai-Quan; Tovar, John D.Bioconjugate Chemistry (2017), 28 (3), 751-759CODEN: BCCHES; ISSN:1043-1802. (American Chemical Society)Self- assembling peptides are extensively exploited as bioactive materials in applications such as regenerative medicine and drug delivery, despite the fact that their relatively weak non-covalent interactions often render them susceptible to mech. destruction and solvent erosion. Herein, we describe how covalent crosslinking enhances the mech. stability of self-assembling π-conjugated peptide hydrogels. We designed short peptide-chromophore-peptide sequences displaying different reactive functional groups that can form crosslinks with appropriately modified bifunctional polyethylene glycol (PEG) based small guest mols. These peptides self-assemble into one dimensional fibrillar networks in response to pH in the aq. environment. The crosslinking reactions were promoted to create a secondary network locked in place by covalent bonds within the phys. crosslinked (pre-assembled) π-conjugated peptide strands. Rheol. measurements were used to evaluate the mech. modifications of the network, and the chem. changes that accompany the crosslinking were further confirmed by IR spectroscopy. Furthermore, we modified these crosslinkable π-conjugates by incorporating extracellular matrix (ECM) derived Ile-Lys-Val-Ala-Val (IKVAV) and Arg-Gly-Asp (RGD) bioactive epitopes to support human neural stem/progenitor cell (hNSCs) differentiation. The hNSCs undergo differentiation into neurons on IKVAV derived π-conjugates while RGD contg. peptides failed to support cell attachment. These findings provide significant insight into biochem. and electronic properties of π-conjugated peptide hydrogelators for creating artificial ECM to enable advanced tissue engineering applications.
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- 33Arslan, E.; Garip, I. C.; Gulseren, G.; Tekinay, A. B.; Guler, M. O. Adv. Healthcare Mater. 2014, 3 (9) 1357– 1376 DOI: 10.1002/adhm.201300491[Crossref], [PubMed], [CAS], Google Scholar33https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhsV2gurfK&md5=83a118e02810982c504989c8a818e00fBioactive Supramolecular Peptide Nanofibers for Regenerative MedicineArslan, Elif; Garip, I. Ceren; Gulseren, Gulcihan; Tekinay, Ayse B.; Guler, Mustafa O.Advanced Healthcare Materials (2014), 3 (9), 1357-1376CODEN: AHMDBJ; ISSN:2192-2640. (Wiley-VCH Verlag GmbH & Co. KGaA)Recent advances in understanding of cell-matrix interactions and the role of the extracellular matrix (ECM) in regulation of cellular behavior have created new perspectives for regenerative medicine. Supramol. peptide nanofiber systems have been used as synthetic scaffolds in regenerative medicine applications due to their tailorable properties and ability to mimic ECM proteins. Through designed bioactive epitopes, peptide nanofiber systems provide biomol. recognition sites that can trigger specific interactions with cell surface receptors. The present Review covers structural and biochem. properties of the self-assembled peptide nanofibers for tissue regeneration, and highlights studies that investigate the ability of ECM mimetic peptides to alter cellular behavior including cell adhesion, proliferation, and/or differentiation.
- 34Loo, Y.; Goktas, M.; Tekinay, A. B.; Guler, M. O.; Hauser, C. A. E.; Mitraki, A. Adv. Healthcare Mater. 2015, 4 (16) 2557– 2586 DOI: 10.1002/adhm.201500402[Crossref], [CAS], Google Scholar34https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhs1CntrzM&md5=ec48e7d289533710903106a991f54864Self-Assembled Proteins and Peptides as Scaffolds for Tissue RegenerationLoo, Yihua; Goktas, Melis; Tekinay, Ayse B.; Guler, Mustafa O.; Hauser, Charlotte A. E.; Mitraki, AnnaAdvanced Healthcare Materials (2015), 4 (16), 2557-2586CODEN: AHMDBJ; ISSN:2192-2640. (Wiley-VCH Verlag GmbH & Co. KGaA)Self-assembling proteins and peptides are increasingly gaining interest for potential use as scaffolds in tissue engineering applications. They self-organize from basic building blocks under mild conditions into supramol. structures, mimicking the native extracellular matrix. Their properties can be easily tuned through changes at the sequence level. Moreover, they can be produced in sufficient quantities with chem. synthesis or recombinant technologies to allow them to address homogeneity and standardization issues required for applications. Here. recent advances in self-assembling proteins, peptides, and peptide amphiphiles that form scaffolds suitable for tissue engineering are reviewed. The focus is on a variety of motifs, ranging from minimalistic dipeptides, simplistic ultrashort aliph. peptides, and peptide amphiphiles to large "recombinamer" proteins. Special emphasis is placed on the rational design of self-assembling motifs and biofunctionalization strategies to influence cell behavior and modulate scaffold stability. Perspectives for combination of these "bottom-up" designer strategies with traditional "top-down" biofabrication techniques for new generations of tissue engineering scaffolds are highlighted.
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- 37Draper, E. R.; Lee, J. R.; Wallace, M.; Jäckel, F.; Cowan, A. J.; Adams, D. J. Chem. Sci. 2016, 7 (10) 6499– 6505 DOI: 10.1039/C6SC02644C[Crossref], [PubMed], [CAS], Google Scholar37https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhtV2nt7nK&md5=ad187780459c71550d478304967290eaSelf-sorted photoconductive xerogelsDraper, Emily R.; Lee, Jonathan R.; Wallace, Matthew; Jackel, Frank; Cowan, Alexander J.; Adams, Dave J.Chemical Science (2016), 7 (10), 6499-6505CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)We show that a perylene bisimide (PBI)-based gelator forms self-sorted mixts. with a stilbene-based gelator. To form the self-sorted gels, we use a slow pH change induced by the hydrolysis of glucono-δ-lactone (GdL) to gluconic acid. We prove that self-sorting occurs using NMR spectroscopy, UV-Vis spectroscopy, rheol., and viscometry. The corresponding xerogels are photoconductive. Importantly, the wavelength dependence of the photoconductive films is different to that of the films formed from the perylene bisimide alone. Transient absorption spectroscopy of the xerogels reveals changes in the spectrum of the PBI on the picosecond timescale in the presence of stilbene with a PBI radical anion being formed within 10 ps when the stilbene is present. The ability to form the PBI radical anion under visible light leads to the enhanced spectral response of the multicomponent gels. These systems therefore have potential as useful visible-active optoelectronics.
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- 53Guo, Z.; Song, Y.; Gong, R.; Mu, Y.; Jiang, Y.; Li, M.; Wan, X. Supramol. Chem. 2014, 26 (5–6) 383– 391 DOI: 10.1080/10610278.2013.844810[Crossref], [CAS], Google Scholar53https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhsleru7zJ&md5=0a1dbc40a15eec5fb08fb66fc0f77a21Assembly of peptide-thiophene conjugates: the influence of peptide content and locationGuo, Zongxia; Song, Yubao; Gong, Ruiying; Mu, Youbing; Jiang, Yi; Li, Ming; Wan, XiaoboSupramolecular Chemistry (2014), 26 (5-6), 383-391CODEN: SCHEER; ISSN:1029-0478. (Taylor & Francis Ltd.)Biomol.-directed self-assembly of π-conjugated oligomers has attracted great attention in the past decade. In this contribution, two conjugates composed of quaterthiophene and tetrapeptide (Gly-Val-Gly-Val) were synthesized, namely peptide-thiophene-peptide (PTP) and thiophene-peptide-thiophene (TPT), to investigate the influence of peptide content ratio and its location in the mol. structures on the nanostructures and properties of the assemblies. Both conjugates formed organogels consisting of left-handed twisted nanostructures; however, anti-parallel β-sheets were obsd. in PTP while parallel β-sheets were obtained for TPT, although in both cases oligothiophenes adopted an H-like stacking mode. Obvious solvent-induced supramol. chirality inversion from the oligothiophene segment was obsd. for PTP while such phenomenon was not clear for TPT. PTP and TPT gels also showed different stabilities towards temp. increase, as evidenced by variable-temp. CD study. From the data, it is suggested that the rational design of the location and ratio of peptide plays a key role in constructing materials with detd. properties based on peptide-thiophene conjugates.
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Abstract
Figure 1
Figure 1. (a) Molecular structures of the peptides studied herein and (b),(c) diagrams of the potential energy (e.g., resonance-energy transfer (RET)) and electron-transfer events occurring within a two-component peptidic nanostructure with three π-electron units for (b) self-sorted and (c) randomly coassembled systems.
Figure 2
Figure 2. Monitoring of peptide assembly and hydrogel formation for 1:1 OPV3 and OT4-Ac solutions prepared with 30 mg/mL GdL via (a) 1H NMR and (b) rheology (G′ = solid circles; G″ = smaller circles). Each peptide component is at 5 mg/mL (total of 10 mg/mL peptides for the 1:1 mixed sample).
Figure 3
Figure 3. Fiber X-ray diffraction data for hydrogels prepared with 30 mg/mL GdL, composed of one peptide component ((a) OPV3 only; (b) OT4-Ac only) and two peptide components ((c) OPV3/OT4-NDI; (d) OPV3/OT4-Ac; both 1:1). Each peptide component is at 5 mg/mL (total of 10 mg/mL peptides for the 1:1 mixed samples).
Figure 4
Figure 4. TEM images of 1:1 mixed assemblies (3 μM peptide concentration for each component) of OPV3 with (a),(b) OT4-NDI or (c),(d) OT4-Ac (1:1) assembled via (a),(c) HCl addition and (b),(d) GdL (10 mg/mL) hydrolysis.
Figure 5
Figure 5. Solution-phase UV–vis absorption (a–c) and steady-state emission (d–f) spectra for GdL- (10 mg/mL) or HCl-assembled peptide homoassemblies of (a) OPV3 (blue), (b) OT4-Ac (red), and (c) OT4-NDI (black); solutions measured under different conditions: molecularly dissolved samples (---), (a),(d) with GdL (empty circles), (b),(e) with HCl (—), and (c),(f) with HCl, aged (...); [peptide] = 3 μM; X= Raman scatter of water.
Figure 6
Figure 6. Solution-phase steady-state PL spectra for mixed peptide structures of OPV3 with OT4-Ac (red) and with OT4-NDI (black) prepared with GdL (10 mg/mL) or HCl, measured under different conditions: (a–c) λexc= 350 nm; (d–f) λexc= 430 nm; (a) with GdL (empty circles), (b) with HCl (—), and (c) with HCl, aged (...); [OPV3] = 3 μM.
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- 14Sapsford, K. E.; Berti, L.; Medintz, I. L. Angew. Chem., Int. Ed. 2006, 45 (28) 4562– 4589 DOI: 10.1002/anie.200503873[Crossref], [CAS], Google Scholar14https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD28Xnt1ehurg%253D&md5=0f8f06ddd155b1555c5209a72a0f34d9Materials for fluorescence resonance energy transfer analysis: beyond traditional donor-acceptor combinationsSapsford, Kim E.; Berti, Lorenzo; Medintz, Igor L.Angewandte Chemie, International Edition (2006), 45 (28), 4562-4588CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)A review. The use of Forster or fluorescence resonance energy transfer (FRET) as a spectroscopic technique has been in practice for over 50 years. A search of ISI Web of Science with just the acronym "FRET" returns more than 2300 citations from various areas such as structural elucidation of biol. mols. and their interactions, in vitro assays, in vivo monitoring in cellular research, nucleic acid anal., signal transduction, light harvesting and metallic nanomaterials. The advent of new classes of fluorophores including nanocrystals, nanoparticles, polymers, and genetically encoded proteins, in conjunction with ever more sophisticated equipment, has been vital in this development. This review gives a crit. overview of the major classes of fluorophore materials that may act as donor, acceptor, or both in a FRET configuration. The authors focus in particular on the benefits and limitations of these materials and their combinations, as well as the available methods of bioconjugation.
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- 19Ghosh, S.; Praveen, V. K.; Ajayaghosh, A. Annu. Rev. Mater. Res. 2016, 46 (1) 235– 262 DOI: 10.1146/annurev-matsci-070115-031557[Crossref], [CAS], Google Scholar19https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XosFyrsbY%253D&md5=eab37970be9dab05cf1a64e7cdafd852The Chemistry and Applications of π-GelsGhosh, Samrat; Praveen, Vakayil K.; Ajayaghosh, AyyappanpillaiAnnual Review of Materials Research (2016), 46 (), 235-262CODEN: ARMRCU; ISSN:1531-7331. (Annual Reviews)π-Gels are a promising class of functional soft materials formed out of short π-conjugated mols. By utilizing the chem. of noncovalent interactions, researchers have created a wide range of π-gels that are composed of supramol. polymers. During the last two decades, supramol. gel chem. has been pursued with the hope of developing new materials for applications in, for example, org. electronics, energy harvesting, sensing, and imaging. The high expectations for π-gels were centered mainly around their electronic properties, such as tunable emission, energy transfer, electron transfer, charge transport, and elec. cond.; such properties are amenable to modulation through size and shape control of mol. assemblies. Although a large no. of exciting publications have appeared, a major technol. breakthrough is yet to be realized. In this review, we analyze the recent advancements in the area of functional π-gels and their scope in future applications.
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- 22Ajayaghosh, A.; Praveen, V. K.; Vijayakumar, C.; George, S. J. Angew. Chem., Int. Ed. 2007, 46 (33) 6260– 6265 DOI: 10.1002/anie.200701925
- 23Ajayaghosh, A.; Praveen, V. K.; Vijayakumar, C. Chem. Soc. Rev. 2008, 37 (1) 109– 122 DOI: 10.1039/B704456A
- 24Vijayakumar, C.; Praveen, V. K.; Kartha, K. K.; Ajayaghosh, A. Phys. Chem. Chem. Phys. 2011, 13 (11) 4942– 4949 DOI: 10.1039/c0cp02110e
- 25Sandeep, A.; Praveen, V. K.; Kartha, K. K.; Karunakaran, V.; Ajayaghosh, A. Chem. Sci. 2016, 7 (7) 4460– 4467 DOI: 10.1039/C6SC00629A[Crossref], [CAS], Google Scholar25https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XksVylsrk%253D&md5=b51a5da1db6f41e605cbd76cb4afc579Supercoiled fibres of self-sorted donor-acceptor stacks: a turn-off/turn-on platform for sensing volatile aromatic compoundsSandeep, Anjamkudy; Praveen, Vakayil K.; Kartha, Kalathil K.; Karunakaran, Venugopal; Ajayaghosh, AyyappanpillaiChemical Science (2016), 7 (7), 4460-4467CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)To ensure the comfortable survival of living organisms, detection of different life threatening volatile org. compds. (VOCs) such as biol. metabolites and carcinogenic mols. is of prime importance. Herein, we report the use of supercoiled supramol. polymeric fibers of self-sorted donor-acceptor mols. as "turn-off/turn-on" fluorescent sensors for the detection of carcinogenic VOCs. For this purpose, a C3-sym. donor mol. based on oligo(p-phenylenevinylene), C3OPV, and a perylene bisimide based acceptor mol., C3PBI, have been synthesized. When these two mols. were mixed together in toluene, in contrast to the usual charge transfer (CT) stacking, supramol. fibers of self-sorted stacks were formed at the mol. level, primarily driven by their distinct self-assembly pathways. However, CT interaction at the macroscopic level allows these fibers to bundle together to form supercoiled ropes. An interfacial photoinduced electron transfer (PET) process from the donor to the acceptor fibers leads to an initial fluorescence quenching, which could be modulated by exposure to strong donor or acceptor type VOCs to regenerate the resp. fluorescence of the individual mol. stacks. Thus, strong donors could regenerate the green fluorescence of C3OPV stacks and strong acceptors could reactivate the red fluorescence of C3PBI stacks. These supercoiled supramol. ropes of self-sorted donor-acceptor stacks provide a simple tool for the detection of donor- or acceptor-type VOCs of biol. relevance, using a "turn-off/turn-on" fluorescence mechanism as demonstrated with o-toluidine, which has been reported as a lung cancer marker.
- 26Krieg, E.; Bastings, M. M. C.; Besenius, P.; Rybtchinski, B. Chem. Rev. 2016, 116 (4) 2414– 2477 DOI: 10.1021/acs.chemrev.5b00369[ACS Full Text], [CAS], Google Scholar26https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhtVCls7Y%253D&md5=f925f8858d9766bad8c978db3870c589Supramolecular Polymers in Aqueous MediaKrieg, Elisha; Bastings, Maartje M. C.; Besenius, Pol; Rybtchinski, BorisChemical Reviews (Washington, DC, United States) (2016), 116 (4), 2414-2477CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)This review discusses one-dimensional supramol. polymers that form in aq. media. First, naturally occurring supramol. polymers are described, in particular, amyloid fibrils, actin filaments, and microtubules. Their structural, thermodn., kinetic, and nanomech. properties are highlighted, as well as their importance for the advancement of biol. inspired supramol. polymer materials. Second, five classes of synthetic supramol. polymers are described: systems based on (1) hydrogen-bond motifs, (2) large π-conjugated surfaces, (3) host-guest interactions, (4) peptides, and (5) DNA. We focus on recent studies that address key challenges in the field, providing mechanistic understanding, rational polymer design, important functionality, robustness, or unusual thermodn. and kinetic properties.
- 27Sanders, A. M.; Dawidczyk, T. J.; Katz, H. E.; Tovar, J. D. ACS Macro Lett. 2012, 1 (11) 1326– 1329 DOI: 10.1021/mz3004665[ACS Full Text], [CAS], Google Scholar27https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhsFygt7rO&md5=61217ed255dc27c5e489890df36b81cdPeptide-based supramolecular semiconductor nanomaterials via Pd-catalyzed solid-phase "dimerizations"Sanders, Allix M.; Dawidczyk, Thomas J.; Katz, Howard E.; Tovar, John D.ACS Macro Letters (2012), 1 (11), 1326-1329CODEN: AMLCCD; ISSN:2161-1653. (American Chemical Society)We report a streamlined method for the synthesis of peptides embedded with complex and easily variable π-conjugated oligomeric subunits from com. available precursors. These modified peptides self-assemble under aq. conditions to form one-dimensional nanomaterials contg. networks of π-stacked conduits, despite the inclusion of π-conjugated oligomers with quadrupoles extended over larger areas. The procedure has circumvented soly. and other synthetic issues to allow for the facile formation of a diverse library of bioelectronic nanomaterials, including a complex sexithiophene-contg. peptide whose nanostructures display gate-induced cond. within field effect transistors.
- 28Besar, K.; Ardoña, H. A. M.; Tovar, J. D.; Katz, H. E. ACS Nano 2015, 9 (12) 12401– 12409 DOI: 10.1021/acsnano.5b05752
- 29Wall, B. D.; Diegelmann, S. R.; Zhang, S.; Dawidczyk, T. J.; Wilson, W. L.; Katz, H. E.; Mao, H.-Q.; Tovar, J. D. Adv. Mater. 2011, 23 (43) 5009– 5014 DOI: 10.1002/adma.201102963
- 30Liyanage, W.; Ardoña, H. A. M.; Mao, H. Q.; Tovar, J. D. Bioconjugate Chem. 2017, 28 (3) 751– 759 DOI: 10.1021/acs.bioconjchem.6b00593[ACS Full Text], [CAS], Google Scholar30https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhvVenu7fO&md5=2a8594256b856fa77d9f034b3abae8c3Cross-Linking Approaches to Tuning the Mechanical Properties of Peptide π-Electron HydrogelsLiyanage, Wathsala; Ardona, Herdeline Ann M.; Mao, Hai-Quan; Tovar, John D.Bioconjugate Chemistry (2017), 28 (3), 751-759CODEN: BCCHES; ISSN:1043-1802. (American Chemical Society)Self- assembling peptides are extensively exploited as bioactive materials in applications such as regenerative medicine and drug delivery, despite the fact that their relatively weak non-covalent interactions often render them susceptible to mech. destruction and solvent erosion. Herein, we describe how covalent crosslinking enhances the mech. stability of self-assembling π-conjugated peptide hydrogels. We designed short peptide-chromophore-peptide sequences displaying different reactive functional groups that can form crosslinks with appropriately modified bifunctional polyethylene glycol (PEG) based small guest mols. These peptides self-assemble into one dimensional fibrillar networks in response to pH in the aq. environment. The crosslinking reactions were promoted to create a secondary network locked in place by covalent bonds within the phys. crosslinked (pre-assembled) π-conjugated peptide strands. Rheol. measurements were used to evaluate the mech. modifications of the network, and the chem. changes that accompany the crosslinking were further confirmed by IR spectroscopy. Furthermore, we modified these crosslinkable π-conjugates by incorporating extracellular matrix (ECM) derived Ile-Lys-Val-Ala-Val (IKVAV) and Arg-Gly-Asp (RGD) bioactive epitopes to support human neural stem/progenitor cell (hNSCs) differentiation. The hNSCs undergo differentiation into neurons on IKVAV derived π-conjugates while RGD contg. peptides failed to support cell attachment. These findings provide significant insight into biochem. and electronic properties of π-conjugated peptide hydrogelators for creating artificial ECM to enable advanced tissue engineering applications.
- 31Du, X.; Zhou, J.; Shi, J.; Xu, B. Chem. Rev. 2015, 115 (24) 13165– 13307 DOI: 10.1021/acs.chemrev.5b00299[ACS Full Text], [CAS], Google Scholar31https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXitVanur7O&md5=ad261f8a2373312f090587b6f6887e5cSupramolecular Hydrogelators and Hydrogels: From Soft Matter to Molecular BiomaterialsDu, Xuewen; Zhou, Jie; Shi, Junfeng; Xu, BingChemical Reviews (Washington, DC, United States) (2015), 115 (24), 13165-13307CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)In this review we intend to provide a relatively comprehensive summary of the work of supramol. hydrogelators after 2004 and to put emphasis particularly on the applications of supramol. hydrogels/hydrogelators as mol. biomaterials. After a brief introduction of methods for generating supramol. hydrogels, we discuss supramol. hydrogelators on the basis of their categories, such as small org. mols., coordination complexes, peptides, nucleobases, and saccharides. Following mol. design, we focus on various potential applications of supramol. hydrogels as mol. biomaterials, classified by their applications in cell cultures, tissue engineering, cell behavior, imaging, and unique applications of hydrogelators. Particularly, we discuss the applications of supramol. hydrogelators after they form supramol. assemblies but prior to reaching the crit. gelation concn. because this subject is less explored but may hold equally great promise for helping address fundamental questions about the mechanisms or the consequences of the self-assembly of mols., including low mol. wt. ones. Finally, we provide a perspective on supramol. hydrogelators. We hope that this review will serve as an updated introduction and ref. for researchers who are interested in exploring supramol. hydrogelators as mol. biomaterials for addressing the societal needs at various frontiers.
- 32Boekhoven, J.; Stupp, S. I. Adv. Mater. 2014, 26 (11) 1642– 1659 DOI: 10.1002/adma.201304606
- 33Arslan, E.; Garip, I. C.; Gulseren, G.; Tekinay, A. B.; Guler, M. O. Adv. Healthcare Mater. 2014, 3 (9) 1357– 1376 DOI: 10.1002/adhm.201300491[Crossref], [PubMed], [CAS], Google Scholar33https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhsV2gurfK&md5=83a118e02810982c504989c8a818e00fBioactive Supramolecular Peptide Nanofibers for Regenerative MedicineArslan, Elif; Garip, I. Ceren; Gulseren, Gulcihan; Tekinay, Ayse B.; Guler, Mustafa O.Advanced Healthcare Materials (2014), 3 (9), 1357-1376CODEN: AHMDBJ; ISSN:2192-2640. (Wiley-VCH Verlag GmbH & Co. KGaA)Recent advances in understanding of cell-matrix interactions and the role of the extracellular matrix (ECM) in regulation of cellular behavior have created new perspectives for regenerative medicine. Supramol. peptide nanofiber systems have been used as synthetic scaffolds in regenerative medicine applications due to their tailorable properties and ability to mimic ECM proteins. Through designed bioactive epitopes, peptide nanofiber systems provide biomol. recognition sites that can trigger specific interactions with cell surface receptors. The present Review covers structural and biochem. properties of the self-assembled peptide nanofibers for tissue regeneration, and highlights studies that investigate the ability of ECM mimetic peptides to alter cellular behavior including cell adhesion, proliferation, and/or differentiation.
- 34Loo, Y.; Goktas, M.; Tekinay, A. B.; Guler, M. O.; Hauser, C. A. E.; Mitraki, A. Adv. Healthcare Mater. 2015, 4 (16) 2557– 2586 DOI: 10.1002/adhm.201500402[Crossref], [CAS], Google Scholar34https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhs1CntrzM&md5=ec48e7d289533710903106a991f54864Self-Assembled Proteins and Peptides as Scaffolds for Tissue RegenerationLoo, Yihua; Goktas, Melis; Tekinay, Ayse B.; Guler, Mustafa O.; Hauser, Charlotte A. E.; Mitraki, AnnaAdvanced Healthcare Materials (2015), 4 (16), 2557-2586CODEN: AHMDBJ; ISSN:2192-2640. (Wiley-VCH Verlag GmbH & Co. KGaA)Self-assembling proteins and peptides are increasingly gaining interest for potential use as scaffolds in tissue engineering applications. They self-organize from basic building blocks under mild conditions into supramol. structures, mimicking the native extracellular matrix. Their properties can be easily tuned through changes at the sequence level. Moreover, they can be produced in sufficient quantities with chem. synthesis or recombinant technologies to allow them to address homogeneity and standardization issues required for applications. Here. recent advances in self-assembling proteins, peptides, and peptide amphiphiles that form scaffolds suitable for tissue engineering are reviewed. The focus is on a variety of motifs, ranging from minimalistic dipeptides, simplistic ultrashort aliph. peptides, and peptide amphiphiles to large "recombinamer" proteins. Special emphasis is placed on the rational design of self-assembling motifs and biofunctionalization strategies to influence cell behavior and modulate scaffold stability. Perspectives for combination of these "bottom-up" designer strategies with traditional "top-down" biofabrication techniques for new generations of tissue engineering scaffolds are highlighted.
- 35Ardoña, H. A. M.; Tovar, J. D. Chem. Sci. 2015, 6 (2) 1474– 1484 DOI: 10.1039/C4SC03122A[Crossref], [CAS], Google Scholar35https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXitV2isbrK&md5=23efcefcbe14835dc44f1c76dfd1c23fEnergy transfer within responsive pi-conjugated coassembled peptide-based nanostructures in aqueous environmentsArdona, Herdeline Ann M.; Tovar, John D.Chemical Science (2015), 6 (2), 1474-1484CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)Steady-state and time-resolved photophys. measurements demonstrate energy transfer within π-conjugated peptide nanostructures composed of oligo-(p-phenylenevinylene)-based donor units and quaterthiophene-based acceptor units in completely aq. environments. These peptide-based assemblies encourage energy migration along the stacking axis, thus resulting in the quenching of donor emission peaks along with the development of new spectral features reminiscent of acceptor emission. These spectral changes were obsd. even at minute amts. of the acceptor (starting at 1 mol%), suggesting that exciton migration is involved in energy transport and supporting a funnel-like energy transduction mechanism. The reversibility of nanostructure formation and the assocd. photophys. responses under different conditions (pH, temp.) were also studied. This unique material design incorporates two different semiconducting units coassembled within peptide nanostructures and offers a new platform for the engineering of energy migration through bioelectronic materials in aq. environments.
- 36Sanders, A. M.; Magnanelli, T. J.; Bragg, A. E.; Tovar, J. D. J. Am. Chem. Soc. 2016, 138 (10) 3362– 3370 DOI: 10.1021/jacs.5b12001
- 37Draper, E. R.; Lee, J. R.; Wallace, M.; Jäckel, F.; Cowan, A. J.; Adams, D. J. Chem. Sci. 2016, 7 (10) 6499– 6505 DOI: 10.1039/C6SC02644C[Crossref], [PubMed], [CAS], Google Scholar37https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhtV2nt7nK&md5=ad187780459c71550d478304967290eaSelf-sorted photoconductive xerogelsDraper, Emily R.; Lee, Jonathan R.; Wallace, Matthew; Jackel, Frank; Cowan, Alexander J.; Adams, Dave J.Chemical Science (2016), 7 (10), 6499-6505CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)We show that a perylene bisimide (PBI)-based gelator forms self-sorted mixts. with a stilbene-based gelator. To form the self-sorted gels, we use a slow pH change induced by the hydrolysis of glucono-δ-lactone (GdL) to gluconic acid. We prove that self-sorting occurs using NMR spectroscopy, UV-Vis spectroscopy, rheol., and viscometry. The corresponding xerogels are photoconductive. Importantly, the wavelength dependence of the photoconductive films is different to that of the films formed from the perylene bisimide alone. Transient absorption spectroscopy of the xerogels reveals changes in the spectrum of the PBI on the picosecond timescale in the presence of stilbene with a PBI radical anion being formed within 10 ps when the stilbene is present. The ability to form the PBI radical anion under visible light leads to the enhanced spectral response of the multicomponent gels. These systems therefore have potential as useful visible-active optoelectronics.
- 38Nakayama, T.; Tashiro, K.; Takei, T.; Yamamoto, Y. Chem. Lett. 2017, 46 (4) 423– 425 DOI: 10.1246/cl.161112
- 39Sugiyasu, K.; Kawano, S.-i.; Fujita, N.; Shinkai, S. Chem. Mater. 2008, 20 (9) 2863– 2865 DOI: 10.1021/cm800043b
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- 41Li, B.; Li, S.; Zhou, Y.; Ardoña, H. A. M.; Valverde, L. R.; Wilson, W. L.; Tovar, J. D.; Schroeder, C. M. ACS Appl. Mater. Interfaces 2017, 9 (4) 3977– 3984 DOI: 10.1021/acsami.6b15068
- 42Morris, K. L.; Chen, L.; Raeburn, J.; Sellick, O. R.; Cotanda, P.; Paul, A.; Griffiths, P. C.; King, S. M.; O’Reilly, R. K.; Serpell, L. C.; Adams, D. J. Nat. Commun. 2013, 4, 1– 6 DOI: 10.1038/ncomms2499
- 43Draper, E. R.; Eden, E. G. B.; McDonald, T. O.; Adams, D. J. Nat. Chem. 2015, 7 (10) 848– 852 DOI: 10.1038/nchem.2347[Crossref], [PubMed], [CAS], Google Scholar43https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhsV2ltrvJ&md5=8304aec51435b06adb41933d377d6fbfSpatially resolved multicomponent gelsDraper, Emily R.; Eden, Edward G. B.; McDonald, Tom O.; Adams, Dave J.Nature Chemistry (2015), 7 (10), 848-852CODEN: NCAHBB; ISSN:1755-4330. (Nature Publishing Group)An approach to forming patterned, spatially resolved multicomponent supramol. hydrogels is reported. A multicomponent gel is first formed from two low-mol.-wt. gelators and consists of two types of fiber, each formed by only one gelator. One type of fiber in this 'self-sorted network' is then removed selectively by a light-triggered gel-to-sol transition. The remaining network has the same mech. properties as it would have done if it initially formed alone. The selective irradn. of sections of the gel through a mask leads to the formation of patterned multicomponent networks, in which either one or two networks can be present at a particular position with a high degree of spatial control.
- 44Wallace, M.; Iggo, J. A.; Adams, D. J. Soft Matter 2015, 11 (39) 7739– 7747 DOI: 10.1039/C5SM01760B
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- 46Draper, E. R.; Wallace, M.; Schweins, R.; Poole, R. J.; Adams, D. J. Langmuir 2017, 33 (9) 2387– 2395 DOI: 10.1021/acs.langmuir.7b00326
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- 52Pratihar, P.; Ghosh, S.; Stepanenko, V.; Patwardhan, S.; Grozema, F. C.; Siebbeles, L. D. A.; Würthner, F. Beilstein J. Org. Chem. 2010, 6 (1) 1070– 1078 DOI: 10.3762/bjoc.6.122[Crossref], [PubMed], [CAS], Google Scholar52https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXhsVOqu77M&md5=679a2d4063f95090d6f1a4d337352562Self-assembly and semiconductivity of an oligothiophene supergelatorPratihar, Pampa; Ghosh, Suhrit; Stepanenko, Vladimir; Patwardhan, Sameer; Grozema, Ferdinand C.; Siebbeles, Laurens D. A.; Wuerthner, FrankBeilstein Journal of Organic Chemistry (2010), 6 (), 1070-1078, No. 122CODEN: BJOCBH; ISSN:1860-5397. (Beilstein-Institut zur Foerderung der Chemischen Wissenschaften)A bis(trialkoxybenzamide)-functionalized quaterthiophene deriv. was synthesized and its self-assembly properties in soln. were studied. In non-polar solvents such as cyclohexane, this quaterthiophene π-system formed fibril aggregates with an H-type mol. arrangement due to synergistic effect of hydrogen bonding and π-stacking. The self-assembled fibers were found to gelate numerous org. solvents of diverse polarity. The charge transport ability of such elongated fibers of quaterthiophene π-system was explored by the pulse radiolysis time resolved microwave cond. (PR-TRMC) technique and moderate mobility values were obtained. Furthermore, initial AFM and UV-vis spectroscopic studies of a mixt. of our electron-rich quaterthiophene deriv. with the electron acceptor [6,6]-phenyl-C61-butyric acid Me ester (PCBM) revealed a nanoscale segregated assembly of the individual building blocks in the blend.
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Supporting Information
Supporting Information
ARTICLE SECTIONSThe Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b04006.
General synthesis procedures and experimental conditions; characterization data for peptides (1H NMR, ESI-MS, HPLC traces); pH titration curves; 1NMR stacked plots and data for monitoring OPV3 and OT4-NDI assembly with GdL; supplementary rheology data, TEM images, UV–vis absorption spectra, and CD spectra (PDF)
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