Dynamic Peptide Library for the Discovery of Charge Transfer

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Dynamic Peptide Library for the Discovery of Charge Transfer Hydrogels Cristina Berdugo, Siva Krishna Mohan Nalluri, Nadeem Javid, Beatriu Escuder, Juan F. Miravet, and Rein V. Ulijn ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b08968 • Publication Date (Web): 05 Nov 2015 Downloaded from http://pubs.acs.org on November 12, 2015

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ACS Applied Materials & Interfaces

Dynamic Peptide Library for the Discovery of Charge Transfer Hydrogels Cristina Berdugo,†, ‡ Siva Krishna Mohan Nalluri,*,†,§ Nadeem Javid,† Beatriu Escuder,‡ Juan F. Miravet,‡ and Rein V. Ulijn*,†, ǁ †

WestCHEM/Department of Pure and Applied Chemistry, University of Strathclyde, Thomas

Graham Building, 295 Cathedral Street, Glasgow G1 1XL, UK, ‡Departament de Química Inorgànica i Orgànica, Universitat Jaume I, Avda. Sos Baynat s/n, 12071 Castelló, Spain, ǁ Advanced Science Research Center (ASRC) and Hunter College, City University of New York (CUNY), 85 St Nicholas Terrace, New York NY10031, USA. KEYWORDS: charge transfer interactions, hydrogels, dynamic combinatorial libraries, peptide derivatives, self-assembly, thermolysin, supramolecular electronics.

ABSTRACT: Coupling of peptide self-assembly to dynamic sequence exchange provides a useful approach for the discovery of self-assembling materials. In here, we demonstrate the discovery and optimization of aqueous, gel-phase nanostructures based on dynamically exchanging peptide sequences that self-select to maximize charge transfer of n-type semiconducting naphthalenediimide (NDI)- dipeptide bioconjugates with various π-electron-rich donors (di-alkoxy/hydroxy/amino-naphthalene or pyrene derivatives). These gel-phase peptide

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libraries are characterized by spectroscopy (UV-vis and fluorescence), microscopy (TEM), HPLC and oscillatory rheology and it is found that, of the various peptide sequences explored (tyrosine Y-NDI with tyrosine Y, phenylalanine F, leucine L, valine V, alanine A or glycine GNH2), the optimum sequence is tyrosine-phenylalanine in each case, however both its absolute and relative yield amplification is dictated by the properties of the donor component, indicating cooperativity of peptide sequence and donor/acceptor pairs in assembly. The methodology provides an in situ discovery tool for nanostructures that enable dynamic interfacing of supramolecular electronics with aqueous (biological) systems.

INTRODUCTION Functional nanomaterials based on electronically active chromophores are of potential value for next-generation opto- and bio-electronic devices.1-3 Supramolecular self-assembly approaches of π-conjugated aromatic chromophores are of particular interest, as their high degree of spatial organization and the face-to-face packing mode facilitate the formation of well-defined one-dimensional (1D) nanowires possessing intermolecular charge delocalization of π-electron cloud.4-7 To improve the electro-conducting properties of 1D nanowires, strategies based on alternate stacking of π-electron rich donors and π-electron deficient acceptors via charge transfer interactions8-10 have also been developed. Employing self-assembling short peptides to spatially organize these molecules provides an attractive approach for the production of such materials.1117

Naphthalenediimide (NDI) derivatives are considered to be promising n-type organic semiconductors and have been extensively utilized as π-electron-deficient acceptors.18-21 These


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species form strong charge transfer complexes with various π-electron-rich donors such as dihydroxy/alkoxy/amino naphthalene derivatives22-26 or pyrene derivatives.27-28 NDI derivatives have shown potential applications in wide-ranging fields such as organic field effect transistors, artificial light-harvesting systems, photovoltaic devices and solar cells.29-31 The self-assembly of NDI-peptide bioconjugates into various nanoscale assemblies such as 1D nanotubes,18,

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ribbons,19 -belts,34 and -fibers,35 as well as supramolecular polymers and microspheres36 has been demonstrated. It is increasingly appreciated that, even in nanostructures based on very short (di- or tri-) peptide sequences, the amino acid sequence can dramatically influence the stability, morphology and dimensions of the final supramolecular structure and hence the material properties.37-38 Despite progress in computational approaches,39 it is currently not possible to predict which peptide sequence will give rise to the most stable nanostructures for self-assembling peptide conjugates. Dynamic combinatorial libraries (DCLs) have been used as a tool for discovering new self-assembling structures with interesting properties40 based on covalent exchange of library components. This approach allows for unexpected41-42 structures to be obtained by screening of a wide range of molecules simultaneously and competitively. Therefore, the dynamic combinatorial chemistry43-44 approach can shed light on what structures are best suited to a desired purpose. In DCL, the library components interconvert continuously by exchanging covalently bonded building blocks through a reversible, yet covalent, chemical reaction. Over the years, DCLs involving peptide derivatives have been developed using disulphide exchange,45-46 metal binding47-48 and amide exchange49 reactions. We have developed dynamic peptide libraries based on fully reversible enzymatic amide bond exchange, enabling thermodynamic sequence selection


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of short peptides with self-assembly capability,50 including those appended to functional aromatic chromophores.51 In this approach, enzymatic amide condensation converts the precursors into self-assembling building blocks in a reversible manner,51-53 as a consequence of the free energy contribution provided by the self-assembly of the peptide reaction product, which overcomes the preference for amide hydrolysis in aqueous media. We have previously demonstrated gelation-driven peptide-based DCLs to identify the most stable aromatic peptide amphiphiles from mixtures.54-55 We also reported on the discovery of efficient energy transfer nanostructures, observed between naphthoxy donor and dansyl acceptor from a library of eight competing amino acid components via self-selection and amplification mechanism.56 In recent work, we showed the fully reversible formation of free-energy optimised aqueous charge transfer nanofibers of naphthalenediimide-tyrosine-phenylalanine amide (NDIYF-NH2) bioconjugates in the presence of 1,5- or 2,6-di-alkoxy/hydroxy naphthalene derivatives57 demonstrating reversibility of the system which enables thermodynamic selfhealing and structure optimisation by avoiding the formation of (route- dependent) kinetic aggregates. We now investigate whether the DCL approach allows for optimization of peptide sequence with different donor/acceptor pairs and whether the peptide sequence selection is dependent on the chemical nature of the donors used. Thus, we present a dynamic library of NDI-peptide bioconjugate acceptors and a variety of π-electron donors, providing a range of free-energy optimised aqueous functional electronic nanostructures combined with new insights into relative effectiveness of several donors in stabilising the nanostructures formed (Figure 1). The present biocatalytic DCLs operate under physiological conditions and results are therefor of direct


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interest for the identification of suitable supramolecular structures for interfacing of electronics with biological systems.5

Figure 1. (a) Thermolysin-catalyzed condensation reaction between NDI-Y and X-NH2 to form NDI-YX-NH2 bioconjugate acceptors. (b) Molecular structures of various π-electron-rich donors used in this study. (c) Schematic representation showing the proposed self-assembly mechanism of the formation of charge transfer nanofibers and hydrogels, as a result of π-π stacking between NDI chromophores, intermolecular H-bonding between peptide motifs and charge transfer interactions between donor-acceptor components. RESULTS AND DISCUSSION The compounds, NDI-tyrosine (NDI-Y) acceptor, di-alkoxy naphthalene donors (1,5-DAN and 2,6-DAN) were synthesized and characterized as previously reported.57 In particular, we selected NDI-Y bioconjugate as a starting material because we previously reported the self-assembly of


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NDI-Y (10 mM) into a yellow-orange self-supporting hydrogel, composed of 1D twisted nanofibers of several micrometers in length as visualized by TEM and AFM, in phosphate buffer (100 mM) at pH 8.57 We previously demonstrated that this gel provides a suitable starting point for the optimization, via amide coupling and exchange, of free energy optimized nanostructures. The first objective of this work was to develop a dynamic peptide library to assess which dipeptide sequence is best suited to stabilize nanostructure formation while simultaneously maximizing charge transfer interactions using a number of different π-electron donors. The system is based on the competitive biocatalytic condensation reaction between NDI-Y and a series of amino acid amide derivatives (X-NH2, where “X” refers tyrosine, Y; phenylalanine, F; leucine, L; valine, V, alanine, A and glycine, G) both in the absence and presence of various πelectron donors. For this purpose, we employed the largely nonspecific endoprotease, thermolysin from Bacillus thermoproteolyticus rokko,54 to catalyze the condensation reaction between NDI-Y and X-NH2 to form NDI-YX-NH2. Before exploring the dynamic peptide library, we first investigated the condensation driven assembly of individual NDI-YX-NH2 bioconjugates (also referred as "YX") from each possible combination of NDI-Y and X-NH2. The time-dependent percentage conversion of NDI-Y into NDI-YX-NH2 was analyzed by reversed-phase HPLC. The addition of thermolysin (1 mg ml-1) to the binary mixture of NDI-Y (10 mM) and a two-fold excess of F-NH2 led to the formation of NDI-YF-NH2 in 65% yield after 48 h.57 Similar experiments were carried out with the other combinations of NDI-Y and X-NH2 and showed that YL derivative was formed in 45% yield, while YY, YA, and YG derivatives were formed in very low yields of 6, 4 and 0.1% respectively (Supporting Information Figure S1).58 The better yields obtained for YF and YL derivatives indicate that the self-assembly of these derivatives is thermodynamically favorable as result of


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the presence of both strong intermolecular π-π stacking interactions between NDI chromophores and intermolecular H-bonding between peptide motifs. The low yields obtained for the other derivatives (YY, YA and YG) are likely due to the poor self-assembly of these derivatives which causes the condensation reaction to be thermodynamically unfavorable and the system thus being unable to overcome the bias for amide hydrolysis normally observed in aqueous systems. It is noteworthy to mention that thermolysin has a kinetic preference particularly for hydrophobic amino acids on N-terminus of the peptide bond (phenylalanine in this case), however, an equilibrium distribution which does not contain kinetic bias should be eventually reached provided that all peptide sequences are catalyzed by the enzyme, as is shown here. Interestingly, the self-supporting hydrogels were retained in all cases after the addition of enzyme, although slight color changes were observed, implying changes in chromophore organization (Supporting Information Figure S2).


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Figure 2. Time-dependent HPLC enzymatic percentage peptide conversion of various NDI-YXNH2 bioconjugates in biocatalytic DCL hydrogels. (a) In the absence of any donor, (b) in the presence of 1,5-DAN donor, (c) the preferential formation and amplification of NDI-YF-NH2 in the presence of a variety of donors , (d) the relative amplification of NDI-YF-NH2 at the expense of NDI-YL-NH2 in the absence (solid traces) and presence (dashed traces) of 1,5-DAN donor. Conditions: [NDI-Y] =10 mM, [each X-NH2] = 20 mM, [donor] = 10 mM and [thermolysin] = 1 mg·mL-1 in 100 mM phosphate buffer at pH 8. Screening for Peptide Sequence. Having established that each of the peptide candidates could be formed using thermolysin, a peptide-based DCL was prepared by mixing the components, NDI-Y (10 mM), all amino acid amides (20 mM for each X-NH2, where X = F, L, Y, A and G)


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and thermolysin (1 mg ml-1), together in one pot and thereby allowing the thermolysin-catalyzed condensation reaction to be carried out in competition. The time-dependent distribution of all possible NDI-dipeptide amide bioconjugates (NDI-YX-NH2), was monitored by reversed-phase HPLC (Figure 2a). After 48 hours, YF was formed preferentially in good yield of 48%, corresponding to 67% of the total peptide yield obtained, which indicates that the self-assembly of YF derivative represents the lowest free energy assembly.56 YL represented 18% out of total peptide yield obtained, while YY, YA and YG derivatives were formed in negligible amounts with only a poor conversion of less than 1% in total. The results clearly demonstrate that the preferred peptide sequence is not intuitive as YY derivatives, alone or within the DCL, based on the aromatic characteristics of this peptide and its similarity to YF give rise to low yields. Similar differences between YY and YF were observed before in a dynamic library based on naphthalene-dipeptides in energy transfer gels.56 Macroscopically, the orange color of gel gradually faded and turned into a pale yellow gel over time (approx. 3 hours), suggesting changes in supramolecular organisation when converting NDI-Y into NDI-YX-NH2 (Figure 3 (top) and Supporting Information Figures S3-S5). The morphological investigation of the dried DCL gel samples by TEM imaging revealed the formation of short nanofibers of 1-2 µm in length (Figure 4a).


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Figure 3. Digital photographs of transparent donor solutions and the corresponding gel/viscous liquid samples of DCLs containing NDI-Y/X-NH2/NDI-YX-NH2 acceptor conjugates both in the absence (top) and presence of various π-electron-rich donors (bottom). The spontaneous color changes observed were monitored over time after the addition of thermolysin to the corresponding samples in 100 mM phosphate buffer (pH 8).


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Figure 4. TEM images (on a carbon-coated copper grid) of dried DCL hydrogels at 24 h after exposing to thermolysin in the absence and presence of charge transfer. (a) In the absence of any donor, (b) in the presence of 1,5-DAN donor and (c) in the presence of 2,6-DAN donor. Inset: Digital photographs of the corresponding DCL hydrogels. Screening for Charge Transfer. The dynamic peptide library approach was then exploited to explore a number of π-electron donors, which are expected to form stable charge transfer interactions with NDI-amino acid/dipeptide bioconjugates. For this purpose, the charge transfer interaction between NDI-Y and NDI-YX-NH2 acceptor and various π-electron donors such as water-soluble di-alkoxy naphthalenes (1,5-DAN and 2,6-DAN), 1,5-diamino naphthalene (1,5DAmN), 1-pyrene acetic acid (1-PAA) and less water-soluble di-hydroxy naphthalenes (1,5DHN and 2,6-DHN) were selected based on our previous results57 (Figure 1). NDI-Y (10 mM), all six amino acid amides (20 mM each for X-NH2) and a particular π-electron donor (10 mM) were mixed together in one pot. In all cases, the charge transfer complexation was evidenced by the clear change in color (Figure 3) and macroscopic appearance (solutions rather than gels of NDI-Y precursors in absence of donors), which was further confirmed by the appearance of a broad charge transfer absorption band between 450 and 700 nm in the corresponding UV-Vis absorption spectra (Supporting Information Figure S6).22, 27, 59, 60


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After the subsequent addition of thermolysin, the charge transfer hydrogels were formed in all cases, obtained with a variety of colors ranging from yellow-orange to green depending on the combination of the charge transfer donor/acceptor pair used (Figure 3 and Supporting Information Figure S3). The changes observed in library composition of all NDI-YX-NH2 derivatives in the presence of each π-electron donor were monitored over time by HPLC (Figures 2b, 2c, Figure 5 and Supporting Information Figures S7-S8). For instance, in the case of 1,5DAN donor system, only YF derivative was preferentially produced in 71% out of 86% of the total peptide yield formed, while YL (13%) was produced in low yield and, YY, YA and YG derivatives were produced in negligible amounts (Figure 2b). Strikingly, the comparison of yields obtained in the absence (Figure 2a) and presence (Figure 2b) of 1,5-DAN donor in DCL suggests that the formation of YF was significantly amplified from 48% (without donor) to 71% (with donor), in agreement with our previous results which were carried out in the absence of competing peptide sequences,57 while YL was reduced from 18% (without donor) to 13% (with

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donor). Significant amplification of YF was also observed for all the other donor systems.57 %D: 0.0 %

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Figure 5. Representative HPLC traces of the charge transfer DCL in the presence of a 1,5-DAN donor at 24 h after the addition of thermolysin and the absorbance was monitored at λ= 383 nm with the retentions times of each compound found.

The relative yields obtained (corresponding to total peptide yield formed in each case) for the amplified YF depends on the type of donor used in DCL, that is 69, 65, 63, 58 and 57% for 2,6DAN, 1,5-DAmN, 1,5-DHN, 2,6-DHN and 1-PAA, respectively (Supporting Information Figures S7 and S8). These observations indicate that the presence of suitable donors and acceptors in DCL provides additional charge transfer interactions, which enhance the biocatalytic self-assembly of NDI-YX-NH2 bioconjugates. The percentage formation of YF in DCL is strictly dependent on the type of the donor and the overall donor to acceptor ratio. For instance, at 1:10, 1:2, 1:1 and 2:1 donor-acceptor ratios, the relative yields of YF in DCL after 48 h are 57, 68, 71 and 73% for 1,5-DAN donor system and 59, 63, 69 and 69% for 2,6-DAN donor system, respectively (Supporting Information Figure S9). In addition to the correlation of percentage amplification of YF in each DCL system with the donor type, it is remarkable that the relative yields of YF over YL are strongly dependant on the nature of the donor, clearly showing a cooperative contribution from peptide and donor/acceptor pair. When we compared the relative ratios of the %conversions of YF to YL formed in each charge transfer DCL system we found the YF/YL ratio was about 2.7 in the absence of any donor, while it was found to be about 2.8, 6.4, 4.2, 4.0, 4.3 and 5.5 for 2,6-DHN, 1-PAA, 1,5DHN, 1,5-DAmN, 2,6-DAN and 1,5-DAN respectively (Figure 6). Remarkably, the high YF/YL ratio obtained in the presence of 1-PAA donor indicates that there is selectivity in the charge


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transfer co-assembly process which enhances its ability to amplify the formation of more suitable YF at the expense of less suitable YL in the DCL system. These observations further confirm that the product distribution in the library is dependent on the nature and the extent of the interactions present which is currently not possible to predict, especially in multicomponent coassemblies.

Figure 6. Graph showing the relative %conversion of YF (blue bars) and the observed YF to YL ratios (empty bars) in the biocatalytic DCL hydrogels both in the absence and in the presence of a variety of charge transfer donors used in this study. Thermodynamically Driven Reconfiguration. In order to explore whether thermodynamically driven reconfiguration from one structure to the other is possible, i.e. to verify the fully reversible nature of thermolysin-catalyzed condensation reaction, the following YL/YF competition experiments were carried out. Firstly, an experiment involving the components, NDI-Y, F-NH2, L-NH2, 1,5-DAN and thermolysin was set up with sequential addition. As shown in Figure 2d, we initially mixed NDI-Y and L-NH2 in the presence of thermolysin and the product YL was obtained in 37% yield after leaving it for 24 h. At this stage, both F-NH2 and 1,5-DAN donor (to induce charge transfer) components were subsequently added on top of the gel. After vortexing and sonication the sample was left for 192 h to reach equilibrium. It was


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observed that YF was preferentially produced in 82% yield replacing YL which was significantly reduced to 12% yield (dashed traces in Figure 2d), suggesting that the process is indeed reversible and further highlights the unique advantage of charge transfer interactions for the selection, amplification and reconfiguration of charge transfer nanostructures. Interestingly, the reconfiguration appeared to be facilitated by formation of the charge transfer complex- with a clear decrease in YL shown as YF formed. In a control experiment involving the addition of only F-NH2 (in the absence of 1,5-DAN donor) at 24 h the YL and YF derivatives formed only in 42 and 24% yields, respectively (solid traces in Figure 2d)- it appears that kinetics (probably related to enzyme access of the assembled NDI-YL-NH2 fibers) prevent effective reconfiguration in the absence of the donor. Hydrogel Characterization. Based on the high condensation yields obtained in charge transfer DCLs, we selected 1,5-DAN and 2,6-DAN donor systems for the further characterization of the DCL materials. Notably, TEM imaging of dried charge transfer DCL gel samples disclosed that a dense network of 1D functional charge transfer twisted nanofibers of several micrometers in length were observed for 1,5-DAN (Figure 4b) as well as 2,6-DAN donor systems (Figure 4c). Also, the mechanical strength for DCL hydrogels as measured by rheology indicated a significant increase in stiffness from 470 Pa (in the absence of any donor) to 850 Pa in the presence of 1,5-DAN donor and 1100 Pa in the presence of 2,6-DAN donor, similar to our previous observations on the formation of reinforced hydrogels in the presence of charge transfer interactions (Supporting Information Figure S10).57 We further employed fluorescence emission spectroscopy to assess the mechanism governing the self-assembly and hydrogelation of NDI-Y and NDI-YX-NH2 bioconjugates in DCLs both in the absence and presence of charge transfer interactions. A dilute solution of the non-assembling


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NDI-Y bioconjugate in DMSO was used as a reference, showing its maxima at 410 nm and a shoulder at 430 nm which is typical for monomeric N,N-dialkyl-substituted NDIs (Supporting Information Figure S11).61-62 We first monitored the changes observed in the emission spectra over time in the absence of charge transfer (that is, without any donor). Before the addition of thermolysin to DCL hydrogel consisting of NDI-Y and X-NH2 showed that a red-shifted (by about 150 nm) broad-band emission maxima at 548 nm accompanied with two weak shoulder peaks at 438 and 467 nm, respectively (Figure 7a). This is consistent with our previous studies on the single peptide system, suggesting the intermolecular excimer emission emerging from closely stacked NDI chromophores.19, 35, 57. As shown in Figure 7a, the addition of thermolysin led to a gradual increase in the relative emission intensity over time which eventually remained constant after 24 h, associated with the formation of NDI-YX-NH2 (Figure 2a) and consequent structural changes (Figure 4a).

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The effect of charge transfer interactions on the emission spectra of the hydrogel systems in the presence of 1,5-DAN and 2,6-DAN donors was also monitored over time. Before the addition of thermolysin to charge transfer DCL system consisting of NDI-Y, X-NH2 and 1,5-DAN revealed that the fluorescence emission was completely quenched and no emission was observed when either NDI acceptor (λexc= 360 nm) or DAN donor (λexc= 296 nm) were excited, attributed to the strong charge transfer complexation between donor and acceptor chromophores, in combination with the morphological changes observed previously (Figure 7b and Supporting Information Figure S11).57 Interestingly, the addition of thermolysin to the charge transfer DCL hydrogels immediately enhanced the emission and the peaks were re-appeared at 467 and 548 nm, respectively (Figure 7b). The emission intensity of both these peaks gradually increased over time, with the intensity of peak at 467 nm dominating the peak at 548 nm. The emission intensity was reduced slightly after 48 h. Also, such an increase in the emission for 24 h and a decrease in the emission after 48 h was observed for the DCL hydrogels in the presence of 2,6-DAN donor (Figure 7c and Supporting Information Figure S11). These dramatic changes observed in the emission intensity of charge transfer DCLs before and after the addition of enzyme are attributed to morphological changes leading to the formation of more ordered and extensive charge transfer aggregates within the self-assembling nanofibers (Figures 4b and 4c). This aggregation induced enhanced emission (AIEE)63-64 of NDI-dipeptide hydrogels is in agreement with our previous results obtained for individual NDI-YF-NH2 hydrogels in the absence and presence of charge transfer,57 further confirming that NDI-YF-NH2 bioconjugate is the preferential product obtained in the gel phase charge transfer DCLs (Supporting Information Figure S11). CONCLUSIONS


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In summary, we have successfully developed peptide-based functional DCLs in the hydrogel phase based on the self-assembly of NDI-amino acid/dipeptide amide bioconjugates, formed via a fully reversible thermolysin-catalyzed condensation reaction under thermodynamic control. We further showed that the presence of suitable charge transfer donors and acceptors in this peptidebased DCL allows for facile self-selection and amplification of most stable functional aqueous charge transfer peptide nanostructures from a library of several competing donor, acceptor and amino acid precursor components. Based on thermolysin-catalyzed condensation yields, we discovered that 1,5-DAN is the best donor for NDI-YF-NH2 acceptor and the co-assembly of these two components form the most stable highly ordered 1D functional aqueous charge transfer peptide nanostructures, as observed by screening a variety of acceptors (NDI-Y and NDI-YXNH2 where X = F, L, V, Y, A and G) and donors (1,5-DAN, 2,6-DAN, 1,5-DAmN, 1-PAA, 1,5DHN and 2,6-DHN) through a self-selection and amplification mechanism. This methodology opens up new possibilities for the discovery of electronically conducting nanomaterials with enhanced long-range charge transfer properties and fewer defects that are inherently selfhealing.65 EXPERIMENTAL SECTION Materials. All commercial reagents were used as supplied, unless otherwise mentioned. All solvents were used as supplied (analytical or HPLC grade) without further purification, unless otherwise mentioned. All reactions were carried out in oven-dried glassware and magnetically stirred. The compounds, NDI-Y acceptor, 1,5-DAN and 2,6-DAN donors were synthesized and characterized as reported by us previously.57


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High resolution mass spectra (ESI-HRMS) were recorded on a Thermo Electron Exactive. 1H and 13C nuclear magnetic resonance (NMR) spectra were recorded on Bruker AV400 spectrometer in the deuterated solvents. All chemical shifts (δ) are quoted in ppm and coupling constants (J) given in Hz. 1H NMR peak multiplicity is represented as singlet (S), doublet (d), triplet (t), broad (br) and quintet (qn). Residual signals from the solvents were used as an internal reference. The precursor NDI-Y (500 mM) was prepared as a stock solution in DMSO. Few microliters of NDI-Y (20 µL for 10 mM) was added to 1 mL of 100 mM phosphate buffer (pH 8). The solution was vortexed for few seconds and sonicated for about 1 min to ensure dissolution which later on developed into a self-supporting hydrogel. Gelation was considered to have occurred when a homogeneous solid-like material was obtained that exhibited no gravitational flow in each case. Enzyme-Triggered Hydrogelation. Similarly, the precursors NDI-Y (10 mM) and each X-NH2 (20 mM, where where “X” refers tyrosine, Y; phenylalanine, F; leucine, L; valine, V; alanine, A and glycine, G; purchased from Bachem, Germany) were mixed (at 1:2 ratio) in a glass vial. The mixture was suspended in 1.0 mL of 100 mM phosphate buffer (pH 8) with the addition of 1 mg mL-1 lyophilized thermolysin powder (bacillus Thermoproteolyticus rokko from Sigma-Aldrich, UK, mol wt 34.6 kDa by amino acid sequence). The mixture was vortexed for few seconds and sonicated for about 1 min to ensure dissolution which later on developed into a self-supporting hydrogel. Samples were incubated at room temperature for 24 h before analysis, unless otherwise stated.


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In the case of dynamic combinatorial library (DCL), NDI-Y (10 mM) and various X-NH2 (20 mM each) derivatives were mixed and the above procedure was followed to form a selfsupporting hydrogel. In the case of mixed charge transfer donor-acceptor system, donor was first dissolved in buffer at the required concentration (10 mM, at 1:1 donor/acceptor ratio, 1 mM at 10:1 ratio, 5 mM at 1:2 ratio and 20 mM at 2:1 ratio respectively) followed by the sequential additions of each X-NH2, NDI-Y and thermolysin, mixed as described above, to form self-supporting charge transfer hydrogels. High-Performance Liquid Chromatography (HPLC). A Dionex P680 HPLC system was used to quantify the percentage conversion of the enzymatic reaction. A 50 µL sample was injected onto a Macherey–Nagel C18 column of 250 mm length with an internal diameter of 4.6 mm and 5 µm fused silica particles at a flow rate of 1 mL per minute (eluting solvent system: linear gradient of 20% (v/v) acetonitrile in water for 4 min, gradually rising to 80% (v/v) acetonitrile in water at 35 min. This concentration was kept constant until 40 min when the gradient was decreased to 20% (v/v) acetonitrile in water at 42 min). Sample preparation involved mixing 20 µL of the sample with 1 mL of acetonitrile-water (50:50 mixture) containing 0.1% trifluoroacetic acid. The intensity of each identified peak was determined by UV detection at 383 nm for (NDI derivatives) and 296 nm (di-alkoxy/hydroxy/amino-naphthalene or pyrene derivatives). The experimental data was acquired in triplicate and the average data was shown. The samples were vortexed before collecting the aliquots for HPLC and the percentage yields are calculated from HPLC integrated peak areas.


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UV-vis Absorption Spectroscopy. UV-vis absorption spectra were recorded on a Jasco V-660 spectrophotometer. Samples were prepared in PMMA cuvettes (from Fisher Scientific). The experimental data was acquired in triplicate and the average data was shown. Fluorescence Emission Spectroscopy. Fluorescence emission spectra were measured on a Jasco FP-6500 spectrofluorometer with light measured orthogonally to the excitation light, at a scanning speed of 200 nm min-1. For NDI acceptor derivatives, the excitation wavelength was 360 nm and emission data were recorded in the range between 380 and 700 nm, while for DAN donor derivatives, the excitation wavelength was 296 nm and emission data were recorded in the range between 320 and 700 nm. The spectra were measured with a bandwidth of 5 nm (or 3 nm) with a low (or medium) response and a 1 nm data pitch. Samples were prepared in PMMA cuvettes (from Fisher Scientific). The freshly prepared samples were directly taken in the cuvette and the time-dependent spectra were recorded immediately. The experiments were carried out directly under the described concentrations and the gels were used under “as it is condition” for the experiments. The experimental data was acquired in triplicate and the average data was shown. Oscillatory Rheology. The mechanical properties of the hydrogels were investigated by dynamic frequency sweep experiments which were carried out by strain controlled rheometer (Kinexus Pro Rheometer) by employing parallel plates of 20 mm diameter with 0.5 mm gap. The experiments were performed at 25°C and this temperature was controlled throughout the experiment using an integrated electrical heater. Additional precautions were taken to minimize solvent evaporation and to keep the sample hydrated: a solvent trap was used and the internal atmosphere was kept saturated. To ensure the measurements were made in the linear viscoelastic regime, an amplitude sweep was performed and the results showed no variation in elastic


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modulus (G’) and viscous modulus (G’’) up to a strain of 1%. The dynamic modulus of the hydrogels was measured as a frequency function, where the frequency sweeps were carried out between 0.1 and 10 Hz. The measurements were repeated three times to ensure reproducibility, with the average data shown. Transmission Electron Microscopy (TEM) (University of Glasgow, UK). Transmission electron microscopy (TEM) images were captured using a LEO 912 energy filtering transmission electron microscope operating at 120kV fitted with 14 bit/2 K Proscan CCD camera. Carboncoated copper grids (200 mesh) were glow discharged in air for 30 seconds. The support film was touched onto the gel surface for 3 seconds and blotted down using filter paper. Each sample was allowed to dry afterwards for few minutes in a dust-free environment prior to TEM imaging. Negative stain (20 µL, 1 % aqueous methylamine vanadate obtained from Nanovan, Nanoprobes) was applied and the mixture blotted again using filter paper to remove excess. The dried specimens were then imaged using the microscope. ASSOCIATED CONTENT Supporting Information. Characterization details for NDI-YX-NH2; HPLC spectra; digital photographs of the hydrogels; stiffness of hydrogels measured by rheology; additional fluorescence and UV-vis measurements; supporting figures and tables. This material is available free of charge via the Internet at http://pubs.acs.org AUTHOR INFORMATION Corresponding Author * [email protected]


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* [email protected] Present Addresses §Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 602083133, United States, ǁAdvanced Science Research Center (ASRC), City University of New York, 85 St Nicholas Terrace, New York, NY 10031, Unites States, ǁǁDepartment of Chemistry, Hunter College, City University of New York, 695 Park Ave, New York, NY 10065, United States Author Contributions All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was funded by the US Air Force (AFOSR, grant 12448RK7359A), the European Research Council under the European Union’s Seventh Framework Programme (FP7/20072013)/ERC grant agreement no. 258775 and the Ministry of Science and Innovation of Spain (CTQ2009-13961 grant). We also thank Margaret Mullin from University of Glasgow for help in TEM imaging. REFERENCES (1)

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(56) Nalluri, S. K. M.; Ulijn, R. V. Discovery of Energy Transfer Nanostructures Using Gelation-Driven Dynamic Combinatorial Libraries. Chem. Sci. 2013, 4, 3699-3705. (57) Nalluri, S. K.; Berdugo, C.; Javid, N.; Frederix, P. W.; Ulijn, R. V. Biocatalytic SelfAssembly of Supramolecular Charge-Transfer Nanostructures Based on n-Type SemiconductorAppended Peptides. Angew. Chem. Int. Ed. 2014, 53, 5882-5887. (58) In the case of YV derivative, the HPLC peaks/retention times of the reactant (unreacted NDI-Y) and the product (NDI-YV-NH2) are completely superimposed as per our HPLC conditions. As a result, the exact conversion of NDI-Y into NDI-YV-NH2 could not be determined and hence we assumed that the formation of YV in DCL is clearly negligible when compared to the high yields obtained for YF and YL derivatives. (59) Das, A.; Molla, M. R.; Banerjee, A.; Paul, A.; Ghosh, S. Hydrogen-Bonding Directed Assembly and Gelation of Donor-Acceptor Chromophores: Supramolecular Reorganization from a Charge-Transfer State to a Self-Sorted State. Chem. Eur. J. 2011, 17, 6061-6066. (60) The charge transfer band for NDI-Y acceptor and 1,5-DAmN donor combination system appeared at relatively higher wavelength region in the UV/Vis absoprtion spectrum, that is at 500-800 nm region (Figure S2). (61) Bell, T. D. M.; Bhosale, S. V.; Forsyth, C. M.; Hayne, D.; Ghiggino, K. P.; Hutchison, J. A.; Jani, C. H.; Langford, S. J.; Lee, M. A. P.; Woodward, C. P. Melt-Induced Fluorescent Signature in a Simple Naphthalenediimide. Chem. Commun. 2010, 46, 4881-4883. (62) Andric, G.; Boas, J. F.; Bond, A. M.; Fallon, G. D.; Ghiggino, K. P.; Hogan, C. F.; Hutchison, J. A.; Lee, M. A. P.; Langford, S. J.; Pilbrow, J. R.; Troup, G. J.; Woodward, C. P.


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Spectroscopy of Naphthalene Diimides and their Anion Radicals. Aust. J. Chem. 2004, 57, 10111019. (63) Hong, Y.; Lam, J. W. Y.; Tang, B. Z. Aggregation-Induced Emission. Chem. Soc. Rev. 2011, 40, 5361-5388. (64) Hong, Y.; Lam, J. W. Y.; Tang, B. Z. Aggregation-Induced Emission: Phenomenon, Mechanism and Applications. Chem. Commun. 2009, 4332-4353. (65) Wei, Z.; Yang, J. H.; Zhou, J.; Xu, F.; Zrinyi, M.; Dussault, P. H.; Osada, Y.; Chen, Y. M. Self-Healing Gels Based on Constitutional Dynamic Chemistry and their Potential Applications. Chem. Soc. Rev. 2014, 43, 8114-8131.


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Dynamic Peptide Library for the Discovery of Charge Transfer

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