Chirality-Driven Parallel and Antiparallel Î²-Sheet Secondary

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Chirality-Driven Parallel and Antiparallel #-sheet Secondary Structures of Phe–Ala Lipodipeptides Shuwei Lin, Jiaming Qin, Yi Li, Baozong Li, and Yonggang Yang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01942 • Publication Date (Web): 01 Aug 2017 Downloaded from http://pubs.acs.org on August 2, 2017

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Chirality-Driven Parallel and Antiparallel β-sheet Secondary Structures of Phe–Ala Lipodipeptides Shuwei Lin, Jiaming Qin, Yi Li, Baozong Li and Yonggang Yang*

Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Polymer Science and Engineering, State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, P.R. China.

Four Phe–Ala lipodipeptides with different stereochemical structures are observed to selfassemble into twisted nanoribbons in water. The handedness of the twisted nanoribbons is controlled by the chirality of the phenylalanine near the alkyl chain, while the stacking handedness of the phenyl and carbonyl groups is determined by the alanine at the C-terminal. The homochiral and heterochiral lipodipeptides self-assemble into parallel and antiparallel βsheet structures, respectively. The 1H NMR, FTIR, X-ray diffraction and circular dichroism characterizations indicate that these phenomena are mainly driven by the interaction between neighboring phenyl groups and H-bonding among the amide groups.

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INTRODUCTION Peptide derivatives have attracted intensive study in recent decades owing to their diverse molecular structures, ease in the modification of the building blocks and their potential application as biomaterials.1-5 By taking advantage of supramolecular non-covalent interactions, peptide derivatives are able to self-assemble to form various nanostructures,6-15 which are of great significance in the design of functional soft materials,14-18 to mimic natural hierarchical nanostructures14 and for biological applications.18-20 It has already been demonstrated that the supramolecular nanostructures of peptides are subtly affected by the sequence,21,22 external environmental factors23,24 and even the pathways taken in the self-assembly process.25 Chiral packings have been found among the supramolecular systems.26 However, the precise control of the chirality of supramolecular nanostructures is still of interest. Early research indicated that the chirality of supramolecular aggregates could be finely tuned by alternating the side groups,27,28 changing the solvent (or solvent composition),28-30 modulating the pH value,31 and by controlling the direction of stirring,32 additives,33,34 ultrasonification35 and temperature.35,36 Moreover, in the case of self-assemblies constructed by achiral molecules, the addition of a small number of chiral molecules is known to induce the formation of chiral supramolecular nanostructures. A similar phenomenon has also been observed for artificial polymers.37,38 A lipopeptide is a kind of effective hydrogelator whose self-assembly process is believed to be thermodynamically favoured.39 It has already been verified that the chirality of the amino acid at the C-terminal plays a decisive role in controlling the handedness of the self-assemblies of homogeneous

lipodipeptides.30,40,41

For

the

heterogeneous

lipodipeptides

containing

phenylglycine and phenylalanine residues, the chirality of the phenylalanine and the steric hindrance of the phenyl group of phenylglycine affected both the molecular packing and the handedness of the organic self-assemblies.42 Generally, these lipodipeptides self-assembled into parallel β-sheet structures.30,40,42 Herein, we report the synthesis of a group of four heterogeneous


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dipeptides derived from phenylalanine and alanine. It was found that the chirality of alanine at the C-terminal played an important role in controlling the molecular packing and the handedness of the self-assemblies was dominated by the chirality of phenylalanine near the alkyl chain. Moreover, the homochiral and heterochiral lipodipeptides self-assembled into a parallel and an antiparallel β-sheet structure, respectively.

RESULTS AND DISCUSSION

Figure 1. Molecular structures of the lipodipeptides.

Figure 2. FE-SEM images of the xerogels of the lipodipeptides prepared in deionized water at a concentration of 27.0 g L-1. (a) (L, L)-7, (b) (L, D)-7, (c) (D, L)-7 and (d) (D, D)-7.


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Figure 1 shows the molecular structures of the four heterogeneous lipodipeptides consisting of phenylalanine and alanine residues. The self-assembly behaviors of Ala–Ala and Phe–Phe lipodipeptides with same alkyl chain have been studied.40,43 Comparison on the self-assembly behaviors of these three series of compounds may help us to better understand the effect of the phenyl group. The minimum gelation concentration of the lipodipeptides was approximately 24 g L-1 at 25 °C, which was determined by the tube inversion method (Figure S1, Supporting Information). The study was carried out by putting hot solutions in water bath at 25 ºC for 12 h. These lipodipeptides could also form physical gels in THF, DMF, DMSO and toluene (Table S1, Supporting Information). Field-emission scanning electron microscopy (FE-SEM) and Transmission electron microscopy (TEM) images of the xerogels of the lipodipeptides are shown in Figure 2 and S2, Supporting Information, respectively. Each sample was prepared in deionized water at a concentration of 27.0 g L-1. All of the lipodipeptides could self-assemble into twisted nanoribbons. (L, L)-7 and (D, D)-7 were feasible to self-assemble into nanobelts which were formed by twisted nanoribbons (Figure 2a and d). A similar structure has been reported previously.44 Decrease of surface energy was proposed to drive this structural transition from twisted nanoribbon to nanobelt. (L, L)-7 and (L, D)-7 self-assembled into right-handed twisted nanoribbons (Figure 2a and b), while (D, L)-7 and (D, D)-7 self-assembled into left-handed twisted nanoribbons (Figure 2c and d). The handedness of the self-assemblies was controlled by the chirality of the phenylalanine near the alkyl chain. However, for the Ala–Ala lipodipeptides, the handedness of the self-assemblies was controlled by the chirality of the alanine near the Cterminal.40 Therefore, for the Phe–Ala lipodipeptides shown here, the interaction between neighboring phenyl rings are proposed to play an important role in this phenylalanine controlled phenomenon. Although L–Ala–L–Ala and L–Phe–L–Phe lipodipeptides self-assembled into lefthanded twisted nanoribbons,40,43 (L, L)-7 self-assembled into right-handed twisted nanoribbons. (L, L)-7 should self-assemble into a different structure.30 The polarity of a dimmer was proposed to drive this handedness inversion.30,45


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Figure 3. FT-IR spectra of the solutions and gels of (L, L)-7 and (L, D)-7 in D2O at a concentration of 30 g L-1. To obtain detailed information of the molecular packing, Fourier transformed infrared (FT-IR) spectra of the solutions and gels of the lipodipeptides in D2O were recorded at a concentration of 30 g L-1 and 25 °C and 85 °C, respectively (Figure 3 and Figure S3, Supporting Information). In the case of the (L, D)-7 aqueous solution, it could be observed that the absorption bands of amide A (νN-H) and amide I (νC=O) appeared at 3439 and 1635 cm-1, respectively. While in the gel state, the absorption bands were identified at 3411 (νN-H), 1636 (νC=O) and 1611 (νC=O) cm-1.46,47 These shifts originated from a β-sheet structure.48 The intermolecular hydrogen bonds among amide groups were formed in the gel state. However, during the solution/gel transition, the absorption band of the carboxylate group at (νC=O) 1590 cm-1 almost did not shift, indicating that there was no strong repulsive interaction among the carboxylate groups. In addition, two absorption bands were observed at 2923 and 2853 cm-1 in the spectrum of the (L, D)-7 aqueous solution. This


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indicated that the alkyl chains tended to maintain a gauche form in aqueous solution.46 In the gel state, these absorption bands shifted to 2918 and 2848 cm-1, respectively. The bandwidths of them became narrow, indicating the formation of an all-trans form.49 This mainly arose from the hydrophobic association among the alkyl chains. Namely, intermolecular H-bonding among the amide groups and an increase of entropy were the two main driving forces for the formation of the self-assemblies. A similar phenomenon was also observed for (D, L)-7 (Figure S1, Supporting Information). For the (L, L)-7 aqueous solution, the absorption bands of amide A (νN-H), amide I (νC=O) and the carboxylate group (νC=O) were identified at 3438, 1624 and 1589 cm-1, respectively. While in the hydrogel state, the absorption bands were identified at 3411, 1642, 1619 and 1580 cm-1. The absorption band shifts were evidence of intermolecular H-bonding formation and repulsive interaction among the carboxylate groups. Moreover, the bandwidth of the carboxylate became narrow. In the gel state, the absorption band of amide I (νC=O) at 1642 cm-1 originated from a βsheet structure.48 Besides, in the solution state, the absorption bands of the C–H of the alkyl chains appeared at 2918 and 2853 cm-1, indicating that the alkyl chains of (L, L)-7 molecules were inclined to adopt the all-trans arrangement.40 In the gel state, these two absorption bands were slightly shifted to 2913 and 2848 cm-1, respectively, suggesting an all-trans form. A similar phenomenon was also observed in (D, D)-7 (Figure S3, Supporting Information).


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Figure 4. 1H NMR spectra of (L, L)-7 and (L, D)-7 hydrogels (25 g L-1) and solutions (5 g L-1) prepared in a D2O/H2O mixed solvent (v/v = 15/85). The 1H nuclear magnetic resonance (1H NMR) spectra of the hydrogels (25 g L-1) and aqueous solutions (5 g L-1) were obtained to study the intermolecular H-bonding (Figure 4). Hence, we recorded the 1H NMR spectra in a D2O/H2O mixed solvent to better simulate an aqueous solution and the hydrogel state while avoiding excessive deuteration. In the case of the (L, D)-7 aqueous solution, the chemical shifts of the amide protons were observed at 7.99 and 7.78 ppm for the N– H of phenylalanine and N–H of alanine, respectively.39 In the hydrogel state, the corresponding chemical shifts were observed at 7.95 and 7.76 ppm, respectively. The change of H-bonding from N–H···O–H to N–H···O=C was proposed to lead to the upfield shifts. The intermolecular H-bonds formed by the NH groups of the phenylalanines contributed more to the formation of the self-assemblies than those formed by the NH groups of the alanines. For the (L, L)-7 aqueous solution, two amide protons were identified at 7.99 and 7.86 ppm for the N–H of phenylalanine and N–H of alanine, respectively. In the gel state, the corresponding chemical shifts were observed at 7.97 and 7.84 ppm, respectively. The intermolecular H-bonding formed by the NH groups of the phenylalanines and alanines seemed to contribute equally for the formation of the self-assemblies. A similar phenomenon was also found for the other lipodipeptides (Figure S4, Supporting Information). Namely, the formation of the H-bonding was different between (L, D)7 (or (D, L)-7) and (L, L)-7 (or (D, D)-7).


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Figure 5. SAXRD patterns of (a) (L, L)-7 and (b) (L, D)-7 xerogels. Each sample was prepared at a concentration of 30 g L-1.

Figure 6. WAXRD patterns of (a) (L, L)-7 and (b) (L, D)-7 xerogels. Each sample was prepared at a concentration of 30 g L-1.


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Figure 7. Proposed molecular packing structure of (L, L)-7. Small and wide angle X-ray diffraction (SAXRD/WAXRD) patterns of the xerogels of the lipodipeptides were recorded to reveal the ordered organization of the self-assemblies (Figure 5 and 6). The geometric structures of (L, L)-7 and (L, D)-7 were calculated using molecular mechanics. The molecular lengths of (L, L)-7 and (L, D)-7 were 29.9 and 30.1 nm, respectively. The proposed molecular packing structures of (L, L)-7 and (L, D)-7 are summarized in Figure 7 and 8, respectively. SAXRD patterns of the xerogels indicated that (L, L)-7 and (L, D)-7 organized in different structures (Figure 5a and 5b). For (L, L)-7, the diffraction peaks were identified at 4.12, 2.08, 1.39, 1.04, 0.83 and 0.59 nm, indicating a lamellar structure with a dspacing of 4.12 nm. Since this d-spacing is longer than the molecular length of 29.9 nm, an interdigitated bilayer structure was proposed to be formed (Figure 7). Ala–Ala and Phe–Phe lipodipeptides with same alkyl chain also packed into this kind of structure.40,43 The peptide segments packed into a parallel β-sheet structure. The repulsive interaction among carboxylate groups was shown in Figure 7, Top view, which was identified in the FT-IR spectra (Figure 3b). Moreover, the alkyl chains exhibited an all-trans form in the xerogel. The WAXRD patterns of


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(L, L)-7 exhibited peaks at 2θ of 23.45° and 19.35°, indicating that the spacing between the stacked phenyl groups and that between adjacent β-strands were approximately 3.8 Å and 4.6 Å, respectively (Figure 6a and 7). The diffraction peak at 2θ of 9.63° was proposed to originate from a 9.2 Å distance between two parallel β-sheets (Figure 6a and 7, Top view). The (D, D)-7 xerogel showed similar diffraction peaks as the (L, L)-7 xerogel (Figure S5 and S6, Supporting Information). Therefore, (L, L)-7 and (D, D)-7 is proposed share a similar molecular arrangement in their xerogels.

Figure 8. Proposed molecular packing structure of (L, D)-7. The (L, D)-7 and (D, L)-7 xerogels showed similar SAXRD and WAXRD patterns (Figure 5, 6 and Figure S5 and S6, Supporting Information). For the (L, D)-7 xerogel, the d-spacings of 2.95, 1.47 and 0.98 nm indicated a periodic structure with a distance of 2.95 nm (Figures 5b). Since this distance was shorter than the molecular length, the molecules should tilt within the organic self-assemblies (Figure 8). The weak diffraction peak at a d-spacings of 2.29 nm was proposed to originate from the dimer with a length of 4.58 nm (Figure 5b and 8, Top view). The peptide


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segments packed into an antiparallel β-sheet structure (Figure 8, Top view). A similar packing structure has been reported previously.50 The carboxylate groups were trapped within the alkyl chains. Therefore, the absorption band of the carboxylate group at (νC=O) 1590 cm-1 did not shift. The d-spacing of 1.30 nm was proposed to be the distance between two parallel lipodipeptides (Figures 8, Top view). The diffraction peak of 0.47 nm was proposed to be the distance of neighboring β-strands those forming H-bonds (Figures 8, Top view). The diffraction peaks at 0.90, 0.56 and 0.45 nm originated from β-sheets (Figure 6b and 8, Front view). The distance of neighboring phenyl rings was about 0.39 nm (Figure S6b, Supporting Information). The results shown here indicated that the molecular packings were controlled by the chirality of the lipodipeptides. It was also reported that both parallel and antiparallel β-sheets could be formed by changing the molecular structures of the aromatic group protected dipeptides.51-53

Figure 9. CD and UV-Vis spectra of the hydrogels prepared at 30 g L-1 and 25 °C. To further understand the molecular packing of the lipodipeptides, the circular dichroism (CD) and ultraviolet-visible (UV-Vis) spectra of the hydrogels were recorded at a concentration of 30 g L-1 at 25 °C (Figure 9). The UV absorption bands at 257 and 200 nm mainly originated from the phenyl and carbonyl groups, respectively. The hydrogels of (L, L)-7 and (D, D)-7 were opaque, and those of (L, D)-7 and (D, L)-7 were translucent (Figure S1, Supporting Information).


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This phenomenon should be caused by the different size of the organic self-assemblies (Figure 2). Since the hydrogels were opaque, the UV-Vis spectra of (L, L)-7 and (D, D)-7 showed strong light scattering. The CD spectra of the lipodipeptides exhibited strong exciton coupling signals of the phenyl groups at wavelengths longer than 230 nm. The CD spectrum of the (L, L)-7 hydrogel showed three positive CD signals at 249, 236 and 231 nm; and that of the (L, D)-7 hydrogel showed two negative CD signals at 272 and 237 nm. The phenyl groups of (L, L)-7 and (D, L)-7 stacked in a right-handed fashion, while those of (D, D)-7 and (L, D)-7 stacked in a left-handed manner.54 The CD signals at 213–217 nm mainly originated from the exciton coupling of carbonyl groups. The CD spectrum of the (L, L)-7 hydrogel showed one negative CD signal at 213 nm; and that of the (L, D)-7 hydrogel showed one positive CD signal at 215 nm. The carbonyl groups of (L, L)-7 and (D, L)-7 stacked in a left-handed manner, while those of (D, D)-7 and (L, D)-7 stacked in a right-handed fashion.55 Namely, both the chiral stacking of the phenyl groups and that of the carbonyl groups were determined by the chirality of alanine at the Cterminal. This phenomenon has been also observed in other lipopeptides.30,40,43 However, the FESEM characterization indicated that the handedness of the self-assemblies of the dipeptides was controlled by the chirality of the phenylalanine. According to the NMR and XRD study above, this difference should arise from the difference of the molecular packings (Figure 7 and 8). The “C-terminal determination” rule has been found in some lipodipeptides and tetrapeptides.56 The handedness of their organic self-assemblies was dominated by the chirality of the amino acid at the C-terminal. The results shown here indicated that the handedness of the organic selfassemblies of Phe-Ala lipopeptides did not follow this rule. The reason was that the homochiral and heterochiral lipodipeptides packed into different structures. Therefore, the molecular packing effect should override the “C-terminal determination” rule.

SUMMARY AND CONCLUSIONS


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The self-assembly behavior of four Phe–Ala lipodipeptides with different stereochemical structures were studied. They were shown to self-assemble into twisted nanoribbons in water. The handedness of the twisted nanoribbons was controlled by the chirality of the phenylalanine near the alkyl chain, while the stacking handedness of the phenyl and carbonyl groups was determined by the alanine at the C-terminal. These phenomena were proposed to be driven by the interaction between neighboring phenyl groups and H-bonding among the amide groups. The homochiral and heterochiral lipopeptides self-assembled into a parallel and an antiparallel βsheet structure, respectively. The interaction between neighboring phenyl groups was proposed to cause this structural difference. The results shown here not only provide us with a better understanding of the effect of aromatic rings, but also shed light on controlling the structures of organic self-assemblies by modifying the molecular structures.

EXPERIMENTAL SECTION Materials. N-(9-Fluorenylmethyloxycarbonyl)-L-alanine (Fmoc-Ala-OH) (≥99.5%), N-(9fluorenylmethyloxycarbonyl)-D-alanine fluorenylmethyloxycarbonyl)-L-phenylalanine

(Fmoc-D-Ala-OH) (Fmoc-Phe-OH)

(≥99.5%), (≥99.5%),

N-(9N-(9-

fluorenylmethyloxycarbonyl)-D-phenylalanine (Fmoc-D-Phe-OH) (≥99.5%), O-benzotriazole-N, N, Nʹ, Nʹ-tetramethyl-uronium-hexafluorophosphate (HBTU) and 2-chlorotrityl chloride resin (loading 1.01 mmol/g) were purchased from GL Biochem Ltd., diisopropylethylamine (DIEA) (≥99.5%) was purchased from Soochiral Co., Ltd., and used as received. N,N-Dimethyl formamide (DMF), dichloromethane (DCM) and tetrahydrofuran (THF) were obtained from Chinasun Specialty Products Co., Ltd. Before use, both DMF and DCM were dehydrated by treatment with calcium hydride for over 24 h, THF was treated with sodium and then redistilled. Deionized water, trifluoroacetic acid, ethanol, diethyl ether, toluene, dimethylsulfoxide (DMSO), deuterium oxide (D2O) and DMSO-d6 were purchased from Sinopharm Chemical Reagent Co., Ltd.


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Synthesis of the heterogeneous dipeptides. All the four heterogeneous dipeptides were synthesized following the manual solid phase peptide synthesis (SPPS) procedure which has already been reported by our group.42 Characterization of C17H35CO-L-Phe-L-Ala-ONa ((L, L)-7). FT-IR (KBr, cm-1): 3294 (νN-H, amide A), 2911, 2849 (νC-H, alkyl chain), 1628 (νC=O, amide I), 1582 (νC=O, carboxylate group), 1541 (νN-H, amide II). 1H NMR (400 MHz, DMSO-d6, TMS): δ =0.85 (t, 3H; J = 6.6 Hz, CH3CH2), 1.08–1.24 (m, 31H; CH3(CH2)14CH2, NHCH(CH3)CO), 1.34 (t, 2H; J = 7.0 Hz, CH2CH2CO), 2.01 (t, 2H; J = 7.0 Hz, CH2CH2CO), 3.02–3.06 (m, 2H; PhCH2CH), 3.63–3.69 (m, 1H; CONHCHCOONa), 4.37–4.42 (m, 1H; CONHCHCONH), 7.12–7.23 (m, 5H; 1Ph), 7.55 (d, 1H; J = 6.6 Hz, CONHCHCOONa), 8.19 (d, 1H; J = 8.0 Hz, CONHCHCO). Elemental analysis for C30H49N2NaO4 calcd (%): C, 68.67; H, 9.41; N, 5.34. Found: C, 67.52; H, 9.31; N, 5.29. Characterization of C17H35CO-L-Phe-D-Ala-ONa ((L, D)-7). FT-IR (KBr, cm-1): 3298 (νN-H, amide A), 2921, 2852 (νC-H, alkyl chain), 1627 (νC=O, amide I), 1585 (νC=O, carboxylate group), 1538 (νN-H, amide II). 1H NMR (400 MHz, DMSO-d6, TMS): δ =0.87 (t, 3H; J = 6.8 Hz, CH3CH2), 1.07–1.25 (m, 31H; CH3(CH2)14CH2, NHCH(CH3)CO), 1.34 (t, 2H; J = 7.0 Hz, CH2CH2CO), 2.03 (t, 2H; J = 7.2 Hz, CH2CH2CO), 3.01–3.06 (m, 2H; PhCH2CH), 3.62-3.68 (m, 1H; CONHCHCOONa), 4.37–4.44 (m, 1H; CONHCHCONH), 7.11–7.25 (m, 5H; 1Ph), 7.62 (d, 1H; J = 6.0 Hz, CONHCHCOONa), 8.19 (d, 1H; J = 8.4 Hz, CONHCHCO). Elemental analysis for C30H49N2NaO4 calcd (%): C, 68.67; H, 9.41; N, 5.34. Found: C, 67.64; H, 9.35; N, 5.43. Characterization of C17H35CO-D-Phe-L-Ala-ONa ((D, L)-7). FT-IR (KBr, cm-1): 3301 (νN-H, amide A), 2923, 2853 (νC-H, alkyl chain), 1627 (νC=O, amide I), 1586 (νC=O, carboxylate group), 1540 (νN-H, amide II). 1H NMR (400 MHz, DMSO-d6, TMS): δ =0.87 (t, 3H; J = 6.8 Hz, CH3CH2), 1.06–1.25 (m, 31H; CH3(CH2)14CH2, NHCH(CH3)CO), 1.33 (t, 2H; J = 7.0 Hz, CH2CH2CO), 2.00 (t, 2H; J = 7.4 Hz, CH2CH2CO), 3.04–3.09 (m, 2H; PhCH2CH), 3.62–3.68 (m, 1H; CONHCHCOONa), 4.35–4.41 (m, 1H; CONHCHCONH), 7.13–7.24 (m, 5H; 1Ph), 7.22 (d,


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1H; J = 6.4 Hz, CONHCHCOONa), 8.20 (d, 1H; J = 8.4 Hz, CONHCHCO). Elemental analysis for C30H49N2NaO4 calcd (%): C, 68.67; H, 9.41; N, 5.34. Found: C, 67.62; H, 9.56; N, 5.23. Characterization of C17H35CO-D-Phe-D-Ala-ONa ((D, D)-7). FT-IR (KBr, cm-1): 3300 (νN-H, amide A), 2918, 2851 (νC-H, alkyl chain), 1626 (νC=O, amide I), 1582 (νC=O, carboxylate group), 1543 (νN-H, amide II). 1H NMR (400 MHz, DMSO-d6, TMS): δ =0.85 (t, 3H; J = 6.8 Hz, CH3CH2), 1.06–1.23 (m, 31H; CH3(CH2)14CH2, NHCH(CH3)CO), 1.32 (t, 2H; J = 7.2 Hz, CH2CH2CO), 2.02 (t, 2H; J = 7.4 Hz, CH2CH2CO), 3.01–3.10 (m, 2H; PhCH2CH), 3.61–3.67 (m, 1H; CONHCHCOONa), 4.32–4.40 (m, 1H; CONHCHCONH), 7.11–7.25 (m, 5H; 1Ph), 7.55 (d, 1H; J = 6.0 Hz, CONHCHCOONa), 8.20 (d, 1H; J = 8.4 Hz, CONHCHCO). Elemental analysis for C30H49N2NaO4 calcd (%): C, 68.67; H, 9.41; N, 5.34. Found: C, 67.53; H, 9.30; N, 5.52. Methods. The CD and UV spectroscopy studies were performed on a Jasco J-815 circular dichroism spectrometer (Tokyo, Japan) under nitrogen atmosphere. The dipeptide hydrogel was loaded into a quartz cell with a 0.01 mm optical path (Hellma, 0.01 mm quartz Suprasil). All the spectra were recorded in the range from 550 to 180 nm with a 0.5 nm interval, a 1.0 nm bandwidth, an average scanning speed of 50 nm/min and 1 scan at 25.0 °C. The morphology study was performed on an Hitachi S-4800 field emission scanning electron microscope instrument (Ibaraki prefecture, Japan) with acceleration voltage of 3.0 kV. All the xerogel samples were prepared by smearing the pre-prepared viscous dipeptide aqueous solution (3 g L-1) on the surface of glass flakes and then allowing to stand for the thorough evaporation of water. Before the FE-SEM images were taken, platinum was sputtered on the surface of the xerogel samples for 30 seconds to improve the conductivity and prevent the charging effect. TEM images were obtained using a TecnaiG220 instrument operated at 200 kV (FEI, USA). 1H nuclear magnetic resonance analysis was performed on a Varian NMR 400 spectrometer (Palo Alto, USA). Aqueous solution and hydrogel samples were prepared in D2O and H2O mixed solvents with a volume ratio of 15/85. Elemental analysis was performed on a PerkinElmer series


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II CHNS/O analyzer 2400 (Waltham, Massachusetts, USA). FT-IR spectra were recorded by using a Thermo Scientific Nicolet 6700 spectrometer (Waltham, Massachusetts, United States) with a resolution of 2 cm-1 and averaged from 64 scans. All of the dipeptides were dissolved in deuterium oxide with a concentration of 30 g L-1 and incubated at 25 °C for 12 hours to obtain stable gel samples. The gel and solution were injected between two BaF2 wafers with an optical path of 50 µm and placed in a hot stage. Small angle X-ray diffraction analysis was conducted on an X’ Pert-Pro MPD X-ray diffractometer (Almelo, Netherland) with Cu-Kα radiation (1.542 Å) and a Ni filter at room temperature. 1 mL of deionized water was mixed with 30 mg of each of the dipeptides and the mixture was sealed in a tube followed by heating to a high temperature to obtain clear solutions. After that, the aqueous solutions were pipetted out and carefully smeared onto the surface of the glass substrates. The SAXRD patterns were recorded after the dipeptide aqueous solutions became film-like self-assemblies. All of the glass flakes were ultrasonicated in nitric acid for 2 h followed by washing with water. Supporting Information.

The FT-IR, 1H NMR, WAXRD and SAXRD spectra of (D, D)-7 and (D, L)-7. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Fax: +86 512 65882052. Tel: +86 512 65880047.

ACKNOWLEDGMENTS


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This work was supported by the Priority Academic Program Development of Jiangsu High Education Institutions (PAPD), the Science and Technology Development Plan (nano special) of Suzhou (ZXG201415) and the National Natural Science Foundation of China (No. 51473106, 51473141 and 21574095).

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Figure 1. Molecular structures of the lipodipeptides. 77x52mm (300 x 300 DPI)

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Figure 2. FE-SEM images of the xerogels of the lipodipeptides prepared in deionized water at a concentration of 27.0 g L-1. (a) (L, L)-7, (b) (L, D)-7, (c) (D, L)-7 and (d) (D, D)-7. 80x80mm (256 x 256 DPI)


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Figure 3. FT-IR spectra of the solutions and gels of (L, L)-7 and (L, D)-7 in D2O at a concentration of 30 g L1. 56x64mm (300 x 300 DPI)


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Figure 4. 1H NMR spectra of (L, L)-7 and (L, D)-7 hydrogels (25 g L-1) and solutions (5 g L-1) prepared in a D2O/H2O mixed solvent (v/v = 15/85). 182x109mm (80 x 80 DPI)


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Figure 5. SAXRD patterns of (a) (L, L)-7 and (b) (L, D)-7 xerogels. Each sample was prepared at a concentration of 30 g L-1. 60x59mm (300 x 300 DPI)


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Figure 6. WAXRD patterns of (a) (L, L)-7 and (b) (L, D)-7 xerogels. Each sample was prepared at a concentration of 30 g L-1. 60x59mm (300 x 300 DPI)


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Figure 7. Proposed molecular packing structure of (L, L)-7. 705x370mm (72 x 72 DPI)


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Figure 8. Proposed molecular packing structure of (L, D)-7. 352x430mm (72 x 72 DPI)


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Figure 9. CD and UV-Vis spectra of the hydrogels prepared at 30 g L-1 and 25 °C. 60x32mm (300 x 300 DPI)


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Chirality-Driven Parallel and Antiparallel Î²-Sheet Secondary

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