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Jul 7, 2016 - the stacking handedness control of dipeptide molecules and the handedness control of the self-assemblies of dipeptides can improve the ...
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Control of the Handedness of Self-assemblies of Dipeptides by the Chirality of Phenylalanine and Steric Hindrance of Phenylglycine Shuwei Lin, 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, People’s Republic of China S Supporting Information *

ABSTRACT: Eight dipeptides, composed of phenylalanine and phenylglycine, that are able to self-assemble into twisted nanoribbons in deionized water are synthesized. The handedness of the nanoribbons is controlled by the chirality of the phenylalanine and the steric hindrance owing to the phenyl group of the phenylglycine. When the phenylalanine is at the C-terminal, π−π stacking by the phenyl groups, hydrogen bonding by the NH group of the phenylalanine, and hydrophobic associations of the alkyl chains control the stacking of the molecules. When phenylglycine is at the Cterminal, the chiral π−π stacking by the phenyl groups of the phenylalanines is suppressed. The hydrogen bonds formed by the NH groups of the phenylalanines had a greater contribution on forming organic self-assemblies than those formed by the NH groups of the phenylglycines.

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assemblies.15,31−33 Molecular dynamics simulation indicated that the hydrogen bonding formed by the NH groups of the amino acids at the C-terminal directed the formation of the helical self-assemblies.34 For some tripeptides, it was observed that the chirality of the amino acids at the centers controlled the stacking handedness of the molecules.35 Further study of the stacking handedness control of dipeptide molecules and the handedness control of the self-assemblies of dipeptides can improve the understanding of the self-assembly of tripeptides and polypeptides. Moreover, these self-assemblies can potentially be used as biomaterials. Herein, eight heterogeneous dipeptides were designed and synthesized using phenylglycine and phenylalanine. Both the steric hindrance owing to the phenyl group of the phenylglycine and the hydrogen bonding formed by the NH group of the phenylalanine play important roles in the handedness of the self-assemblies.

INTRODUCTION Peptides and their derivatives are ideal supramolecular materials for many applications, especially in biomaterial manufacturing and biomimetic device design.1−5 Owing to the variety and the sequence of the amino acids, the self-assembly mechanism of peptides has been intensively investigated.6−12 The selfassembly process has already been regarded as arising from a synergetic effect of supramolecular weak interactions, which eventually lead to the formation of various nanostructures in different solvents,13−26 for example, nanofibers,16−18 nanoribbons/nanobelts,19,20,25 nanotubes,21,22 and hierarchical architectures.23 The morphologies are sensitive to external factors such as solvent composition,14,15 aging time,19 pH value,22,24,25 and exposure to ultraviolet light.26 Usually, in the case of aromatic peptides, the major driving forces of self-assembly include the π−π stacking of the aromatic groups and the intermolecular hydrogen bonding of the amide groups.8,27−29 For a peptide with a long alkyl chain, the hydrophobic association of the alkyl chains plays an important role in the self-assembly.9,30,31 A mechanistic study of this process has shown a transformation from a kinetic control toward a more stable thermodynamic-controlled self-assembly system.19 It is already widely accepted that chirality is vital in advanced material manufacturing, life systems, and pharmaceutical design. The mechanism of chirality control and chirality transfer is of great significance. For a homodipeptide with a long alkyl chain at the N-terminal, it was observed that the chirality of the amino acid at the C-terminal controlled the stacking handedness of molecules and the handedness of the self© 2016 American Chemical Society

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RESULTS AND DISCUSSION The molecular structures of the dipeptides constructed by phenylalanine and phenylglycine are shown in Figure 1. The molecules with phenylglycine at the C-terminal are named as 5, and those with phenylalanine at the C-terminal are named as 6. All of the dipeptides can produce physical gels at a concentration of ∼8 g L−1 in deionized water. The fieldemission scanning electron microscopy (FE-SEM) images of Received: May 17, 2016 Revised: June 20, 2016 Published: July 7, 2016 7420

DOI: 10.1021/acs.langmuir.6b01874 Langmuir 2016, 32, 7420−7426

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Figure 1. Molecular structures of the dipeptides.

For 6, the handedness of the self-assemblies of (D, D)-6 and (L, D)-6 was left and that of the self-assemblies of (L, L)-6 and (D, L)-6 was right (Figure 2e−h). Therefore, the chirality of the amino acid at the C-terminal becomes important. Similar to 5, the handedness of the self-assemblies was also controlled by the chirality of the phenylalanine. Because the handedness of the self-assemblies of (L, L)-6 was also opposite to that of the selfassemblies of the homodipeptide constructed by phenylalanines, the steric hindrance of the phenyl group of the phenylglycine should play an important role in the handedness of the organic self-assemblies. For 5 and 6, this phenylalaninedriven handedness was proposed to originate from the steric hindrance by the phenyl group of the phenylglycine and the hydrogen bonds formed by the NH group of the phenylalanine. To understand the chiral stacking by the phenyl and carbonyl groups of the dipeptides, circular dichroism (CD) and ultraviolet (UV) spectra of the dipeptide hydrogels were obtained at a concentration of 15 g L−1 at 25 °C (Figure 3). The UV spectra of the hydrogels are all almost identical. The absorption bands at approximately 260 and 200 nm mainly originated from the phenyl ring and carbonyl group, respectively. For 5 with phenylglycines at the C-terminal, ultraweak CD signals were identified at 239−293 nm, indicating weak chiral π−π stacking of the phenyl rings (Figure S1, Supporting Information). The self-assemblies of the homodipeptides derived from phenylalanines have also been characterized using CD.32 A stronger chiral π−π stacking was identified. Herein, there were no strong chiral π−π interactions among the dipeptides. The CD spectrum of (L, L)-5 showed two strong positive signals at 225 and 194 nm. The positive CD signal at 225 nm originated from the right-handed stacking of the carbonyl groups.36 This handedness was the same as that of the nanoribbons shown in Figure 2a. The CD spectrum of (D, D)-5 showed opposite signals, indicating that the carbonyl groups stacked with left handedness. The CD spectrum of (L, D)-5 showed two stronger positive signals at 224 and 201 nm, which is similar to that of (L, L)-5. The handedness of the organic self-assemblies followed the stacking handedness of the carbonyl groups (Figure 2b). The CD spectrum of (D, L)-5 showed opposite signals, indicating that the carbonyl groups

the xerogels of the dipeptides prepared at a concentration of 15 g L−1 are shown in Figure 2. All of the xerogels could self-

Figure 2. FE-SEM images of the xerogels of the dipeptides prepared at a concentration of 15 g L−1. (a) (L, L)-5, (b) (L, D)-5, (c) (D, L)-5, (d) (D, D)-5, (e) (L, L)-6, (f) (L, D)-6, (g) (D, L)-6, and (h) (D, D)-6. The scale bars inserted indicate 400 nm.

assemble into twisted nanoribbons in deionized water. For 5, the handedness of the self-assemblies of (L, L)-5 and (L, D)-5 was right and that of the self-assemblies of (D, D)-5 and (D, L)-5 was left (Figure 2a−d). The handedness of the nanoribbons was controlled by the chirality of the phenylalanine near the alkyl chain and not controlled by the chirality of the phenylglycine. We have previously reported that the handedness of the self-assemblies of the dipeptides constructed by the same amino acids is controlled by the chirality of the amino acids at the C-terminal.31 Apparently, the handedness of the self-assemblies of 5 does not follow this rule. The steric hindrance of the phenyl group of the phenylglycine was proposed to suppress the intermolecular hydrogen bonding formed by the NH group of the phenylglycine between neighboring dipeptide molecules. Although the homodipeptide constructed by L-phenylalanine self-assembled into left-handed twisted nanoribbons in deionized water,32 (L, L)-5 selfassembled into right-handed helical nanoribbons. This handedness inversion was also proposed to be driven by the steric hindrance of the phenyl group of the phenylglycine. 7421

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Langmuir Table 1. λmax and θ Values at 220−225 nm and the Handedness of the Nanoribbons (L, L)-5 (D, D)-5 (L, D)-5 (D, L)-5 (L, L)-6 (D, D)-6 (L, D)-6 (D, L)-6

λmax (nm)/θ (mdeg)

nanoribbon

225/+28.0 224/−21.5 225/+26.7 224/−8.7 220/−9.0 221/+8.6 221/+56.2 221/−29.3

right left right left right left left right

To study the organization of the dipeptides, the Fourier transform infrared spectroscopy (FT-IR) spectra of the solutions and gels of 5 and 6 in D2O were obtained at 80 and 25 °C, respectively (Figure 4 and Figure S2, Supporting

Figure 3. CD and UV spectra of (a) 5 and (b) 6 hydrogels at a concentration of 15 g L−1 at 25 °C.

stacked with left-handedness. For this series, it should be noted that the stacking handedness of the carbonyl groups is controlled by the chirality of the phenylalanine. For 6 with phenylalanine at the C-terminal, strong π−π interactions among the phenyl rings were identified (Figure 3b). CD spectra of (L, L)-6 and (D, D)-6 showed exciton coupling signals at 224−400 nm. The stacking handedness of phenyl37 and that of carbonyl groups36 can be identified from the sign of the exciton coupling CD signals. When the sign of the first CD is positive, phenyl or carbonyl groups stack with right handedness. On the contrary, they stack with left handedness. The CD spectrum of (L, L)-6 showed a negative signal at 257 nm and a positive CD signal at 230 nm, indicating a left-handed stacking of the phenyl groups. A negative CD signal at 220 nm was also identified, which originated from a left-handed stacking of the carbonyl groups. Although both phenyl and carbonyl groups stacked with left handedness, (L, L)-6 self-assembled into right-handed twisted nanoribbons. For (D, L)-6, two negative CD signals were identified at 240 and 221 nm, indicating that both phenyl and carbonyl groups stacked with left handedness. Similar to (L, L)-6, (D, L)-6 also selfassembled into right-handed twisted nanoribbons (Figure 2g). For (L, D)-6 and (D, D)-6, both phenyl and carbonyl groups stacked with right handedness. For this series, the stacking handedness of phenyl and that of the carbonyl groups were controlled by the chirality of the phenylalanine at the Cterminal, which are opposite to the handedness of the selfassemblies. Handedness inversion has been found in polymeric systems.38,39 The stacking handedness of the polymer main chains was controlled by the helical pitch/diameter (P/D) ratio of single polymer chain. When the P/D ratio was smaller than a given value, handedness inversion would occur.39 Herein, although the carbonyl groups of (L, L)-6 stacked with lefthandedness, their self-assemblies in nanoscale were reasonable to twist in right handedness. The relationship between the handedness of the nanoribbon and the stacking handedness of the carbonyl groups was summarized Table 1. The signals at 220−225 nm mainly originated from the chiral stackings of the carbonyl groups.

Figure 4. FT-IR spectra of the aqueous solutions and hydrogels of (D, −1 D)-5 and (D, D)-6 in D2O at a concentration of 15 g L .

Information). For the (D, D)-5 hydrogel, the absorption bands of amide A (νN−H), amide I (νCO), and the carboxylate group (νCO) were identified at 3400, 1608, and 1579 cm−1, respectively. In the solution state, the absorption bands of amide A (νN−H), amide I (νCO), and the carboxylate group (νCO) were identified at 3433, 1619, and 1583 cm−1, respectively.40 Similar shifts of the absorption bands were also observed for the other 5 compounds, which were driven by intermolecular hydrogen bonding among the amide groups and electrostatic interactions of the carboxylate groups.41 For the (D, D)-5 aqueous solution, two absorption bands at 2919 and 2852 cm−1 were identified, corresponding to a partially all-trans form alkyl chain. In the hydrogel, these bands did not shift, indicating that the conformation of the alkyl chains did not greatly change. For the (D, D)-6 hydrogel, the absorption bands of amide A (νN−H), amide I (νCO), and carboxylate group (νCO) in the gel state were identified at 3401, 1627, and 1582 cm−1, respectively. In an aqueous solution, the absorption bands shifted to 3433, 1631, and 1595 cm−1. These shifts also suggest that intermolecular hydrogen bonding among the amide groups and electrostatic interactions of the carboxylate groups were the main driving forces for the formation of the self-assemblies. Moreover, a hydrophobic association among the alkyl chains was observed. For the (D, D)-6 hydrogel, two absorption bands were identified at 2916 and 2846 cm−1, corresponding to an all-trans form alkyl chain. In the aqueous solution, these bands shifted to 2921 and 2850 cm−1, indicating the presence of a gauche form alkyl chain. This phenomenon appeared in the rest of the 6 compounds (Figure S2, Supporting Information). Therefore, the hydrophobic associa7422

DOI: 10.1021/acs.langmuir.6b01874 Langmuir 2016, 32, 7420−7426

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Langmuir tion also plays an important role in the formation of the selfassemblies of 6. To reveal the intermolecular hydrogen bonds, 1H NMR spectra of (D, L)-5 and (L, D)-6 were recorded for both gel (15 g L−1) and solution (5 g L−1) states in DMSO-d6 (Figure 5). For

Figure 5. 1H NMR spectra of (D, L)-5 and (L, D)-6 gels (15 g L−1) and solutions (5 g L−1) in DMSO-d6.

the (D, L)-5 solution, two protons of the amide groups were observed at 8.03 ppm for the HN of the phenylglycine at the C-terminal and 8.28 ppm for the HN of the phenylalanine near the alkyl chain.19 In the gel state, the corresponding chemical shifts appeared at 8.04 and 8.31 ppm, respectively. These downfield shifts were driven by the change of the hydrogen bonding from NH···OS to NH···OC. The results shown here indicated that the hydrogen bonds formed by the NH groups of the phenylalanines had a greater contribution than those formed by the NH groups of the phenylglycines. For the (L, D)-6 solution, two protons of the amide groups were observed at 7.62 ppm for the HN of the phenylalanine and 8.41 ppm for the HN of the phenylglycine. In the gel state, the chemical shifts appeared at 7.65 and 8.42 ppm. The hydrogen bonds formed by the NH groups of the phenylalanines also played an important role in the formation of the twisted nanoribbons. This phenomenon was also observed in the other dipeptides (Figure S3, Supporting Information). Namely, in the self-assembling process of 5 and 6 the hydrogen bonds formed by the NH groups of the phenylalanines had a greater contribution than those formed by the NH groups of the phenylglycines. Small angle X-ray diffraction (SAXRD) patterns of the xerogels of 5 and 6 are shown in Figure 6. The SAXRD patterns of the xerogels of (L, L)-5 and (D, D)-5 did not show clear diffraction peaks. Only one broad peak at 2θ of 2.28° was identified in the SAXRD patterns of the xerogels of (L, D)-5 and (D, L)-5. A short-range ordered lamellar structure with a dspacing of 3.87 nm was proposed. The above shows that the alkyl chains had a partially trans conformation (Figure 4 and Figure S2, Supporting Information). Therefore, the molecules of 5 should not organize well in the hydrogels and xerogels owing to this irregular conformation. SAXRD patterns of the xerogels of 6 were approximately identical (Figure 6). Two diffraction peaks at 2θ of 2.42° and 4.85° were identified, suggesting a lamellar structure with a d-spacing of 3.64 nm. The hydrophobic association among the all-trans form alkyl chains should be the driving force for the formation of this lamellar structure. According to previous results, the molecular lengths of 5 and 6 are approximately 3.0 nm.31 Therefore, interdigitated bilayer structures are proposed to be formed in the hydrogels (Figure

Figure 6. Small angle X-ray diffraction patterns of the xerogels of (a) 5 and (b) 6.

7). For (L, D)-5, the alkyl chain shows a partially all-trans form. This loose organization of the alkyl chains drives the construction of a short-range ordered lamellar structure with a longer d-spacing. Owing to the steric hindrance of the phenyl group of the phenylglycine, the NH group of the phenylglycine forms a hydrogen bond with water, and the chiral stacking of the phenyl groups of the phenylalanines is suppressed in the gel state. Hence, the stacking of (L, D)-5 molecules is mainly controlled by the intermolecular hydrogen bonds formed by the NH group of the phenylalanine. Additionally, the stacking handedness of (L, D)-5 molecules is controlled by the chirality of the phenylalanine. For (L, D)-6, the alkyl chain shows an almost all-trans form. A long-range ordered lamellar structure with a shorter d-spacing is formed in the hydrogel. The steric hindrance of the phenyl group of the phenylglycine also drives the NH group of the phenylglycine to form a hydrogen bond with water in the gel state. Because the phenylalanine is at the C-terminal, the phenyl group of the phenylalanine is able to form intermolecular π−π stacking. With the hydrogen bonding formed by the NH group of the phenylalanine, the stacking handedness of (L, D)-6 molecules is also controlled by the chirality of the phenylalanine.

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SUMMARY AND CONCLUSIONS Eight dipeptides, composed of phenylalanine and phenylglycine, were synthesized that are able to self-assemble into twisted nanoribbons in deionized water. The handedness of the nanoribbons was controlled by the chirality of the phenylalanine and the steric hindrance of the phenyl group of the phenylglycine. When phenylalanine was at the C-terminal, π−π stacking among the aromatic rings, hydrogen bonds formed by the NH groups of the phenylalanines, and hydrophobic associations of the alkyl chains controlled the stacking of the molecules. When phenylglycine was at the C-terminal, the chiral π−π stacking among phenylalanines was suppressed. The hydrogen bonds formed by the NH groups of the phenylalanines had a greater contribution on forming organic self7423

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Figure 7. Simulated interdigitated bilayer structures of (L, D)-5 and (L, D)-6. Trifluoroacetic acid was applied to cleave the peptide derivatives from the resin. The obtained crude products were precipitated in anhydrous diethyl ether and filtered to achieve the peptides. Characterization of C17H35CO-L-Phe-L-Phg-ONa ((L, L)-5). FT-IR (KBr, cm−1): 3299 (νN−H, amide A), 2927, 2844 (νC−H, alkyl chain), 1625 (νCO, amide I), 1540 (νN−H, amide II). 1H NMR (400 MHz, DMSO-d6, TMS): δ = 0.85 (t, 3H; J = 6.8 Hz, CH3CH2), 1.07−1.25 (m, 28H; CH3(CH2)14CH2), 1.37 (t, 2H; J = 7.4 Hz, CH2CH2CO), 2.04 (t, 2H; J = 7.4 Hz, CH2CH2CO), 3.01−3.07 (m, 2H; PhCH2CH), 4.43−4.51 (m, 1H; CONHCHCONH), 4.65−4.70 (m, 1H; CONHCHCOONa), 7.08−7.30 (m, 10H; 2Ph), 8.00−8.05(m, 1H; CONHCHCOONa), 8.28 (d, 1H; J = 8.6 Hz, CONHCHCO). Elemental analysis for C35H51N2NaO4 calcd (%): C, 71.64; H, 8.76; N, 4.77. Found: C, 69.92; H, 8.89; N, 4.49. Characterization of C17H35CO-L-Phe-D-Phg-ONa ((L, D)-5). FT-IR (KBr, cm−1): 3303 (νN−H, amide A), 2924, 2839 (νC−H, alkyl chain), 1629 (νCO, amide I), 1540 (νN−H, amide II). 1H NMR (400 MHz, DMSO-d6, TMS): δ = 0.85 (t, 3H; J = 6.8 Hz, CH3CH2), 1.07−1.24 (m, 28H; CH3(CH2)14CH2), 1.38 (t, 2H; J = 7.4 Hz, CH2CH2CO), 2.03 (t, 2H; J = 7.2 Hz, CH2CH2CO), 3.02−3.06 (m, 2H; PhCH2CH), 4.43−4.53 (m, 1H; CONHCHCONH), 4.66−4.71 (m, 1H; CONHCHCOONa), 7.07−7.26 (m, 10H; 2Ph), 8.01−8.05(m, 1H; CONHCHCOONa), 8.29 (d, 1H; J = 8.0 Hz, CONHCHCO). Elemental analysis for C35H51N2NaO4 calcd (%): C, 71.64; H, 8.76; N, 4.77. Found: C, 69.72; H, 8.90; N, 4.46. Characterization of C17H35CO-D-Phe-L-Phg-ONa ((D, L)-5). FT-IR (KBr, cm−1): 3299 (νN−H, amide A), 2919, 2854 (νC−H, alkyl chain), 1624 (νCO, amide I), 1544 (νN−H, amide II). 1H NMR (400 MHz, DMSO-d6, TMS): δ = 0.85 (t, 3H; J = 6.8 Hz, CH3CH2), 1.08−1.25 (m, 28H; CH3(CH2)14CH2), 1.38 (t, 2H; J = 7.2 Hz, CH2CH2CO), 2.04 (t, 2H; J = 7.2 Hz, CH2CH2CO), 3.02-−3.07 (m, 2H; PhCH2CH), 4.43−4.51 (m, 1H; CONHCHCONH), 4.64−4.69 (m, 1H; CONHCHCOONa), 7.09−7.30 (m, 10H; 2Ph), 8.01−8.05(m, 1H; CONHCHCOONa), 8.28 (d, 1H; J = 8.0 Hz, CONHCHCO). Elemental analysis for C35H51N2NaO4 calcd (%): C, 71.64; H, 8.76; N, 4.77. Found: C, 69.23; H, 8.69; N, 4.34. Characterization of C17H35CO-D-Phe-D-Phg-ONa ((D, D)-5). FT-IR (KBr, cm−1): 3305 (νN−H, amide A), 2926, 2847 (νC−H, alkyl chain), 1624 (νCO, amide I), 1537 (νN−H, amide II). 1H NMR (400 MHz, DMSO-d6, TMS): δ = 0.85 (t, 3H; J = 6.8 Hz, CH3CH2), 1.07−1.24 (m, 28H; CH3(CH2)14CH2), 1.37 (t, 2H; J = 7.4 Hz, CH2CH2CO), 2.03 (t, 2H; J = 7.2 Hz, CH2CH2CO), 3.02−3.07 (m, 2H; PhCH2CH), 4.43−4.51 (m, 1H; CONHCHCONH), 4.64−4.69 (m, 1H; CONHCHCOONa), 7.09−7.30 (m, 10H; 2Ph), 8.00−8.04(m, 1H; CONHCHCOONa), 8.27 (d, 1H; J = 8.4 Hz, CONHCHCO).

assemblies than those formed by the NH groups of the phenylglycines. Further research on heterogeneous dipeptides will provide a better understanding of the driving forces of selfassembly. These dipeptides can be potentially applied as biomaterials and for chiral recognition.

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EXPERIMENTAL SECTION

Materials. N-(9-Fluorenylmethyloxycarbonyl)-L-phenylalanine (Fmoc-Phe-OH) (≥99.5%), N-(9-fluorenylmethyloxycarbonyl)-D-phenylalanine (Fmoc-D-Phe-OH) (≥99.5%), N-(9-fluorenylmethyloxycarbonyl)-L-phenylglycine (Fmoc-Phg−OH) (≥99.5%), N-(9-fluorenylmethyloxycarbonyl)-D-phenylglycine (Fmoc-D-Phg−OH) (≥99.5%), diisopropylethylamine (DIEA) (≥99.5%), O-Benzotriazole-N,N,N′,N′tetramethyl-uronium-hexafluorophosphate (HBTU), and 2-chlorotrityl chloride resin (loading 1.01 mmol/g) were purchased from GL Biochem Ltd. (Shanghai, China) and used as received. N,NDimethylformamide (DMF) and dichloromethane (DCM) were obtained from Chinasun Specialty Products Co., Ltd. (Changshu, China). Before use, both DMF and DCM were dehydrated by treating with calcium hydride for over 24 h and then redistilled. Trifluoroacetic acid, ethanol, and diethyl ether were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Synthesis of the Peptide Derivatives. Figure 1 shows the molecular structures of each of the eight peptide derivatives. All the eight peptide derivatives were synthesized following the manual solid phase peptide synthesis (SPPS) procedure. Before the reaction, the 2chlorotrityl chloride resin was swelled in anhydrous DCM for 30 min. Then, the resin was washed with anhydrous DMF four times. The first Fmoc-protected amino acid was dissolved in DMF in the presence of DIEA. The mixture was added into the reactor and the coupling process was kept for 1 h to ensure the amino acid was fully loaded onto the resin. The resin was washed by DMF four times and the possible unreacted sites of the resin were deactivated by using a block solution made from DCM, methanol, and DIEA with the ratio as DCM/MeOH/DIEA = 80:15:5 two times (30 and 15 min each). After washing with DMF five times, the resin was treated with 20% piperidine in DMF solution two times (30 and 15 min each) to deprotect the Fmoc group and then washed with DMF five times. The second protected amino acid was also dissolved in DMF in the presence of DIEA and the coupling reagent HBTU. The coupling and deprotection processes were operated as described above. The stearyl tail was conjugated by using a mixed solution of stearic acid, DIEA, and HBTU. After all the elongation steps were carried out, the resin was successively washed in DMF, DCM, methanol, and n-hexane. 7424

DOI: 10.1021/acs.langmuir.6b01874 Langmuir 2016, 32, 7420−7426

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Langmuir Elemental analysis for C35H51N2NaO4 calcd (%): C, 71.64; H, 8.76; N, 4.77. Found: C, 69.52; H, 8.70; N, 4.34. Characterization of C17H35CO-L-Phg-L-Phe-ONa ((L, L)-6). FT-IR (KBr, cm−1): 3305 (νN−H, amide A), 2927, 2845 (νC−H, alkyl chain), 1630 (νCO, amide I), 1531 (νN−H, amide II). 1H NMR (400 MHz, DMSO-d6, TMS): δ = 0.85 (t, 3H; J = 6.8 Hz, CH3CH2), 1.08−1.26 (m, 28H; CH3(CH2)14CH2), 1.45 (q, 2H; J = 7.4 Hz, CH2CH2CO), 2.12−2.19 (m, 2H; CH2CH2CO), 2.78−3.09 (m, 2H; PhCH2CH), 3.88−4.01 (m, 1H; CONHCHCOONa), 5.34−5.45 (m, 1H; CONHCHCONH), 6.86−7.30 (m, 10H; 2Ph), 7.59−7.67 (m, 1H; CONHCHCOONa), 8.38−8.46 (m, 1H; CONHCHCO). Elemental analysis for C35H51N2NaO4 calcd (%): C, 71.64; H, 8.76; N, 4.77. Found: C, 69.72; H, 8.81; N, 4.37. Characterization of C17H35CO-L-Phg-D-Phe-ONa ((L, D)-6). FT-IR (KBr, cm−1): 3303 (νN−H, amide A), 2929, 2843 (νC−H, alkyl chain), 1633 (νCO, amide I), 1535 (νN−H, amide II). 1H NMR (400 MHz, DMSO-d6, TMS): δ = 0.85 (t, 3H; J = 6.8 Hz, CH3CH2), 1.08−1.23 (m, 28H; CH3(CH2)14CH2), 1.45 (q, 2H; J = 7.2 Hz, CH2CH2CO), 2.11−2.20 (m, 2H; CH2CH2CO), 2.78−3.08 (m, 2H; PhCH2CH), 3.88−3.99 (m, 1H; CONHCHCOONa), 5.35−5.45 (m, 1H; CONHCHCONH), 6.87−7.30 (m, 10H; 2Ph), 7.53−7.66 (m, 1H; CONHCHCOONa), 8.37−8.45 (m, 1H; CONHCHCO). Elemental analysis for C35H51N2NaO4 calcd (%): C, 71.64; H, 8.76; N, 4.77. Found: C, 69.52; H, 8.82; N, 4.48. Characterization of C17H35CO-D-Phg-L-Phe-ONa ((D, L)-6). FT-IR (KBr, cm−1): 3305 (νN−H, amide A), 2927, 2845 (νC−H, alkyl chain), 1626 (νCO, amide I), 1537 (νN−H, amide II). 1H NMR (400 MHz, DMSO-d6, TMS): δ = 0.85 (t, 3H; J = 7.0 Hz, CH3CH2), 1.07−1.24 (m, 28H; CH3(CH2)14CH2), 1.45 (q, 2H; J = 7.0 Hz, CH2CH2CO), 2.12−2.20 (m, 2H; CH2CH2CO), 2.78−3.08 (m, 2H; PhCH2CH), 3.90−4.03 (m, 1H; CONHCHCOONa), 5.36−5.46 (m, 1H; CONHCHCONH), 6.87−7.30 (m, 10H; 2Ph), 7.60−7.69 (m, 1H; CONHCHCOONa), 8.39−8.47 (m, 1H; CONHCHCO). Elemental analysis for C35H51N2NaO4 calcd (%): C, 71.64; H, 8.76; N, 4.77. Found: C, 69.17; H, 8.80; N, 4.46. Characterization of C17H35CO-D-Phg-D-Phe-ONa ((D, D)-6). FT-IR (KBr, cm−1): 3305 (νN−H, amide A), 2926, 2855 (νC−H, alkyl chain), 1633 (νCO, amide I), 1537 (νN−H, amide II). 1H NMR (400 MHz, DMSO-d6, TMS): δ = 0.84 (t, 3H; J = 6.8 Hz, CH3CH2), 1.07−1.27 (m, 28H; CH3(CH2)14CH2), 1.45 (q, 2H; J = 7.2 Hz, CH2CH2CO), 2.12−2.19 (m, 2H; CH2CH2CO), 2.88−3.07 (m, 2H; PhCH2CH), 3.89−4.01 (m, 1H; CONHCHCOONa), 5.35−5.46 (m, 1H; CONHCHCONH), 6.86−7.31 (m, 10H; 2Ph), 7.59−7.68 (m, 1H; CONHCHCOONa), 8.38−8.46 (m, 1H; CONHCHCO). Elemental analysis for C35H51N2NaO4 calcd (%): C, 71.64; H, 8.76; N, 4.77. Found: C, 69.24; H, 8.82; N, 4.38. Methods. FE-SEM images were obtained using an Hitachi S-4800 instrument (Ibaraki prefecture, Japan) with acceleration voltage of 3.0 kV. Each sample was prepared by smearing the prepared hydrogel on the surface of glass flakes. After evaporating the water, the platinum was sputtered on the surface of the xerogel samples for 30 s to improve the conductivity and prevent the charging effect. CD and UV spectra were obtained by using a Jasco J-815 circular dichroism spectrometer (Tokyo, Japan) under nitrogen atmosphere with the wavelength ranging from 180 to 400 nm, a 0.5 nm interval, a 1.0 nm bandwidth, and an average scanning speed of 50 nm/min at 25.0 °C. 1H NMR spectra were recorded using a Varian NMR 400 spectrometer (Palo Alto, United States). Solution and gel samples were prepared in DMSO-d6 with tetramethylsilane as the internal standard. FT-IR spectra were recorded on a Thermo Scientific Nicolet 6700 spectrometer (Waltham, United States) with a resolution of 2 cm−1 averaged from 64 scans. All of the peptide derivatives were dissolved in deuterium oxide with a concentration of 15 mg·mL−1 and incubated at 25 °C for 12 h to obtain stable gel samples. The gel and solution were injected between two BaF2 wafers with a 0.05 mm optical path and placed in a hot stage. SAXRD patterns were recorded using an X’ PertPro MPD X-ray diffractometer (Almelo, Netherland) with Cu−Kα radiation (1.542 Å) and a Ni filter at room temperature. Dipeptide (15 mg) was dissolved in deionized water at a high temperature. Then, the aqueous solution was pipetted out and carefully smeared onto the

surface of the glass substrate. The pattern was recorded after the water was evaporated. All of the glass flakes were ultrasonicated in nitric acid for 2 h followed by washing with water.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b01874. The FT-IR spectra of the hydrogels and aqueous solutions of the dipeptides and the 1H NMR spectra of the gels (15 g L−1) and solutions (5 g L−1) of the dipeptides in DMSO-d6(PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 86 512 65882052. Tel: 86 512 65880047. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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 (ZXG20145) and the National Natural Science Foundation of China (Nos. 51473106 and 21574095).

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REFERENCES

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