Solvent-Induced Handedness Inversion of Dipeptide Sodium Salts

Jul 15, 2013 - Hongkun Li , Wei Yuan , Hexiang He , Zhihui Cheng , Canlong Fan , Yonggang Yang , Kam Sing Wong , Yongfang Li , Ben Zhong Tang...
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Solvent-Induced Handedness Inversion of Dipeptide Sodium Salts Derived from Alanine Yi Li, Baozong Li, Yitai Fu, Shuwei Lin, and Yonggang Yang* Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Polymer Science and Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, P. R. China S Supporting Information *

ABSTRACT: The relationship between amino acid sequences and their resulting nanostructure has been well studied, but that between amino acid chirality and nanostructure handedness has not. Four dipeptide sodium salts with long alkyl chains derived from L- and D-alanines were synthesized. The behavior of their self-assembly into physical gels in water and THF was studied using field-emission scanning electron microscopy, circular dichroism (CD), FT-IR spectroscopy, 1H NMR spectroscopy, and X-ray diffraction. The dipeptide salts organized into twisted nanoribbons, whose handedness was controlled by the terminal alanine chirality. The handedness of nanoribbons formed in water was opposite to that of those formed in THF. The dipeptide salts self-assembled into similar interdigitated bilayer structures in water and THF, but CD spectra of the gels indicated that the stacking of carbonyl groups was opposite. The formation of this handedness inversion is proposed to arise from the difference in interlayer distance and chiral stacking of carbonyl groups near the C-terminals.

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INTRODUCTION Helical structures are widely observed in biomolecules such as DNA and proteins. Much focus has been on studying the formation and applications of artificial helical oligomers,1−3 polymers,4−9 and organic self-assemblies.10−20 Helicity is important in functionality, and the conformation of helical polymers and chiral organization of small molecules have been widely studied. In artificial polymers, both the “sergeants− soldiers” principle5 and “majority rule”6 are observed. The handedness of these polymers can be controlled using chiral guest molecules. Such chirality transfer and amplification phenomena are usually driven by hydrogen bonding7 and electrostatic8 and van der Waals forces.9 Small molecules can organize into helical nanostructures because of their configuration and conformational chirality and their surrounding conditions. The handedness of small molecule self-assemblies has been tuned by changing substituents and tuned by changing surrounding conditions such as temperature, solvent, pH, stirring direction, and cooling rate.11−13 The self-assembly behavior and applications of peptides have been widely studied.14−25 They generally self-assemble into helical nanostructures through hydrogen bonding. The growth and transition of helical nanostructures have been reported.15 Dipeptides are usually linked with aromatic rings and can then self-assemble into straight or twisted nanoribbons through electrostatic interaction, hydrogen bonding, and π−π stacking. The relationship between amino acid sequence and nanostructure has been well studied,19 but the relationship between amino acid chirality and nanostructure handedness has not.13,20 © 2013 American Chemical Society

A series of dipeptide sodium salts derived from L- and Dalanines were recently synthesized and could self-assemble into twisted nanoribbons in water.20 The nanoribbon handedness could be controlled by the terminal alanine chirality. Herein, we study the relationship between nanostructure handedness and solvent. Another series of dipeptide sodium salts derived from L- and D-alanines is synthesized. These selfassemble into twisted nanoribbons in water and THF. The nanoribbon handedness is controlled by the alanine terminal chirality. However, the handedness of nanoribbons formed in water is opposite to that in THF.

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RESULTS AND DISCUSSION The molecular structures of the dipeptides are shown in Figure 1. They can gelate in water, toluene, THF, DMSO, DMF, 1,4dioxane, and cyclohexanone at 25 °C. However, they are highly soluble in methanol, ethanol, and n-propanol. Figure 2 shows FE-SEM images of LMWG xerogels, prepared from viscous aqueous solutions at concentrations of 2.0 g L−1. Twisted nanoribbons are apparent, which are 10−50 nm wide, 20−30 nm thick, and ca. 130 nm in helical pitch. The handedness of nanoribbons of (L,L)-2 and (D,L)-2 is left (Figure 2a,c), and that of nanoribbons of (L,D)-2 and (D,D)-2 is right (Figure 2b,d). The FE-SEM images indicate that the handedness of these organic self-assemblies is controlled by the alanine terminal Received: June 8, 2013 Revised: July 15, 2013 Published: July 15, 2013 9721

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Figure 1. Molecular structures of the LMWGs.

Figure 3. FE-SEM images of LMWG xerogels prepared from organogels in THF at concentrations of 30 g L−1: (a) (L,L)-2; (b) (L,D)-2; (c) (D,L)-2; (d) (D,D)-2.

were 9:1 and 7:3, right-handed twisted nanoribbons were identified, which are similar as those prepared in pure water (Figure 2d). When the H2O/THF volume ratios were 5:5 and 3:7, membranes and shrank particles were identified, respectively. No helical morphologies were identified. When the H2O/THF volume ratio was 1:9, straight belts were identified. The results shown here indicated that the LMWGs are more feasible to form helical morphologies in water. The hydrophobic association among the alkyl chains was proposed to play an important role in the formation of the helical morphologies. To study the chiral organization of the LMWGs in water and THF, CD and UV absorption spectra were measured at concentrations of 30.0 g L−1 at 25 °C (Figure 4). UV absorption bands of the hydrogels are observed at 195 nm, which originate from carbonyl groups. λθ=0 values at 195 and 216 nm are observed in the CD spectra. The CD signs of (L,L)2 and (D,L)-2 at 236 nm are negative, indicating that carbonyl groups stack in left-handedness. Carbonyl groups of (L,D)-2 and (D,D)-2 stack in right-handedness.26 The CD spectral data are consistent with the FE-SEM images (Figure 2), confirming that the handedness of the organic self-assemblies is controlled by the terminal alanine chirality. THF absorbs strongly in the UV region at short wavelengths, so UV absorption and CD spectra of the organogels in THF were measured at 200−400 nm. The CD signs of (L,L)-2 and (D,L)-2 at 217 nm are negative, while those of (L,D)-2 and (D,D)-2 at 217 nm are positive, indicating that the terminal alanine chirality is important in controlling the organic self-assembly handedness. Comparison of Figures 4a and 4b suggests that the LMWGs should organize into different structures upon changing solvent. The CD sign for the (L,L)-2 hydrogel at 202 nm is positive (Figure 4a), while that of the organogel in THF at 217 nm is negative (Figure 4b), indicating the carbonyl groups organize in opposite handedness in water and THF.

Figure 2. FE-SEM images of LMWG xerogels prepared from viscous aqueous solutions at concentrations of 2.0 g L−1: (a) (L,L)-2; (b) (L,D)2; (c) (D,L)-2; (d) (D,D)-2.

chirality. The molecular structures of the LMWGs are shown in Figure 1. Hydrogen bonding and electrostatic interaction of the terminal alanine residues are expected to play an important role in controlling the handedness of the organic self-assemblies. Similar results have been previously observed. For example, when the alkyl chain was changed to n-C17H35-, the dipeptide self-assembly handedness was controlled by the terminal alanine chirality.20 Single-handed twisted nanoribbons are also apparent in FESEM images of xerogels prepared from organogels in THF at concentrations of 30 g L−1. Nanoribbons are 50−200 nm wide, 20−30 nm thick, and 100−600 nm in helical pitch (Figure 3). The helical pitch increases with increasing nanoribbon width, and these nanoribbons are larger than those formed in water, perhaps because they are formed at higher concentration. The handedness of nanoribbons of (L,L)-2 and (D,L)-2 is right (Figure 3a,c), while that of nanoribbons of (L,D)-2 and (D,D)-2 is left (Figure 3b,d). Nanoribbon handedness is controlled by the terminal alanine chirality, and the handedness is opposite to that of those formed in water. The FESEM images of the xerogels of (D,D)-2 prepared at different H2O/THF volume ratios are shown in Figure S1 (Supporting Information). When the H2O/THF volume ratios 9722

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formed at 70 °C and hydrogels at 25 °C. For aqueous (L,L)-2, the amide group absorbed at 3427 (νN−H) and 1633 cm−1 (νCO) and the carboxylate group at 1591 cm−1 (νCO).27,28 For the (L,L)-2 hydrogel, absorption bands were observed at 3407 (νN−H) and 1617 cm−1 (νCO), and the carboxylate group absorption was at 1587 cm−1 (νCO). Wavenumber shifts of the amide and carboxylate absorptions indicate that intermolecular hydrogen-bonding and electrostatic interaction affects the stacking of molecules. Hydrophobic association among alkyl chains in the hydrogel is also identified from wavenumber shifts between 2800 and 2950 cm−1. For aqueous (L,L)-2, absorption bands at 2925 and 2854 cm−1 indicate a gauche form alkyl chain.27 In the hydrogel spectrum, these bands shift to 2917 and 2849 cm−1, indicating an all-trans form alkyl chain.27 In the FT-IR spectrum of (L,L)-2 in the solid state, absorptions are observed at 2918 and 2849 cm−1. Thus, the alkyl chain conformations in the solid and hydrogel states are similar. The FT-IR spectra of (L,L)-2 and (D,L)-2 hydrogels are similar, as are those of (L,L)-2 and (D,L)-2 aqueous solutions (Figure 5a). Therefore, the driving forces for gelation are the same. 1 H NMR spectra were recorded to better understand the hydrogen bonding. Hydrogen−deuterium exchange made it unfeasible to observe amide groups in the spectra, even in a 9:1 volume ratio of H2O:D2O. To understand hydrogen bonding among the molecules in THF, 1H NMR spectra were recorded in the gel (30.0 g L−1) and solution (4.0 g L−1) states in THFd8 (Figure 6). For the (L,L)-2 solution, two amide protons form

Figure 4. CD spectra of (a) hydrogels and (b) organogels in THF at concentrations of 30 g L−1.

To further understand the organization of the LMWGs in water and THF, FT-IR spectra of (L,L)-2 and (D,L)-2 were recorded in the gel and solution states at concentrations of 30.0 g L−1 in D2O and THF-d8 (Figure 5). Aqueous solutions were

Figure 6. 1H NMR spectra of (a) (L,L)-2 and (D,L)-2 gels (30 g L−1) and solutions (5.0 g L−1) in DMSO-d6 and (b) (L,L)-2 and (D,L)-2 gels (30 g L−1) and solutions (4.0 g L−1) in THF-d8.

hydrogen bonds with THF-d8 and are observed at 7.65 and 7.89 ppm for H−N near the terminal and alkyl chain, respectively. Upon forming the gel, two protons are observed at 7.24 and 7.65 ppm. The chemical shifts move upfield, indicating hydrogen bonding among the molecules, and similar results are found for the (D,L)-2 solution and gel. Upon forming the gel, the chemical shift of the H−N near the terminal moves further downfield than that of H−N near the alkyl chain. Thus, hydrogen bonding of the H−N near the terminal appears to be important in controlling the handedness of the organic selfassemblies. SAXRD patterns of the xerogels of the LMWGs (L,L)-2, (L,D)-2, (D,L)-2, and (D,D)-2 prepared at concentrations of 30.0 g L−1 in water were recorded to investigate the organization of the molecules (Figure 7a). The SAXRD patterns of the four xerogels are nearly identical, and peaks at 2θ of 3.06° originate from glass flakes.20 The samples exhibit three diffraction peaks

Figure 5. FT-IR spectra of (a) solutions at 70 °C and hydrogels at 25 °C in D2O and (b) solutions at 40 °C and organogels at 25 °C in THF-d8, at concentrations of 30 g L−1. 9723

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Figure 7. SAXRD patterns of xerogels prepared from (a) hydrogels and (b) organogels in THF.

Figure 8. Formation of the interdigitated bilayer structure.

organization of the organic self-assemblies shown in Figure 8. A similar packing model has been also suggested by Cui et al.11

at 2θ of 2.46°, 4.85°, and 7.18°, indicating a lamellar structure with d-spacing of 3.6 nm. Hydrophobic association among the alkyl chains is proposed to be important in the formation of this lamellar structure. The gelators are expected to organize in an interdigitated bilayer structure.28 SAXRD patterns of xerogels prepared at concentrations of 30.0 g L−1 in THF are shown in Figure 7b and are nearly identical. Three diffraction peaks at 2θ of 2.25°, 4.58°, and 6.66° indicate a lamellar structure with dspacing of 3.9 nm. The interlayer distance of the organic selfassemblies formed in THF is longer than that of those formed in water. Thus, the LMWGs organized into different structures in different solvents. Recent results have also indicated that the handedness of the twisted nanoribbons was controlled by the terminal alanine chirality.20 The chiral stacking of the amide group and electrostatic interaction of the carboxylate group at the terminal play important roles in the handedness of the nanoribbons. A proposed mechanism for the origin of the handedness in water and THF is shown Figure 8. The LMWG (L,L)-2 forms interdigitated bilayer structures in the solvents. However, SAXRD indicates that the interlayer distance of the organic selfassembly formed in THF is longer than that of the one formed in water. Alkanes are highly soluble in THF, but the FT-IR spectra of the organogels in THF indicates that THF did not penetrate into the alkyl chain layers.27 The handedness of a series of bola-type gelators was shown to be tunable by changing the length of the central alkylene group.30,31 A pair of LMWG (L,L)-2 molecules can be considered as a basic building block. As the lengths of the central hydrophobic regions differ, so too does the formation of hydrogen bonding between the building blocks. CD spectra show that the stacking of carbonyl groups is opposite in THF and water (Figure 4), so it is reasonable that the organization of the basic building blocks is also opposite. It was proposed that the handedness inversion of the nanoribbons shown in Figures 2 and 3 was driven by the

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CONCLUSIONS Four dipeptide sodium salts with long alkyl chains derived from L- and D-alanines were synthesized. In water and THF, they organized into twisted nanoribbons, whose handedness was controlled by the terminal alanine chirality. The nanoribbon handedness formed in water was opposite to that of those formed in THF. The change in interlayer distance between different solvents was proposed to drive the opposing organization of the basic building blocks. This study aids our understanding of the chirality−structure relationship and the chiral organization of dipeptides.

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EXPERIMENTAL SECTION

Materials. Stearic acid, methanol, sodium hydroxide, and alanines were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. Characterization of C15H31CO-L-Ala-L-Ala-ONa ((L,L)-2). FT-IR (KBr, cm−1): 3300 (vN−H, amide A), 1634 (vCO, amide I), 1535 (vN−H, amide II). 1H NMR (400 MHz, DMSO-d6, TMS, 25 °C): δ = 0.85 (t, 3H; J = 7.0 Hz, CH3(CH2)12CH2), 1.12−1.23 (m, 30H; CH3(CH2)12CH2, 2CH3CH), 1.48 (t, 2H; J = 6.8 Hz, CH2CH2CO), 2.09 (t, 2H; J = 8.8 Hz, CH2CH2CO), 3.61−3.66 (m, 1H; CONHCHCOONa), 4.17−4.21 (m, 1H; CONHCHCONH), 7.50− 7.54 (m, 1H, CONHCHCOONa), 8.10 (d, 1H, J = 6.6 Hz, CONHCHCONH). Elemental analysis for C15H31CO-L-Ala-L-AlaONa·1/2H2O calcd (%): C, 61.51; H, 9.85; N, 6.52. Found: C, 61.21; H, 10.35; N, 6.32. Characterization of C15H31CO-L-Ala-D-Ala-ONa ((L,D)-2). FT-IR (KBr, cm−1): 3300 (vN−H, amide A), 1634 (vCO, amide I), 1535 (vN−H, amide II). 1H NMR (400 MHz, DMSO-d6, TMS, 25 °C): δ = 0.85 (t, 3H; J = 6.6 Hz, CH3(CH2)12CH2), 1.12−1.24 (m, 30H; CH3(CH2)12CH2, 2CH3CH), 1.48 (t, 2H; J = 7.0 Hz, CH2CH2CO), 2.09 (t, 2H; J = 7.4 Hz, CH2CH2CO), 3.64−3.69 (m, 1H; CONHCHCOONa), 4.16−4.22 (m, 1H; CONHCHCONH), 7.48− 7.52 (m, 1H, CONHCHCOONa), 8.09 (d, 1H, J = 6.0 Hz, 9724

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Notes

CONHCHCONH). Elemental analysis for C15H31CO-L-Ala-D-AlaONa·H2O calcd (%): C, 60.25; H, 9.88; N, 6.39. Found: C, 60.37; H, 10.27; N, 6.18. Characterization of C15H31CO-D-Ala-L-Ala-ONa ((D,L)-2). FT-IR (KBr, cm−1): 3300 (vN−H, amide A), 1635 (vCO, amide I), 1535 (vN−H, amide II). 1H NMR (400 MHz, DMSO-d6, TMS, 25 °C): δ = 0.85 (t, 3H; J = 6.6 Hz, CH3(CH2)12CH2), 1.12−1.23 (m, 30H; CH3(CH2)12CH2, 2CH3CH), 1.48 (t, 2H; J = 6.4 Hz, CH2CH2CO), 2.09 (t, 2H; J = 9.4 Hz, CH2CH2CO), 3.60−3.64 (m, 1H; CONHCHCOONa), 4.16−4.20 (m, 1H; CONHCHCONH), 7.49− 7.54 (m, 1H, CONHCHCOONa), 8.10 (d, 1H, J = 8.0 Hz, CONHCHCONH). Elemental analysis for C15H31CO-D-Ala-L-AlaONa·1/2H2O calcd (%): C, 61.51; H, 9.85; N, 6.52. Found: C, 61.22; H, 9.87; N, 6.38. Characterization of C15H31CO-D-Ala-D-Ala-ONa ((D,D)-2). FT-IR (KBr, cm−1): 3300 (vN−H, amide A), 1634 (vCO, amide I), 1535 (vN−H, amide II). 1H NMR (400 MHz, DMSO-d6, TMS, 25 °C): δ = 0.85 (t, 3H; J = 7.0 Hz, CH3(CH2)12CH2), 1.12−1.23 (m, 30H; CH3(CH2)12CH2, 2CH3CH), 1.48 (t, 2H; J = 6.8 Hz, CH2CH2CO), 2.09 (t, 2H; J = 8.8 Hz, CH2CH2CO), 3.61−3.66 (m, 1H; CONHCHCOONa), 4.17−4.21 (m, 1H; CONHCHCONH), 7.50− 7.54 (m, 1H, CONHCHCOONa), 8.10 (d, 1H, J = 6.6 Hz, CONHCHCONH). Elemental analysis for C15H31CO-D-Ala-D-AlaONa·H2O calcd (%):C, 60.25; H, 9.88; N, 6.39. Found: C, 60.48; H, 10.49; N, 6.29. Methods. FT-IR spectra of LMWGs were recorded from KBr pellets of solids, on a Nicolet 6700 FT-IR spectrometer over the 4000−400 cm−1 range, with a resolution of 2.0 cm−1 and after accumulation of 32 scans. FT-IR spectra of gels and solutions in D2O were recorded from CaF2 windows, with a 50 μm spacer within a hot stage. A D2O spectrum was used as the background and subtracted from all sample spectra. FT-IR spectra of gels and solutions in THF-d8 were recorded from CaF2 windows, with a 50 μm spacer within a hot stage. A THF-d8 spectrum was also recorded but not subtracted from other spectra. 1H NMR spectra were recorded using a Varian NMR 400 spectrometer, in DMSO-d6 and THF-d8 solutions or gels using tetramethylsilane (TMS) as an internal standard. Elemental analysis was performed on a PerkinElmer series II CHNS/O analyzer 2400. Field emission scanning electron microscopy (FESEM) images were taken on a Hitachi S-4800 operating at 3.0 kV. Viscous aqueous solutions were prepared at concentrations of 2.0 g L−1. FE-SEM samples were prepared by dropping solutions on microscopy cover glass. Samples were dried in air and then coated with platinum for 25 s at 3.0 kV and 10 μA. THF organogel samples were prepared at concentrations of 30 g L−1. FE-SEM samples were prepared by placing a small amount of the gels on microscopy cover glass. Samples were dried in air and then coated with platinum for 45 s at 3.0 kV and 10 μA. Small-angle X-ray diffraction (SAXRD) patterns were taken on an X′Pert-Pro MPD X-ray diffractometer using Cu Kα radiation with a Ni filter (1.542 Å). Hydrogels and organogels were prepared at concentrations of 30 g L−1. Samples were prepared by drying the gels on the surface of microscopy cover glass. The CD spectra were obtained using an AVIV 410 spectrophotometer with a 1.0 nm bandwidth, 1 scan, and an averaging time of 0.1 s at 25 °C using a 0.01 mm cell. Hydrogels and organogels were prepared at concentrations of 30 g L−1.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of Jiangsu Province (No. BK2011354), the Priority Academic Program Development of Jiangsu High Education Institutions (PAPD), and the National Natural Science Foundation of China (No. 21104053, 21071103, and 21074086).

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ASSOCIATED CONTENT

S Supporting Information *

FESEM images of the xerogels prepared using (D,D)-2 at different H2O/THF volume ratios. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Fax 86 512 65882052; Tel 86 512 65880047; e-mail ygyang@ suda.edu.cn (Y.Y.). 9725

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dx.doi.org/10.1021/la402174w | Langmuir 2013, 29, 9721−9726