Aggregation-Induced Chirality: Twist and Stacking Handedness of the

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Aggregation-Induced Chirality: Twist and Stacking Handedness of the Biphenylene Groups of n-C12H25O-BP-CO-Ala-Ala Dipeptides Lianglin Zhang, Jiaming Qin, Shuwei Lin, Yi Li, Baozong Li, and Yonggang Yang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b02576 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017

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Aggregation-Induced Chirality: Twist and Stacking Handedness of the Biphenylene Groups of nC12H25O-BP-CO-Ala-Ala Dipeptides Lianglin Zhang, Jiaming Qin, 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, P.R. China.

In mixtures of water and dimethyl sulfoxide, 4ʹ-(n-dodecyloxy)-1,1ʹ-biphenyl-4-carbonyl Ala– Ala dipeptides can self-assemble into tubular structures that are formed by coiled nanoribbons. The twist and stacking handedness of biphenylene groups were studied using circular dichroism and confirmed by theoretical chemical calculations. The handedness of the coiled nanoribbons and the stacking handedness of the biphenylene groups are controlled by the chirality of the alanine at the C-terminus, while the twist handedness of the biphenylene is determined by the chirality of the alanine at the N-terminus. The 1H NMR spectra indicated that the hydrogen bond formed by the N–H group of alanine at the N-terminus plays an important role in the formation

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of organic self-assemblies. Based on small-angle X-ray scattering characterization, a dimer structure was proposed to form through the terminal COOH groups.


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INTRODUCTION The chiral transfer phenomenon has been widely studied in chemistry, biochemistry and materials science. Chiral transfer is well known in chiral compound synthesis and asymmetric catalysis. For helical copolymers, it has also been found that a small number of chiral monomers could drive the whole polymer chain to adopt a single-handedness.1,2 This chiral transfer proceeds through covalent bonding and steric hindrance. The chirality of the metal-organic frameworks can also be controlled by adding chiral dopants.3,4 This chiral transfer generally progresses through non-covalent interactions. Helical metal oxide nanotubes have also been prepared by transcribing the helical morphologies of organic self-assemblies.5,6 This chiral morphological transfer is caused by H-bonding and/or electrostatic interactions. The optical activity of the nanotubes has been proposed to originate from the chiral defects formed by the organic self-assemblies.5 4,4ʹ-Biphenylene-bridged polybissilsesquioxane nanotubes have also been prepared through this external templating approach.7 Both the twist and stacking handedness of the biphenylene groups can be controlled. Aggregation-induced stacking chirality has been found in helical polymers and supramolecular assemblies. Chiral polymeric aggregates8,9 and organic self-assemblies10,11 are formed by the packing of helical polymer main chains and low-molecular-weight compounds, respectively. Chiral stacking of the chromophores can be identified from circular dichroism (CD) and vibrational CD spectra.12 To date, the chiral stacking of carbonyl and aromatic groups have been well studied.7,12,13 Aggregation can also induce conformational chirality. However, the research on this phenomenon is limited. The chiral conformations of tetraphenylethene in aggregates and single crystals have been preliminary reported.14,15 Fluorocarbon chains adopt helical conformations at room temperature.16 The handedness of the fluorocarbon chain has been controlled in chiral aggregates.17,18 The twist handedness of biphenyl groups should also be controllable in chiral aggregates.19 However, this has not been well studied.


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Lipopeptides can self-assemble into twisted nanoribbons, coiled nanoribbons and nanotubes,2025

and may find applications as biomaterials.26-29 Recently, the self-assembly behavior of several

series of homogeneous lipodipeptide sodium salts with different stereochemical sequences were studied.30,31 The handedness of the self-assemblies was dominated by the chirality of the amino acid at the C-terminus. Hydrogen bonds, formed by the N-H group of the amino acid at the Cterminus, were shown to play an important role in controlling the molecular stacking.32 This “Cterminal determination” rule was also identified in a series of tetrapeptides.33 These lipodipeptides have been used for nanomaterial preparation.34 Biphenyl usually adopts twist conformations with twist angles of 30–45°. Since the handedness is sensitive to the surrounding chiral conditions, one of its derivatives has been used to detect the chirality of the solid surfaces.35 Herein, a biphenyl group is linked to an Ala-Ala dipeptide through covalent bonding. The obtained compounds can self-assemble into tubes on the nano or micro scale. It was found that the twist and stacking handedness of the biphenyl groups were controlled by the chirality of the alanines at the N- and C-termini, respectively.

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

(Fmoc-D-Ala-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),

n-propanol,

1,4-dioxane,

n-hexane,

cyclohexane,

ethyl

acetate,

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, and THF was treated with sodium and then redistilled. Deionized water, trifluoroacetic acid, ethanol, diethyl ether, toluene, dimethyl sulfoxide (DMSO), deuterium oxide


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(D2O) and DMSO-d6 were purchased from Sinopharm Chemical Reagent Co., Ltd. 1Bromododecane was obtained from Shanghai Aladdin Bio-Chem Technology Co., Ltd. 4ʹHydroxy-1,1ʹ-biphenyl-4-carboxylic acid was purchased from Shanghai Macklin Biochemical Co., Ltd. Synthesis of the heterogeneous dipeptides. C12H25O-BP-COOH was synthesized according to the literature.36 All the four dipeptides were synthesized following the manual solid phase peptide synthesis (SPPS) procedure, which has already been reported by our group (Supporting Information).37 Characterization of n-C12H25O-BP-CO-L-Ala-L-Ala-OH ((L, L)-13). FT-IR (KBr, cm-1): 3286 (νN-H, amide A), 2923, 2852 (νC-H, methylene), 1716, (νC=O, -COOH), 1633 (νC=O, amide I), 1542 (νN-H, amide II). 1H-NMR (400 MHz, DMSO-d6, TMS, 25 °C, δ): 0.85 (t, 3H; J = 6.2 Hz, CH3CH2), 1.13 – 1.51 (m, 24H; CH3(CH2)9CH2CH2O, 2CH3CH), 1.67 – 1.78 (m, 2H; (CH2)9CH2CH2O) 4.01 (t, 2H; J = 6.2 Hz, (CH2)9CH2CH2O), 4.22 (q, 1H; J = 7.2 Hz , CH3CHCOOH), 4.54 (q, 1H; J = 7.1 Hz ,CH3CHCONH), 7.03 (d, 2H; J = 7.7 Hz, Ar-H), 7.62– 7.75 (m, 4H; Ar-H), 7.95 (d, 2H; J = 7.2 Hz, Ar-H), 8.20 (d, 1H; J = 7.0 Hz, CONHCHCONH), 8.48 (d, 1H; J = 7.0 Hz, NHCHCOOH), 12.50 (s, 1H; NHCHCOOH). Elemental analysis calcd (%): C, 70.96; H, 8.45; N, 5.34. Found: C, 69.77; H, 8.28; N, 5.24. Characterization of n-C12H25O-BP-CO-L-Ala-D-Ala-OH ((L, D)-13). FT-IR (KBr, cm-1): 3284 (νN-H, amide A), 2921, 2852 (νC-H, methylene), 1716, (νC=O, -COOH), 1634 (νC=O, amide I), 1542 (νN-H, amide II). 1H-NMR (400 MHz, DMSO-d6, TMS, 25 °C, δ): 0.85 (t, 3H; J = 6.2 Hz, CH3CH2), 1.13 – 1.51 (m, 24H; CH3(CH2)9CH2CH2O, 2CH3CH), 1.67 – 1.77 (m, 2H; (CH2)9CH2CH2O) 4.01 (t, 2H; J = 6.2 Hz, (CH2)9CH2CH2O), 4.24 (q, 1H; J = 7.3 Hz,CH3CHCOOH), 4.55 (q, 1H; J = 7.2 Hz ,CH3CHCONH), 7.03 (d, 2H; J = 7.7 Hz, Ar-H), 7.63-7.75 (m, 4H; Ar-H), 7.95 (d, 2H; J = 7.2 Hz, Ar-H), 8.13 (d, 1H; J = 7.0 Hz,


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CONHCHCONH), 8.42 (d, 1H; J = 7.0 Hz, NHCHCOOH), 12.50 (s, 1H; NHCHCOOH). Elemental analysis calcd (%): C, 70.96; H, 8.45; N, 5.34. Found: C, 70.42; H, 8.26; N, 5.30. Characterization of n-C12H25O-BP-CO-D-Ala-L-Ala-OH ((D, L)-13). FT-IR (KBr, cm-1): 3284 (νN-H, amide A), 2922, 2852 (νC-H, methylene), 1715, (νC=O, -COOH), 1632 (νC=O, amide I), 1542 (νN-H, amide II). 1H-NMR (400 MHz, DMSO-d6, TMS, 25 °C, δ): 0.85 (t, 3H; J = 6.2 Hz, CH3CH2), 1.09 – 1.51 (m, 24H; CH3(CH2)9CH2CH2O, 2CH3CH), 1.67 – 1.77 (m, 2H; (CH2)9CH2CH2O) 4.01 (t, 2H; J = 6.2 Hz, (CH2)9CH2CH2O), 4.23 (q, 1H; J = 7.3 Hz, CH3CHCOOH), 4.57 (q, 1H; J = 7.3 Hz,CH3CHCONH), 7.02 (d, 2H; J = 7.7 Hz, Ar-H), 7.63– 7.76 (m, 4H; Ar-H), 7.95 (d, 2H; J = 7.2 Hz, Ar-H), 8.16 (d, 1H; J = 7.0 Hz, CONHCHCONH), 8.44 (d, 1H; J = 7.0 Hz, NHCHCOOH), 12.51 (s, 1H; NHCHCOOH). Elemental analysis calcd (%): C, 70.96; H, 8.45; N, 5.34. Found: C, 70.45; H, 8.18; N, 5.21. Characterization of n-C12H25O-BP-CO-D-Ala-Ala-OH ((D, D)-13). FT-IR (KBr, cm-1): 3285 (νN-H, amide A), 2921, 2851 (νC-H, methylene), 1714, (νC=O, -COOH), 1634 (νC=O, amide I), 1541 (νN-H, amide II). 1H-NMR (400 MHz, DMSO-d6, TMS, 25 °C, δ): 0.85 (t, 3H; J = 6.2 Hz, CH3CH2), 1.13 – 1.51 (m, 24H; CH3(CH2)9CH2CH2O, 2CH3CH), 1.67 – 1.77 (m, 2H; (CH2)9CH2CH2O) 4.01 (t, 2H; J = 6.2 Hz, (CH2)9CH2CH2O), 4.23 (q, 1H; J = 7.2 Hz,CH3CHCOOH), 4.54 (q, 1H; J = 7.2 Hz,CH3CHCONH), 7.02 (d, 2H; J = 7.7 Hz, Ar-H), 7.63 – 7.75 (m, 4H; Ar-H), 7.94 (d, 2H; J = 7.2 Hz, Ar-H), 8.16 (d, 1H; J = 7.0 Hz, CONHCHCONH), 8.44 (d, 1H; J = 7.0 Hz, NHCHCOOH), 12.48 (s, 1H; NHCHCOOH). Elemental analysis calcd (%): C, 70.96; H, 8.45; N, 5.34. Found: C, 70.57; H, 8.18; N, 5.13. Methods. The methods are provided in the Supporting Information.

RESULTS AND DISCUSSION


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Figure 1. Molecular structures of the dipeptides.

Figure 2. FE-SEM images of the nanotubes of (a) (L, L)-13 and (b) (D, D)-13 prepared in a mixture of DMSO and water (9/1, v/v) at a concentration of 2.0 g L-1. FE-SEM images of the microtubes and nanoribbons of (c) (L, D)-13 and (d) (D, L)-13 prepared in a mixture of DMSO and water (8/2, v/v) at a concentration of 2.0 g L-1. Figure 1 shows the molecular structures of the dipeptides consisting of alanine residues. The dipeptides can be dissolved in hot n-propanol, DMSO, THF and 1,4-dioxane at a concentration of 30 g L-1 and precipitate out at 25 °C. They are insoluble in water, acetone, DCM, toluene, n-


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hexane, cyclohexane and ethyl acetate. Although (L, L)-13 and (D, D)-13 were insoluble in a mixture of DMSO and water (8/2, v/v) at a concentration of 2.0 g L-1, they could dissolve in a mixture of DMSO and water (9/1, v/v) at a concentration of 2.0 g L-1 and high temperature. After cooling to 25 °C, they self-assembled into nanotubes (Figure 2a and b). The transmission electron microscopy (TEM) image of (L, L)-13 indicated that the wall thickness was approximately 36 nm (Figure 3a). A few twisted and coiled nanoribbons were also identified in the field-emission scanning electron microscopy (FE-SEM) images (Figure S1, Supporting Information). The nanotubes prepared using (L, L)-13 and (D, D)-13 should be formed by rightand left-handed coiled nanoribbons, respectively. It was reported that C13H27CO-L-Ala-L-Ala self-assembled into left-handed helical nanofibers.25 The handedness is opposite to that of (L, L)13. Therefore, interactions among biphenylene groups should play an important role in the formation of organic self-assemblies.

Figure 3. TEM images of (a) the nanotubes of (L, L)-13 prepared in a mixture of DMSO and water (9/1, v/v) at a concentration of 2.0 g L-1 and (b) a microtube of (L, D)-13 prepared in a mixture of DMSO and water (8/2, v/v) at a concentration of 2.0 g L-1. Although (L, D)-13 and (D, L)-13 formed homogeneous solutions in a mixture of DMSO and water (9/1, v/v) at a concentration of 2.0 g L-1, they could self-assemble into nanoribbons in a


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mixture of DMSO and water (8/2, v/v) at a concentration of 2.0 g L-1, and a few microtubes were also identified (Figure 2c and d). The microtubes prepared using (L, D)-13 and (D, L)-13 were formed by left- and right-handed coiled nanoribbons, respectively. The wall thickness of the microtubes was approximately 100 nm (Figure 3b). The FE-SEM images, shown in Figure 2, also indicated that the handedness of the nanoribbons was controlled by the chirality of the alanine at the C-terminus. A similar phenomenon has also been found in several series of homogeneous lipodipeptides.30,31 The molecular packing structure and the hydrogen-bond formed by the N-H group of the amino acid at the C-terminus were proposed to drive this “Cterminal determination” rule.

Figure 4. (a) CD and UV-Vis spectra of the suspensions prepared in a mixture of DMSO and water (9/1, v/v) at a concentration of 2.0 g L-1 and 25 °C, and (b) CD and UV-Vis spectra of the suspensions prepared in a mixture of DMSO and water (8/2, v/v) at a concentration of 2.0 g L-1 and 25 °C. CD and ultraviolet-visible (UV-Vis) spectra of the suspensions were recorded at a concentration of 2.0 g L-1 and 25 °C using a 2.0-mm pathlength cell (Figure 4). The UV


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absorption bands at 250–340 nm originated from the biphenylene group. Owing to the UV absorption of DMSO, the absorption bands of the carbonyl groups of the dipeptides could not be identified. (L, L)-13 and (D, D)-13 showed opposite CD signals (Figure 4a). For (L, L)-13, two positive CD signals at 380 and 258 nm, and three negative CD signals at 317, 303 and 278 nm were identified. The neighboring biphenylene groups should exhibit π-π interactions. Since the first CD signal at 380 nm was positive, the biphenylene groups should stack in a right-handed fashion.7 The negative CD signal at 317 nm originated from the right-handed twisted biphenylene group. (L, D)-13 and (D, L)-13 also showed opposite CD signals (Figure 4b). Broad CD signals were identified at 340–600 nm. The CD spectrum of (L, D)-13 also showed two positive signals at 329 and 254 nm, and two negative signals at 319 and 277 nm. The broad and negative CD signal at a long wavelength originated from left-handed stacked biphenylene rings. The negative CD signal at 319 nm originated from the right-handed twisted biphenylene group. Table 1. Twist of the biphenylene group, stacking handedness of the biphenylene groups and the handedness of the nanoribbons. Sample (L, L)-13 (D, D)-13 (L, D)-13 (D, L)-13

Twist Right Left Right Left

Stacking Right Left Left Right

Nanoribbon Right Left Left Right

The twist of the biphenylene group, stacking handedness of the biphenylene groups and the handedness of the nanoribbons are summarized in Table 1. The relationships among three levels chirality, including molecular chirality of the dipeptide, supramolecular chirality of the stacked biphenylene groups, and morphological chirality of the nanoribbons, are clearly shown in this table. It seems that the conformational chirality of the biphenylene group is determined by the molecular chirality of the neighboring amino acid, while the supramolecular chirality based on the stacking of the biphenylene groups should be determined by the molecular chirality of the


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terminal amino acid. Moreover, the handedness of the nanotube is also determined by the molecular chirality of the terminal amino acid.

Figure 5. TD-DFT simulated CD spectra of a N-methyl 4'-methoxy-1,1'-biphenyl-4-amide dimer. (a) Twisted and stacked in a right-handed fashion, and (b) twisted in right-handed and stacked in left-handed fashion. To confirm the origin of the CD signals, the CD spectra of the self-assemblies of (L, L)-13 and (L, D)-13 at a long wavelength were simulated using a N-methyl 4'-methoxy-1,1'-biphenyl-4amide dimer. The optimized conformation of N-methyl 4'-methoxy-1,1'-biphenyl-4-amide was calculated using time-dependent density functional theory calculations (TD-DFT) with B3LYP/6-31G(d) (Figure S2, Supporting Information).38 The twist angle of the biphenylene group was 36.3°. The stacking angle of the dimer was fixed at 3°. The calculated CD spectra were based on the 100 lowest energy singlet excited states (Figure 5). The highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) are shown in Figures S3 and S4, Supporting Information. The different orbital colors indicate the different


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wavefunction phases. When the biphenylene groups twist and stack in a right-handed fashion, one positive signal at 394 nm and two negative signals at 351 and 284 nm are observed (Figure 5a). This spectrum is similar as that of the self-assembled (L, L)-13. The signals at 394, 351 and 284 nm are mainly caused by the transitions from HOMO to LUMO, HOMO-1 to LUMO and HOMO-1 to LUMO+1, respectively (Figure S3, Supporting Information). When the biphenylene groups twist in a right-handed manner and stack in a left-handed way, one positive signal at 289 nm and three negative signals at 351, 311 and 268 nm are found (Figure 5b). This spectrum is similar as that of the self-assembled (L, D)-13. The signals at 351, 311, 289 and 268 nm are mainly caused by the transitions from HOMO to LUMO, HOMO-2 to LUMO, HOMO-3 to LUMO and HOMO-2 to LUMO+1, respectively (Figure S4, Supporting Information). The electron transitions between neighboring biphenylene groups and within biphenylene groups are clearly identified. The organization and simulated CD spectra of the enantiomers of the dimmers were shown in Figure S5 and S6, Supporting Information. Exactly opposite CD signals were obtained.

Figure 6. FT-IR spectra of the (L, L)-13 and (L, D)-13 assemblies. (a) 2800–3800 cm-1 and (b) 1500–1800 cm-1.


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Fourier transformed infrared (FT-IR) spectra of the xerogels of (L, L)-13 and (L, D)-13 were shown in Figure 6, which were almost same. In the case of the (L, L)-13, the absorption band of OH groups were identified at 3462 cm-1, indicating that free COOH groups should exist at the surfaces of the nanotubes (Figure 6a). The absorption band of C=O groups at 1744 cm-1 also indicated the existence of free COOH groups (Figure 6b). The absorption band of C=O groups at 1728 cm-1 originated from the COOH dimmers.39,40 it could be observed that the absorption bands and appeared at 3439 and 1635 cm-1, respectively. The absorption band of amide A (νN-H) was identified at 3270 (νN-H) cm-1. The absorption bands of amide I (νC=O) were identified at 1660, 1632 and 1606 cm-1. A β-sheet structure should be formed.41 Two sharp absorption bands were observed at 2918 and 2848 cm-1 (Figure 6a). An all-trans form was formed due to the hydrophobic association among the alkyl chains.42 Namely, the self-assemblies were formed due to hydrophobic association and H-bonding.

Figure 7. 1H NMR spectra of (L, L)-13 in DMSO (10.0 g L-1) and DMSO-d6/H2O (v/v = 9/1, 2.0 g L-1) mixed solvent; and those of (L, D)-13 in DMSO (10.0 g L-1) and DMSO-d6/H2O (v/v = 8/2, 2.0 g L-1) mixed solvent. The 1H nuclear magnetic resonance (1H NMR) spectra of the (L, L)-13 and (L, D)-13 suspensions and solutions are shown in Figure 7. In DMSO solutions, a N–H···O=S hydrogen bond was formed. In self-assemblies, the N–H···O=C hydrogen bond was proposed to be formed.


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In the case of the (L, L)-13 solution in DMSO, the chemical shifts of the amide protons were observed at 8.45 and 8.16 ppm for the N–H groups of alanines at the C- and N-terminus, respectively. For the suspension, the corresponding chemical shifts were observed at 8.47 and 8.23 ppm, respectively. Both of the chemical shifts moved downfield. The ∆δ of the N–H group of alanine at the N-terminus was larger than that at the C-terminus. The π-π interaction between neighboring biphenylene rings was proposed to drive this difference. For the (L, D)-13 solution in DMSO, two amide protons were identified at 8.42 and 8.13 ppm for the N–H groups of the alanine at C- and N-termini, respectively. In aggregates, the corresponding chemical shifts were observed at 8.40 and 8.16 ppm, respectively. The intermolecular H-bond formation between neighboring dipeptides should drive these shifts. A similar phenomenon was also found for the other dipeptides (Figure S7, Supporting Information). Since the chemical shift of the N–H group of alanine at the C-terminus was shifted to the upfield, the intermolecular H-bond should be weak. The hydrogen bond formed by the N–H group of alanine at N-terminus seemed to dominate the formation of organic self-assemblies.

Figure 8. SAXS patterns of (a) the (L, L)-13 suspensions prepared in a mixture of DMSO/H2O (9/1, v/v) at a concentration of 2.0 g L-1 and 25 °C, and (b) (L, D)-13 suspensions prepared in a mixture of DMSO/H2O (8/2, v/v) at a concentration of 2.0 g L-1 and 25 °C.


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Since DMSO is difficult to be removed, we failed to obtain the X-ray diffraction patterns of the organic self-assemblies of the dipeptides. Therefore, small angle X-ray scattering (SAXS) patterns of the suspensions of the dipeptides were recorded to reveal the ordered organization of the self-assemblies (Figure 8). The SAXS patterns of (L, L)-13 and (L, D)-13 suspensions showed the same diffraction peak at 5.87 nm. A lamellar structure with a d-spacing of 5.87 nm was proposed. The proposed molecular packing structure of (L, L)-13 is shown in Figure 9. Since the solvents were positive, the COOH groups should point out from the surfaces of the nanoribbons. The alkoxy chain exhibited an all-trans form, and a dimer structure was formed through the terminal COOH groups. The peptide segments packed into a parallel β-sheet structure. The distances between neighboring biphenylene rings and β-strands were approximately 3.7 and 4.3 Å, respectively. Moreover, the biphenylene group twisted in a right-handed manner.

Figure 9. Proposed molecular packing structure of (L, L)-13. Table 1 shows that the twist handedness of the biphenylene followed the chirality of the alanine at the N-terminus. Since the chiral center of the N-terminal alanine is far away from the axial of the biphenylene group, it is hard to believe that the twist handedness of the biphenylene was controlled by the chirality of the N-terminal alanine in an isolated state. A series of lowmolecular-weight gelators (LMWGs) with fluorocarbon chains have been reported.17 These


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LMWGs could self-assemble into helical nanofibers, and it was found that the handedness of the fluorocarbon chains was controlled by the handedness of the nanofibers. Herein, the twist handedness of the biphenylene groups was shown not to be controlled by the handedness of nanofibers. Therefore, the π-π stacking of neighboring biphenylene groups and the H-bonding formed by the N-H group of alanine at the N-terminus were proposed to drive this phenomenon.

SUMMARY AND CONCLUSIONS The aggregation-induced conformation and stacking chirality of biphenylene groups were studied based on dipeptides. The CD spectra of the organic self-assemblies indicated that the twist and stacking handedness of biphenylene groups were controlled by the chirality of the alanine at N- and C-termini, respectively. This result was confirmed by the simulated CD spectra. The π-π stacking of neighboring biphenylene groups and the H-bonding formed by the N-H group of alanine at the N-terminus were proposed to dominate the twist handedness of the biphenylene group. The stacking handedness of the neighboring biphenylene groups seems to be dominated by the H-bonding formed by the N-H group of alanine at the C-terminus. The results shown here provide a method to control the conformational chirality of the compounds with axial chirality. Supporting Information.

FESEM images of (L, L)-13 and (D, D)-13, molecular packings and simulated CD spectra of Nmethyl 4'-methoxy-1,1'-biphenyl-4-amide dimer, view of HOMOs and LUMOs and 1H NMR (D, D)-13 and (D, L)-13. This material is available free of charge via the Internet at

http://pubs.acs.org.

AUTHOR INFORMATION


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

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 (ZXG201415) and the National Natural Science Foundation of China (No. 51473106, 51473141 and 21574095).

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TOC


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Figure 1. Molecular structures of the dipeptides. 68x57mm (300 x 300 DPI)

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Figure 2. FE-SEM images of the nanotubes of (a) (L, L)-13 and (b) (D, D)-13 prepared in a mixture of DMSO and water (9/1, v/v) at a concentration of 2.0 g L-1. FE-SEM images of the microtubes and nanoribbons of (c) (L, D)-13 and (d) (D, L)-13 prepared in a mixture of DMSO and water (8/2, v/v) at a concentration of 2.0 g L-1. 80x80mm (256 x 256 DPI)


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Figure 3. TEM images of (a) the nanotubes of (L, L)-13 prepared in a mixture of DMSO and water (9/1, v/v) at a concentration of 2.0 g L-1 and (b) a microtube of (L, D)-13 prepared in a mixture of DMSO and water (8/2, v/v) at a concentration of 2.0 g L-1. 335x164mm (72 x 72 DPI)


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Figure 4. (a) CD and UV-Vis spectra of the suspensions prepared in a mixture of DMSO and water (9/1, v/v) at a concentration of 2.0 g L-1 and 25 °C, and (b) CD and UV-Vis spectra of the suspensions prepared in a mixture of DMSO and water (8/2, v/v) at a concentration of 2.0 g L-1 and 25 °C. 60x65mm (300 x 300 DPI)


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Figure 5. TD-DFT simulated CD spectra of a N-methyl 4'-methoxy-1,1'-biphenyl-4-amide dimer. (a) Twisted and stacked in a right-handed fashion, and (b) twisted in right-handed and stacked in left-handed fashion. 60x65mm (300 x 300 DPI)


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Figure 6. FT-IR spectra of the (L, L)-13 and (L, D)-13 assemblies. (a) 2800–3800 cm-1 and (b) 1500–1800 cm-1. 58x64mm (300 x 300 DPI)


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Figure 7. 1H NMR spectra of (L, L)-13 in DMSO (10.0 g L-1) and DMSO-d6/H2O (v/v = 9/1, 2.0 g L-1) mixed solvent; and those of (L, D)-13 in DMSO (10.0 g L-1) and DMSO-d6/H2O (v/v = 8/2, 2.0 g L-1) mixed solvent. 51x31mm (300 x 300 DPI)


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Figure 8. SAXS patterns of (a) the (L, L)-13 suspensions prepared in a mixture of DMSO/H2O (9/1, v/v) at a concentration of 2.0 g L-1 and 25 °C, and (b) (L, D)-13 suspensions prepared in a mixture of DMSO/H2O (8/2, v/v) at a concentration of 2.0 g L-1 and 25 °C. 63x61mm (300 x 300 DPI)


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Figure 9. Proposed molecular packing structure of (L, L)-13. 97x46mm (300 x 300 DPI)


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Aggregation-Induced Chirality: Twist and Stacking Handedness of the

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