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Supramolecular Chirality of the Two-component Supramolecular Copolymer Gels: Who Determines the Handedness? Yaqing Liu, Chunfeng Chen, Tianyu Wang, and Minghua Liu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b03938 • Publication Date (Web): 11 Dec 2015 Downloaded from http://pubs.acs.org on December 17, 2015
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Supramolecular Chirality of the Two-component Supramolecular Copolymer Gels: Who Determines the Handedness? Yaqing Liu, Chunfeng Chen, Tianyu Wang,* Minghua Liu* Beijing National Laboratory for Molecular Science (BNLMS), CAS Key Laboratory of Colloid, Interface and Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, People’s Republic of China KEYWORDS: Supramolecular chirality, chiral conflict, supramolecular copolymer, twocomponent gels, bolaamphiphiles
ABSTRACT: Natural supramolecular systems typically contain a wide variety of chiral molecules. Studying the chiral conflict within different supramolecular assemblies can not only be very helpful for understanding the inherent principles of supramolecular chirality, but also can guide the preparation of many functional chiral soft matters. For assemblies containing only structurally similar molecules, supramolecular chirality is determined by enantiomeric excess of molecular building blocks. For supramolecular systems assembled by structurally different chiral molecules, however, the optical activity of the systems and the chiral conflict among different chiral molecules can be very complex. We found rather unexpected results regarding the chiral conflict within two-component supramolecular copolymer gels in this study. The handedness of
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the chirality of supramolecular copolymer gels, which were formed by the co-assembly of bolaamphiphilic L-histidine derivatives and tartaric acids, was found to be dependent on the ordering molecular packing, instead of the preponderance of certain chiral molecules.
INTRODUCTION Supramolecular polymers, which are polymeric materials constructed through noncovalent interactions, are innovative soft matters with considerable potential applications.1,2,3,4 The nature of supramolecular polymers is not only similar to that of synthetic polymers, but further show many
very
special
properties,
such
as
self-healing1,5,6,7,8,9
or
highly
ordered
nanostructures.1,10,11,12,13 Certain supramolecular polymers can be used as gelators to form supramolecular gels in different solvents.11,14,15 Their reversible properties, as well as the hierarchical self-assembly of small organic molecules within these supramolecular polymer gels, provide the interesting models for the study of soft matters. Chirality is one of the fundamental and essential properties of nature.16 And supramolecular chirality in self-assembled systems, including transfer,17-19 amplification,20,21 memory22-24 and handedness control of chiral information,25,26 have been investigated. The emergence of supramolecular chirality from the self-assembly of achiral molecules upon symmetry breaking was also studied.16,27-30 Because many natural or artificial supramolecular systems contain different chiral molecules, understanding the chiral conflict within these systems is crucial
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issue.31 Structural differences between diverse chiral molecular building blocks play a very significant role in the supramolecular chirality of mixed systems. As of now, several important rules have been established for understanding supramolecular chirality. These include the “sergeants-and-soldiers” principle and the “majority-rules” principle, which were first developed by M. Green et al. based on covalent polymers.32,33 The “sergeantsand-soldiers” principle suggests that a few chiral monomers can control the helix sense of polymers, even though most units of the polymers are achiral.32 The “majority-rule” reflects the fact that the chirality of polymers containing enantiomerism monomers should be determined nonlinearly by the slight majority of R over S monomers (or vice versa).33 As far as covalentlylinked synthetic polymers, however, the chirality of systems polymerized by monomers with totally different molecular structure and chirality is more complicated. M. Green and his coworkers thoroughly investigated the “chiral conflict” within these synthetic polymers, and found that the supramolecular chirality of the polymers is not simply decided by the molar ratios of different competitive chiral groups.34,35 The effect of temperature, which changes the selfassembly properties of these polymers, also plays a crucial role.34 Both “sergeants-and-soldiers” and “majority-rules” principles have been proven equally effective at representing the supramolecular chirality of supramolecular polymers, even though supramolecular polymers are based on hierarchical self-assembly of small organic molecules via non-covalent interactions.36-38 However, depending on the distinct molecular packing mode, noncovalent interactions and nanostructures of supramolecular polymers, the supramolecular chirality of supramolecular polymers do differ from that of covalently-linked synthetic polymers. In general, the chiral supramolecular polymers are the assemblies containing either identical/enantiomeric molecules or structural different molecular building blocks. And the
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optical activity of the supramolecular copolymers formed by several structural different chiral molecular components is particularly important as it is more close to the situation of natural supramolecular chiral systems. This type of system is more complicated, and relatively rare in the literature. One valuable study by Meijer and his colleagues based on benzene-1,3,5tricarboxamide derivatives did show that the optical activity of these supramolecular polymers containing structurally different enantiomers is dependent on the number of stereocenters within the assemblies, which is consistent with the majority-rules principle.39,40 This paper presents our findings on the chiral conflict of supramolecular copolymer gels formed by the co-assembly of two totally disparate small organic chiral molecular building blocks (Figure 1A). Previous studies have shown that bolaamphiphiles containing L-histidine methyl ester head-groups (BolaHis) can form supramolecular copolymer and two-component hydrogels41,42 with different dicarboxylic acids via non-covalent interactions.11,43 It is worth mentioning that the self-aggregation of BolaHis and the interactions between dicarboxylic acids are important features of these assemblies.11,43 Inspired by these results, we prepared BolaHis/tartaric acids supramolecular copolymer gels. Because tartaric acid molecules also have two chiral centers, chiral conflict between BolaHis and tartaric acids can be expected. Interestingly, we found that the optical activity of BolaHis/tartaric acids supramolecular copolymer does not simply rely on the number and ratios of different chiral monomers, but instead, on the chirality of well-organized components with fewer stereocenters. To the best of our knowledge, this is the first study to observe nanostructure and packing mode dependent chirality of supramolecular polymer gels.
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Figure 1: (A) Schematic diagram showing chiral conflict within BolaHis/dicarboxylic acids supramolecular copolymer gels; (B) molecular structures of BolaHis and tartaric acids; (C, D) TEM images of 1a/BolaHis=1/1 (C) and 1a/BolaHis=2/1 (D) supramolecular copolymers. RESULTS AND DISCUSSION The co-assembly of BolaHis with tartaric acids (TA) can form hydrogels upon a “heating and cooling” process in water. When the molar ratio of TA/BolaHis systems are greater than or equal to 1/1, two-component transparent hydrogels can be obtained. We measured the critical gelation concentration (CGC) of BolaHis in 1a/BolaHis two-component hydrogels: the CGC values of BolaHis for the formation of 1a/BolaHis=2/1 and 1a/BolaHis=1/1 hydrogels are 0.5mg/ml and 0.4mg/ml, respectively. Accordingly, 1mg/ml (0.1wt%) BolaHis was consistently applied for the subsequent preparation of TA/BolaHis supramolecular copolymers. The morphologies of these supramolecular copolymers were then analyzed by TEM measurements (Figure 1C, 1D). In the
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1a/BolaHis supramolecular copolymer hydrogels with different molar ratios, faintly twisted nanoribbons with widths around 20-80 nm were observed. The formation of TA/BolaHis supramolecular copolymers can be proved by FT-IR spectra, which suggest the non-covalent interactions between BolaHis and tartaric acids, as well as the ordered molecular packing mode of BolaHis (Figure S1a). The broad band around 1960 cm-1 should be associated with N-H---O stretching bands of imidazolium carboxylate salts. When the histidine methyl ester head-groups interacted with the dicarboxylic acids, protons on the COOH were transferred onto the pyridine nitrogen of the imidazole to form salts. In addition, asymmetric and symmetric stretching vibrations of CH2 on the alkyl chains consistently appeared at 2920 and 2850 cm-1, respectively, suggesting that the alkyl spacers of BolaHis were highly ordered and packed in an all-trans manner. Amide I and II bands from BolaHis appeared at about 1646 cm-1 and 1541 cm-1, respectively, indicating strong hydrogen bonding between amide groups. Therefore, within these supramolecular copolymers, BolaHis not only interacted with dicarboxylic acids but also self-assembled into monolayer lipid membranes (MLM). The interactions between different tartaric acid molecules in the supramolecular copolymers were also determined through FT-IR measurements. The shoulder peak of 3440 cm-1 indicated the hydrogen bonding mode of the OH groups between different tartaric acid molecules.
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Figure 2: (A, B) CD spectra of 1a/BolaHis (A), 1b/BolaHis (B) supramolecular copolymers with different molar ratios (CBolaHis = 1 mg/ml); (C) statistical analysis of CD signal g value at 215nm of 1a/BolaHis and 1b/BolaHis supramolecular copolymers with different molar ratios. The optical activity of these supramolecular copolymers was evaluated by measuring their circular dichroism (CD) spectra. Notably, although both 1a and BolaHis (Figure 1B) have two S chiral centers, they showed totally opposite Cotton effects upon CD spectral measurement. In contrast, 1b (Figure 1B) showed negative Cotton effects, similar to BolaHis. For understanding chiral conflict of supramolecular copolymers, the CD signals from different 1a/BolaHis and 1b/BolaHis assemblies were also investigated. The results showed that 1b/BolaHis hydrogels with different molar ratios had consistently negative CD signals (Figure 2B). When more 1b molecules were introduced into the assemblies, the intensity of the negative CD signal increased (Figure 2C). Remarkable observations related to supramolecular chirality were provided by the CD spectra of 1a/BolaHis supramolecular copolymers. Specifically, the handedness and intensity of CD signals were found to be independent on the number of chiral centers of the molecular building blocks. When the molar ratio of 1a/BolaHis was changed from 1/1 to 2/1 (and, further, to 3/1 or 4/1), the measured CD signal showed positive Cotton effect, then negative Cotton effect, then positive Cotton effect again (Figure 2A, 2C). Considering 1a produces consistently positive Cotton effect, and BolaHis produces negative Cotton effect, the supramolecular chirality of 1a/BolaHis assemblies with different molar ratios are really quite interesting. For the 1a/BolaHis≥2/1 assemblies, adding more 1a increased positive Cotton effect (analogous to majority rules). However, when 1a/BolaHis≤2/1, the optical activity of the supramolecular
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copolymers is not dependent on the number or ratio of different chiral centers within the assemblies.
Figure 3: (A) Molecular structures of CD silent dicarboxylic acids with (1c) or without (2) chiral centers; (B-D) CD spectra of 2/BolaHis (B), 1c/BolaHis (C) and (1a+1c)/BolaHis=1/1 (D) supramolecular copolymers with different molar ratios (CBolaHis = 1 mg/ml). In order to understand this unusual supramolecular chirality within supramolecular polymer gels, mesomeric tartaric acid (1c) and succinic acid (2) were used as control dicarboxylic acids to form co-assembly with BolaHis. The CD spectra of 1c/BolaHis and 2/BolaHis assemblies with different molar ratios were then measured, as shown in Figure 3. It is worth mentioning that both the individual 1c and 2 molecules are CD silence. However, each 1c molecule has two chiral centers while molecule 2 has no chiral center. These features yielded valuable observation results regarding the chiral conflict of supramolecular polymer gels. A strong negative CD signal was detected in different measurements for 1c/BolaHis=1/1 supramolecular copolymer reflecting the
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supramolecular chirality of the BolaHis assembly (Figure 3C). Interestingly, when more and more 1c was added into the system, the CD spectra became silent (Figure 3C). Presumably, when 1c molecules were introduced into these systems, the chiral centers from the CD silent 1c molecules provided contradictory information that drowned out other CD signals in the supramolecular polymer gels. By contrast, the CD spectra of 2/BolaHis assemblies showed consistently negative Cotton effect (Figure 3B). And the g values of corresponding CD signals decreased gradually as molar ratios of 2/BolaHis systems increased (Figure 3B). Because the CD silent molecule 2 does not have chiral centers, no chiral conflict was identified in the 2/BolaHis assemblies. These results altogether indicate that the chiral centers from different molecular building blocks play very important roles as far as chiral conflict within two-component supramolecular copolymer gels. The handedness of certain chiral supramolecular copolymers, such as 1a/BolaHis systems, is not simply dependent on the number and ratios of different stereocenters. Although 1a/BolaHis hydrogels can only be obtained with 1a/BolaHis≥1/1, the changes in CD signals among 1a/BolaHis supramolecular copolymers with different molar ratios encouraged us to study the supramolecular chirality of 1a/BolaHis assemblies containing very small amounts of 1a, such as 1a/BolaHis
