Chiral Packing of Cholesteryl Group as an Effective Strategy To Get

Article pubs.acs.org/Macromolecules

Chiral Packing of Cholesteryl Group as an Effective Strategy To Get Low Molecular Weight Supramolecular Hydrogels in the Absence of Intermolecular Hydrogen Bond Fangming Xu, Haibo Wang, Jie Zhao, Xiangsheng Liu, Dandan Li, Chaojian Chen, and Jian Ji* Key Laboratory of Macromolecular Synthesis and Functionalization, Ministry of Education, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: Although the most commonly reported low molecular weight hydrogelators (LMWG) are based on the strong and highly directional hydrogen bond between polar groups, the expansion of novel species of LMWG is quite necessary to fulfill different requirements of practical applications. Herein, we demonstrated that using an inherently chiral groupcholesterol as the hydrophobic part is a quite effective strategy to get phosphocholine-based LMWG, in which strong hydrogen bonds cannot be directly formed. A series of phospholipid hydrogelators, in which hydrophilic phosphocholine was connected to the chiral hydrophobic cholesteryl group through an alkyl chain, were designed and synthesized. Cholesteryl group greatly promoted the formation of one-dimensional supramolecular structures: helical nanofibril, twisted nanoribbon, helical nanotube, and spindle shaped vesicles were formed, and they showed drastic variation with just simple minor change on the molecular structures. All of them can further organize into cross-linked three-dimensional networks and form hydrogels. The hydrophobic interaction between cholesteryl groups also greatly improved the performance of hydrogel.

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INTRODUCTION Hydrogels have being attracted great attention because of their intriguing properties and wide applications (biomedical service, food industry, agriculture, and cosmetic, etc.) for decades.1 Supramolecular self-assembly not only helps people to get better understanding of biological system but also became a very important strategy to fabricate various functional materials.2 Recently, low molecular weight hydrogelator (LMWG) is becoming one of the most attractive subjects in supramolecular chemistry due to its superior properties over polymeric hydrogelators: reversibility of gelation process, better tunability over properties, degradability, and greater potential to fabricate a “smart material” due to a higher sensitivity to stimulus.3 LMWG first self-assembled into some kind of onedimensional (1D) supramolecular structures like nanofibers, twisted nanoribbons, et al. and further entangled into threedimensional networks to ensnare a great amount of water molecules inside, and then a hydrogel could form.4,5 This demands the the structure of LMWG should greatly facilitate the formation of one-dimensional assemblies.6 Biocompatibility is the basic criteria that must be met for the biomedical applications of LMWG. Choosing biological components as structural building block is a widely adopted strategy to prepare biocompatible materials, not only for low toxicity of the constituent elements but also for structural similarity to the living organism.7 The route of chemical synthesis greatly depends on the molecular structure of the target compound (like hyperbranched polymers and dendrimers).8 A simple structure is the prerequisite to simplify a relatively complicated © XXXX American Chemical Society

synthetic process, which greatly hinders the development and practical application of LMWG.9 However, the exploration of rational structures which will endow LMWG with strong gelation ability, biocompatibility, and good availability still remains very challenging but quite an urgent task at present. Phosphocholine (PC) is the polar group of phosphatidylcholine, which is one of the major components of biomembranes.10 Polymeric PC hydrogels were extensively studied for their attractive biocompatibility.11 However, unlike most adopted hydrophilic polar groups of LMWG (such as carboxyl, hydroxyl et al.): the strong hydrogen bond, which serves as very important intermolecular association for the formation of supramolecular hydrogel,12 cannot be formed between PC groups.13a So LMWG with PC as polar group can rarely be found in the literature.13 Blume et al. reported the formation of a hydrogel by a bipolar lipid PC-C32-PC. To optimize the hydrophobic interaction, the adjacent molecules slightly twisted relative to each other and self-assembled into nanofibers.13a Barthélémy et al. prepared hydrogelators by replacing glycerol with nucleobase in conventional lipids.13c The internucleobase stacking played an important role in stabilizing the helical nanofibers. Herein, we present a novel and effective strategy to get phosphocholine LMWG by simply connecting PC groups to cholesterol which is a chiral motif with strong hydrophobic interactions. Owing to the structural simplicity, chemical Received: February 6, 2013 Revised: February 21, 2013

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dx.doi.org/10.1021/ma400276u | Macromolecules XXXX, XXX, XXX−XXX

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groups, which can only form spherical micelles in water,17a the hydrophilic proportion of a single PC group in the present design is just in the range of where 1D supramolecular structures could form, as predicted by the packing parameter, P.18 Some reports about liposomes had Ch0CP2C involved but they did not investigate its potential in the formation of a hydrogel.17b The method reported by Eibl was used to convert the hydroxyl group into a phosphocholine group (Scheme 1b).19 This route was simple, the condition was mild, and the yield was satisfactory. Most important, there was no need for chromatographic purification to get pure product. The reported bipolar structure of PC-C32-PC is crucial for the formation of nanofibers and hydrogel.13a Herein, we specially chose half of the bipolar lipid, -16CP2C, as a control to show the role of the cholesteryl moiety. The intermediate oxazaphospholanes have five membered rings in Eibl’s report, but usually the stability of the six membered rings is better due to less stress. So there might be great possibilities to obtain a final product with three carbon atoms between the charged centers via intermediates with six membered rings. With this in mind, we tried to use 3amino-1-propanol instead of ethanol amine and followed the same procedures. Two kinds of phospholipid with three carbon atoms between two charged centers (Ch0CP3C and Ch10CP3C) were successfully prepared. Data in the Supporting Information proved the synthesis was successful and the obtained products were pure. The retardation factors (Rf) of Ch0CP3C and Ch10CP3C were lower than those of Ch0CP2C and Ch10CP2C, respectively (Figure S1, Supporting Information). This obviously indicated the increase of distance between the charged centers from two to three carbon atoms raised the polarity of zwitterionic molecules.20 Preparation of Hydrogel. The solid of all six kinds of Chol-Alkyl-PC are quite hygroscopic, and can be easily dissolved in water with the help of thermo-treatment. All of the six Chol-Alkyl-PC phospholipids can form hydrogel when the concentration was above critical gelation concentration (Cgel) and the temperature was below critical gelation temperature (Tgel). The thermo-induced sol−gel transition is reversible, and a typical abrupt change of transmittance can be observed during the transition (Figure S6, Supporting Information). The hydrogel looks turbid while the solution looks transparent. Strong mechanical force can also turn the elastic hydrogel into highly viscous turbid liquid. But different from polymeric hydrogels which rely on covalent cross-link, the mechanical force disrupted LMWG hydrogel can be easily recovered to its original state by thermo-treatment. The hydrogel samples can be prepared either by directly dissolving the bulk solid of Chol-Alkyl-PC into water or by film rehydration just like the way to prepare liposomes. The filmrehydration usually cost shorter time and the sample is more homogeneous as characterized by TEM, probably because the water contact area of thin film is larger than the bulk solid. So all the samples used for characterization were prepared by film rehydration method. Microscopic Structure in Hydrogel. (a). Basic Assemblies. In order to determine the structure of networks inside hydrogel, a bottom-up investigation was carried out. First, the basic assemblies in dilute aqueous solution of Chol-Alkyl-PC were investigated (Figure 1). Both Ch0CP2C and Ch0CP3C self-assembled into nanofibers, and the length was over several micrometers. The nanofibers of Ch0CP2C and Ch0CP3C showed very strong tendency to aggregate into bundles. Single free nanofiber can hardly be found. The nanofibers aligned

synthesis of traditional linear single polar amphiphile is usually easier, as compared with bipolar or diacyl architecture. Cholesterol is specially chosen as hydrophobic part for the fact that derivatives of cholesterol have shown a strong ability to pack into various elegant 1D structures due to the chiral nature of the cholesteryl moiety.14 More importantly, the hydrophobic interaction between cholesterol molecules is quite strong. The melting point of cholesterol is higher than linear alkane and polyethylene. Cholesterol can enhance the stability of bilayer and promote a liquid condensed state in lipid mixtures.15 In the present work, we demonstrated that the introduction of the cholesteryl group not only facilitated the formation of 1D structures, but also greatly improved the properties of hydrogel. The overall properties of the best hydrogelator Ch0CP3C surpassed the previously reported phosphocholine LMWG, and it is very promising for practical applications.

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RESULTS AND DISCUSSION Synthesis of Cholesteryl Phospholipid LMWG. Several excellent reviews have summarized the cholesteryl gelators; however, they will precipitate or cannot be dissolved in water and almost none of them was able to form a hydrogel effectively.16 The rational design of molecular structure is still a great challenge to get LMWG which relay on the chiral packing of cholesteryl groups: how to give full play to the strong hydrophobic interaction of the cholesteryl part, and how to choose the hydrophilic part to reach a balance with the hydrophobic part. In this work, a family of hydrogelators, in which hydrophilic phosphocholine was connected to the chiral hydrophobic cholesteryl group through an alkyl chain (CholAlkyl-PC), were designed and synthesized (Scheme 1). Different from our previously reported cholesteryl polymeric phosphocholine (PMPC) compound with multiple PC side Scheme 1. Chemical Structures, Abbreviations (a), and Synthesis Route (b) of Chol-Alkyl-PC, 16CP2Ca

a Reaction conditions: (a) p-TsCl, DMAP, pyridine/CH2Cl2, room temperature, 72 h; (b) dihydroxy alkanol, 1.4-dioxane, 120 °C, 72 h; (c) POCl3, TEA, THF, 0 °C, 5 h; (d) ethanol amine (n = 2) or 3amino-1-propanol (n = 3), TEA, THF, 0 °C, 5 h; (e) HAc, THF/H2O, 70 °C, 5 h; (f) CH3I, NaOH, CH3OH/CH2Cl2, room temperature, 72 h.

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Figure 1. TEM images of basic supramolecular assemblies of (a) Ch0CP2C, (b) Ch0CP3C, (c) Ch6CP2C, (d) Ch10CP2C, (e) Ch10CP3C, and (f) Ch12CP2C. SEM images of basic supramolecular assemblies of (g) Ch6CP2C, (h) Ch10CP2C, (i) Ch10CP3C, and (j) Ch12CP2C. (k) Helical twist formed by nanofibers of Ch0CP2C. (l) Intermediate morphology of Ch12CP2C. The samples of Ch0CP2C and Ch0CP3C were prepared from 0.1 mg/mL aqueous solution, and the other samples were prepared from 2 mg/mL aqueous solution.

and closely jointed nanofibrils (Figure 2a). The average width of each nanofibril was measured to be 5.1 nm for Ch0CP2C and 4.9 nm for Ch0CP3C (Figure 2, parts d and e). Helical pitch can be found on the nanofibrils, and was measured to be 8.1 nm for Ch0CP2C and 6.5 nm for Ch0CP3C on average. The angle between those helical patterns and the main axis of nanofibril was measured to be 45° for Ch0CP2C and 50° for Ch0CP3C on average. Ch6CP2C formed a one-dimensional structure quite different from Ch0CP2C and Ch0CP3C, it showed distinct periodic wavy contour (Figures 1c and 2c). A

parallel with each other and densely packed inside the bundles. In diluted samples (0.1 mg/mL), relative loosely packed nanofibers can be found at the terminus of bundles. At this area, the DNA-like helical twist structure formed by nanofibers of Ch0CP2C intertwining with each other can be observed (Figure 1a inset and Figure 1k). But a similar structure was not observed in the sample of Ch0CP3C. High-resolution TEM (HR-TEM) images showed the substructure of nanofibers formed by Ch0CP2C and Ch0CP3C looks all the same, and each of a single nanofiber is composed of two parallelly aligned C


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knowledge, this is the first example to simply get this unique assembly by a single ingredient formula of phospholipid. The molecular structure showed great influence on the morphology of these assemblies. Nanofibers, twisted nanoribbons, nanotubes and spindled shaped vesicles appeared successively as the length of alkyl chain increased from zero to 12 carbon atoms (Figure 1 and Figure S2Supporting Information). The length of alkyl chain showed great influence on the self-assemblies. While the longer spacer between two charged centers did not cause fundamental morphological difference. The cooling speed of preparation process also showed great influence on the morphology and size of obtained assembles, and some more intriguing assemblies can be obtained (Figure 1l). These irregular unique morphologies might be thermo-kinetically favored.23 This causes more complicated morphological evolution and great effort is needed to investigate the specific mechanism. So herein we just cool the samples slowly (ca. 0.3 °C/min) and keep the samples tested below corresponding transition temperatures in order to study the basic rules. (b). Organizational Patterns of Basic Assemblies. As revealed by TEM, two parallelly aligned and closely jointe nanofibrils formed a single nanofiber of Ch0CP2C and Ch0CP3C. Then several nanofibers densely packed with their main axis oriented parallel with each other and formed a bundle. The bundles further entangled and cross-linked to form a 3D network (Figure 3, a and c). From the SEM images of xerogels, the 3D network formed by bundle shaped structures is quite clear. The bundles of Ch0CP2C tend to align closely with each other rather than get cross-linked (Figure 3b). However, the bundles of Ch0CP3C are highly cross-linked. The network of Ch0CP3C is apparently more homogeneous and the density of cross-linkage is much higher (Figure 3d). The twisted ribbons of Ch6CP2C exhibited similar organization manner, as discussed above (Figure 3e,f and Figure S4, Supporting Information). But the bundles of Ch6CP2C are much wider than bundles of Ch0CP2C and Ch0CP3C. The organization in the hydrogel of Ch0CP2C, Ch0CP3C and Ch6CP2C are very similar: several nanofibers or twisted nanoribbons form a bundle, and then the bundles further entangle with each other to form the 3D network (Figure 3g). TEM images clearly showed the tree-like branching structure between nanotubes of Ch10CP2C and Ch10CP3C (Figure 4a and 5b). At each branching point, the wider main stem divided into narrower branches. These branching points can serve as cross-linkage for the interconnection of these tubes. The SEM images revealed the existence of cross-linked 3D-network in the hydrogel of Ch10CP2C and Ch10CP3C (Figure 4c,d and Figure S4, Supporting Information). Some nanotubes protruded out from the network. This clearly proved the network is formed by these nanotubes. The protruded nanotubes also indicated that not all of the ends of nanotubes were connected to the branching point, and entanglement of nanotubes was also involved in the network. So the network in the hydrogel of Ch10CP2C and Ch10CP3C was formed by branching and entanglement of nanotubes (Figure 4e). SEM images also showed a very obvious 3D network in the hydrogel of Ch12CP2C (Figure 5a and Figure S4, Supporting Information). TEM images showed more detailed information about the organization between the spindle shaped vesicles in the network of Ch12CP2C. Several different organization patterns can be found. First, the tips of the spindle shaped vesicles joined together, and the axes of spindles converged at

Figure 2. HR-TEM images of nanofibers of (a) Ch0CP2C and (b) Ch0CP3C. The inset of part a showed there is always a pale line in the middle of nanofibers indicates each nanofiber is composed of two individual nanofibrils. (c) Twisted nanoribbons of Ch6CP2C. The size of nanofibrils of (d) Ch0CP2C, (e) Ch0CP3C, and twisted nanoribbons of (f) Ch6CP2C are labeled in the image.

saddle-like curvature can be recognized and they are usually termed as twisted nanoribbons in references.21 The width was measured to be 17.9 nm on average, and the pitch was 61.6 nm on average. The angle between tangent direction of the ribbon rim and main axis was 25° on average (Figure 2f). The length of the whole ribbon was also over several micrometers. The twisted nanoribbons also tend to entangle with each other. The bundles formed by convergent twisted nanoribbons are quite like pine tree branches in SEM images (Figure 1g). Ch10CP2C and Ch10CP3C self-assembled into nanotubes with length over several micrometers, and did not show obvious morphological difference between them. But the average width of nanotubes was measured to be 610 nm for Ch10CP2C and 330 nm for Ch10CP3C. The narrower width of Ch10CP3C nanotube may be caused by relatively larger polar groups.18 From TEM photos, these tubes were enclosed 1D structure. The holes occasionally found on the nanotubes in SEM image proved the hollow interior of tubular structures (inset of Figure 1h,i and Figure S2, Supporting Information). It is interesting to find that Ch12CP2C self-assembled into spindle shaped vesicles. It is an enclosed extended vesicular structure with sharp tips at two ends and a bulge in the middle. The average width of the bulge was 185 nm, and the length of the whole spindle was 1400 nm on average. The TEM and SEM images clearly proved the spindle shape and vesicular structure (Figure 1j and Figure S2, Supporting Information). The preparation of spindle shaped vesicles is very simple: just dissolve the solid of Ch12CP2C into water and heat above transition temperature with stirring to make a homogeneous solution, and then cool the solution slowly below transition temperature. Unlike some reports to get ovoid vesicles, it did not need specially controlled preparation conditions or radiation of polarized light.22 Moreover, they were very stable and no obvious changes were observed by TEM after being stored at 4 °C for months. To the best of our D


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Figure 4. (a, b) TEM images of tree-like branching structure of nanotubes formed by Ch10CP2C. (c, d) SEM images of networks formed by Ch10CP2C. The white circle marked a nanotube protruded out from the network. (e) 3D network formed by combination of branching and entanglement of nanotubes.

dimensional structures by molecules with phosphocholine group has been not an inconceivable phenomenon. Lots of research focused on the self-assemble behavior of diacyl phosphocholine molecules and found that either a singlecomponent formula of diacetylene phospholipid or a multicomponent formula with cholic acid or other kinds of lipids can yield various elegant and fascinating one-dimensional structures.24 However, none of them reported the formation of hydrogel. This might mostly be because of the absence of effective organization between assemblies to build the 3D networks. It is quite interesting and unusual for our system that so many different morphologies and different organizational patterns can be found, while just minor changes were made on the molecular structure. The control sample, 16CP2C, which consisted of a long alkyl chain and a single phosphocholine group, cannot form hydrogel either at high concentrations (as high as 100 mg/mL) or after long time storage at low temperature (as long as two months under 4 °C) (Figure S8, Supporting Information). White crystals of 16CP2C slowly precipitated out from solution after long time of storage under 4 °C. In the aqueous solution of 16CP2C, only spherical micelles can be observed by TEM. Apparently, cholesteryl group played key role in the formation of one-dimensional structures and the resulting hydrogel. Chirality of the Assemblies. The helical twist of nanofibers of Ch0CP2C and the periodic wavy contour of twisted nanoribbon of Ch6CP2C indicated these structures might be chiral. The circular dichroism (CD) spectra of water solutions of these Chol-Alkyl-PC assemblies were examined, and indeed it was found that all of the supramolecular structures were chiral (Figure 6 and Figure S3, Supporting

Figure 3. (a) TEM and (b) SEM images of network formed by nanofibers of Ch0CP2C. (c) TEM and (d) SEM images of network formed by nanofibers of Ch0CP3C. (e) SEM images of bundles of Ch6CP2C formed by convergent twisted nanoribbons. (f) SEM image of network formed by Ch6CP2C. (g) Schematic illustration of 3D network in hydrogels of Ch0CP2C, Ch0CP3C, and Ch6CP2C.

the crossover point of the tips. Sometimes, this pattern can form a star-shaped cross-link point (Figure 5c). Second, the spindle shaped vesicles were juxtaposed with each other. The end parts of spindle shaped vesicles joined together from lateral, and the axes of spindles were parallel. This pattern can form a long-range one-dimensional structure (Figure 5d). Third, the end parts of spindle shaped vesicles also joined together from lateral. But the axes of spindles were bent. This pattern can also form a long-range one-dimensional structure, but exhibited a sinuous shape (Figure 5e). Fourth, the central part of spindles joined together from lateral, but their tips bent outward (Figure 5f). These four patterns were the supramolecular structures at the same level, and each of them may appear either at the cross-link points or at the connection part between cross-link points. The combination of them formed the cross-linked 3D network in the hydrogel of Ch12CP2C (Figure 5g and Figure S5, Supporting Information). Apparently, the networks consisted of supramolecular structures at different levels. In fact, the formation of oneE


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Figure 6. (a) CD spectra of Chol-Alkyl-PC water solution. The value of signals of Ch0CP2C was multiplied by 0.15 for clarity. T = 10 °C. All the samples were cooled slowly from 85 °C (ca. 0.3 °C/min) to avoid a cooling speed induced change of chirality. (b) Specific rotation of Chol-Alkyl-PC in dichloromethane/methanol = 1/1 (v/v). T = 25 °C. (c) SEM image of a left-handed bundle formed by Ch6CP2C.

Figure 5. Supramolecular organization in the hydrogel of Ch12CP2C. (a) SEM image of networks formed by Ch12CP2C. (b) TEM image of networks formed by spindle shaped vesicles. (c) SEM image of star shaped cross-link between spindle shaped vesicles. The upper surfaces of vesicles were broken under the vacuum in the equipment. (d−f) TEM images of the second, third, and fourth types of organization between spindle shaped vesicles. (g) Schematic illustration of networks formed by the combination of the four organization patterns.

of those structures might be all left-handed. Many researchers believe that the handedness of supramolecular structures is the reflection of the chirality of underlying constituent molecules.25 The specific rotation values of Chol-Alkyl-PC in mixed organic solvent (CH2Cl2/MeOH = 1/1, v/v) were all negative, indicated levorotatory rotation. The absolute values of specific rotation of Chol-Alkyl-PC were in the order of: Ch0CP2C (−20.2°) > Ch0CP3C (−20.0°) > Ch6CP2C (−16.0°) > Ch10CP2C (−15°) > Ch10CP3C (−14.3°) > Ch12CP2C (−13.9°). This means the molecules of all six Chol-Alkyl-PC are chiral inherently, and they have chirality of the same nature. 16CP2C again did not show specific rotation. This proved that the alkyl chain and phosphocholine parts were not chiral, and the inherent chirality of molecules originated from the cholesterol moiety. The reaction process did not change the charity of cholesteryl part. The formation of those onedimensional structures was closely related to the strong tendency of cholesteryl moieties to pack in the chiral manner.26 Cgel and Tgel. The lowest concentration needed to form hydrogel (Cgel) and sol−gel transition temperature (Tgel) are two main parameters to depict the performance of hydro-

Information). The values of CD signals in the range of 190− 272 nm were all negative and the intensity was strong; the values in the range of 272−375 nm were positive but relatively weak, Ch0CP2C and Ch10CP3C were silent in 272−375 nm. The intensity of CD spectra signals gradually decreased as the temperature elevated above corresponding transition temperatures. This can be ascribed to the chiral packing was disturbed under elevated temperature (Figure S3, Supporting Information). The solution of 16CPC was completely CD silent. So the formation of those 1D structures can be attributed to the chiral packing of Chol-Alkyl-PC molecules. A similar pattern of the CD spectrum indicated the chirality of six kinds of supramolecular assemblies might be of the same nature. The SEM image clearly showed the bundles formed by Ch6CP2C nanoribbons are left-handed (Figure 6c), so the handedness F


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gelators. Cgel is also the lowest concentration needed to form the 3D network which is strong enough to interlock the whole volume.27 So the hydrogelator with a lower Cgel value has a stronger gelation ability. The values of Cgel are measured by the bottle-invert method (Figure S7, Supporting Information), and they are in the sequence Ch0CP2C (0.5 mg/mL) < Ch0CP3C (0.6 mg/mL) < Ch6CP2C (8 mg/mL) < Ch10CP2C (40 mg/ mL) < Ch12CP2C (60 mg/mL) < Ch10CP3C (70 mg/mL) (Figure 7a). The intermolecular force is usually disrupted above

Fortunately, the hydrophobic parts of these cholesteryl phospholipids are the intermediate products. The melting point, Mp, and melting enthalpies, ΔH, of Chol-Alkyl-OH were measured by DSC (Figure 7b). The values of Mp are in the sequence Ch0COH (150.5 °C) > Ch6COH (82.1 °C) > Ch12COH (80.7 °C) > Ch10COH (69.2 °C); and the values of ΔH are in the sequence Ch0COH (73.9 J/g) > Ch12COH (61.8 J/g) > Ch6COH (41.4 J/g) > Ch10COH (38.5 J/g). Mp and ΔH of Chol-Alkyl-OH first decrease as the length of the alkyl chain increases from zero to ten carbon atoms and then rise again at 12 carbon atoms but are still lower than those of Ch0COH. This agreed well with the trend of Tgel, and proved the hydrophobic part of the molecule is crucial to the thermostability of Chol-Alkyl-PC hydrogel. This variation can be explained as the combination of two factors: the attachment of alkyl chains weakens the hydrophobic interaction between cholesteryl moieties, but the overall intermolecular hydrophobic interactions tend to be reinforced as the length of alkyl chain increased. The gelation ability was also affected by the spacer between two charged centers. The gelation abilities of Ch0CP2C and Ch0CP3C are close, but Ch0CP3C is a little lower than Ch0CP2C. In the case of Ch10CP2C and Ch10CP3C, in which there are long alkyl chains between cholesteryl and phosphocholine groups, the gelation ability of Ch10CP3C is obviously lower than Ch10CP2C. As discussed above, the hydrophilicity of P3C group is higher.20 This enhanced the tendency of molecules to disperse, but weakened the tendency to aggregate. Thus, the gelation ability decreased. Apparently, Ch0CP2C and Ch0CP3C have very strong gelation ability. The Tgel values for them were all over 60 °C. This suggests they can keep the solid state at 37 °C for tissue engineering related applications. Ch0CP3C can form a hydrogel at 0.6 mg/mL (one hydrogelator trapped around 52 390 water molecules), and Cgel of Ch0CP2C is even lower at 0.5 mg/mL (one hydrogelator trapped around 61 310 water molecules) (Figure S7, Supporting Information). These are almost in the same level of the most effective hydrogelators appeared in literatures.30 So both of them can be regarded as super hydrogelators since the values of Cgel are below 1 mg/mL (0.1 wt %). Performance of Hydrogel. (a). Rheological Measurements. A minimum storage modulus (G′) of approximately 100 Pa is the prerequisite to support the mass of cell for tissue engineering, and the ideal storage loss modulus (G″) varies depending on the specific cell type.7c,31 Most reported supramolecular hydrogels exhibited concentration-dependent rheological behavior, and the modulus of hydrogel can be simply adjusted by the concentration of hydrogelators. However, the transport of substance such as nutrient and oxygen, which are essential to sustain the growth of living cells, is a critical challenge facing the research of using hydrogel for tissue engineering. If the concentration of hydrogelator is too high, the substance diffusion will be hindered and the cells can only spread over the surface layer of the hydrogel. So the mechanical performance of hydrogel at a relatively lower concentration is very important.31 Because both Ch0CP2C and Ch0CP3C can form hydrogels at very low concentrations and they can be very easily prepared from simple raw materials, we specially chose them to investigate the influence of different phosphocholine groups on the rheological performance. Dynamic modulus G′ (storage modulus) and G″ (loss modulus) were measured at 25 °C. In the nondestructive frequency sweep (Figure 8c), both G′ and G″ declined as the

Figure 7. (a) Tgel and Cgel of Chol-Alkyl-PC. All the samples for Tgel test were 100 mg/mL. (b) Mp and ΔH of Chol-Alkyl-OH, which were the hydrophobic parts of cholesteryl phospholipids.

Tgel, which lead to breakdown of the 3D network; thus, the whole volume exhibits as the solution state. Higher Tgel means that more energy is needed to break the 3D network.28 The Tgel of Chol-Alkyl-PC tested by DSC measurement did not show dependence with the sample concentration, and the values are in the sequence Ch0CP2C (61.6 °C) > Ch0CP3C (60.3 °C) > Ch6CP2C (41.2 °C) > Ch12CP2C (30.5 °C) > Ch10CP2C (16.6 °C) > Ch10CP3C (13.2 °C). Apparently, the gelation abilities of the six cholesteryl phospholipid hydrogelators are closely related to their molecular structures. In most published work about LMWG, the hydrogen bond between polar groups played a key role in the formation of one-dimensional selfassemblies. This mainly benefits from the selectivity, highly directional features, and strong force of hydrogen bond.29 While Chol-Alkyl-PC bears a great resemblance to PC-C32-PC: there is no hydrogen-donor atom in the molecule at neutral pH. 13a The formation of hydrogel is not driven by intermolecular hydrogen bonds but solely by hydrophobic interactions. So it is conceivable that the gelation ability is closely related to the interaction between hydrophobic parts. G


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be 0.093 Pa·s for Ch0CP2C and 0.0091 Pa·s for Ch0CP3C when the temperature rose up to 75 °C (1 order of magnitude lower than Ch0CP2C). This means the solution of Ch0CP3C formed above Tgel will be easier to fill in a certain shaped space like a mold or somewhere where a hydrogel is required;35 thus, the processability of the Ch0CP3C hydrogel is better. For small molecules with similar structure, the lower viscosity is usually related to the higher hydrophilicity of polar group.20,36 So the rheological properties were greatly improved by the higher hydrophilicity of P3C group at the expense of a little bit of loss of gelation ability. The measurement of static viscosity during two continuous heating and cooling cycles showed only small gradual increase was tested when the samples were cooled from hot solution (Figure S9, Supporting Information). However, after the samples were stored at 25 °C for 1 h, the viscosity increased to the original value again. This is closely related to structural change: highly ordered 3D network of nanofibers is disrupted under elevated temperature, but the rebuilding of the supramolecular network requires the sample to be stand still for a certain period of time below Tgel. This reveals the packing of LMWG into highly ordered structures cannot be accomplished immediately and it takes time.37 All of the six Chol-Alkyl-PC can form hydrogel below Tgel within 5 h, but gelation time can be reduced from hours to just several minutes by increasing the sample concentration or accelerating the cooling speed. (b). Stability at Low Temperature. In many cases, for the purpose of convenience, the hydrogel products such as jelly dessert and solid medium used for bacteria culture are preserved at low temperature for instant use. This requires that the hydrogel should be stable at low temperature. So hydrogel samples of Ch0CP2C and Ch0CP3C were stored at 4 °C for the comparison of low temperature stability. To our surprise, the hydrogel of Ch0CP2C will shrink when being stored at a low temperature (Figure 9a). The samples of Ch0CP2C can diminish to less than 50% of their original volume after being stored at 4 °C for 16 h, and became stiffer. More than 50% of the water was expelled from the samples. In our repeated test, the expelled free water can be as much as 70% of the originally added volume. The shrunken hydrogel did not swell when being stored at 35 °C or even higher. Only heating the container above Tgel to form a homogeneous solution and cooling it again can recover the hydrogel to its original state. No pronounced shrinkage was observed when the freshly prepared hydrogel of Ch0CP2C was stored at 37 °C. The speed of the shrinking process can be slowed down by being kept at elevated temperature (still below Tgel). In the network of shrunken hydrogel, the bundles were apparently aggregated, and the network was denser than the freshly prepared hydrogel as shown in SEM images (Figure 9b,c). Obviously, the aggregation of the microscopic structure was the immediate cause. Ishihara et al. found that the temperature dependence of hydration degree of PC polymer is positive.38 So the aggregation of microscopic structure may be ascribed to the decrease of hydrophilicity of phosphocholine at low temperature. Such shrinkage was not found in Ch0CP3C and other kinds of hydrogels. The better stability of Ch0CP3C hydrogel can be attributed to the higher hydrophilicity of P3C group, which keeps the network still dispersed well under low temperature. The absence of shrinkage of the other four kinds of hydrogels may be attributed to the relatively weak aggregation tendency of their hydrophobic parts (Figure 7b).

Figure 8. (a) Frequency sweep of dynamic modulus measurement. T = 25 °C. (b) Temperature dependence of dynamic modulus. f = 10 Hz. The speed of heating was 3 °C/min. All the hydrogel samples were freshly prepared, and the concentration was 6 mg/mL.

frequency of oscillatory shear decreased, and then they reached a plateau right after the cross point. In the plateau range, G′ (ca. 2460 Pa for Ch0CP2C and 10860 Pa for Ch0CP3C at 10 Hz) was higher than G″ (ca. 780 Pa for Ch0CP2C and 2130 Pa for Ch0CP3C at 10 Hz), which indicated the volume exhibited as an elastic solid.32 According to the theoretical models of transient networks, G′ at low frequency is proportional to the density of cross-linkages.33 The G′ of Ch0CP3C was higher than Ch0CP2C, which indicated that the density of crosslinkage was higher in the network of Ch0CP3C hydrogel. This agreed well with the SEM observation of their 3D networks (Figure 3). Amplitude sweep showed that the yield stress of 6 mg/mL Ch0CP2C and Ch0CP3C hydrogel was ca. 5.6 and 62 Pa, respectively (Figure S9, Supporting Information). Obviously, the mechanical performance of Ch0CP3C hydrogel was better than that of Ch0CP2C. This can be ascribed to the higher hydrophilicity of the phosphocholine group of Ch0CP3C, which made the bundles easier to be dispersed in aqueous medium. The temperature dependence of dynamic modulus was also measured (Figure 8d). When the temperature rose to around 60 °C, both G′ and G″ decreased drastically, and the curves got crossed at 63 °C for Ch0CP2C and 62 °C for Ch0CP3C, which indicated that a sol−gel transition occurred.34 This was basically consistent with the Tgel tested by DSC. Then the value of G″ became higher than G′ above Tgel, which indicated that the mixture was in the liquid state. At 80 °C the storage modulus and loss modulus of Ch0CP3C (ca. 10 and 52 Pa) were lower than that of Ch0CP2C (ca. 33 and 111 Pa), indicated the lower dynamic viscosity. The static viscosity also showed similar change (Figure S9, Supporting Information). Before the sharp decline at Tgel, the static viscosity of Ch0CP2C was 0.80 Pa·s and that of Ch0CP3C was 0.92 Pa·s; while the value changed to H


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[100] direction of a 2D hexagonal close-packing of nanofibrils (Figure 10a). Thus, the radius of nanofibrils r has a relation

Figure 10. (a) The radius of Ch0CP2C nanofibrils is calculated to be 26.4 Å by d/2r = sin 60°. (b) The semimajor axis of Ch0CP3C nanofibrils rma is calculated to be 27.5 Å, and semiminor axis rmi is 24.6 Å of the 2D mesophase close packing (a = 49.3 Å, b = 53.6 Å, γ = 117.3°). (c) Four types of arrangement for molecules within a unit cell.

Figure 9. Stability comparison at low temperature. (a) The hydrogel refrigerated at 4 °C for 48 h then stored at 35 °C for 72 h. (A, C, and E) Same sample of Ch0CP2C. (B, D, and F) Same sample of Ch0CP3C. The concentration of each sample was 3 mg/mL and the volume was 5 mL at beginning. SEM images of (b) freshly prepared and (c) shrunken hydrogel of Ch0CP2C. The freshly prepared and shrunken hydrogel samples were quickly frozen by liquid nitrogen and lyophilized before SEM observation. (d) The shrinkage of Ch0CP2C hydrogel was caused by aggregation of network at low temperature.

with the observed repeating distance d: d/2r = sin 60°.39 By this equation, the radius of Ch0CP2C can be calculated as 26.4 Å. In the sample of Ch0CP3C, there are two repeating distances, 47.7 and 43.8 Å, observed. The intensity ratio between corresponding peaks was almost constant when we make comparison between the samples with different thermotreatment during our repeated test. This suggests they are likely to be the periodic lengths within the same morph. As observed in HR-TEM, the nanofibrils of Ch0CP3C also form a 2D mesophase close packing. The repeating distance 47.7 Å can be indexed as (010), and 43.8 Å can be indexed as (100) of the 2D mesophase close-packing (a = 49.3 Å, b = 53.6 Å, γ = 117.3°) (Figure 10b). The different length of a and b crystalline axes might be caused by that: the unit cells of Ch0CP3C tilted by θ0 from the equatorial plane of nanofibrils, and made an ellipse shaped cross-section. All the unit cells are tilted in the same direction relative to the main axis. Then the length of unit cells (|A|) of Ch0CP3C is calculated to be 55.0 Å, and θ0 is 26.5°. There are two different repeating distances in samples of Ch6CP2C, Ch10CP2C, Ch10CP3C, and Ch12CP2C. During our repeated test, the values of distances calculated from the peaks are constant in each kind of hydrogel; but the intensity ratio between the corresponding peaks of different distances is

Ch0CP3C is the best in the family of Chol-Alkyl-PC LMWG from the above discussions. All of its superiorities can be ascribed to its molecular structure: a stronger hydrophobic part leads to higher gelation ability, a more hydrophilic polar group renders better rheological performance and better stability, simple molecular structure makes it can be easily synthesized from basic materials. X-ray Diffraction Studies. In order to obtain the information about molecular organization within those assemblies, both small (from 0.5° to 10°) and wide angle (from 3° to 60°) X-ray diffraction (XRD) studies were carried out (results listed in Table 1). Diffraction peaks were observed in each kind of sample, and this suggests the existence of wellordered aggregate structure in the assemblies (Supporting Information). And it is unlikely for the cholesteryl phospholipid molecules packed up in the fluidic state. The peaks found in the sample of Ch0CP2C can be interpreted as reflections from

Table 1. Repeating Distance Calculated from Modeling and Observed from XRD Data calculated value of |A|b (Å) compound

contour length (Å)a type Ic

type II type III type IV

Ch0CP2C Ch0CP3C

24.6 23.4

49.2 46.9

41.4 39.1

30.7 28.4

24.6 23.4

Ch6CP2C Ch10CP2C CH10CP3C Ch12CP2C

33.6 38.6 38.8 40.9

67.2 77.2 77.7 81.8

59.6 69.6 70.1 74.3

49.0 59.0 59.4 63.8

41.3 51.5 51.8 56.3

observed repeating distance (Å), and ratio (%)d

index

type,f tilt angle θ0 (deg)

45.7 (100) 47.7 (100) 43.8 45.9, (88.9), 42.6, (11.1) 55.5, (100), 52.9e 47.2, (87.5), 50.0, (12.5) 55.3, (82.4), 66.5, (17.6)

(100) (010) (100) (100) (100) (100) (100)

I, 0 I, 26.5 III, III, IV, IV,

20.5 19.8 24.3 10.8

Contour length of a single cholesteryl phospholipid molecule which was measured from molecular models. b|A|is the length of unit cell, and the thickness of phospholipid membrane is |A| cos θ0. cThe value of these four types is corresponding to the arrangement of two molecules in a unit cell which is illustrated in Figure 10c. dValues in parentheses showed the percentage of a given morph, roughly calculated from peak intensity. eObserved in some other samples which were not well thermo-treated. fCalculated for the arrangement of main morph. The arrangement of two molecules in a unit cell is shown in the upper part of Figure 11. a

I


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Figure 11. Proposed structural model of assemblies. Upper part of each model is proposed molecular arrangement within unit cells. (a) Helical nanofibril of Ch0CP2C. (b) Twisted nanoribbon of Ch6CP2C, the middle is the arrangement along a lateral axis, and the lower part is arrangement between different lateral axes within an entire nanoribbon. (c) Helical nanotube of Ch10CP2C. Left part of the middle is the arrangement along a single spiral line of helical packing, right part of the middle is the top view of this single spiral line. The lower part is the arrangement between different spiral lines within an entire nanotube. (d) Spindle shaped vesicle of Ch12CP2C.46.

orientation to be energetically preferable in a solid bilayer membrane, and this makes the molecules pack at a certain angle with respect to the adjacent molecule. This chiral packing contributes to the helical arrangement or twisting of the bilayer membrane, which finally leads to the formation of chiral supramolecular assemblies.25,41 It is well-known that many biological systems (cell membrane, DNA, and proteins etc.) are in the form of liquid crystals.42 The molecule arrangement within liquid crystals is quite sensitive to molecular structure, solvent, heat, electric field and so forth. The subtle transition can even be used for optical sensing applications.43 The morphological transition in our system also might be molecular structure induced variation of molecule arrangement in liquid crystal phase. Helfrich et al. developed the theory of curvature elasticity of lipid bilayers from classic theories of liquid crystalline phases, and it soon became very important to theoretical foundations for the research of lipid assemblies.44 Ou-Yang and Liu derived the tilt and surface shape-equilibrium equations for tilted chiral lipid bilayers in analogy with cholesteric liquid crystals. With this improvement of the symmetry, not only wound-ribbon helices but also twisted strips and vesicles of tilted chiral lipid bilayers can be successfully treated.45 The arrangement and orientation of molecules in these assemblies are illustrated by four different models based on the experimental data and calculation of theory of elasticity. The computer generated packing models are shown in Figure 11. In our proposed models, two phospholipid molecules aligned antiparallelly beside each other in a single unit cell (Figure 10c). Generally, the helical packing caused by inherent chirality can be illustrated as the unit cell rotates a certain angle to the energetically favorable position, when it moves a distance along the rotation axis. Then it is conceivable that the variation of rotation axis might cause a series change on the morphology of obtained assemblies. If we examine two adjacent unit cells in each of the models, we can find a continuous positional change of the rotation axis during the evolution from nanofibrils to spindle shaped vesicles. In nanofibrils, the rotation axis is right through the center of the unit cell. In twisted nanoribbon, a gradual transition can be observed: for the unit cells located at

not constant from the comparison between the samples with different thermo-treatment. This suggests that the two different repeating distances might originate from lipid polymorphism. As discussed above, the assemblies of Ch6CP2C, Ch10CP2C, Ch10CP3C, and Ch12CP2C were formed by lamellar structures, so the repeating distances found by XRD were corresponding to the thickness of the phospholipid membrane and they were all indexed as (100). The surface of phospholipid bilayer membrane is covered by the hydrophilic phosphocholine groups, and the hydrophobic parts are embedded in the middle. The phospholipid molecules on two opposite surfaces would adopt antiparallel orientation to build such a structure. In lots of reports about bilayer films and crystal structures of cholesteryl compounds, there are usually four typical arrangements for two cholesteryl molecules lying antiparallel as the distance between them getting closer (Figure 10c):40 type I, two molecules simply placed head to head, without any part overlapped or interdigitated; type II, two molecules laid head to head, and the C-17 cholesteryl side chains are interdigitated; type III, two molecules oriented antiparallel, and their cholesteryl groups are overlapped; type IV, the cycloalkane rings part of the cholesteryl groups overlapped. For types III and IV, the cholesteryl groups might also be placed beside the other one. In some cases, the two molecules may be between two types.40 These different arrangements will yield different thickness of the bilayer film. Their long axis may also tilt away from the normal of bilayer membrane by an angle θ0 as discussed by theory of elasticity. If the long axis aligns parallel with the normal of membrane (θ0 = 0), the membrane will be flat in mechanical equilibrium state; but if the long axis tilt (θ0 ≠ 0), the membrane tends to bend.41 This makes the accurate calculation of specific arrangement of molecules inside bilayer membrane almost impossible merely from the repeating distance indexed as (100). However, the tilt direction would be locked to the crystalline axes, if the membrane is in a crystalline phase.25b So the arrangement of molecules in the assemblies can still be roughly illustrated by these models, with the assumptions for molecular arrangement within unit cells. Proposed Structural Model of Supramolecular Assemblies. The chirality of molecules induces one particular J


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evolution can be attributed to the position of rotation axis of chiral packing moved gradually from the center of unit cells to the outside, and the driving force might be the need of the longer alkyl chains to reduce their contact with water. Our findings on the relationship between molecular structure and hydrogel properties might give insights into the design of hydrogelators. The best hydrogelator, Ch0CP3C, will be very promising for practical applications, not only for its simplicity in preparation but also for its superior performance.

main axis, the rotation axis passes through the center of these unit cells; for the unit cells at the rim, the rotation axis is out of the unit cells. The rotation axes are out of the unit cells in both helical nanotubes and spindle shaped vesicles, but in spindle shaped vesicles the unit cell rotates around two axes simultaneously. The fundamental reason for the morphological variation might be the influence of alkyl chain. When there’s no alkyl chain in the molecule, the influence of cholesterol part is predominant, and the bulky phosphocholine groups can also be well placed peripherally by the helical packing. So the hydrophobic interaction between cholesteryl parts can be maximized when the rotation axis is place at the center of unit cells in nanofibrils. But longer alkyl chain needs to be stabilized by having close contact with hydrophobic part of another molecule. So the alkyl chain will exert more and more influence on the helical packing when its length is increased, and this cause a positional movement of the rotation axis. The final rotation axis of helical packing is the result of the contest between cholesterol and alkyl chain. From the models of lamella structures in Figure 11, the contact between alkyl chain and water is effectively reduced in the inward bent leaflet of a curved phospholipid membrane, and both of the cholesteryl and alkyl chain parts can be stabilized. But for the outward bent leaflet of the curved phospholipid membrane, the stabilization originated from changing the ration axis is not as effective as the inward bent leaflet, because the alignment of long axes of molecules is divergent. This indicates the outer surface of curved phospholipid membrane is still not perfectly stabilized. Those nanofibrils also remain large area uncovered by phosphocholine groups. So there is still driving force for these supramolecular assemblies to combine with each other and form the 3D networks which eventually lead to the formation of hydrogel.

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ASSOCIATED CONTENT

* Supporting Information S

Chemical synthesis and characterization, nomenclature of chemicals, experimental procedures, digital photos, other relevant TEM and SEM photos, other rheological test results, rotation temperature dependence of CD, and XRD charts. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*(J.J.) Telephone/Fax: (+86)-571-87953729. E-mail: jijian@ zju.edu.cn. Funding Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We sincerely appreciate the help of Prof. Jun Ling, Doc. Hongli Zhu and Doc. Qicheng Zhang (Zhejiang University) on the computer modeling and programing of cholesteryl phospholipid assemblies. This work was financially supported from the National Science Fund for Distinguished Young Scholars (Grant 51025312), the Natural Science Foundation of China (Grant 50830106) and Open Project of State Key Laboratory of Supramolecular Structure and Materials (Grant SKLSSM201103)

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CONCLUSIONS In the present work, we demonstrated the effective strategy, which is based on chiral packing of hydrophobic groups, to obtain low molecular weight supramolecular hydrogels. A family of LMWG, in which the chiral cholesteryl group was con-nected to the hydrophilic phosphocholine through an alkyl chain, was designed and synthesized. Both supramolecular assemblies and properties of obtained hydrogel can be easily controlled by changing the alkyl chain length and spacer between two charged centers. Abundant supramolecular structures, including nanofibers, twisted nanoribbons, helical nanotubes, and even spindle shaped vesicles, were obtained with the length of alkyl chain between cholesteryl group and phosphocholine group increased from zero to 12 carbon atoms. However, the distance of spacer between two charged centers did not cause fundamental morphological changes. The gelation ability first gradually decreased as the alkyl chain length increased from zero to ten carbon atoms, and then rose again as the length increased to 12 carbon atoms. The increase of spacer between two charged centers from two to three carbon atoms lowered the gelation ability. But the comparison between Ch0CP2C and Ch0CP3C showed that the rheological properties were greatly improved because of higher hydrophilicity of P3C groups. CD spectra and XRD data showed the existence of helical organization and well-ordered structures in the assemblies. Structural models to illustrate the molecule arrangement were proposed based on experimental data and theory of elasticity. From these models, the morphological

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REFERENCES

(1) (a) Balakrishnan, B.; Banerjee, R. Chem. Rev. 2011, 111, 4453− 4474. (b) Paul, F.; Morin, A.; Monsan, P. Biotechnol. Adv. 1986, 4, 245−259. (2) (a) Cini, N.; Tulun, T.; Decher, G.; Ball, V. J. Am. Chem. Soc. 2010, 132, 8264. (b) Decher, G. Science 1997, 277, 1232−1237. (c) Grunze, M. Nature 2008, 454, 585. (d) Bowden, N.; Terfort, A.; Carbeck, J.; Whitesides, G. M. Science 1997, 276, 233−235. (3) (a) Weiss, R. G.; Terech, P. Molecular Gels - Materials with SelfAssembled Fibrillar Networks; Springer: Berlin, 2006; (b) Komatsu, H.; Matsumoto, S.; Tamaru, S.; Kaneko, K.; Ikeda, M.; Hamachi, I. J. Am. Chem. Soc. 2009, 131, 5580−5585. (c) Zhang, Y.; Kuang, Y.; Gao, Y.; Xu, B. Langmuir 2011, 27, 529−537. (4) (a) Loos, M.; Fèringa, B. L.; Esch, J. H. Eur. J. Org. Chem. 2005, 2005, 3615−3631. (b) Estroff, L. A.; Hamilton, A. D. Chem. Rev. 2004, 104, 1201−1218. (5) (a) Yang, Z. M.; Gu, H. W.; Fu, D. G.; Gao, P.; Lam, J. K.; Xu, B. Adv. Mater. 2004, 16, 1440−1444. (b) Kiyonaka, S.; Sugiyasu, K.; Shinkai, S.; Hamachi, I. J. Am. Chem. Soc. 2002, 124, 10954−10955. (c) Liu, Y.; Yu, Y.; Gao, J.; Wang, Z.; Zhang, X. Angew. Chem., Int. Ed. 2010, 49, 6576−6579. (6) (a) Zhao, F.; Gao, Y.; Shi, J. F.; Browdy, H. M.; Xu, B. Langmuir 2011, 27, 1510−1512. (b) Oda, R.; Huc, I.; Candau, S. J. Angew. Chem., Int. Ed. 1998, 37, 2689−2691. (c) Gao, Y.; Yang, Z. M.; Kuang, Y.; Ma, M. L.; Li, J. Y.; Zhao, F.; Xu, B. Peptide Sci. 2010, 94, 19−31. K


Macromolecules

Article

(7) (a) Langer, R.; Tirrell, D. A. Nature 2004, 428, 487−492. (b) Sanchez, C.; Arribart, H.; Guille, M. M. G. Nat. Mater. 2005, 4, 277−288. (c) Ryan, D. M.; Nilsson, B. L. Polym. Chem. 2012, 3, 18− 33. (8) Jikei, M.; Kakimoto, M. Prog. Polym. Sci. 2001, 26, 1233−1285. (9) (a) Vemula, P. K.; Li, J.; John, G. J. Am. Chem. Soc. 2006, 128, 8932−8938. (b) Zhang, S. Nat. Mater. 2004, 3, 7−8. (10) Gennis, R. B. Biomembranes: Molecular Structure and Function: Springer-Verlag: New York, 1989. (11) (a) Uchiyama, T.; Kiritoshi, Y.; Watanabe, J.; Ishihara, K. Biomaterials 2003, 24, 5183−5190. (b) Willis, S. L.; Court, J. L.; Redman, R. P.; Wang, J. H.; Leppard, S. W.; O’Byrne, V. J.; Small, S. A.; Lewis, A. L.; Jones, S. A.; Stratford, P. W. Biomaterials 2001, 22, 3261−3272. (12) (a) Hartgerink, J. D.; Beniash, E.; Stupp, S. I. Science 2001, 294, 1684−1688. (b) Godeau, G.; Barthélémy, P. Langmuir 2009, 25, 8447−8450. (c) Hamley, I. W. Soft Matter 2011, 7, 4122−4138. (13) (a) Köhler, K.; Förster, G.; Hauser, A.; Dobner, B.; Heiser, U. F.; Ziethe, F.; Richter, W.; Steiniger, F.; Drechsler, M.; Stettin, H.; Blume, A. Angew. Chem., Int. Ed. 2004, 43, 245−247. (b) Köhler, K.; Förster, G.; Hauser, A.; Dobner, B.; Heiser, U. F.; Ziethe, F.; Richter, W.; Steiniger, F.; Drechsler, M.; Stettin, H.; Blume, A. J. Am. Chem. Soc. 2004, 126, 16804−16813. (c) Moreau, L.; Barthélémy, P.; Maataoui, M. E.; Grinstaff, M. W. J. Am. Chem. Soc. 2004, 126, 7533− 7539. (d) Menger, F. M.; Peresypkin, A. V. J. Am. Chem. Soc. 2003, 125, 5340−5345. (14) (a) Snip, E.; Koumotoa, K.; Shinkai, S. Tetrahedron 2002, 58, 8863−8873. (b) Jung, J. H.; Kobayashi, H.; Masuda, M.; Shimizu, T.; Shinkai, S. J. Am. Chem. Soc. 2001, 123, 8785−8789. (c) Qiao, Y.; Lin, Y.; Wang, Y.; Yang, Z.; Liu, J.; Zhou, J.; Yan, Y.; Huang, J. Nano Lett. 2009, 9, 4500−4504. (15) Huang, Z.; Szoka, F. C., Jr. J. Am. Chem. Soc. 2008, 130, 15702− 15712. (16) Ž inić, M.; Vögtle, F.; Fages, F. Top. Curr. Chem. 2005, 256, 39− 76. (17) (a) Zidovska, A.; Evans, H. M.; Ahmad, A.; Ewert, K. K.; Safinya, C. R. J. Phys. Chem. B 2009, 113, 5208−5216. (b) Xu, J. P.; Ji, J.; Chen, W. D.; Shen, J. C. Macromol Biosci. 2005, 23, 164−171. (18) (a) Shimizu, T.; Masuda, M.; Minamikawa, H. Chem. Rev. 2005, 105, 1401. (b) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: New York, 1991. (19) Eibl, H. Proc. Natl. Acad. Sci. U.S.A. 1978, 75, 4074−4077. (20) Weers, J. G.; Rathman, J. F.; Axe, F. U.; Crichlow, C. A.; Foland, L. D.; Scheuing, D. R.; Wiersema, R. J.; Zielske, A. G. Langmuir 1991, 7, 854−867. (21) Oda, R.; Huc, I.; Schmutz, M.; Candau, S. J.; MacKintosh, F. C. Nature 1999, 399, 566−569. (22) (a) Chiruvolu, S.; Warriner, H. E.; Naranjo, E.; Idziak, S. H. J.; Räddler, J. O.; Piano, R. J.; Zasadzinski, J. A.; Safinya, C. R. Science 1994, 266, 1222−1225. (b) Li, N.; Ye, G.; He, Y.; Wang, X. Chem. Commun. 2011, 47, 4757−4759. (23) Murata, K.; Aoki, M.; Suzuki, T.; Harada, T.; Kawabata, H.; Komri, T.; Ohseto, F.; Ueda, K.; Shinkai, S. J. Am. Chem. Soc. 1994, 116, 6664−6667. (24) (a) Lin, K. C.; Weis, R. M.; McConnell, H. M. Nature 1982, 296, 164−165. (b) Konikoff, F. M.; Cohen, D. E.; Carey, M. C. J. Lipid Res. 1994, 35, 60−70. (c) Zastavker, Y. V.; Asherie, N.; Lomakin, A.; Pande, J.; Donovan, J. M.; Schnur, J. M.; Benedek, G. B. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 7883−7887. (25) (a) Messmore, B. W.; Sukerkar, P. A.; Stupp, S. I. J. Am. Chem. Soc. 2005, 127, 7992−7993. (b) Selinger, J. V.; Spector, M. S.; Schnur, J. M. J. Phys. Chem. B 2001, 105, 7157−7169. (26) (a) James, T. D.; Kawabata, H.; Ludwig, R.; Murata, K.; Shinkai, S. Tetrahedron 1995, 51, 555−556. (b) Jung, J. H.; Ono, Y.; Shinkai, S. Langmuir 2000, 16, 1643−1649. (c) Wang, R.; Geiger, C.; Chen, L.; Swanson, B.; Whitten, D. G. J. Am. Chem. Soc. 2000, 122, 2399−2400. (27) Suzuki, M.; Owa, S.; Shirai, H.; Hanabusa, K. Tetrahedron 2007, 63, 7302−7308.

(28) Dhruv, H. D.; Draper, M. A.; Britt, D. W. Chem. Mater. 2005, 17, 6239−6245. (29) Sijbesma, R. P.; Beijer, F. H.; Brunsveld, L.; Folmer, B. J. B.; Hirschberg, J. H. K. K.; Lange, R. F. M.; Lowe, J. K. L.; Meijer, E. W. Science 1997, 278, 1601−1604. (30) (a) Qiao, Y.; Lin, Y.; Yang, Z.; Chen, H.; Zhang, S.; Yan, Y.; Huang, J. J. Phys. Chem. B 2010, 114, 11725−11730. (b) van Bommel, K. J. C.; van der Pol, C.; Muizebelt, I.; Friggeri, A.; Heeres, A.; Meetsma, A.; Feringa, B. L.; van Esch, J. Angew. Chem., Int. Ed. 2004, 43, 1663−1667. (c) Kobayashi, H.; Friggeri, A.; Koumoto, K.; Amaike, M.; Shinkai, S.; Reinhoudt, D. N. Org. Lett. 2002, 4, 1423−1426. (31) Hule, R. A.; Nagarkar, R. P.; Altunbas, A.; Ramay, H. R.; Branco, M. C.; Schneider, J. P.; Pochan, D. J. Faraday Discuss. 2008, 139, 251− 264. (32) Haines, L. A.; Rajagopal, K.; Ozbas, B.; Salick, D. A.; Pochan, D. J.; Schneider, J. P. J. Am. Chem. Soc. 2005, 127, 17025−17029. (33) (a) Abdala, A. A.; Tonelli, A. E.; Khan, S. A. Macromolecules 2003, 36, 7833−7841. (b) Zhu, L. Z.; Shang-guan, Y. G.; Sun, Y. X.; Ji, J.; Zheng, Q. Soft Matter 2010, 6, 5541−5546. (34) Liu, J.; Chen, G.; Guo, M.; Jiang, M. Macromolecules 2010, 43, 8086−8093. (35) Yu, L.; Ding, J. Chem. Soc. Rev. 2008, 37, 1473−1481. (36) Tsuchiya, K.; Orihara, Y.; Kondo, Y.; Yoshino, N.; Ohkubo, T.; Sakai, H.; Abe, M. J. Am. Chem. Soc. 2004, 126, 12282−12283. (37) Maeda, H. Chem.Eur. J. 2008, 14, 11274−11282. (38) Ishihara, K.; Ueda, T.; Nakabayashi, N. Polym. J. 1990, 22, 355− 360. (39) Molinier, V.; Kouwer, P. J. J.; Fitremann, J.; Bouchu, A.; Mackenzie, G.; Queneau, Y.; Goodby, J. W. Chem.Eur. J. 2006, 12, 3547−3557. (40) (a) Bhattacharya, S.; Krishnan-Ghosh, Y. Langmuir 2001, 17, 2067−2075. (b) Krishnan-Ghosha, Y.; Gopalanb, R. S.; Kulkarnib, G. U.; Bhattacharya, S. J. Mol. Struct. 2001, 560, 345−355. (c) Park, Y. J. Bull. Korean Chem. Soc. 2007, 28, 299−302. (d) Shieh, H. S.; Hoard, L. G.; Nordman, C. E. Acta Cryst. B 1981, 37, 1538−1543. (e) Gao, Q.; Craven, B. M. J. Lipid Res. 1986, 27, 1214−1221. (f) Abrahamsson, S.; Dahlén, B. Chem.Phys. Lipids 1977, 20, 43−56. (41) Spector, M. S.; Selinger, J. V.; Singh, A.; Rodriguez, J. M.; Price, R. R.; Schnur, J. M. Langmuir 1998, 14, 3493−3500. (42) (a) Rey, A. D. Soft Matter 2010, 6, 3402. (b) Decher, G.; Ringsdorf, H. Liq. Cryst. 1993, 13, 57. (43) Shah, R. R.; Abbott, N. L. Science 2001, 293, 1296−1299. (44) Fuhrhop, J. H.; Helfrich, W. Chem. Rev. 1993, 93, 1565. (45) (a) Ou-Yang, Z.; Liu, J. Phys. Rev. Lett. 1990, 65, 1679. (b) OuYang, Z.; Liu, J. Phys. Rev. A 1991, 43, 6826. (46) In order to clearly demonstrate the arrangement of molecules, some parameters are not the observed values. But the basic principles of those structural models are rigorously kept unchanged.

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