Steric and Temperature Control of Enantiopurity of Chiral Mesoporous

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J. Phys. Chem. C 2008, 112, 1871-1877

1871

Steric and Temperature Control of Enantiopurity of Chiral Mesoporous Silica Huibin Qiu,† Shuguang Wang,† Wanbin Zhang,† Kazutami Sakamoto,‡ Osamu Terasaki,§ Yoshihisa Inoue,*,¶ and Shunai Che*,† School of Chemistry and Chemical Technology, State Key Laboratory of Composite Materials, Shanghai Jiao Tong UniVersity, 800 Dongchuan Road, Shanghai, 200240, People’s Republic of China, Department of Pure and Applied Chemistry, Tokyo UniVersity of Science, 2641 Yamazaki, Noda, Chiba 278-85, Japan, Structural Chemistry, Arrhenius Laboratory, Stockholm UniVersity, S-10691 Stockholm and Berzelii Center EXSELENT, Sweden, and Entropy Control Project (JST) and Department of Applied Chemistry, Osaka UniVersity, 2-1 Yamada-oka, Suita 565-0871, Japan ReceiVed: October 7, 2007; In Final Form: NoVember 14, 2007

To elucidate the factors and mechanisms that control the chiral mesoporous silica (CMS) formation, we employed a series of chiral amphiphilic molecules derived from nine different amino acids as templates and quantitatively investigated the effects of the substituent attached to the chiral center of amino acid upon CMS synthesis at various temperatures. The enantiomeric excess (ee) of the CMS obtained was a critical function of both the substituent’s steric bulk and the temperature, and eventually exceeded 90% ee by performing the CMS synthesis at 288 K in the presence of amphiphilic N-palmitoyl-Phe or Met. The temperature dependence study of the product’s ee not only gave the high ee’s but also enabled us to determine the differential enthalpy (∆∆H) and entropy (∆∆S) changes for antipodal CMS formation, which simultaneously increased with increasing steric bulk of the amino acid’s substituent, indicating their critical roles in determining the CMS’s enantiopurity. The present results also indicate that the CMS synthesis is a convenient tool for taking a snapshot of an average image of the dynamically fluctuating supramolecular aggregates with quantitative information (ee).

Introduction Self-assembly of small molecules into single and/or bundled helical architecture is amply found in nature.1 Controlling the sense of chirality (handedness) and the enantiopurity of such helical architectures are of prime interest in the fields of materials science, chemical sensing, and asymmetric catalysis.2-5 However, the detailed mechanisms and controlling factors, as well as the roles of incorporated molecules that enable the formation of such chiral structures, are not fully understood yet. It was believed that the given chirality of the molecules that form the chiral arrays has a decisive effect on the resulting supramolecular structure and hence leads invariably to a single chiral sense upon stacking under any conditions.6-10 Nevertheless, many exceptions from this simple belief that relates the molecular chirality to the supramolecular helicity have also been observed; indeed, almost racemic mixtures of left-handed and right-handed helices and tubules were formed from enantiomerically pure building blocks.11 These findings suggest that the molecular chirality is not the only driving force but some other opposing force(s) against the formation of chiral structure should exist to cancel the inherent chirality. In the studies on the self-assembly of small molecules into helical architecture, much attention has been paid to helical lyotropic liquid crystal due to its unique macroscopic chirality originating from the helical arrangement of the micelles formed * Authors to whom correspondence should be addressed. E-mail: [email protected]; [email protected]. † Shanghai Jiao Tong University. ‡ Tokyo University of Science. § Stockholm University and Berzelii Center EXSELENT. ¶ Osaka University and JST.

from surfactants or amphiphilic molecules. Also, it is wellknown that the mesoporous inorganic materials can be synthesized through self-assembly of various surfactants or amphiphilic molecules, i.e., the lyotropic liquid crystal templating route.12 By employing this method, we have recently achieved the first synthesis of chiral mesoporous silica (CMS) crystals through the self-assembly of amphiphilic L-alanine derivative and inorganic precursors.13,14 From the observation of high-resolution transmission electron microscope (HRTEM) and scanning electron microscope (SEM) images, we have revealed that mesoporous silica crystals have two-dimensional hexagonal chiral channels and that the handedness of the channels is reflected to the crystal morphology in these systems.13-15 The enantiomeric excess (ee) of the CMS was evaluated by counting the characteristic morphologies from 500 randomly chosen crystals in the SEM images, and the maximum left-handed/righthanded ratios were 3:1 (or 50% ee). It seemed that the exertion of the molecular chirality was diminished by some other forces operating in the synthesis. Furthermore, the racemic helical mesoporous silicas were also formed by using achiral amphiphilic molecules,15-20 which are usually not thought to be able to form helical lyotropic liquid crystal. These results prompted us to closely investigate the effects of the template’s chirality and the other important environmental factors that control the CMS formation. To elucidate the effects of molecular chirality on the enantiopurity of CMS, we employed nine N-acylamino acids (Scheme 1), i.e., N-palmitoyl-L-Ala (C16-L-Ala), N-palmitoylL-Val (C16-L-Val), N-palmitoyl-L-Ile (C16-L-Ile), N-palmitoylL-Met (C16-L-Met), N-palmitoyl-L-Phe (C16-L-Phe), N-palmitoylL-Pro (C16-L-Pro), N-palmitoyl-D-Phe (C16-D-Phe), N-palmitoyl-

10.1021/jp709798q CCC: $40.75 © 2008 American Chemical Society Published on Web 01/18/2008

1872 J. Phys. Chem. C, Vol. 112, No. 6, 2008 SCHEME 1: Molecular Structures of the N-Acylamino Acids Used in This Work

Qiu et al. Synthesis. The synthesis of the CMS samples was performed as follows: N-acylamino acid and NaOH were dissolved in deionized water with stirring at a given temperature. After the compounds were dissolved, a mixture of TMAPS and TEOS was added to the solution with stirring in 10 min. Then, the mixture was allowed to react at the same temperature for 1-3 days. The products were collected by centrifugal separation and dried in the air at 313 K. All the amphiphilic molecules were removed by extraction. The molar ratios of different Nacylamino acids based synthesis mixtures are shown below:

C16-L-Ala:NaOH:TMAPS:TEOS:H2O ) 1.0:0.90:0.5:5.8:1817 C16-L-Val:NaOH:TMAPS:TEOS:H2O ) 1.0:1.00:0.5:5.8:1978 C16-L-Ile:NaOH:TMAPS:TEOS:H2O ) 1.0:1.05:0.5:5.8:2056 C16-L-Met:NaOH:TMAPS:TEOS:H2O ) 1.0:1.14:0.5:5.8:2156 C16-L-Phe:NaOH:TMAPS:TEOS:H2O ) 1.0:1.20:0.5:5.8:1778 C16-L-Pro:NaOH:TMAPS:TEOS: H2O ) 1.0:0.90:0.5: 5.8:1967 C16-D-Phe:NaOH:TMAPS:TEOS:H2O ) 1.0:1.20:0.5:5.8:1778 C16-2-AIBA:NaOH:TMAPS:TEOS:H2O ) 1.0:0.95:0.5:5.8:1900 C16-rac-Phe:NaOH:TMAPS:TEOS:H2O ) 1.0:1.20:0.5:5.8:1778 2-aminoisobutyric acid (C16-2-AIBA), and N-palmitoyl-rac-Phe (C16-rac-Phe) in the present study. Taking into account the geometrical effect of the molecules on the helix formation, we chose chiral amphiphilic molecules from N-acylamino acids with different substituents attached to the chiral center and investigated the relationship of the enantiopurity of the CMS with the amphiphile’s molecular structure as an important internal factor and also with the reaction temperature as a crucial environmental factor. From the temperature dependence study of the product’s ee, we determined the differential enthalpy (∆∆H) and entropy (∆∆S) changes for the antipodal CMS formation for the first time and could obtain the CMS in unprecedentedly high ee’s over 90%. A plausible mechanism, involving the transformation of the molecular conformation of N-acylamino acids in the micelles, is proposed for accounting the CMS formation and the observed dependence of the enantiopurity upon the steric bulk of the substituent in amphiphile as well as the temperature. Experimental Section Chemicals. C16-L-Ala, C16-L-Val, C16-L-Ile, C16-L-Met, C16C16-L-Pro, C16-D-Phe, C16-2-AIBA, and C16-rac-Phe were synthesized according to the previous report.21 Tetraethoxylsilane (TEOS) was purchased from TCI and N-trimeth- oxysilylpropyl-N,N,N-trimethylammonium chloride (TMAPS, 50% in methanol) from Azmax. All chemicals were used as received without further purification.

L-Phe,

The minimum reaction temperatures of the CMS synthesis were limited to about 313 K for C16-L-Ala and C16-2-AIBA, 303 K for C16-L-Pro, and 288 K for the other chiral N-acylamino acids, which may be related to the Kraft point. Characterization. The morphologies of all samples were observed with SEM (JEOL JSM-7401F) at 1.0 kV. HRTEM observation was performed with a JEOL JEM-2100 microscope operated at 200 kV. Images were recorded with a CCD camera at 25000 magnification under low-dose conditions. For HRTEM measurements, all samples were dispersed in ethanol and deposited on a microgrid. XRD patterns were recorded on a Rigaku D/Max 2000 powder diffractometer equipped with Cu KR radiation. The nitrogen adsorption-desorption isotherms were obtained at 77 K on a Quantachrome NOVA 4200E surface area and pore size analyzer. The 13C NMR spectra were obtained on a 500 MHz Bruker AVANCE DRX NMR spectrometer; the acquisition time was set at 30 s or more to ensure that the spectral intensities were proportional to the trans/cis population. The samples (free acid) were first neutralized to the corresponding sodium salt form before being dissolved in D2O to take 13C NMR. Results and Discussion Synthesis of CMS. The characteristic morphologies of the CMSs synthesized with different N-acylamino acids at proper temperatures (see figure captions) are shown by SEM images in Figure 1. Low-magnification SEM images (Figure S1)

Steric and Temperature Control of CMS Enantiopurity

Figure 1. SEM images of the CMSs synthesized by using various N-acylamino acids as template: (a) C16-L-Ala (313 K), (b) C16-L-Val (293 K), (c) C16-L-Ile (293 K), (d) C16-L-Met (293 K), (e) C16-L-Phe (293 K), (f) C16-L-Pro (323 K), (g) C16-D-Phe (293 K), (h) C16-2-AIBA (323 K), and (i) C16-rac-Phe (293 K). Scale bars indicate 200 nm.

Figure 2. TEM images of the CMSs shown in Figure 1. Scale bar indicates 100 nm. Zoom in the images for better observing the fringes indicated by the arrows.

confirmed that all the samples are homogeneous in morphology with right- or left-handedness. All of these samples were composed of particles uniform in shape: the particles have welldefined twisted rodlike morphology with a hexagonal cross section. All of these particles were confirmed to have hexagonally ordered channels twisted from two-dimensional (2d) hexagonal p6mm by HRTEM images (Figure 2).13-16,22 The fringes indicated by the arrows correspond to the interplanar spacing of {10} planes. Between two sets of {10} fringes, the rod is twisted by 60°, which means that the distance between two sets of {10} fringes is one-sixth of one pitch length.13-16,22 XRD patterns (Figure 3) of all extracted samples revealed the three well-resolved 10, 11, and 20 reflections of the 2d hexagonal p6mm in the range 2θ ) 1.5-5°, indicating that these samples have highly ordered 2d hexagonal structure. Nitrogen sorption isotherms showed that all of the samples possess

J. Phys. Chem. C, Vol. 112, No. 6, 2008 1873

Figure 3. XRD patterns of the extracted CMSs shown in Figure 1.

uniform mesopores with a BJH (Barrett-Joyner-Halenda) pore diameter of 2.1-3.2 nm (Table S1). Temperature Dependence of the Enantiopurity of the CMS. As mentioned above, we have confirmed the presence of 2d hexagonal chiral channels in CMS and that the handedness of the channels reflect the same handedness of crystal morphology by using TEM analysis.13-15 Therefore, we can identify left- or right-handed CMS through the SEM images. Here, to express enantiopurity, the ee is defined by 100% × [(l - r)/ (l + r)], where l and r are the amounts of the left- and righthanded rods of the CMS. Therefore, the ee was statistically estimated by counting the characteristic morphologies of more than 500 randomly chosen crystals in the SEM images obtained in over 10 different regions on the sample holder. It gave a best semiquantified evaluation of the enantiopurity of the CMS obtained, which is reliable from the statistical consideration. Figures 4a-4c show the SEM images of the CMSs synthesized by using C16-L-Phe as template at 288, 293, and 313 K, respectively. It can be seen that the contents of the left-handed rods, denoted by the blue circles, increased with decreasing reaction temperature. The sample synthesized at 288 K was composed of almost exclusively left-handed rods. Figure 5 shows the temperature dependence of the ee’s of the CMSs synthesized by using C16-L-Phe, C16-D-Phe, C16-LMet, C16-L-Ile, C16-L-Val, C16-L-Ala, and C16-L-Pro as templates. The following was found: (i) the absolute ee values decreased with increasing the temperature of the reaction mixture from the maximum value to zero, which means that the resulting CMS is racemic; (ii) all of the CMSs synthesized with L-form N-acylamino acids are predominantly left-handed and the antipodal amphiphilic molecule (C16-D-Phe) gave the mirrorimaged profiles; (iii) the CMSs of over 90% ee were obtained by using C16-L-Phe and C16-L-Met as chiral templates at lower temperature; and (iv) the maxima of the absolute ee values and the degree of the temperature dependence depend on the substituent attached to the chiral center of the amphiphilic molecule in the order C16-L-Phe ∼ C16-L-Met > C16-L-Ile > C16-L-Val > C16-L-Ala. The absolute ee value of the CMS formed with C16-L-Pro was much smaller than those obtained with the other amphiphiles. In contrast to the above, all of the CMS samples synthesized with racemic (C16-rac-Phe) and achiral (C16-2-AIBA) N-acylamino acids under various reaction conditions were shown to be racemic; that is, the left-to-righthanded rod ratios were proved to be ∼1:1. The crystallinity and ee of the CMS depended also on the NaOH/surfactant ratio

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Figure 4. SEM images of the CMSs synthesized by using C16-L-Phe as template at (a) 288 K, (b) 293 K, and (c) 313 K. The left- and right-handed CMS crystals with characteristic twisted morphology are denoted by blue and red circles, respectively.

Figure 5. Temperature dependence of the ee’s of the CMSs synthesized by using different chiral N-acylamino acids as template.

but its effect was much smaller than that of temperature, which will be reported elsewhere. Differential Thermodynamic/Activation Parameters. Although the detailed mechanism or origin of the chiral CMS formation is not fully elucidated at present, it is unquestionable that the ratio of right-handed and left-handed CMS (r- and l-CMS) is controlled either by kinetics or by thermodynamics in one of the most critical stages in the overall silica synthesis trajectory starting from the amphiphilic molecule aggregation. Thus, the ee of the CMS is determined by the relative rate constant for l- and r-CMS formation (kl/kr) or by the relative stability of l- and r-CMS (Kl/Kr), which is experimentally equivalent to the relative abundance of l- and r-CMS, i.e., (100 + ee)/(100 - ee), by the definition of ee ) 100% × [(l - r)/ (l + r)]. Then, the differential activation free energy change, or the differential free energy change, of the most critical stage is given by a common equation,

∆∆G ) -RT ln(l/r)

(1)

where R and T represent the gas constant and temperature, respectively, and the superscripts (° and ‡) are omitted for simplicity purposes. The ∆∆G value is related to the differential

Figure 6. Temperature dependence of the ln(l/r) value for the CMSs synthesized by using different chiral N-acylamino acids as template.

enthalpy and entropy changes by the Gibbs-Helmholtz equation:

∆∆G ) ∆∆H - T∆∆S

(2)

Combining the two equations, we obtain

ln(l/r) ) -∆∆H/RT + ∆∆S/R

(3)

According to eq 3, we plotted the ln(l/r) values (derived form the average values of the ee’s shown in Figure 5), obtained in the CMS synthesis using five representative N-acylamino acids, as a function of the reciprocal temperature (Figure 6). Each plot shows a good linear relationship, indicating that both the ∆∆H and ∆∆S values can be considered to be constant over the temperature range studied. The ∆∆H and ∆∆S values were calculated from the slope and intercept of each plot; note that ∆∆H ) ∆Hl - ∆Hr and ∆∆S ) ∆Sl - ∆Sr. The differential parameters are summarized in Table 1. It is crucial that the absolute ∆∆H and ∆∆S values simultaneously decrease in the following order: C16-L-Met > C16-L-Phe > C16-L-Ile > C16-LVal > C16-L-Ala. Enthalpy-Entropy Compensation. The fact that the ∆∆H and ∆∆S values show synchronized decreases with decreasing substituent size prompted us to examine the validity of the

Steric and Temperature Control of CMS Enantiopurity

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TABLE 1: Differential Enthalpy (DDH) and Entropy (∆∆S) Changes in the CMS Synthesis with Different N-Acyl-L-amino Acids

a

N-acylamino acid

∆∆Ha

∆∆Sb

C16-L-Ala C16-L-Val C16-L-Ile C16-L-Met C16-L-Phe

-21 -23 -42 -75 -61

-58 -66 -128 -235 -191

Unit in kJ mol-1. b Unit in J mol-1 K-1.

widespread enthalpy-entropy compensation23-29 in the present system, although the number of the data sets is limited. Such an examination is particularly useful for demonstrating the identical enantiodifferentiation mechanism operating in different supramolecular systems. The ∆∆H values were plotted against the corresponding ∆∆S values to give excellent straight lines with almost negligible intercepts, as illustrated in Figure 7. Such compensatory enthalpy-entropy relationships have been observed in a wide variety of supramolecular systems.27,30,31 The linear relationship between ∆∆H and ∆∆S (and the almost zero intercepts) leads us to the empirical eq 4,

∆∆H ) β∆∆S

Figure 7. Compensation plot of the differential enthalpy (∆∆H) against the differential entropy change (∆∆S) upon CMS synthesis in the presence of various chiral N-acylamino acids.

(4)

where the proportional coefficient β is the slope of the compensation plot in Figure 6 and has a dimension of temperature. Merging eq 4 into the differential form of the GibbsHelmholtz equation (eq 2) gives eq 5:

∆∆G ) (1 - T/β)∆∆H

(5)

It is readily recognized from eq 5 that when the temperature is equal to β (which is called “equipodal” temperature,27 if enantiodifferentiation is concerned), any change in ∆∆H never affects the ∆∆G and the original product selectivity survives throughout the alteration in internal or external variants, such as surfactant, substituent, and solvent, as far as the same mechanism is operating. From the slope of the straight line (Figure 7), the equipodal temperature (T0) was calculated as 305 K. The helical handedness of CMS should rely on the chiral molecular structure of the amphiphilic molecule, and therefore l-CMS is preferred over the temperature range examined in the present cases where L-amino acids were used as the chiral source. Conformational Analysis of the N-Acylamino Acid. As mentioned above, the ee of produced CMS is a critical function of not only the molecular structure of amphiphile but also the reaction temperature. This fact indicates that the silica synthesis process, and the product’s properties as well, are a critical function of dynamic conformational changes of amphiphilic molecule rather than the static structure. Hence, it is essential to include the dynamic structural aspects of organic template for illustrating the formation mechanism of the CMSs of varying enantiopurities. So far, the helical micelle’s self-assembly phenomenon in helical lyotropic liquid crystal systems has been analyzed and discussed in relation to the chiral molecular shape. Thus, because of the molecular chirality, the amphiphilic molecules in cylinderlike micelles may also prefer helical propeller-like packing, handedness of which is dominated by the chirality of the amphiphilic molecule (Figure 8).32,33 Such supramolecular handedness is thought to be determined by the handedness of the above-mentioned helical molecular arrangement. Closest packing of the propeller-like hexagonal rod assembly of single

Figure 8. Molecular origin of the left-handed (left) and right-handed (right) helical structure of the CMS derived from the helical propellerlike packing of the chiral amphiphilic molecules.

handedness is achieved by twisting adjacent propeller-like rod micelles to aggregate into twisted larger helical assemblies. So a strand of closely packed propeller-like rod micelles resembles a twisted rod, whose handedness is determined by the handedness of the propeller-like rod micelles. From these observations, it is reasonable to assume that closest packing of the twisted micelles of the same chirality makes the helices, which in turn self-assemble to form the highly ordered helical 2d hexagonal mesostructure, and that the chiral sense of packing of the amphiphilic molecules essentially controls the enantiopurity of the resulting CMS. To make such helical arrangement more stable, the cylinder-like micelles may deform to two chiral curved shapes (Figure 8), which are thought to be determined by the handedness of the helical molecular arrangement. The remarkable temperature dependence of the ee may be interpreted thermodynamically by the equilibrium shift between two antipodal helical aggregates triggered by the temperaturedriven conformational changes of amphiphilic molecules.34,35 By rotation of the CR-N single bond of the amphiphilic molecule in micelle, a new conformer, which is diastereomeric to the original one, is formed with greatly altered topology around the head of the amphiphilic molecule. Usually, these rotational isomers (rotamers), which are diastereomeric to each other relative to the chiral center, are quickly equilibrated mutually at ambient temperatures, but in the stacked micellar structure the rate of equilibriation is decelerated and thus the

1876 J. Phys. Chem. C, Vol. 112, No. 6, 2008 two diastereomeric rotamers can survive to form the antipodal helical structures independently, as illustrated in Figure 8. For the L-form N-acylamino acids studied in this work, the conformer of the lowest energy may have the chiral sense upon packing, leading to left-handed CMS, while the less stable conformer of smaller proportion gives right-handed CMS, and vice versa for the D-form N-acylamino acids. Obviously, the proportion of the conformer with higher energy is increased at higher temperature, and hence the ee of the CMS decreases. As mentioned above, both the absolute ee maxima and the temperature dependence of the ee are critically affected by the substituent attached to the chiral center. The absolute ∆∆H and ∆∆S values gradually decrease by changing the substituent from CH2CH2SMe to benzyl to sec-Bu to iso-Pr, and then to methyl. This means that the ability (represented by ∆∆H) of the chiral amphiphilic molecule to form CMS with high ee, as well as its sensitivity (represented by ∆∆S) toward the reaction temperature, becomes smaller as the substituent size decreases. It is likely that the conformational freedoms of the chiral amphiphilic molecules in aggregate are determined by the intra- and/or intermolecular steric hindrance of the substituent attached to the chiral center. Thus, the introduction of a bulky substituent at the chiral center should reduce the conformational freedoms through destabilization of less-favored diastereomeric conformation, leading to the formation of CMS of higher ee. At the same time, this destabilization makes the population to the lessfavored conformation more sensitive to the reaction temperature. Further elucidation of the quantitative relationship between the energy difference of the diastereomeric pair and the bulkiness of the substituent is in progress. The present results also indicate that the organic template and its conformational transformation play an important role in the formation of CMS, rather than the morphology transformation.36 Other effects including the racemization37,38 and trans-cis diastereorotamerization39-42 of the chiral N-acylamino acids during the CMS synthesis may also account for the temperature dependence of ee. However, CD analysis showed that there is no discernible racemization of the chiral N-acylamino acids even when the reaction mixtures were heated to 353 K (in the absence of TEOS) for 3 days (data not shown). The 13C NMR spectra (Figure S3), with no doublet peaks, also eliminate the possibility of trans-cis diastereorotamerization of Ala-, Val-, Ile-, Met-, and Phe-based N-acylamino acids.41-43 The trans form is preferred because it is much more stable than the cis form.43 Only for C16-L-Pro, the cis form was detected with a proportion of about 19% (Figures S4 and S5), which may account for its low ability to form CMS with high ee. In the case of the racemic amphiphilic molecule, only the racemic CMS was formed, indicating that the homochiral D-D and L-L interactions are predominant in the micelles.44 This is very different to explain if the heterochiral D-L interaction is favored to give nematic phase in lyotropic liquid crystal systems. Generally, achiral amphiphilic molecules are believed to lack the ability to form helical lyotropic liquid crystal without adding chiral dopant.34,45,46 The formation of solid helical lyotropic liquid crystal replica (CMS) may be taken as unambiguous evidence in support of the idea that achiral amphiphilic molecules also form the helical lyotropic liquid crystal which is hard to be detected by the traditional optical methods since it is racemic at the macroscopic level. Conclusions Various types of N-acylamino acids were successfully used to form CMS. The enantiopurity of the CMS obtained was well-

Qiu et al. controlled by the temperature of the reaction mixture, depending on the substituent attached to the chiral center. CMS of >90% ee was synthesized at low temperature by properly choosing the N-acylamino acid with a bulky substituent as template. A general mechanism, which is compatible with the thermodynamic analysis, was proposed to explain the CMS formation and the temperature control of its enantiopurity by taking into account the amphiphile’s conformation transformation. We believe that the present study promotes our in-depth understanding of the CMS formation mechanism at the molecular level and stimulates similar endeavors in the related systems. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant Nos. 20425102 and 20521140450), the China Ministry of Education, and Shanghai Science Foundation (05XD14010). Financial support from JST (Y.I. and O.T.) and VR and Vinnova (O.T.) are also acknowledged. Supporting Information Available: Low-magnification SEM images, N2 sorption data, and CD spectrum of CMS; 13C NMR spectra of C16-L-Phe and C16-L-Pro. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Klug, A. Angew. Chem., Int. Ed. Engl. 1983, 22, 565. (2) Spector, M. S.; Price, R. R.; Schnur, J. M. AdV. Mater. 1999, 11, 337. (3) Tang, K.; Green, M. M.; Cheon, K. S.; Selinger, J. V.; Garetz, B. A. J. Am. Chem. Soc. 2003, 125, 7313. (4) Jung, J. H.; Kobayashi, H. K.; Masuda, M.; Shimizu, T.; Shinkai, S. J. Am. Chem. Soc. 2001, 123, 8785. (5) Numata, M.; Sugiyasu, K.; Hasegawa, T.; Shinkai, S. Angew. Chem., Int. Ed. 2004, 43, 3279. (6) Tachibana, T.; Kambara, H. J. Am. Chem. Soc. 1965, 87, 3015. (7) Fuhrhop, J.-H.; Schnieder, P.; Boekema, E.; Helfrich, W. J. Am. Chem. Soc. 1988, 110, 2861. (8) Frankel, D. A.; O’Brien, D. F. J. Am. Chem. Soc. 1994, 116, 10057. (9) Messmore, B. W.; Sukerkar, P. A.; Stupp, S. I. J. Am. Chem. Soc. 2005, 127, 7992. (10) Brizard, A.; Oda, R.; Huc, I. Top. Curr. Chem. 2005, 256, 167. (11) Thomas, B. N.; Lindemann, C. M.; Corcoran, R. C.; Cotant, C. L.; Kirsch, J. E.; Persichini, P. J. J. Am. Chem. Soc. 2002, 124, 1227. (12) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (13) Che, S.; Liu, Z.; Ohsuna, T.; Sakamoto, K.; Terasaki, O.; Tatsumi, T. Nature 2004, 429, 281. (14) Ohsuna, T.; Liu, Z.; Che, S.; Terasaki, O. Small 2005, 1, 233. (15) Wu, X.; Jin, H.; Liu, Z.; Ohsuna, T.; Terasaki, O.; Sakamoto, K.; Che, S. Chem. Mater. 2006, 18, 241. (16) Wu, X.; Ruan, J.; Ohsuna, T.; Terasaki, O.; Che, S. Chem. Mater. 2007, 19, 1577. (17) Trewyn, B. G.; Whitman, C. M.; Lin, V. S.-Y. Nano Lett. 2004, 4, 2139. (18) Wang, B.; Chi, C.; Shan, W.; Zhang, Y.; Ren, N.; Yang, W.; Tang, Y. Angew. Chem., Int. Ed. 2006, 45, 2088. (19) Zhang, Q.; Lu¨, F.; Li, C.; Wang, Y.; Wan, H. Chem. Lett. 2006, 35, 190. (20) Yang, S.; Zhao, L.; Yu, C.; Zhou, X.; Tang, J.; Yuan, P.; Chen, D.; Zhao, D. J. Am. Chem. Soc. 2006, 128, 10460. (21) Takehara, M.; Yoshimura, I.; Takizawa, K.; Yoshida, R. J. Am. Oil Chem. Soc. 1972, 49, 157. (22) Jin, H.; Liu, Z.; Ohsuna, T.; Terasaki, O.; Inoue, Y.; Sakamoto, K.; Nakanishi, T.; Ariga, K.; Che, S. AdV. Mater. 2006, 18, 593. (23) Leffler, J. E. J. Org. Chem. 1955, 20. 1202. (24) Leffler, J. E.; Grunwald, E. Rates and Equilibria of Organic Reactions; Wiley: New York, 1963; reprinted version from Dover: New York, 1989. (25) Exner, O. Correlation Analysis of Chemical Data; Plenum: New York, 1988. (26) Chen, R. T. Correlation Analysis in Coordination Chemistry; Anhui Educational Publishing: Hefei, 1995 (in Chinese). (27) Inoue, Y.; Wada, T. In AdVances in Supramolecular Chemistry; Gokel, G. W., Ed.; JAI Press: Greenwich, CT, 1997; Vol. 4, pp 55-96. (28) Grunwald, E.; Steel, C. J. Am. Chem. Soc. 1995, 117, 5687.

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