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Stereospecific Morphogenesis of Aspartic Acid Helical Crystals through Molecular Recognition Yuya Oaki and Hiroaki Imai* Department of Applied Chemistry, Faculty of Science and Technology, Keio UniVersity, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, 223-8522, Japan ReceiVed December 18, 2006. In Final Form: March 2, 2007 Helical morphologies were generated from aspartic acid (Asp) crystals in agar gel matrix. The morphogenesis stereospecifically proceeded in the helical crystal growth: D- and L-Asp provided left- and right-handed structures, respectively. The backbone of the helical morphology was twisted twins of tilted unit crystals, as was the case with inorganic helical crystals. The molecular recognition between the Asp crystals and agar matrix molecules resulted in the stereospecific morphogenesis. The chirality in Asp and agar molecules, the enantiomorph of unit crystals, and the resultant macroscopic helix were exquisitely associated with each other.
Introduction Chirality, which exists in various fields and scales, has fascinated many researchers because of its scientific importance and relevance to natural homochirality. Molecules, macromolecules, and crystals are typical chiral building blocks leading to higher organized architectures. In the nanoscopic scale, helical molecular assemblies were fabricated from chiral and achiral components and were applied to chiral functional materials and templates.1-9 The formation process and the chirality in the helical aggregates were well investigated by the researchers in supramolecular chemistry. As well as the supramolecular assemblies, it is well-known that some polymers and inorganic crystals induce helical crystalline materials in a certain growth condition, even though the building blocks are not chiral.10-19 While much effort has been devoted to the investigation of the molecular helices, understanding of helical crystals is a current challenge in chemistry. Moreover, transfer and amplification of * Corresponding author. E-mail:
[email protected]. (1) Rowan, A. E.; Nolte, R. J. M. Angew. Chem., Int. Ed. 1998, 37, 63-67. (2) Berthier, D.; Buffeteau, T.; Leger, J. M.; Oda, R.; Huc, I. J. Am. Chem. Soc. 2002, 124, 13486-13491. (3) Sumiyoshi, T.; Nishimura, K.; Nakano, M.; Handa, T.; Miwa, Y.; Tomioka, K. J. Am. Chem. Soc. 2003, 125, 12137-12141. (4) Kato, T.; Mizoshita, N.; Kishimoto, K. Angew. Chem., Int. Ed. 2006, 45, 38-68. (5) Solladie´, G.; Zimmermann, R. G. Angew. Chem., Int. Ed. Engl. 1984, 23, 348-362. (6) van Bommel, K. J. C.; Friggeri, A.; Shinkai, S. Angew. Chem., Int. Ed. 2003, 42, 980-999. (7) Hanabusa, K.; Yamda, M.; Kimura, M.; Shirai, H. Angew. Chem., Int. Ed. Engl. 1996, 35, 1949-1951. (8) Jung, J. H.; Ono, Y.; Hanabusa, K.; Shinkai, S. J. Am. Chem. Soc. 2000, 122, 5008-5009. (9) Che, S.; Liu, Z.; Ohsuna, T.; Sakamoto, K.; Terasaki, O.; Tatsumi, T. Nature 2004, 429, 281-284. (10) Li, C. Y.; Cheng, S. Z. D.; Ge, J. J.; Bai, F.; Zhang, J. Z.; Mann, I. K.; Harris, F. W.; Chien, L. C.; Yan, D.; He, T.; Lotz, B. Phys. ReV. Lett. 1999, 83, 4558-4561. (11) Suda, J.; Nakayama, T.; Matsushita, M. J. Phys. Soc. Jpn. 1998, 67, 2981-2983. (12) Yu, S. H.; Co¨lfen, H.; Tauer, K.; Antonietti, M. Nat. Mater. 2005, 4, 51-55. (13) Li, M.; Mann, S. Langmuir 2000, 16, 7088-7094. (14) Ozin, G. A.; Yang, H.; Sokolov, I.; Coombs, N. AdV. Mater. 1997, 9, 662-667. (15) Wang, Z. L.; Kong, X. Y.; Ding, Y.; Gao, P.; Hughes, W. L.; Yang, R.; Zhang, Y. AdV. Funct. Mater. 2004, 14, 943-956. (16) Imai, H.; Oaki, Y. Angew. Chem., Int. Ed. 2004, 43, 1363-1368. (17) Oaki, Y.; Imai, H. J. Am. Chem. Soc. 2004, 126, 9271-9275. (18) Oaki, Y.; Imai, H. Langmuir 2005, 21, 863-869. (19) Oaki, Y.; Imai, H. AdV. Funct. Mater. 2005, 15, 1407-1414.
chirality between different hierarchies are a significant topics to a broad range of chemistry disciplines. Aspartic acid (Asp), a fundamental chiral amino acid in nature, is an essential building block for the higher organized bioorganic materials. In nature, chiral architectures in different scales and hierarchies are associated with each other. Chiral amino acids are building blocks of peptides and proteins, and the folding structures of proteins are closely related to versatile biological activities. In this paper, we show a new family of hierarchical chiral architecture having a definite crystalline material. While the R-helix structure of protein is an essential helical material consisting of amino acids, no macroscopic crystalline helix has been found on any amino acid molecules. We found that helical morphologies emerged from D- and L-Asp crystals grown in an agar matrix. Furthermore, the chirality of Asp molecules perfectly directed the handedness of macroscopic helical crystalline architecture with the assistance of an agar matrix: the left- and right-handed helices were dominantly obtained from D- and L-Asp, respectively. Experimental Section Materials. Monoclinic crystals of D-Asp (Wako Pure Chemical, 98.0%), L-Asp (Tokyo Kasei Kogyo, 98.0%), and agar powder (Junsei Chemical) were used without further purification. Experimental Procedure. Stock solutions containing 4 g dm-3 of D- or L-Asp were prepared by using purified water at room temperature. While 0.5 g dm-3 of agar powder was added in the 30 cm3 of stock solution, the precursor solution was heated to approximately 100 °C by a hot stirrer with sealing. After the agar was completely dissolved, the 2 cm3 of precursor solution was immediately poured into polystyrene vessels (35 mm in diameter and 8 mm in height) and maintained at 25 °C over a day without sealing. Characterization. The resultant material was observed by field emission scanning electron microscopy (FESEM) (FEI, Sirion operated at 2.0 kV) without washing. The sample vessels were sputtercoated with Au/Pd and then directly put in the sample chamber. The structural analysis was also carried out by X-ray diffraction (XRD) (Bruker, D8 Advance with Cu KR radiation). The powdered samples and the vessels without the wall were placed on the sample holder.
Results and Discussion Emergence of Helical Morphologies. After water evaporated from the precursor solution, helical morphologies were observed
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Figure 1. Macroscopic morphology of the helical crystals: (a) the macroscopic appearance in a sample vessel; (b-d) typical FESEM images of the right- (b and d) and left-handed (c) helical morphologies generated from L- and D-Asp crystals, respectively.
on the branches of a spherulitic morphology in a vessel (Figure 1a). In contrast, D- and L-Asp formed clear, platelike crystals from aqueous solution without an agar molecule regardless of the molecular chirality. Figure 1b-d shows the typical FESEM images of the helical D- and L-Asp crystals. The diffusioncontrolled growth in the agar gel matrix resulted in the emergence of helical crystals, as was the case with our previous works.16,17 The pitch of the helices was varied within a range of 50 µm to 1 mm in response to the local growth condition, as shown in Figure 1, parts b and d. The helical materials in several vessels were powdered and then analyzed by XRD. The patterns revealed that both the architectures consisting of D- and L-Asp were not helical molecular assemblies but crystalline materials (Figure 2). The peak positions were the same in the both powdered samples of D- and L-Asp helical crystals and were assigned to that of the diffraction data (Joint Committee of Powder Diffraction Standards (JCPDS) card nos. 39-1523 and 23-1519; space group, P21; monoclinic system). The slight differences in the peak intensity ratio were not clear because the detailed crystal structure analysis was not performed in both the D- and L-Asp. The handedness was never changed on a branch or in a spherulitic morphology: one spherulitic form in a sample vessel was occupied by either left- or right-handed helices. In the 70 samples counted, the morphological chirality was absolutely determined by the D- or L-Asp crystals, as depicted in Figure 1. The left-handed helices were dominantly generated from D-Asp crystals, whereas L-Asp provided only right-handed forms. Structure of Helical Crystals. The helical Asp crystals showed backbone structure consisting of tilted units with twinned connections (Figure 3). According to the results of our previous study, the backbone of inorganic helical crystals was the twisted twins of the tilted unit crystals.16,17 The morphogenesis was achieved through the diffusion-controlled growth of the tilted unit crystals in a polymer matrix.16-19 The lower symmetry of the crystal system, such as triclinic and orthorhombic, induced the formation of tilted unit crystals.16,18 Thus, the monoclinic system of D- and L-Asp crystals was naturally expected to form the tilted units. In this study, the magnified image suggests that the tilted units were regularly accumulated in the backbone structure (Figure 3, parts a and b). When the XRD pattern was
Figure 2. XRD patterns of the helical crystals: (a) the peak positions of D- and L-Asp crystals according to the JCPDS cards; (b and c) the powdered L- and D-Asp crystals, respectively.
collected from the samples pressed on the vessel, the peaks of (100), (200), (300), (-3 0 1), and (-1 3 1) only were observed (Figure 3c). The diffraction peaks of (h00) planes were enhanced in the pattern, whereas the peaks perpendicular to (h00) planes were extinct. The results suggest that the platy unit crystals mainly exhibited (100) faces, as shown in Figure 3, parts d and e. In addition to the XRD analysis, the morphology and the growth direction including the Miller index were estimated using several FESEM images of an Asp crystal grown without agar and previous literature about the crystal structure.20-22 In consequence, the unit crystals were surrounded with (100), (001), (111), and (110) faces and had a three-dimensionally tilted form (Figure 3, parts d and e). The growth of the tilted units proceeded in the [010] direction because the dihedral angle in the tilted unit crystal was consistent with that calculated from the model (Figure 3f). Through several FESEM images, the tilted units were aligned in the [010] direction and were twisted with a rotation of ca. 3.0° on the (110) faces as the twin plane (Figure 4a-e). Therefore, the helical backbone structures of the Asp unit crystals were also regarded as twisted twins, as was the case with the inorganic helices.16 Although the pitch of the helical crystals was varied in the samples as shown in Figure 1b-d, the rule of connections between each unit was constant regardless of the chirality in the molecules. The rotated angles are determined by the coincident lattice on the twin plane, and the local growth condition around the tilted unit crystals contributes the variation of the helical pitches.16 Since the helical backbone formed from crystal growth under diffusion-controlled condition, the fundamental structures of the twisted twins were not different in both the D- and L-Asp helical crystals. The morphogenesis would be substantially different from that of a supramolecular assembly of chiral organic (20) Asahi, T.; Takahashi, M.; Kobayashi, J. Acta Crystallogr., Sect. A 1997, A53, 763-771. (21) Derissen, J. L.; Endeman, H. J.; Peerdeman, A. F. Acta Crystallogr., Sect. B 1968, B24, 1349. (22) Kishihata, A.; Kishimoto, S.; Nagashima, N. J. Cryst. Growth 1996, 167, 729-733.
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Figure 3. Backbone structure of the Asp helical crystal: (a and b) FESEM and the magnified images of the twisted twins of the tilted unit crystals; (c) XRD patterns recorded from the samples pushed on the vessel; the broadened hallo around 20° was caused by the polystyrene vessel; the filled circles with broken lines represent the peak positions by JCPDS card no. 23-1519; (d) the morphology of tilted units embedded in the unit cell; (e) a tilted unit; (f) FESEM image viewed perpendicular to the platy unit crystals, indicating that the dihedral angle between the two faces corresponded to that calculated from the unit cell structure.
molecules.22 While the interactions at the molecular and mesoscopic levels involved the formation helical molecular assembly, the crystalline helices were controlled through crystal growth. However, further investigation is needed to interaction styles at various scales. Proposed Model for the Stereospecific Morphogenesis. On the basis of the model, the left- and right-handed helices were supposed to be produced from the opposite sides of the primary unit crystal (Figure 4f), indicating that an equal amount of leftand right-handed forms is eventually generated.16,17 The leftand right-handed forms would be grown from the primary unit regardless of the chirality in the Asp molecules (Figure 4f): righthanded helices form on the (110) face in the [010] direction, and left-handed ones accumulate on the (-1 -1 0) face in the [0 -1 0] direction. However, the growth of both right- and left-handed structures was never observed in this study (Figure 5). Whereas the left-handed helices were grown from the starting point, the growth of the right-handed structure did not proceed in the case of D-Asp (Figure 5, parts a and b). Interestingly, the results imply that the stereospecific morphogenesis involved the growth behavior from the unit crystals in the initial stage. When the ratio of D- and L-Asp was varied, the helical morphologies were obtained within the limited conditions. Herein, the percentage of L-Asp contained in the total weight of Asp is denoted as RL for the following explanations: for example, only the left- and right-handed structures were formed at RL ) 0 and RL ) 100 as discussed above, respectively. The left- and right-handed types were clearly recognized at the conditions of RL ) 1 and RL ) 99, respectively. When the RL values were changed to 3 and 97,
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Figure 4. FESEM images (a-d) and the schematic representation (e) of the twisted twins with the constant rotated angle: (a and b) D-Asp; (c and d) L-Asp. The twisted angles were estimated from the half-pitch of the helix (a and c) and the size of a plate (b and d). Since about 60 units made up the half (180°) of the helical backbone, a rotated angle on a unit was deduced to be about 3.0°. We carried out the same calculation in many samples.
the left- and right-handed types respectively formed and the winding morphology was also observed at these conditions. It was difficult to find the helical architectures with the further changes of RL values within 5 to 95. However, detailed investigations are required for the clarification of these phenomena. At least, the small amount of enantiomer (RL ) 1, 3, 97, 99) did not influence the macroscopic chirality in the helical crystals. The homochirality in macroscopic shape can be associated with molecular recognition between the Asp crystal surface and the agar molecules (Figure 6). Since the helical architecture is not a molecular assembly but a crystalline material in this report, the interaction on the crystal face would mainly concern the stereospecific morphogenesis. The interaction at the molecular level, in contrast, is significant for the morphogenesis of supramolecular assemblies. Recently, it has been well studied that stereochemical molecular recognition takes place at the interface between the crystal face and the chiral organic molecule. Chiral organic molecules recognize chiral crystal faces, such as positive (010) and negative (0 -1 0), made by achiral molecules.24-26 We precisely controlled the macroscopic chirality in the helical crystals through stereochemical recognition.17 On the other hand, the stereospecific morphogenesis of Asp crystals was not attributed to the stereochemical molecular recognition because the concept of positive and negative faces was not adapted to the crystals of chiral molecules. (23) Selinger, J. V.; Spector, M. S.; Schnur, J. M. J. Phys. Chem. B 2001, 105, 7157-7169. (24) Hazen, R. M.; Sholl, D. S. Nat. Mater. 2003, 2, 367-374 and references therein. (25) Cody, A. M.; Cody, R. D. J. Cryst. Growth 1991, 113, 508-519. (26) Orme, C. A.; Noy, A.; Wierzbicki, A.; McBridge, M. T.; Grantham, M.; Teng, H. H.; Dove, P. M.; DeYoreo, J. J. Nature 2001, 411, 775-779.
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Figure 5. FESEM images (a and b) and the schematic illustration (c) of the starting point: (a and b) the growth and the inhibition of the left- and right-handed helices in the case of D-Asp crystals, respectively; (c) the schematic representation based on the twisted twin model.
Figure 6. Proposed model for the stereospecific morphogenesis: (a) the mirror image of D- and L-Asp molecules represented by left and right hands, respectively; (b and c) the model of D- and L-Asp unit crystals illustrated with left and right hands, respectively; (d) the structural formula of an agar molecule (D-polysaccharide); (e) the left- and right-handed growth behaviors from the primary unit with molecular recognition.
The stereochemistry regarding crystallization of a chiral organic molecule has been well studied in previous papers.27,28 According to the reports, the enantiomeric D- and L-Asp form enantiomorph unit crystals, as depicted by right- and left-handed models (Figure 6a-c). A pair of positive and negative faces is exhibited on the opposite sides of polyhedral crystal, even though the absolute assignment of (110) and (-1 -1 0) faces could not be demonstrated in the analysis. The (110) and (-1 -1 0) faces in one enantiomorph expose the different structure, corresponding to the relationship between the fingers and wrist in one hand (Figure 6b). The pair of (110) and (-1 -1 0) faces in one enantiomorph is the mirror image to that in another enantiomorph, as compared to right and left hands (Figure 6, parts b and c). For example, the L-Asp unit crystal exhibiting (110) and (-1 -1 0) faces assimilates to right hands including the fingers and wrists (upper panel in Figure 6, parts b and c). The illustration of left hands, in contrast, represents the unit crystal of D-Asp (lower panel in Figure 6, parts b and c). Therefore, the faces of (-1 -1 0) in L-Asp and (110) in D-Asp expose the similar chemical structure because the both faces correspond to the wrist side of the hands (Figure 6c). In a similar manner, the faces of (110) in L-Asp and (-1 -1 0) in D-Asp anticipate the similar interaction behavior with additives. When the additives selectively adsorb on the (-1 -1 0) face, the growth inhibition in the [0 -1 0] direction results in the (27) Weissbuch, I.; Popovitz-Biro, R.; Lahav, M.; Leiserowitz, L. Acta Crystallogr., Sect. B 1995, B51, 115-148. (28) Weissbuch, I.; Addadi, L.; Lahav, M.; Leiserowitz, L. Science 1991, 253, 637-645.
domination of right-handed growth (Figure 6c-e). In this experimental system, L- or D-Asp crystals were grown in agar molecules consisting of polysaccharide as a growth matrix. The stereospecific morphogenesis may be deduced to be the specific interaction between agar molecules and the crystal faces as represented by fingers or wrists (Figure 6b-d). The molecular recognition influences the growth behavior of left- and righthanded forms (Figure 6e). As previously discussed, the rightand left-handed helices would be generated from the opposite sides of unit crystals (Figures 4f and 6c). If the (-1 -1 0) face of L-Asp and the (110) face of D-Asp accepted the interaction of polysaccharide, the left- and right-handed growth patterns of L- and D-Asp, respectively, were inhibited (Figure 6b-d). Therefore, the left-handed helices of D-Asp were grown from the (-1 -1 0) face in the [0 -1 0] direction in association with agar molecules. The right-handed growth of L-Asp, in contrast, occurred on the (110) face in the [010] direction. In this way, it is inferred that the molecular recognition at the interface leads to the stereospecific morphogenesis of helical Asp crystals. However, further experimental demonstration is required for the understanding of molecular recognition and the subsequent stereospecific morphogenesis.
Conclusions We found the stereospecific morphogenesis of helical Asp crystals in an agar matrix. Interestingly, D- and L-Asp crystals perfectly formed left- and right-handed structures in an agar gel matrix, respectively. The helical morphologies stereospecifically emerged from the diffusion-controlled growth of the tilted unit
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crystals, indicating that the twisted twin model could be well adapted to organic crystals. Moreover, the stereospecific morphogenesis was realized through molecular recognition between Asp unit crystals and agar molecules. The chiral molecules formed enantiomorphic unit crystals, and then the subsequent growth with association of the agar matrix induced the helical morphologies through molecular recognition and the diffusioncontrolled condition. The chirality in different hierarchies, including Asp molecules, the enantiomorph of unit crystals, agar matrix, and the macroscopic morphologies exquisitely cooperated with each other. According to our previous reports, the techniques
Oaki and Imai
designing helical crystals promise to develop chiral functional materials having unique optical, magnetic, and catalytic properties. Furthermore, these findings may be related to chirality in nature. Acknowledgment. This work was supported by the 21st Century COE program “KEIO Life Conjugated Chemistry” from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. One of the authors (Y.O.) thanks the JSPS for a research fellowship for young scientists. LA063663O
