Chiral recognition at the air–water interface

Chiral recognition at the air–water interface

Current Opinion in Colloid & Interface Science 13 (2008) 23 – 30 www.elsevier.com/locate/cocis Chiral recognition at the air–water interface Katsuhik...

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Current Opinion in Colloid & Interface Science 13 (2008) 23 – 30 www.elsevier.com/locate/cocis

Chiral recognition at the air–water interface Katsuhiko Ariga a,⁎, Tsuyoshi Michinobu b , Takashi Nakanishi c,d , Jonathan P. Hill a a

International Center for Materials Nanoarchitectonics (MANA) and Supermolecules Group, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044, Japan b Institute of Symbiotic Science and Technology, Tokyo University of Agriculture and Technology, Japan c MPI-NIMS International Joint Laboratory, Max Planck Institute of Colloids and Interfaces, Germany d Organic Nanomaterials Center, National Institute for Materials Science (NIMS), Japan Received 9 May 2007; accepted 22 August 2007 Available online 1 September 2007

Abstract Chiral recognition within monolayer components and of aqueous guests has been investigated at the air–water interface. For the first topic, supramolecular chirality related to the formation of chiral structures from achiral components is a current topic of importance. A marked development of the latter subject involved tuning of chiral selectivity upon dynamic changes of monolayer structures. © 2007 Elsevier Ltd. All rights reserved. Keywords: Air–water interface; Chiral recognition; Dynamic motion; Monolayer; Supramolecular chirality

1. Introduction The air–water interface supplies an ideal surface possessing molecular-level flatness, where theoretical considerations of molecular recognition may be simplified so that a logical design of recognition sites becomes feasible. For example, molecular recognition between amphiphilic components can be investigated readily through observation of surface pressure (π)– molecular area (A) isotherms together with related measurements on the corresponding mixed monolayers. Arnett et al. extensively researched chiral discrimination within a monolayer at the air–water interface in the 1980s and 1990s [1•]. They clearly demonstrated the greater expansion of a racemic monolayer of N-α-methylbenzylstearamide over the more condensed structure for its pure enantiomers (R or S) on a subphase with appropriate acidity [2]. Systematic analyses of monolayer behavior including π–A isotherms, thermodynamics of spreading, and surface shear viscosities of diastereomeric mixtures of various N-acylamino acid methyl esters have been reported [3].

⁎ Corresponding author. Tel.: +81 29 860 4597; fax: +81 29 852 4832. E-mail address: [email protected] (K. Ariga). 1359-0294/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.cocis.2007.08.010

Molecular recognition processes between monolayer components and an aqueous guest have been also extensively researched [4•] since Kitano and Ringsdorf reported the first example of nucleobase recognition at the air–water interface based on the effect of nucleobase binding to a host monolayer [5]. This pioneering work was followed by several research groups, among which Ariga and Kunitake made a systematic investigation of a great variety of combinations of the host monolayers and aqueous guests [6•]. Most importantly they found that molecular recognition at the air–water interface generally occurs with enhanced binding constants relative to those in bulk media [7]. This was also theoretically interpreted using quantum chemical calculations [8,9]. Unfortunately, chiral discrimination was not seriously discussed in Kunitake's reports although they demonstrated recognition of guest substances with chiral structures, such as sugars [10], nucleotides [11], amino acids [12], and peptides [13–16]. In this review, recent developments in chiral recognition at the air–water interface are summarized with classification into two major categories: (i) chiral recognition within monolayer components and (ii) chiral recognition between monolayers and aqueous guests. In the first category, concepts of supramolecular chirality related to formation of chiral structures from achiral components have been recently emphasized. For the

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latter category, in addition to examples of simple chiral recognition, conversion of chiral selectivity upon dynamic changes of monolayer structure has been very recently pioneered. 2. Chiral recognition within monolayer components The synthesis of chiral compounds is an attractive yet challenging task in organic chemistry, with synthetic amphiphilic analogues sometimes subjected to simple π–A isotherm measurements. Somfai et al. researched enantioselective synthesis of sugar-like polyhydroxylated amphiphiles whose monolayer properties were investigated using π–A isotherm measurements [17]. Very recently, Vögtle et al. synthesized chiral molecular knots that were demonstrated to form monolayers on water [18]. Apart from these pioneering reports, specialized techniques for analysis of molecular structure at the air–water interface should provide deep insights into chiral discrimination within a single monolayer phase. One of the most powerful methods in high resolution structural analyses is grazing-incidence X-ray diffraction (GIXD) analysis, which has

been studied intensively for the analysis of monolayer structures on water at the Weizmann Institute. For example, Weissbuch et al. investigated precise chiral discrimination and phase separation of α-amino acid amphiphiles using GIXD measurements [19]. Chiral discrimination within a monolayer phase sometimes leads to the formation of controlled structures. Kimura et al. investigated assembly structures formed in monolayer and Langmuir–Blodgett (LB) films of metallophthalocyanines with chiral phytanyl chains (Fig. 1A) [20]. The Zn-complex assembled into one-dimensional columns through π–π interactions between phthalocyanine rings and rotated along the columnar axis under the effect of the chiral bridging segment. Chirality in the phytanyl chains was manifested as a chiral twisting of the columns (Fig. 1B(a)). On the other hand, the Cocomplex formed an alternating multilayer structure consisting of phthalocyanine dimer layers and a dense layer of phytanyl chains (Fig. 1B(b)). Similarly, Haufe demonstrated the formation of controlled three-dimensional structures as the result of interaction of chiral components within a twodimensional monolayer plane [21]. They investigated the

Fig. 1. (A) Metallophthalocyanine with phytanyl chains. (B) Structures formed in monolayers: (a) twisted columns from Zn-complex; (b) flat layer from Co-complex. Reprinted with permission from Ref. [20], ©2006, American Chemical Society.

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morphologies of collapsed phases of monolayers of enantiomeric compounds as well as diastereomeric mixtures and racemic/diastereomeric mixtures of ethyl 2-azido-4-fluoro-3hydroxystearates. The concept of creation of chiral structures from achiral components, so called supramolecular chirality, is important in pure science because it relates to the origin of chirality and the symmetry breaking of a system. Liu et al. first reported formation of spiral (chiral) structures in transferred monolayers of a barbituric acid derivative (Fig. 2A) [22••]. Although barbituric acid derivatives have been widely investigated from the point-of-view of their complementary hydrogen bonding with melamine at the air–water interface [23–25], the hydrogen bonding between barbituric acid derivatives within monolayers was utilized in this case. When the film was compressed to the inflection point in the π–A isotherm, spiral nanofibers were observed by atomic force microscopic (AFM) observation. The spirals observed were wound in both clockwise and anticlockwise directions. The formation mechanism of the supramolecularly chiral amphiphilic barbituric acid in the transferred monolayers is illustrated in Fig. 2B. Compression of the barbituric acid monolayer spread on the water surface induced close packing of the aromatic rings. The neighboring aromatic rings probably tilt from the same plane due to certain steric hindrance between the relatively large aromatic groups when they were hydrogen-bonded. The nanofiber structures then curved further in a certain direction to form spirals because of the clear directional nature of the hydrogen bonding. As suggested from CD spectral data, handedness of aggregation was maintained after initial selection at the start of structure formation because of a cooperative interaction. However, handedness of the starting aggregate was determined by chance. For further understanding on the formation mechanism of such supramolecular chiral assemblies, some achiral molecules, such as octadecanoic acid and octadecylamine were mixed with the barbituric acid derivative [26]. If the concept can be extended to π-conjugate poly(p-phenylenevinylene) derivatives then photoelectronic applications such as light-emitting diodes,

Fig. 2. (A) Barbituric acid derivative for supramolecular chirality in monolayer. (B) Models of molecular stacking in chiral spiral assemblies. Reprinted with permission from Ref. [22••], ©2004, American Chemical Society.

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solar cells, and field effect transistors [27] can be expected. Supramolecular chirality of coordination complexes at the air– water interface has also been examined [28]. Formation of chiral structures was demonstrated in monolayers of Schiff base type ligands that are capable of coordinating to metal ions. Surprisingly, supramolecular chirality can be obtained from achiral organic molecules lacking long chain structures that were embedded at the air–water interface. Superstructure formation of some arylbenzimidazoles with 2-substituted aromatic groups such as phenyl, naphthyl, anthryl and pyrenyl at a water surface or on a subphase containing AgNO3 was investigated [29]. Additionally, chiral assemblies have been obtained from achiral porphyrin derivatives [30]. 3. Chiral recognition between monolayers and aqueous guests In order to discriminate between the chiralities of guests in the subphase, chiral amphiphiles have to be used as host molecules to introduce energy differences between the complexed diastereomers. Vodyanoy et al. used the chiral phospholipid, L-α-1,2-dipalmitoyl-sn-glycero-3-phosphocholine (L-DPPC), for discrimination of the chiral odorant, (+)and (−)-carvone [31]. This can be regarded as olfactory detection. Higashi et al. synthesized a dialkyl poly(L-glutamic acid) amphiphile, which was mixed with a dialkyl acetamide amphiphile, and used for chiral recognition of amino acids at the air–water interface [32]. Binding constants of tryptophan enantiomers (D or L) were found to depend on the secondary structure and lateral packing density of the poly(L-glutamic acid) segments. Tamura et al. synthesized an amphiphilic ruthenium complex for discrimination of the chiral anions, bis [(+)-tartrato][diantimonato(III)]dipotassium [33]. The host complex possess two alkyl chains at the 2,2′-bipyridine ligands, and its polar headgroup is chiral due to the asymmetric coordination structure (Δ- or Λ-isomer). π–A Isotherms of a racemic mixture and a pure enantiomer of the ruthenium complex host were measured on aqueous guest solution. Cyclic hosts such as cyclodextrin, crown ether, and calixarenes can be used for specific recognition of guest molecules from the membrane phase [34,35] and aqueous media [36•] at appropriate interfacial environments. Connection of a chiral moiety to these recognition sites should realize chiral recognition at the air–water interface. In a pioneering report, Kawabata and Shinkai demonstrated chiral recognition of α-amino acids using a conjugate host composed of cholesterol and crown ether (18-crown-6) moieties (Fig. 3A) [37]. A plausible binding mechanism is shown in Fig. 3A, in which the ammonium group of the guest is complexed by crown ether and hydrophobic side chains interact with the hydrophobic cholesterol plane. In this binding mode, the cholesterol skeleton with a wide chiral plane can more effectively enforce the orientation of α-amino acid derivative than a conventional chiral amphiphile with point chirality. Rogalska et al. synthesized chiral amphiphiles containing an 18-crown-6 moiety through 4,6-di-O-alkylation of cyclic chiral diol [38]. Surface pressure and surface potential measurements performed

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Fig. 3. (A) Chiral recognition of aqueous amino acid methyl ester by a monolayer of the crown ether–cholesterol conjugate. (B) Chiral recognition of aqueous phenylalanine by monolayer of calix[4]resorcinarene derivative. Reprinted with permission from Ref. [39], ©2005, American Chemical Society.

on the subphases containing L or D enantiomers of alanine, valine, phenylglycine, and tryptophane indicated that the crown ethers forming the monolayer interact with the amino acids. Shahgaldian et al. synthesized a chiral amphiphilic calix[4] resorcinarene, tetrakis(N-methylprolyl)tetraundecylcalix[4] resorcinarene, bearing four L-prolyl moieties at the macrocyclic upper rim and four undecyl chains at the lower rim [39]. The monolayer prepared from this cyclic amphiphile showed enantioselective binding to phenylalanine with the aid of copper ion chelation, which is reflected in differences in monolayer stabilities. The proposed binding motif depicted in Fig. 3B consists of the ternary complex, where the copper ion is chelated by the carboxylato oxygen atoms of the prolyl moiety of the host and that of phenylalanine and by the two nitrogen atoms of these amino acids, forming a planar square. Enantioselectivity may arise from favorable or unfavorable interactions of the side chain of phenylalanine with the hydrophobic layer formed by aromatic functions of the

calixresorcinarene with a possible inclusion of the aromatic side chain in the macrocyclic cavity. Monolayer structures formed at the air–water interface are excellent media for molecular organization, where binding sites composed of assembled interacting groups can be formed spontaneously. Lahav et al. investigated the intercalation of amino acid molecules contained in the subphase to the monolayer of a cholesteryl-L-glutamate using GIXD technique (Fig. 4A) [40•]. The presence of hydrophobic amino acids (L-leucine or Lphenylalanine) drastically changed the GIXD patterns of the monolayer. It was also revealed that D-leucine cannot be as easily incorporated as L-leucine between the glutamate moieties of the host monolayer. Miyashita et al. introduced a chiral moiety to a monolayer of alkylated polyacrylamide and realized enantiometric detection of 1-phenylethylamine (Fig. 4B) [41]. Greater binding was observed for the R-isomer guest. The R-guest can be incorporated with lesser steric hindrance because the guest methyl group is remote from the host naphthyl group

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Fig. 4. (A) Chiral recognition of aqueous amino acids through intercalation into monolayer of choresteryl-L-glutamate. Reprinted with permission from Ref. [40•], ©2001, American Chemical Society. (B) Binding motifs of 1-phenylethylamine to the copolymer of binaphthyl-functionalized poly(methacrylate) and poly(dodecyl acrylamide) at the air water interface: (a) R-isomer binding; (b) S-isomer binding. Reprinted with permission from Ref. [41], ©1993, American Chemical Society.

(Fig. 4B(a)). In contrast, the S-isomer presents a certain steric hindrance between the methyl and naphthyl groups (Fig. 4B(b)), resulting in less favorable binding. Recently, Molt and Schrader reported use of an enzyme-like receptor molecule for specific molecular recognition at the air–water interface [42]. Their concept was not extended to chiral recognition, but their molecular design should be adaptable for development of chiral recognition systems at the air–water interface. Tuning of molecular recognition capability can be achieved using dynamic changes in monolayer structures at the air–water interface. Ariga et al. realized dynamically controlled molecular recognition using a steroid cyclophane, that contains a

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1,6,20,25-tetraaza[6.1.6.1]-paracyclophane cyclic core connected to four steroid (cholic acid) moieties through a flexible L-lysine spacer, as monolayer components (Fig. 5A) [43•,44••,45•,46]. The cholic acid moiety possesses both hydrophobic and hydrophilic faces so that, when contained in a monolayer at low pressures, it forms an open conformation. Compression of the monolayer induces shrinkage of the steroid cyclophane molecule resulting in a cavity conformation. Using this conformational change, the reversible binding (capture and release) of an aqueous fluorescent guest, 6-(p-toluidino) naphthalene-2-sulfonate (TNS), was demonstrated. In order to extend this concept to dynamic chiral recognition at the air–water interface, an armed cyclen receptor synthesized by Tuskube and Shinoda [47] was applied for molecular recognition at the air–water interface [48••]. The host molecule was composed of a 1,4,7,10-tetraazacyclododecane core with four cholesteric side arms as N-substituents (Fig. 5B). The four side arms are expected to stand up and are bundled to form quadruple helical structures. This host can be of Δ- or Λ-type enantiomer based on the helicity of the side arm arrangement, in which the chiral sense and its extent can be modified through molecular packing in the monolayer. This armed cyclen has guest binding capability, and accommodation of chiral guest molecules within the hydrophobic cavity can affect the helicity of the host. Therefore, packing control of the armed cyclen receptor upon its monolayer compression should provide controlled modification of enantioselectivity in binding of aqueous guests. Addition of amino acids such as leucine or valine into the subphase induced expansion of the monolayer with a definite dependence on side chain structure and chirality of the guest amino acids. Interestingly, an inversion of the magnitude of the binding constant (K) between L- and Denantiomers was observed in the case of valine guest (Fig. 6). In contrast, the binding constants of D-leucine were always greater than those of L-leucine (Fig. 6). These results were also supported by FT-IR investigations on films transferred to a solid substrate. Such switching of chiral recognition had not been accomplished except in a rare example of pH-controlled switching of chiral recognition in assemblies in bulk solution [49]. 4. Conclusion The air–water interface can be considered an excellent medium for chiral molecular recognition. However, research progress to date remains at a challenging stage with further investigations being essential. Advanced analyses of surface monolayers with high sensitivity techniques such as polarized infrared reflection–adsorption spectroscopy [50] and Brewster angle microscopy combined with GIXD technique [51] will assist the progress of this research field especially if combined with theoretical approaches based on computational modeling and energy calculations [52,53]. Several possible outcomes other than the often proposed sensor uses must be given serious consideration, while purposes related to nonlinear optics [54] and surface reactions [55,56] have already been reported for chiral monolayers. However, an important goal for subsequent

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Fig. 5. (A) Dynamic molecular capture by monolayer of a steroid cyclophane. (B) Dynamic chiral recognition of aqueous amino acids by a monolayer of the armed cyclen. Reprinted with permission from Ref. [48••], ©2006, American Chemical Society.

research is chiral recognition involving the unique characteristics of the air–water interface. Molecular assembly and organization achieved with certain motional freedom under

well-defined control can be achieved at the air–water interface, and cannot be currently attained at solid interfaces or at microscopically dispersed interfaces in bulk solution. Research

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Fig. 6. Plots of the surface pressure of the monolayer and the ratio of the binding constants (K) with enantiomeric leucine and valine. Reprinted with permission from Ref. [48••], ©2006, American Chemical Society.

on supramolecular chirality and dynamic molecular recognition well reflects the unique characteristics of the air–water interface. Acknowledgements The research described in this paper was partially supported by Grant-in-Aid for Scientific Research on Priority Areas “Chemistry of Coordination Space” and a Grant-in-Aid for Science Research in a Priority Area “Super-Hierarchical Structures” from the Ministry of Education, Science, Sports, and Culture, Japan, and a Grants-in-Aid for Scientific Research (B) from Japan Society for the Promotion of Science. References and recommended readings•,•• [1] Arnett EM, Harvey NG, Rose PL. Stereochemistry and molecular • recognition in “two dimensions”. Acc Chem Res 1989;22:131–8. Well authorized review for chiral recognition at the air–water interface. [2] Arnett EM, Chao J, Kinzig B, Stewart M, Thompson O. Chiral aggregation phenomena. 1. Acid dependent chiral recognition in a monolayer. J Am Chem Soc 1978;100:5575–6. [3] Heath JG, Arnett EM. Chiral molecular recognition in monolayers of diastereomeric N-acylamino acid methyl esters at the air/water interface. J Am Chem Soc 1992;114:4500–14. [4• ] Leblanc RM. Molecular recognition at Langmuir monolayers. Curr Opin Chem Biol 2006;10:529–36. Informative review on recent research on molecular recognition at the air–water interface. [5] Kitano H, Ringsdorf H. Surface behaviors of nucleic-acid base-containing lipids in monolayer and bilayer systems. Bull Chem Soc Jpn 1985;58:2826–8. Pioneering report on recognition of aqueous guest by monolayer host through complementary hydrogen bonding. [6• ] Ariga K, Kunitake T. Molecular recognition at air–water and related interfaces: complementary hydrogen bonding and multisite interaction. Acc Chem Res 1998;31:371–8. Well authorized review on molecular recognition at the air–water interface through hydrogen bonding. [7] Onda M, Yoshihara K, Koyano H, Ariga K, Kunitake T. Molecular recognition of nucleotides by the guanidinium unit at the surface of aqueous micelles and bilayers. A comparison of microscopic and macroscopic interfaces. J Am Chem Soc 1996;118:8524–30. • ••

Of special interest. Of outstanding interest.

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