Chemistry Publications Chemistry 8-2005 Solvent-Tunable Binding of Hydrophilic and Hydrophobic Guests by Amphiphilic Molecular Baskets Yan Zhao Iowa State University, [email protected] Eui-Hyun Ryu Iowa State University Follow this and additional works at: http://lib.dr.iastate.edu/chem_pubs Part of the Chemistry Commons The ompc lete bibliographic information for this item can be found at http://lib.dr.iastate.edu/ chem_pubs/197. For information on how to cite this item, please visit http://lib.dr.iastate.edu/ howtocite.html. This Article is brought to you for free and open access by the Chemistry at Iowa State University Digital Repository. It has been accepted for inclusion in Chemistry Publications by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. Solvent-Tunable Binding of Hydrophilic and Hydrophobic Guests by Amphiphilic Molecular Baskets Abstract Responsive amphiphilic molecular baskets were obtained by attaching four facially amphiphilic cholate groups to a tetraaminocalixarene scaffold. Their binding properties can be switched by solvent changes. In nonpolar solvents, the molecules utilize the hydrophilic faces of the cholates to bind hydrophilic molecules such as glucose derivatives. In polar solvents, the molecules employ the hydrophobic faces of the cholates to bind hydrophobic guests. A water-soluble basket can bind polycyclic aromatic hydrocarbons including anthracene, pyrene, and perylene. The binding free energy (−ΔG) ranges from 5 to 8 kcal/mol and is directly proportional to the surface area of the aromatic hosts. Binding of both hydrophilic and hydrophobic guests is driven by solvophobic interactions. Disciplines Chemistry Comments Reprinted (adapted) with permission from Journal of Organic Chemistry 70 (2005): 7585, doi:10.1021/ jo051127f. Copyright 2005 American Chemical Society. This article is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/chem_pubs/197 Solvent-Tunable Binding of Hydrophilic and Hydrophobic Guests by Amphiphilic Molecular Baskets Yan Zhao* and Eui-Hyun Ryu Department of Chemistry, Iowa State University, Ames, Iowa 50011-3111 [email protected] Received June 3, 2005 Responsive amphiphilic molecular baskets were obtained by attaching four facially amphiphilic cholate groups to a tetraaminocalixarene scaffold. Their binding properties can be switched by solvent changes. In nonpolar solvents, the molecules utilize the hydrophilic faces of the cholates to bind hydrophilic molecules such as glucose derivatives. In polar solvents, the molecules employ the hydrophobic faces of the cholates to bind hydrophobic guests. A water-soluble basket can bind polycyclic aromatic hydrocarbons including anthracene, pyrene, and perylene. The binding free energy (-∆G) ranges from 5 to 8 kcal/mol and is directly proportional to the surface area of the aromatic hosts. Binding of both hydrophilic and hydrophobic guests is driven by solvophobic interactions. Introduction erties as well. Proteins may denature, or undergo drastic unfolding of the peptide chains, when pH, ionic strength, Rigid supramolecular hosts with minimal conforma- temperature, or other environmental properties are tional flexibility have traditionally been favored by altered. chemists because of their perceived benefits in binding In addition, solvent polarity also has profound influ- affinities. Most biomolecules, on the other hand, can ence on the conformations of biomolecules, as hydropho- respond to environmental stimuli by changing their bic interaction3 (or, more generally, solvophobic interac- conformations. As suggested by the induced-fit model,1 tion) is a major driving force for the folding of polypeptide the substrate of an enzyme can cause necessary confor- chains. One class of biomolecules that adopts dramati- mational change of the active site (to bring the catalytic cally different conformations with the change of envi- groups into proper alignment), but nonsubstrates cannot. ronmental polarity is R-helical antimicrobial peptides.4 Allosteric proteins change their conformations, and in These peptides tend to assume random conformations in turn their binding or catalytic functions, upon binding water but change to amphipathic R-helical structures with effectors or inhibitors.2 Conformational responses when they come in contact with bacterial membranes, a may result from changes of general environmental prop- (3) (a) Tanford, C. The Hydrophobic Effect: Formation of Micelles * To whom correspondence should be addressed. Phone: 515-294- and Biological Membranes, 2nd ed.; Wiley: New York, 1980. (b) Ben- 5845. Fax: 515-294-0105. Mailm, A. Hydrophobic Interactions; Plenum Press: New York, 1980. (1) (a) Koshland, D. E. Proc. Natl. Acad. Sci. U.S.A. 1958, 44,98- (c) Dill, K. A. Biochemistry 1990, 29, 7133-7155. (e) Blokzijl, W.; 104. (b) Koshland, D. E. Nat. Med. 1998, 4, 1112-1114, and references Engberts, J. B. F. N. Angew. Chem., Int. Ed. Engl. 1993, 32, 1545- therein. 1579. (2) (a) Regulation of Enzyme Activity and Allosteric Interactions; (4) (a) Hwang, P. M.; Vogel, H. J. Biochem. Cell Biol. 1998, 76, 235- Kvamme, E., Pihl, A., Eds.; Academic Press: New York, 1968. (b) 246. (c) Hancock, R. E. W.; Chapple, D. S. Atimicrob. Agents Chemother. Allosteric Regulation; Tokushige, M., Ed.; University Park Press: 1999, 43, 1317-1323. (d) Epand, R. M.; Vogel, H. J. Biochim. Biophys. Baltimore, MD, 1971. (c) Allosteric Enzymes; Herve´, G., Ed.; CRC Acta 1999, 1462,12-28. (e) Tossi, A.; Sandri, L.; Giangaspero, A. Press: Boca Raton, FL, 1989. (d) Perutz, M. F. Mechanisms of Biopolymers 2000, 55,4-30. (f) van’t Hof, W.; Veerman, E. C.; Cooperativity and Allosteric Regulation in Proteins; Cambridge Uni- Helmerhorst, E. J.; Amerongen, A. V. Biol. Chem. 2001, 382, 597- versity Press: Cambridge, U.K., 1990. 619. 10.1021/jo051127f CCC: $30.25 © 2005 American Chemical Society Published on Web 08/19/2005 J. Org. Chem. 2005, 70, 7585-7591 7585 Zhao and Ryu much less polar environment. In fact, polarity-induced sufficiently large to bind guest molecules and that its conformational change is important to many biological conformational flexibility will allow it to bind either processes including the translocation of proteins across hydrophilic or hydrophobic guests in a solvent-dependent membranes.5 fashion. In this paper, we report the dual binding Design of synthetic molecules with controllable con- properties of 1 in different solvents. We also find that a formations has received much attention in recent years water-soluble version of 1 indeed acts as a unimolecular and is highlighted in foldamer research.6 Foldamers are micelle to solubilize hydrophobic molecules in aqueous synthetic oligomers with biomolecule-like, ordered con- solutions. formations. Because their conformational flexibility al- lows their folding and unfolding (and in turn their properties) to be controlled by physical or chemical stimuli, they are very attractive as responsive materials. However, using weak, noncovalent forces to stabilize desired conformations in foldamers (and in synthetic molecules in general) remains as difficult challenges.6 We previously reported an amphiphilic molecular basket 1a constructed from cholic acid.7 Cholic acid8 is an example of facial amphiphiles.9 The cone-shaped aminocalix[4]arene is used as a scaffold to promote intramolecular aggregation among the cholates. In polar solvents, the hydrophilic (R) faces of the cholates point outward and the molecule resembles a unimolecular micelle. In nonpolar solvents, the hydrophobic (â) faces turn outward, giving a reversed-micelle-like conforma- 10,11 tion. We hypothesize that the internal cavity of 1a is Results and Discussion (5) Cserhåti, T.; Szo¨gyi, M. Int. J. Biochem. 1994, 26,1-18, and Binding Properties of the Reversed-Micelle-like references therein. Conformer in Nonpolar Solvents. Similar to surfac- (6) For several recent reviews, see: (a) Gellman, S. H. Acc. Chem. 12 Res. 1998, 31, 173-180. (b) Kirschenbaum, K.; Zuckerman, R. N.; Dill, tant reversed micelles, the reversed-micelle-like con- D. A. Curr. Opin. Struct. Biol. 1999, 9, 530-535. (c) Hill, D. J.; Mio, former of 1a requires a small amount of a polar solvent M. J.; Prince, R. B.; Hughes, T. S.; Moore, J. S. Chem. Rev. 2001, 101, for stability. A typical solvent mixture is carbon tetra- 3893-4012. (d) Cubberley, M. S.; Iverson, B. L. Curr. Opin. Chem. Biol. 2001, 5, 650-653. (e) Sanford, A. R.; Gong, B. Curr. Org. Chem. chloride/methanol (90/10). Carbon tetrachloride is a 2003, 7, 1649-1659. better solvent than chloroform for the reversed-micelle- (7) Ryu, E.-H.; Zhao, Y. Org. Lett. 2004, 6, 3187-3189. like conformer, which has a nonpolar exterior. In the (8) For some examples of supramolecular systems constructed from cholic acid, see: (a) Davis, A. P.; Bonar-Law, R. P.; Sanders, J. K. M. reversed-micelle-like conformer, all the hydroxyl groups In Comprehensive Supramolecular Chemistry; Atwood, J. L., Davis, turn inward to create a binding pocket, which should be J. E. D., MacNicol, D. D., Vo¨gtle, F., Eds.; Elsevier: Oxford, U.K., 1996; mostly filled with the polar solvent. We expect that 1a Vol. 4, Chapter 7, and references therein. (b) Li, Y.; Dias, J. R. Chem. Rev. 1997, 97, 283-304, and references therein. (c) Maitra, U. Curr. should bind a hydrophilic guest of appropriate size. Sci. 1996, 71, 617-624. (d) Smith, B. D.; Lambert, T. N. Chem. Because cholate groups are totally aliphatic, we choose Commun. 2003, 2261-2268, and references therein. (e) Davis, A. P.; - hydrophilic guests with an aromatic substituent, hoping Joos, J.-B. Coord. Chem. Rev. 2003, 240, 143 156, and references 1 therein. (f) Burrows, C. J.; Sauter, R. A. J. Inclusion Phenom. 1987, 5, to monitor the binding event by complexation-induced H 117-121. (g) Janout, V.; Lanier, M.; Regen, S. L. J. Am. Chem. Soc. NMR chemical shifts. Also, during NMR titrations, both 1996, 118, 1573-1574. (h) Ariga, K.; Terasaka, Y.; Sakai, D.; Tsuji, the host and the guest need to be sufficiently soluble in H.; Kikuchi, J.-I. J. Am. Chem. Soc. 2000, 122, 7835-7836. (i) Werner, F.; Schneider, H.-J.