Molecular Recognition Study on a Supramolecular System. 10.¹ Inclusion Complexation of Modified β-Cyclodextrins with Amino Acids: Enhanced Enantioselectivity for L/D-Leucine

The novel β-cyclodextrin (β-CD) derivatives bearing a m-toluidinyl or (9-fluorenyl)alkylamino moiety have been synthesized by a convenient method in 45% and 66% yields, respectively. The stability constants (K_s) and Gibbs free energy changes (-ΔG) for inclusion complexation of mono-[6-(m-toluidinyl)-6-deoxy]-β-cyclodextrin 1 and mono-[6-[(9-fluorenyl)alkylammo]-6-deoxy]-β-cyclodextrin 2 with various L/D-amino acids have been examined by the fluorescence spectrum method in buffered aqueous solution (pH = 7.20) at 20-23°C. The modified β-cyclodextrins, possessing a toluidinyl or fluorenyl moiety as fluorescent probe, can recognize not only the size and shape but also the chirality of L/D-amino acids, giving fairly good enantioselectivity up to 33 for L/D-leucine. β-Cyclodextrin derivative 1 gave the highest K_s for L-leucine and the lowest for D-leucine among the amino acid series, eventually showing the highest enantioselectivity for L/D-amino acids. The molecular recognition ability and enantioselectivity for amino acids of the modified β-cyclodextrins 1 and 2 are discussed from the viewpoint of the size/shape-fit relationship between the host cavity and the guest molecules.