Enantioselective Recognition of Aliphatic Amino Acids by Organoselenium Modified β-Cyclodextrins

A series of novel organoselenium modified β-cyclodextrins (CDs) bearing an aromatic group 1-7 have been synthesized by a convenient method in satisfactory yields. Spectrophotometric titrations have been performed in aqueous buffer solution (pH 7.20) at 25.0°C to give the complex stability constants (K_s) and Gibbs free energy change (-ΔG°) for the 1:1 inclusion complexation of the six organoselenium modified β-cyclodextrins with some selected aliphatic amino acids. Inclusion complexation of mono-[6-(naphthylseleno-6-deoxy]-β-cyclodextrin 7 with aliphatic amino acids was too weak to be observed, which is attributable to the stronger self-inclusion of naphthylseleno moiety attached to the primary side of cyclodextrin into the cavity. However, the other modified β-cyclodextrins carrying one arylseleno moiety as a probe for differential UV spectrometry were found to recognize not only the size and shape but also the chirality of amino acids. Among arylseleno CDs 1-6, 3 showed the highest enantioselectivity of 27 for L-Ala over the antipodal D-Ala. The molecular recognition ability and enantioselectivity for amino acids of these seven modified β-cyclodextrins are discussed from the viewpoints of the size/shape-fit concept, substituent effect and the multipoint recognition mechanism. The inclusion complexation of these modified β-cyclodextrins with L/D-amino acids may be more explicitly understood in terms of the complementary geometrical relationship and the induced-fit interaction between the host and the guest.