Molecular recognition study of a supramolecular system .4. Molecular recognition thermodynamics of amino acids by modified beta-cyclodextrins

The novel beta-cyclodextrin derivative mono-(6-anilino-6-deoxy)-beta-cyclodextrin 1 bearing a single anilino moiety has been synthesized by a convenient method in 45% yield. Spectrophotometric titrations have been performed in buffer aqueous solution (pH 7.20) at 25.0-40.0 degrees C in order to obtain the complex stability constants (K-s) and the thermodynamic quantities (Delta H degrees and Delta S degrees) for the stoichiometric 1:1 inclusion complexation of various amino acids with the host compound 1 and mono-(6-1-pyridinio-6-deoxy)-beta-cyclodextrin 2. The molecular recognition abilities and enantioselectivity for guest L-or D-amino acids of the host beta-cyclodextrin derivatives 1 and 2 are discussed from the thermodynamic point of view, The thermodynamic parameters obtained indicate that the modified beta-cyclodextrins 1 and 2 carrying one chromophoric anilino or pyridinio moiety as a probe for differential UV spectrometry can recognize not only differences between the molecular size and shape of amino acids, but also the chirality of the L- or D-amino acid isomer. The host compound 1 possesses higher molecular inclusion ability as well as enantioselectivity than the beta-cyclodextrin derivative 2 as the size and shape of the L- or D-amino acids is varied, which is probably attributable to the increased enthalpic gain, The inclusion complexation is mainly enthalpy driven for 1, while for 2 it is mainly entropy driven, The larger enthalpic and entropic gains are attributed to the complexes' stabilities as a consequence of compensation effects.