Molecular recognition study of a supramolecular system. Part 4.1 Molecular recognition thermodynamics of amino acids by modified β-cyclodextrins

Abstract

The novel β-cyclodextrin derivative mono-(6-anilino-6-deoxy)-β-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 °C in order to obtain the complex stability constants (K_s) and the thermodynamic quantities (ωH° and ωS°) for the stoichiometric 1∶1 inclusion complexation of various amino acids with the host compound 1 and mono-(6-1-pyridinio-6-deoxy)-β-cyclodextrin 2. The molecular recognition abilities and enantioselectivity for guest L- or D-amino acids of the host β-cyclodextrin derivatives 1 and 2 are discussed from the thermodynamic point of view. The thermodynamic parameters obtained indicate that the modified β-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 β-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.