Molecular recognition on supramolecular systems (XXXV)

A novel β-cyclodextrin derivative 4 bearing a pyridinio group on the primary side was synthesized by the reaction of 2-aminopyridine with 6-β-cyclodextrin monoaldehyde 3, and its complexation stability constants with several aliphatic amino acids have been determined in phosphate buffer solution ( pH = 7.2, 0.1 mol L) at 25 by using spectrofluormetric titrations. The stoichiometry is 1 1 for the inclusion complexation of amino acids with compound 4. Circular dichroism study indicates that the aromatic moiety was embedded shallowly into the cyclodextrin cavity. As a spectral probe, the pyridinio group in the modified cyclodextrin can recognize not only differences of the size and shape of amino acid molecules, but also the L/D-amino acid chiral isomer. As compared with mono-[6-(1-pyridinio)-6-deoxy]-β-cyclodextrin 5, compound 4 switched the enantiomer preference for Lto D-isomer, and showed the highest enantioselectivity of 5.4 for D/L-serine. These results are discussed from the viewpoints of geometric compensation, induced-fit concept and cooperation of several weak interactions.