Molecular recognition study on supramolecular systems. 20. Molecular recognition and enantioselectivity of aliphatic alcohols by L-tryptophan-modified beta-cyclodextrin

L-Tryptophan-modified beta-cyclodextrin (L-Trp-beta-CD) has been synthesize d and its molecular recognition behavior investigated through fluorescence and circular dichroism spectrometry, as well as fluorescence lifetime measu rement in the presence and absence of various alcohols as guest molecules. Employing the indolyl group as a spectral probe, spectrofluorometric and sp ectropolarimetric titrations have been performed in aqueous phosphate buffe r solution at pH 7.20 to calculate the complex stability constants for 1:1 inclusion complexation of L-Trp-beta-CD with several series of alcohols at 25 degrees C. The results obtained indicate that L-Trp-beta-CD can recogniz e not only size/shape and hydrophobicity but also the enantiomeric and geom etrical isomers of the guest alcohols, showing a 230-fold molecular selecti vity (13.5 kJ mol(-1)) for 2-adamantanol over cyclopentanol among the cycli c alcohols examined. Moderate enantiomeric selectivities of 1.2 and 1.9 for (-)-isomers of borneol and menthol, respectively, and geometrical selectiv ity of 2.0 for geraniol over nerol have also been observed.