Studies on molecular recognition in supramolecular systems. Part 31: Circular dichroism spectral studies of molecular and chiral recognition of aliphatic alcohols by 6-modified β-cyclodextrins

The stability constants (K_S) for the inclusion complexation of novel mono[6-O-(1- benzotriazole)]-β-cyclodextrin (2) and mono(6-benzylseleno-6-deoxy)-β-cyclodextrin (3) with a series of chiral and achiral (cyclo)alkanols have been determined at 25°C in aqueous phosphate buffer solution at pH 7.20 by circular dichroism spectral titration. It was revealed that 2 and 3 can fairly strictly recognize not only the size/shape but also the chirality of guest molecules. Thus, the log K_S value, or the Gibbs free energy change (-ΔG⁰), increases linearly with increasing number of carbon atoms (N_C) in cycloalkanol, affording comparable increments per methylene unit: -dΔG⁰/dN_C=2.0 and 2.2 kJ mol^-1 for 2 and 3, respectively. Furthermore, 2 and 3 displayed moderate to excellent isomer selectivities of up to 15.3 for the guest alcohols examined, while moderate enantioselectivities of 1.1-1.4 were obtained with chiral borneol and menthol guests upon complexation with 2 and 3. The present results obtained with 2 and 3 elucidate the effects of substituents on the complexation behavior as well as some of the factors governing size, shape, and chiral selectivities.