Molecular Recognition of Aliphatic Alcohols and Carboxylic Acid by Chromophoric Cyclodextrins

Molecular recognition behavior of eight cyclodextrin derivatives, i.e. mono(6-pyridinio-6-deoxy)-α-cyclodextrin (1α), mono(6-pyridinio-6-deoxy)-β-cyclodextrin (1β), mono(6-pyridinio-6-deoxy)-γ-cyclodextrin (1γ), mono[6-(p-picolinio)-6-deoxy]-β-cyclodextrin (2β), mono(6-anilino-6-deoxy)-β-cyclodextrin (3β), mono[6-(m-toluidino)-6-deoxy]-β-cyclodextrin (4β), mono[6-O-(8-quinolyl)]-β-cyclodextrin (5β), and novel mono[6-(2-naphthylamino)-6-deoxy]-β-cyclodextrin (6β), with a series of aliphatic alcohols and carboxylic acid has been investigated spectroscopically. Using the appended aromatic group as a spectral probe, spectrofluorometric or spectropolarimetric titrations have been performed at 25°C in aqueous phosphate buffer solution (pH 7.20, 0.1 M) to determine the complex stability constants (K_s) and Gibbs free energy changes (-ΔG^o) for the stoichiometric 1:1 inclusion complexation of cyclodextrin derivatives with the guests. The results obtained demonstrate that the modified cyldodextrins are highly sensitive to the size/shape and hydrophobicity of guest molecules, and particularly 5β gives an excellent molecular selectivity up to 215 for 1-adamantanol/cyclohexanol. The binding ability and selectivity of the modified cyclodextrins (1α, 1γ, and 1β-6β) are discussed from the view points of size/shape-fit concept, induced-fit interaction, and the multiple recognition mechanisms.