Polymerized cyclomaltoheptaose (beta-cyclodextrin, beta-CDn) inclusion complex formation with chlorogenic acid: solvent effects on thermochemistry and enthalpy-entropy compensation.

Abstract

The inclusion of chlorogenic acid (CA) by epichlorohydrin-polymerized cyclomaltoheptaose (beta-cyclodextrin, beta-CDn) was studied with regard to temperature, solvent, and water activity aH2O approximately mole fraction = XH2O = 0.8-1 using MeOH as the diluent; 0.1 M sodium phosphate buffer). We discovered that the extreme convex curvature in K (the apparent stability constant) as a function of temperature was nearly eliminated at the lowest XH2O. The latter finding argues that this unusual CD behavior in aqueous media was due to perturbations in beta-CD's spatial organization in the polymeric matrix with temperature. Related to this we found, from the dependence of K on XH2O (K = K'XzH2O), that the beta-CDn.CA complex's stoichiometric coefficient, z, for water, varied between 5 and 8, depending on the temperature of the solution (K' = 400-800 M-1; T approximately 295-315 K). Our determinations of z were similar to those reported previously for beta-CD.(+)-limonene (z approximately 7), soluble beta-CD.CA (z approximately 6) or obtained by molecular dynamics calculations for beta-CD.CA reported herein (z approximately 5). However, beta-CDn.CA's z values did show a significant positive correlation with temperature not evident in equivalent solution experiments. Calculations of delta H and delta S at various XH2O values show linear enthalpy-entropy compensation (delta H plotted against delta S) but with a slope (Tc = theta delta H/theta delta S approximately 228 K) significantly less than Tc values determined from either standard aqueous thermodynamic experiments (Tc approximately 305 K on either beta-CD or beta-CDn) or variable XH2O (Tc approximately 272 K) experiments. To the best of our knowledge, this is the smallest Tc value detected in a multitude of CD.guest studies. This evident solvent effect on Tc strongly argues that the chemical part process of inclusion complex formation involves changes in the solvation of the beta-CDn's binding site.