POLYMERIZED CYCLOMALTOHEPTAOSE (BETA-CYCLODEXTRIN, BETA-CDN) INCLUSION COMPLEX FORMATION WITH CHLOROGENIC ACID - SOLVENT EFFECTS ON THERMOCHEMISTRY AND ENTHALPY ENTROPY COMPENSATION

摘要

The inclusion of chlorogenic acid (CA) by epichlorohydrin-polymerized cyclomaltoheptaose (beta-cyclodextrin, beta-CDn) was studied with regard to temperature, solvent, and water activity (a(H2O) similar to mole fraction X(H2O) = 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 The latter finding argues that a function of temperature was nearly eliminated at the lowest X(H2O). 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. dependence of K on X(H2O) (K = K'X(H2O)(z)), 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 similar to 295-315 K). Our determinations of z were similar to those reported previously for beta-CD .(+)-limonene (z similar to 7), soluble beta-CD . CA (z similar to 6) or obtained by molecular dynamics calculations for beta-CD . CA reported herein (z similar to 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 X(H2O) values show linear enthalpy-entropy compensation (Delta H plotted against Delta S) but with a slope (T-c = partial derivative Delta H/partial derivative Delta S similar to 228 K) significantly less than T-c values determined from either standard aqueous thermodynamic experiments (T-c similar to 305 K on either beta-CD or beta-CDn) or variable X(H2O) (T-c similar to 272 K) experiments. To the best of our knowledge, this is the smallest T-c value detected in a multitude of CD . guest studies. This evident solvent effect on T-c strongly argues that the chemical part process of inclusion complex formation involves changes in the solvation of the beta-CDn's binding site. [References: 23]