Thermodynamics of the interaction of alpha-cyclodextrin with alpha,omega-dicarboxylic acids in aqueous solutions. A calorimetric study at 25 degrees C.

The interaction in water of alpha-cyclodextrin with alpha,omega-dicarboxylic acids from C-5 to C-11 has been studied calorimetrically at 25 degrees C in phosphate buffer at pH 11.3 and 1.3. When a complex forms, calorimetry enables the calculation of both enthalpy and association constant, from which the free energy and the entropy of the process can be obtained. At pH 11.3, 1:1 complexes are formed, the association occurring through the interaction of the charged carboxylate groups with the hydroxyl groups on the exterior of the dextrin. The alkyl chain should point toward the interior of the cavity. This 'capping' mechanism relies on the impossibility of the carboxylate group to reside within the cavity. At pH 1.3, the values of the association constants do not follow a regular trend with increasing alkyl chain length. Evidences are presented for a variation of the stoichiometry of association for longer chain substances. A 2:1 stoichiometry would be possible because of the ability of the uncharged carboxyl group to be included in the cavity. The forces involved in the association process are discussed in the light of the analysis of the signs and values of the thermodynamic parameters. (C) 1997 Elsevier Science B.V.