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-dicarb oxylic 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 c an be obtained. At pH 11.3, 1:1 complexes are formed, the association occurring through the interaction of the charged carboxylate groups wi th the hydroxyl groups on the exterior of the dextrin. The alkyl chain should point toward the interior of the cavity. This 'capping' mechan ism relies on the impossibility of the carboxylate group to reside wit hin the cavity. At pH 1.3, the values of the association constants do not follow a regular trend with increasing alkyl chain length. Evidenc es are presented for a variation of the stoichiometry of association f or longer chain substances. A 2:1 stoichiometry would be possible beca use of the ability of the uncharged carboxyl group to be included in t he cavity. The forces involved in the association process are discusse d in the light of the analysis of the signs and values of the thermody namic parameters. (C) 1997 Elsevier Science B.V.