THERMODYNAMICS OF THE INTERACTION OF ALPHA-CYCLODEXTRIN WITH MONOCARBOXYLIC ACIDS IN AQUEOUS-SOLUTIONS - A CALORIMETRIC STUDY AT 25-DEGREES-C

The interaction in water of cyclomaltohexaose (alpha-cyclodextrin) wit h monocarboxylic acids from C-3 to C-12, has been studied calorimetric ally at 25 degrees C in phosphate buffer, pH 11.3, When a complex form s, calorimetry enables the calculation of both the enthalpy and the as sociation constant, from which the free energy and the entropy of the process can be obtained. For these acids, 1:1 inclusion complexes form , characterized by values of the association constants which do not fo llow a regular trend with increasing length of the alkyl chain. A mode l is proposed to explain this unusual behaviour. The association occur s through the insertion of the guest's alkyl chain into the host's cav ity. However, for longer alkyl chains two forms of the acid are possib le, each one associated with a different constant and enthalpy. The fo rces involved in the association process are discussed in the light of the analysis of the signs and values of the thermodynamic parameters. (C) 1996 Elsevier Science Ltd.