Thermodynamics of the interaction of alpha-cyclodextrins with carboxylic acids in aqueous solutions. A calorimetric study at 25 °C.

The interaction in water of cyclomaltohexaose (alpha-cyclodextrin) with monocarboxylic acids from C-3 to C-12, has been studied calorimetrically at 25 degrees C in phosphate buffer, pH 11.3, When a complex forms, calorimetry enables the calculation of both the enthalpy and the association 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 follow a regular trend with increasing length of the alkyl chain. A model is proposed to explain this unusual behaviour. The association occurs through the insertion of the guest's alkyl chain into the host's cavity. However, for longer alkyl chains two forms of the acid are possible, each one associated with a different constant and enthalpy. 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) 1996 Elsevier Science Ltd.