Thermodynamics of the interaction of cyclodextrins with aromatic and α, ω-amino acids in aqueous solutions: a calorimetric study at 25°C

The interaction in water of α- and β-cyclodextrins with l-phenylalanine, l-tyrosine, l-tryptophan, and l-histidine has been studied calorimetrically at 25°C in pure water and in a phosphate buffer (pH 11.3). The interaction in water of α-cyclodextrin with some α, ω-amino acids was also studied. When a complex forms, calorimetry allows the calculation of both the enthalpy and the association constant, from which the free energy and the entropy of the process can be obtained. Aromatic amino acids form 1:1 inclusion complexes, characterized by low values of the association constants. The association occurs through the insertion of the guestʹs aromatic ring into the hostʹs cavity, and is stronger at pH 11.3 than in pure water. For α, ω-amino acids the association constants increase with increasing lengths of the alkyl chains between the functional groups. For this class of substances the association is supposed to occur through an interaction mechanism different from inclusion, which involves mainly the exterior of cyclodextrin. The analysis of the signs and values of the thermodynamic parameters obtained permits hypotheses to be made about the forces involved in the association process.