Interactions of beta- and hydroxypropyl-beta-cyclodextrin with some purine alkaloids: thermodynamic study.

The effect of native and hydroxypropylated beta-cyclodextrin on the solubility and activity of some purine alkaloids was examined. For this purpose, the solubility of purine alkaloids in pure water and in aqueous solutions of mentioned beta-cyclodextrins was determined at 298.15 K. Stability constants of inclusion complexes and their stoichiometry were obtained from solubility diagrams. Enthalpic characteristics of interactions occurring between beta-cyclodextrins and purine alkaloids in aqueous solution were calculated from the direct calorimetric measurements. It was found, that beta-cyclodextrin forms with purine alkaloids weak complexes which are stabilized only by the entropy term. Due to very low complexing affinity of both beta-cyclodextrins to studied purine alkaloids their solubilizing effect is insignificant. The influence of structure of purine alkaloids and beta-cyclodextrin on the thermodynamic parameters of interaction was discussed.