Elucidation of the complexation mechanism between ( )-usnic acid and cyclodextrins studied by isothermal titration calorimetry and phase-solubility diagram experiments

作者：Segura Sanchez Freimar; Bouchemal Kawthar^*; Lebas Genevieve; Vauthier Christine; Santos Magalhaes Nereide S; Ponchel Gilles

来源：Journal of Molecular Recognition, 2009, 22(3): 232-241.

DOI：10.1002/jmr.936

• isothermal titration calorimetry
• complexation thermodynamics
• cyclodextrins
• ( )-usnic acid
• solubilization studies

In the present work the complexation mechanism between ( )-usnic acid (UA) and cyclodextrins (CDs) has been investigated by isothermal titration calorimetry (ITC) and phase-solubility diagrams using pH as a tool for modifying the molecule ionization. ITC experiments have been employed to evaluate the stoichiometry of interaction (N), affinity constants (K), and thermodynamic parameter variation associated with complexation between ( )-UA and alpha-, beta-, HP-beta-, SBE-beta-, and gamma-CD. It was shown that ( )-UA did not interact with alpha-CD and tended to interact more favorably with gamma-CD (K = 1030 M(-1), Delta G = -17.18 kJ . mol(-1)) than beta-CD (K = 153 M(-1), Delta G = -12.46 kJ .mol(-1)) forming 1:1 complexes. It was also demonstrated using ITC and solubilization experiments that chemical modifications of the parent beta-CD resulted in stronger and more spontaneous interactions (K = 281 M(-1), Delta G = -13.97 kJ . mol(-1) for SBE-beta-CD and K = 405 M(-1), Delta G = -14.87 kJ . mol(-1) for HP-beta-CD). Analysis of the thermodynamic data suggested that van der Waals forces and hydrogen bonds were responsible for the formation of complexes with a predominance of van der Waals forces. Finally, pH induced modifications of ( )-UA ionization provided important informations relative to the topology of the interaction between ( )-UA molecule and the gamma-CD cavity, which were confirmed by molecular modeling.

• 出版日期2009-6