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

✍ Scribed by Freimar Segura-Sanchez; Kawthar Bouchemal; Geneviève Lebas; Christine Vauthier; Néréide S. Santos-Magalhaes; Gilles Ponchel

• Publisher: John Wiley and Sons
• Year: 2009
• Tongue: English
• Weight: 548 KB
• Volume: 22
• Category: Article
• ISSN: 0952-3499
• DOI: 10.1002/jmr.936

✦ Synopsis

Abstract

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 α‐, β‐, HP‐β‐, SBE‐β‐, and γ‐CD. It was shown that (+)‐UA did not interact with α‐CD and tended to interact more favorably with γ‐CD (K = 1030 M^−1^, Δ__G__ = −17.18 kJ · mol^−1^) than β‐CD (K = 153 M^−1^, Δ__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 β‐CD resulted in stronger and more spontaneous interactions (K = 281 M^−1^, Δ__G__ = −13.97 kJ · mol^−1^ for SBE‐β‐CD and K = 405 M^−1^, Δ__G__ = −14.87 kJ · mol^−1^ for HP‐β‐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 γ‐CD cavity, which were confirmed by molecular modeling. Copyright © 2009 John Wiley & Sons, Ltd.