Elucidation of the complexation mechanism between (+)-usnic acid and cyclodextrins studied by isothermal titration calorimetry and phase-solubility diagram experiments.
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), DeltaG = -17.18 kJ x mol(-1)) than beta-CD (K = 153 M(-1), DeltaG = -12.46 kJ x 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), DeltaG = -13.97 kJ x mol(-1) for SBE-beta-CD and K = 405 M(-1), DeltaG = -14.87 kJ x 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.
Segura-Sanchez
Freimar
F
Université Paris Sud-11, UMR CNRS 8612, 92296 Châtenay-Malabry, France.
Bouchemal
Kawthar
K
Lebas
Geneviève
G
Vauthier
Christine
C
Santos-Magalhaes
Néréide S
NS
Ponchel
Gilles
G
D001572:Benzofurans / Q000737:chemistry*; D002151:Calorimetry / Q000379:methods*; D003505:Cyclodextrins / Q000737:chemistry*; D019277:Entropy; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008958:Models, Molecular; D012995:Solubility
D001572:Benzofurans; D003505:Cyclodextrins; C073339:usnic acid