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

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 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.

Preparation of novel antimicrobial polymer colloids based on (+)-usnic acid and poly(vinylbenzyl chloride)

“…Depending on medium effect, the positive zeta potential values were observed at lower pH as these hydroxyl groups in the (+)-usnic acid were probably surrounded by H + ions because of ion-dipole attraction in a strongly acidic medium. In addition, negative zeta potential values were observed in basic conditions because of the increasing ionization of the hydroxyls [24]. The IEP for the PVBC particles also fell in a similar pH range.…”

“…To test the interaction/bond between usnic acid and polymer, usnic acid bound colloidal particles were washed with 10 mL of 5 M NaOH (aq) and precipitated at 3000 rpm for 20 min [24]. This procedure was repeated three times.…”

A comprehensive study of the spontaneous formation of nanoassemblies in water by a “lock-and-key” interaction between two associative polymers

“…The ITC instrument was periodically calibrated either electrically using an internal electric heater, or chemically by measuring the dilution enthalpy of methanol in water [18,20]. This standard reaction was in excellent agreement (1-2%) with MicroCal constructor data.…”

“…A heat flow was recorded as a function of time, which corresponds to any processes able to produce or consume energy following each injection. The concentration of the titrant and the sample were used to fit the heat flow per injection to an equilibrium binding equation, providing best fit values of the stoichiometry N, the binding constant K and the change in enthalpy DH [15,19,20,23]. From these results, the free energy DG and the entropy DS were deducted according:…”

“…Further, the dreiding force field was minimized with the software DS ViewerPro 6.0 (Accelrys Software Inc.) leading to the more likely supramolecular assembly. Bump monitorization, minimization of the dreiding force field [25] and molecular rendering (solvent accessible surface) were also achieved with DS Viewer Pro 6.0 in the case of these optimized structures [18,20].…”