Molecular interactions and thermodynamic aspects of the complexation reaction between gentian violet and several cyclodextrins

The complexation process between gentian violet (CV+) and four different cy clodextrins (alpha-, beta-, gamma-, and HP-beta-CDs) has been investigated under different reaction conditions (pH, solvent, and temperature) by elect ronic absorption and H-1 NMR (NOE and NOESY) spectroscopies. All the bindin g constants were determined by the direct spectroscopic method. The Delta H and Delta S complexation values have been evaluated and discussed accordin g to the diverse factors that affect the chemical interactions in these sys tems. A simple association takes place between the secondary hydroxyl or th e hydroxypropyl groups of alpha and HP-beta-cycloamyloses, respectively, wi th the amine group of the gentian violet, while the binding between CV+ and beta- or gamma-CDs corresponds to a real inclusion. Also, a CV22+ dimmeric species within the gamma-CD cavity was detected in aqueous solution, while two molecules of alpha-CD react with one molecule of gentian violet in DMS O at 294 K. In all the reaction media the beta-CD forms 1 : 1 complexes, bu t in the buffered aqueous solution at pH 7.5 the inclusion is deeper than i n the other solvents. It is important to point out that the solvophobic eff ect is the most important binding factor in the complexation of the CV+ wit h the alpha- and HP-beta-CDs, while the complexes with beta-, and gamma-cyc lodextrins are mainly stabilized by van der Waals interactions between the guest and the host cavity. In all cases, the inclusion orientation is proba bly determined by the ion-dipole interactions between gentian violet and th e solvent.