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 cyclodextrins (alpha-, beta-, gamma-, and HP-beta-CDs) has been investigated under different reaction conditions (pH, solvent, and temperature) by electronic absorption and H-1 NMR (NOE and NOESY) spectroscopies. All the binding constants were determined by the direct spectroscopic method. The Delta H and Delta S complexation values have been evaluated and discussed according to the diverse factors that affect the chemical interactions in these systems. A simple association takes place between the secondary hydroxyl or the hydroxypropyl groups of alpha and HP-beta-cycloamyloses, respectively, with 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 DMSO at 294 K. In all the reaction media the beta-CD forms 1 : 1 complexes, but in the buffered aqueous solution at pH 7.5 the inclusion is deeper than in the other solvents. It is important to point out that the solvophobic effect is the most important binding factor in the complexation of the CV+ with the alpha- and HP-beta-CDs, while the complexes with beta-, and gamma-cyclodextrins are mainly stabilized by van der Waals interactions between the guest and the host cavity. In all cases, the inclusion orientation is probably determined by the ion-dipole interactions between gentian violet and the solvent.