The detection of cyclodextrin self-association by titration with dyes

A simple procedure is proposed for the detection of association of cyclodextrins (CDs) and their derivatives in aqueous solutions by analyzing the curves of titration of these substances with dyes. Dissociation constants of cyclodextrin-dye complexes and concentrations of sites involved in the complexation are determined by the nonlinear regression analysis. The processes of CD association were characterized using deviations of the function of CD saturation with ligands from a simple scheme CD + ligand ? complex. Two association mechanisms were disclosed: cooperative association (for α-CD and dihydroxypropyl-β-CD) and step association (for β-CD, dimethyl-β-CD, hydroxypropyl-β-CD, and γ-CD). In the case of stepwise-associating compounds, association constants for head groups of cyclodextrins are found to vary from 3 × 10³ to 8 × 10³ M^?1. The titration data on CD association are confirmed by studying solutions of the above compounds using dynamic light scattering.