Studies on the interaction of diflunisal ion with cyclodextrins using ion-selective electrode potentiometry

The interaction of diflunisal ion (DF) with β-cyclodextrin (βCD), γ-cyclodextrin (γCD), and hydroxypropyl-β-cyclodextrin (HPβCD) was studied in phosphate buffer, pH 7.4, at 5–37°C and various CD concentrations using a home-made diflunisal ion-selective electrode. Typical direct binding plots and Scatchard plots were obtained with HPβCD. The Scatchard model for one class of binding sites was used for the estimation of binding parameters for the DF/HPβCD interaction. The estimates for n (number of binding sites per CD molecule) were in all cases very close to unity, indicating 1:1 complexation. The association constant ( K) estimates decrease with increasing temperature. Sigmoidal direct binding plots and concave-downwards Scatchard plots were obtained with various βCD or γCD concentrations. The Hill model was used for the estimation of the binding parameters for the DF/βCD and DF/γCD interactions. Both the Hill coefficients and the binding constants were markedly dependent on the CD concentration. These findings indicate the cooperative character of DF/βCD and DF/γCD interactions. The free energy change, Δ G, and the thermodynamic parameters, Δ H and Δ S, were estimated for each of the interactions studied using the Van't Hoff equation.