Abstract
The inclusion complexes of selected imidazoline-derived drugs, namely Antazoline (AN), Naphazoline (NP) and Xylometazoline (XM) with beta-cyclodextrin (beta-CD) were investigated using steady-state fluorescence spectroscopy, differential scanning calorimetry (DSC), and molecular mechanics (MM) calculations and modeling. The modified form of the Benesi-Hildebrand relation was employed for estimating the formation constant (K-f) of the 1:1 inclusion complexes, which was applied based on measuring the variation in the fluorescence intensity of the guest molecule as a function of growing beta-CD concentration. On the other hand, the formation of the inclusion complexes was verified by analyzing solid samples of the complexes using DSC. The thermodynamics of the inclusion complexation, standard enthalpy (Delta H degrees) and entropy changes -(Delta S degrees) were obtained from the temperature-dependence of K-f. Obtained values of Delta H degrees and Delta S degrees indicated that the inclusion process favorably proceeds through enthalpy changes that was sufficiently predominant to compensate for the unfavorable entropy changes. MM calculations revealed that the proposed drugs molecules can form 1:1 inclusion complexes with beta-CD that are stabilized predominantly through van der Waals forces. In addition, MM calculation provided the energetically favored configuration of the inclusion complexes, where NP and XM can be included inside the beta-CD cavity through its wide rim, whereas AN can penetrate through the narrow rim of the beta-CD cavity.