Improved solubility and stability of 7-hydroxy-4-methylcoumarin at different temperatures and pH values through complexation with sulfobutyl ether-β-cyclodextrin

The inclusion complex of 7-hydroxy-4-methylcoumarin (7H4MC) with sulfobutyl ether-p-cyclodextrin (SBE-beta-CD) was investigated by means of UV-vis, circular dichroism and H-1 NMR spectroscopy in phosphate buffer solutions at different temperatures and pH values. The stoichiometric ratio of the complexation was found to be 1:1 and the stability constants (K-C) were estimated from phase solubility analysis. The thermodynamic parameters of standard Gibbs free energy change, Delta G degrees, enthalpy change, Delta H degrees, and entropy change, Delta S degrees, for the complexation process were obtained by using the van't Hoff equation and Gibbs-Helmholtz equation. The large negative Delta H degrees and the small negative or positive Delta S degrees (vertical bar Delta H degrees vertical bar > vertical bar T Delta S degrees vertical bar) demonstrated that the inclusion interaction was an enthalpy-driven process. The positive signal of circular dichroism indicated that 7H4MC penetrated the cavity in such a way that the transition moment of the guest chromophore was parallel to the long axis of SEE-beta-CD cavity. Moreover, the H-1 NMR spectrum showed that the entire 7H4MC molecule, except the hydroxyl group, was included in the SBE-beta-CD cavity. (C) 2014 Elsevier Ltd. All rights reserved.