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

Abstract

The inclusion complex of 7-hydroxy-4-methylcoumarin (7H4MC) with sulfobutyl ether-β-cyclodextrin (SBE-β-CD) was investigated by means of UV–vis, circular dichroism and 1H 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 (KC) were estimated from phase solubility analysis. The thermodynamic parameters of standard Gibbs free energy change, ΔGo, enthalpy change, ΔHo, and entropy change, ΔSo, for the complexation process were obtained by using the van’t Hoff equation and Gibbs–Helmholtz equation. The large negative ΔHo and the small negative or positive ΔSo (|ΔHo|>|TΔSo|) 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 SBE-β-CD cavity. Moreover, the 1H NMR spectrum showed that the entire 7H4MC molecule, except the hydroxyl group, was included in the SBE-β-CD cavity.