Physicochemical characterization of coumestrol/β-cyclodextrins inclusion complexes by UV–vis and FTIR-ATR spectroscopies

The inlcusion behaviour of unsubstituted β-cyclodextrin (β-CyD) and (2-hydroxypropyl)-β-cyclodextrin (HP-β-CyD), in solution and solid state was studied with regards to a poorly water-soluble bioflavonoid, coumestrol (Coum), namely 7,12-dihydroxycoumestan, well-known for its anti-oxidant, anti-inflammatory, anti-fungal and anti-viral activities. Phase-solubility measurements were performed to evaluate in solution the complexation of the two cyclodextrins, i.e. β-CyD and HP-β-CyD. The stoichiometry and stability constants of the Coum/β-CyD and Coum/HP-β-CyD complexes were calculated by the phase-solubility method, after which drug–cyclodextrin solid systems were prepared by co-precipitation. In solid phase, the formation of inclusion complexes was confirmed by Fourier transform infrared spectroscopy in attenuated total reflectance (FTIR-ATR) geometry. In particular, complexation mechanisms were explained by the significant differences revealed in the FTIR-ATR spectra of physical mixtures with respect to those of the complexes; the use of deconvolution and curve fitting was determinant.