“Host-guest” interactions in Captisol®/Coumestrol inclusion complex: UV-vis, FTIR-ATR and Raman studies

The ability of Captisol® (sulphobutylether-beta-cyclodextrin, SBE-beta-CD), to form inclusion complexes, both in solution and in the solid state, has been tested in order to improve some unfavorable chemicalphysical characteristics, such as poor solubility in water, of a bioflavonoid, Coumestrol (Coum), well known for its anti-oxidant, anti-inflammatory, anti-fungal and anti-viral activity. In pure water, a phasesolubility study was carried out to evaluate the enhancement of the solubility of Coum and, therefore, the occurred complexation with the macrocycle. The stoichiometry and the stability constant of the SBE-beta-CD/Coum complex were calculated with the phase solubility method and through the Job's plot. After that, the solid SBE-beta-CD/Coum complex was prepared utilizing a kneading method. The spectral changes induced by complexation on characteristic vibrational band of Coum were complementary investigated by Fourier transform infrared spectroscopy in attenuated total reflectance geometry (FTIR-ATR) and Raman spectroscopy, putting into evidence the guest chemical groups involved in the “host-guest” interactions responsible of the formation and stabilization of the complex. Particular attention was paid to the C=O and O-H stretching vibrations, whose temperature-evolution respectively furnished the enthalpy changes associated to the binding of host and guest in solid phase and to the reorganization of the hydrogen bond scheme upon complexation. From the whole set of results, an inclusion geometry is also proposed.