Influence of the "host-guest" interactions on the mobility of genistein/beta-cyclodextrin inclusion complex

Inclusion complexes of cyclodextrins with nonpolar drugs are a topic of current interest in pharmaceutical science, because they increase the aqueous solubility, chemical stability and bioavailability of poorly water-soluble drugs. By means of elastic incoherent neutron scattering (EINS) technique on the backscattering spectrometer IN13, we measured, in the 150-340 K temperature range, the mean square displacements (MSD) of hydrogen atoms derived from elastic spectra of inclusion complex of beta-cyclodextrin (beta-CyD) with genistein (Gen), a phytoestrogen of great interest for its antioxidant, anticarcinogenic and antiosteoporotic activities. The mobility of the complex has been compared with the single components and the physical mixture. The elastic intensity has been interpreted in terms of the double-well model in the whole temperature range. In the case of Gen, a mainly vibrational dynamics is revealed, while for the other samples, the elastic intensity becomes non-Gaussian above a temperature T(d) congruent with 220-230 K. This dynamical activation, which is represented by an Arrhenius trend, seems to be promoted by the crystallization water molecules. The dynamics of the Gen/beta-CyD inclusion complex is restricted with respect to the physical mixture, due to the action of specific "host-guest" interactions upon complexation. Finally, the dynamical response of our systems to temperature is put in relationship with their thermal stability.