Spectroscopic characterization of the system β-cyclodextrin plus propafenone hydrochloride plus water

Propafenone hydrochloride (PRF) and the complex that forms with beta-cyclodextrin (beta-CD) in water have been investigated by different spectroscopies: 1D and 2D H-1 NMR, steady-state fluorescence, and absorption spectroscopy. Fluorescence measurements show the formation of a complex with a quantum yield lower than that of the pure PRF, although still fluorescent. The resultant quenching is mainly static in nature, as deduced from the temperature dependence of the emission and from the absorption spectra. The decrease in fluorescence has been used to assess the formation constants by nonlinear regression analysis and, from their dependence on the temperature, to obtain the DeltaH and DeltaS of the binding. All the resonances of the proton NMR spectrum of PRF in D2O have been assigned. The plot of the induced chemical shifts for certain protons of both molecules versus the mole ratio indicate a 1: 1 stoichiometry, and their analysis yields a formation constant that is in perfect agreement with that obtained using molecular luminescence. ROESY experiments, assisted by molecular modeling strategies, reveal the most likely structure of the complex in solution, in which both phenyl groups are buried into the beta-CD, with the aliphatic part of PRF protruding the cavity. A slight inclusion of the terminal methyl group is also detected.