Solute-Solvent Interaction Effects on Protonation Equilibrium of Some Water-Insoluble Flavonoids

The protonation constants of three different flavonoids (naringenin, chrysin, and daidzein) were determined in water-DMSO mixed solvents using a potentiometric method at 25.0 (+/- 0.1)degrees C and 1.00 mol.dm(-3) tetra-n-butylammonium chloride as supporting electrolyte. The protonation constants were characterized and were analyzed in various solvent media in terms of the Kamlet, Abboud and Taft (KAT) parameters. Single-parameter correlations of the protonation constants versus alpha (hydrogen-bond donor acidity), beta (hydrogen-bond acceptor basicity) or pi* (dipolarity/polarizability) are poor for all compounds, but the dual-parameter alpha and pi* correlation presents significant improvement with regard to the single- and multi-parameter models. A linear correlation is observed when the logarithm of the experimental protonation constants is plotted versus the calculated ones when the KAT parameters are considered. The protonation constants of the flavonoids in water were also calculated by the Yasuda-Shedlovsky extrapolation method at zero percent organic solvent. Finally, the results are discussed in terms of the effect of solvent composition on the protonation constants.