Spectral analysis of naringenin deprotonation in aqueous ethanol solutions

The deprotonation of 5,7-dihydroxy-2-(4-hydroxyphenyl)chroman-4-one (naringenin) was studied in aqueous solutions of ethanol and 0.1 mol L^?1 sodium perchlorate at 25°C. The chemical species that contributed to deprotonation were evaluated together with their pure spectral characteristics and concentration profiles by some chemometric methods. The deprotonation constants assigned by pK ₁, pK ₂, and pK ₃ were determined by multivariate curve analysis of spectral data at different pcH values. The pure spectral analysis concordant with the theoretical prediction of deprotonation constants indicates that the acidity of hydroxyl groups in naringenin decreases in the order: 7-OH, 4′-OH, 5-OH. The effects of the solvent on deprotonation were analysed in terms of the linear solvation energy relationships using the model of Kamlet, Abboud, and Taft (KAT). Multiple linear regressions were aimed towards correlating the deprotonation constants with the microscopic parameters containing hydrogen-bond acidity (α), dipolarity/polarisability (π*), and hydrogen-bond basicity (β). The most significant parameter was found to be the hydrogen-bond acidity of binary mixtures.