Determination of the pKa values of trans-Resveratrol, a Triphenolic Stilbene, by Singular Value Decomposition. Comparison with Theory.

Several independent determinations of the pKa values of trans-resveratrol in water have led to conflicting results. Singular value decomposition analysis of UV absorption spectra of trans-resveratrol (t-Resv) in N2-outgassed aqueous solutions buffered to pH values in the 7.0-13.6 range yielded the UV spectra of the three anionic forms and the corresponding pKa values: pKa1 = 9.16 pKa2 = 9.77 and pKa3 = 10.55 in very good agreement with calculated theoretical values. The analysis of the absorption spectra guided the assignment of the fluorescence spectrum of each anionic form. With the resolved spectra on hand, we applied the Forster equation to estimate pKa* values of 2.5 and 0, respectively, for the p- and m-OH substituents of t-Resv in S1. Theory supports a proposed mechanism for the reaction of t-Resv anions with O2.