Aggregation state and pKa values of (E)-resveratrol as determined by fluorescence spectroscopy and UV-visible absorption

Whether or not (E)-resveratrol can be used as a functional ingredient in foods depends on its structure at different concentrations and pH values. For this reason, the aggregation state of (E)-resveratrol at acidic and basic pH is investigated here for the first time. The data show that (E)-resveratrol forms aggregates above a certain critical concentration, and this concentration is lower at acid (12.5 mu M at pH 5.5) than at basic pH values (37 mu M at pH 10.5), Moreover, although several papers have focused on the effect of the protonation state of (E)-resveratrol on its biological activity, different data concerning the acidic dissociation constants of this natural antioxidant have been reported in the literature. The present paper shows a way of determining the acidic dissociation constants (pK(a1) = 8.8, pK(a2) = 9.8, pK(a3) 11.4) of (E)-resveratrol in aqueous medium using both absorbance and fluorescence spectroscopy. Furthermore, the spectroscopic study of (E)-resveratrol under different ionization states corresponding to the deprotonation of the hydroxyl groups present in the molecule shows significant differences among the absorbance, excitation, and emission spectra of (E)-resveratrol that have not previously been reported.