Determination of Dissociation Constants of Resverastrol and Polydatin by Capillary Zone Electrophoresis

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

Several independent determinations of the pK_a 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-outgased 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 pK_a values: pK_a1 = 9.1₆, pK_a2 = 9.7₇, and pK_a3 = 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 Förster equation to estimate pK_a* 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.

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 microM at pH 5.5) than at basic pH values (37 microM 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 (pKa1 = 8.8, pKa2 = 9.8, pKa3 = 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.

Determination of resveratrol in wine by photochemically induced second-derivative fluorescence coupled with liquid-liquid extraction

Basic studies on the photochemical behaviour of trans-resveratrol and its photoproduct are reported. Photometrically and fluorimetrically calculated acidity constants of the former were determined. The usefulness of the determination of resveratrol by photochemically induced fluorescence and second-derivative photochemically induced fluorescence was also examined. The very weakly fluorescent trans-resveratrol is converted into a highly fluorescent photoproduct by irradiating hydroethanolic solutions of trans-resveratrol containing 40% v/v of ethanol for 60 s with intense UV radiation. The photoproduct presents excitation and emission maxima centred at 260 nm, and 364 and 382 nm, respectively. Under these conditions, a linear relationship between fluorescence intensity and trans-resveratrol concentration was found between 6.6 and 66 ng mL-1. Optimum conditions for the extraction of trans-resveratrol from an aqueous phase at pH 5.0 with diethylether were a phase ratio (aqueous/organic) of 2, a shaking time of 60 s and a buffer concentration of 0.15 mol L-1. An extraction recovery of 100% was reached under these conditions. The optimized extraction procedure was applied to the analysis of resveratrol in wine samples, employing the amplitude between 356 and 364 nm of the second-derivative photoinduced emission spectrum as analytical signal. It was found that there is not matrix effect and recoveries around 100% were obtained at different fortification levels.