文摘
In the present work, the proton-transfer reactions of the methylpyranomalvidin-3-O-glucoside pigment in water with different pH values was studied by NMR and UV鈭抳isible spectroscopies. The results showed four equilibrium forms: the methylpyranomalvidin-3-O-glucoside cation, the neutral quinoidal base, the respective anionic quinoidal base, and a dianionic base unprotonated at the methyl group. According to the NMR data, it seems that for methylpyranomalvidin-3-O-glucoside besides the acid鈭抌ase equilibrium between the pyranoflavylium cation and the neutral quinoidal base, a new species is formed at pD 4.88鈭?.10. This is corroborated by the appearance of a new set of signals in the NMR spectrum that may be assigned to the formation of hemiketal/cis-chalcone species to a small extent. The two ionization constants (pKa1 and pKa2) obtained by both methods (NMR and UV鈭抳isible) for methylpyranomalvidin-3-O-glucoside are in agreement (pKa1 = 5.17 卤 0.03; pKa2 = 8.85 卤 0.08; and pKa1 = 4.57 卤 0.07; pKa2 = 8.23 卤 0.04 obtained by NMR and UV鈭抳isible spectroscopies, respectively). Moreover, the fully dianionic unprotonated form (at the methyl group) of the methylpyranomalvidin-3-O-glucoside is converted slowly into a new structure that displays a yellow color at basic pH. On the basis of the results obtained through LC-MS and NMR, the proposed structure was found to correspond to the flavonol syringetin-3-glucoside.
