Network of carboxypyranomalvidin-3-O-glucoside (vitisin A) equilibrium forms in aqueous solution

The equilibrium forms of carboxypyranomalvidin-3-O-glucoside (vitisin A) were studied in aqueous solution at different pH values by NMR and UV–Visible spectroscopy. The results obtained revealed the presence of three deprotonation reactions at positions 10-COOH, 7-OH and 4′-OH: pKa1 = 1.09 ± 0.09; pKa2 = 4.93 ± 0.02; pKa3 = 9.14 ± 0.05, data obtained from NMR titration. The titration by NMR confirmed the absence of hydration reactions since no characteristic peaks of the hemiketal form were observed in the NMR spectrum at any pD value studied (pD 0.67–10.99).