Equilibrium forms of vitisin B pigments in an aqueous system studied by NMR and visible spectroscopy.

The main species in the acid-base equilibria of two pyranoanthocyanins (vitisins B), pyranomalvidin-3-glucoside I and pyranomalvidin-3-O-coumaroylglucoside II, and the respective pK(a) were determined using NMR, visible spectroscopy, and mass spectrometry techniques. The three equilibria involve protonation of the pyranoflavylium cation of vitisin B (pK(a1)) and two deprotonations (pK(a2) and pK(a3)). For pigment I, the values obtained through the titration curves of the chemical shift of some protons versus pH were (pK(a1) < 0.68; pK(a2) = 4.40 +/- 0.08; pK(a3) = 7.45 +/- 0.09) very close to the values obtained by visible spectroscopy (pK(a1) < 0.63; pK(a2) = 4.43 +/- 0.02; pK(a3) = 7.34 +/- 0.03). For pigment II, it was only possible to calculate the pK(a) by visible spectroscopy (pK(a1) < 0.75; pK(a2) = 4.66 +/- 0.10; pK(a3) = 6.76 +/- 0.10). NMR studies have shown that pigment I does not undergo hydration, and the hypothesis of the occurrence of hemiacetal forms in equilibrium was discarded.