Grape anthocyanin oligomerization: A putative mechanism for red color stabilization?

The equilibrium forms of malvidin-3-O-glucoside trimer present in grape skins were studied in aqueous solution at different pH values through UV-Visible spectroscopy. It was observed that the reactivity of this compound is strongly dominated by acid-base chemistry (pK(a1) = 3.61 +/- 0.03; pK(a2) = 6.83 +/- 0.06), with the reaction sequence hydration-tautomerization-isomerization accounting less than 10% of the overall reactivity. The decrease of hydration of this flavylium cation derivative when compared to the original anthocyanins results from the formation of a cluster of the pigment with a high-energy of solvation that inhibits the access of water molecules to the flavylium cation core preventing by this way the hydration reactions. Overall of these results raise the hypothesis that polymerization may be a natural stabilization mechanism for the red color of anthocyanins.