Reactivity of Anthocyanins and Pyranoanthocyanins. Studies on Aromatic Hydrogen–Deuterium Exchange Reactions in Methanol

Reactivity studies involving anthocyanin structures and their equilibrium forms will lead to better understanding of the properties of these antioxidants. Hydrogen–deuterium (H → D) exchange reactions at various sites of the 3-glucosides of delphinidin (1), petunidin (2), malvidin (3), and the corresponding 3-glucosides of carboxypyranodelphinidin (4), carboxypyranopetunidin (5), carboxypyranomalvidin (6), and the flavonol quercetin 3-O-(6-α-rhamnopyranosyl-β-glucopyranoside) (7) have been examined at room temperature in pure CD3OD and in CD3OD acidified with CF3CO2D. The H → D exchange rate constants of H-6 and H-8 of 2 determined from 1H NMR integration data were found to be independent upon pigment concentration (up to 4 × 10−2 M) and trifluoroactic acid concentration (0–15%, v/v), respectively. This suggest that these reactions follow first-order kinetics and unexpectedly to be independent of the acid concentration. H-6 and H-8 of the flavylium cation A-rings of 1–3, and in the corresponding hydrogens of the hemiketal forms, exchanged with half-lives of ∼100 h (1) and ∼50 h (2 and 3), respectively. The pyranoanthocyanins (4–6) experienced no H → D exchange for the analogous hydrogens, but H → D exchange of H-β (H-4) (t1/2 ≈ 25 h) for these compounds was observed. Only H-8 underwent significant H → D exchange in 7. It is concluded that a stabilization of the σ-complexes, assumed to be the intermediates in the reactions, takes place for the common anthocyanins (1–3) contrary to the pyranoanthocyanins (4–6).