Characterization of Hemiacetal Forms of Anthocyanidin 3-O-β-Glycopyranosides

The 3-O-β-glucopyranosides of delphinidin, petunidin, and malvidin (1−3) and cyanidin 3-O-β-galactopyranoside (4) dissolved in deuterated methanolic solutions without and with acid (5%, CF3COOD) were identified by homo- and heteronuclear NMR techniques. The hemiacetal forms of all the four anthocyanins were characterized as two epimeric 2-hydroxy-hemiacetals on the basis of assignments of both proton and carbon NMR signals together with chemical shift considerations. This is the first report of 13C NMR assignments of two epimeric anthocyanin hemiacetal forms. No 4-hydroxy-hemiacetal form was detected for any of the pigments. For each anthocyanin dissolved in deuterated methanol, the equilibrium between each of the two epimeric hemiacetals and the corresponding flavylium cation was confirmed by the observed positive exchange cross-peaks in the 2D 1H NOESY spectra. The molar proportions of the flavylium cation and the two hemiacetals of 1−4 in deuterated methanol were very similar for all pigments, even during storage for weeks. The majority of the anthocyanins reported to occur in fruits have the same or similar structures as 1−4. These pigments have been proposed to exist predominantly as hemiacetals in slightly acidic to neutral solvents, which is a relevant pH range in plants and in the human gastrointestinal tract. Keywords: Anthocyanidin 3-O-β-monoglycopyranosides; hemiacetal forms; equilibrium proportions; 1H and 13C NMR assignments