Formation of anthocyanin-flavanol adducts in model solutions

The mechanism of direct reactions between malvidin 3-O-glucoside (Mv3glc) and a flavanol monomer, (-)-epicatechin, was studied in model solutions in the pH range 2.0-6.0. The direct reactions are strongly related to pH. The rate of Mv3glc and epicatechin disappearance increased with pH. Nucleophilic addition of epicatechin onto the flavylium form of Mv3glc took place at all pH values. At pH 2.0, the resulting intermediate flavene yielded a colourless Mv3glc-epicatechin adduct with an A-type linkage. In the range pH 3.2-6.0, two new products with UV-vis spectra characteristic of those of xanthylium ions and mass signals at m/z 633 and 453, respectively, in the positive ion mode were detected. These products are postulated to be degradation products of the flavylium form of Mv3glc-epicatechin arising from the intermediate flavene. Moreover, above pH 4.0, oxidation of epicatechin promoted the formation of new pigments detected at m/z 78 1, in the positive ion mode. These ions were attributed to epicatechin-Mv3glc adducts formed by addition of Mv3glc onto the epicatechin o-quinone as the loss of a 138 amu fragment indicated a biphenyl or biphenyl-ether linkage between the flavanol B-ring and the anthocyanin A-ring. (c) 2005 Elsevier B.V. All rights reserved.