A pigment derived from the acetaldehyde-mediated condensation between (+)-catechin and malvidin 3-O-glucoside has been prepared and isolated by semipreparative HPLC, and its characteristics of color and stability have been studied and compared with that of malvidin glucoside in aqueous solutions. When the pH was increased from 2.2 to 5.5, the solution of the pigment became progressively more violet (lambda(max) = 560 nm at pH 5.5), whereas similar solutions of the anthocyanin were almost colorless at pH 4.0. This behavior indicated that the anthocyanin moiety of the pigment was more protected against water attack, and thus the formation of its quinonoidal forms was favored. The color of the pigment also showed more stability with regard to bleaching by SO(2) than that of malvidin glucoside. Nevertheless, the pigment was more sensitive to degradation in aqueous solution than the anthocyanin. The cleavage of the ethyl bridge that links the anthocyanin and the catechin constituted the first step in its degradation, as demonstrated by the formation of malvidin glucoside as a major product.

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In order to model natural polymeric pigments present in old red wines, new covalent adducts have been synthesized upon condensation of synthetic flavylium ions (models of anthocyanins) with catechin (model of tannins) in the presence and in the absence of acetaldehyde. These new pigments have been investigated by 1D and 2D NMR, HPLC, FAB-mass and UV-visible spectroscopies and molecular modelling. The two flavylium salts used in this work (3,4'-dimethoxy-7-hydroxyflavylium chloride and 5,7-dihydroxy-3,4'-dimethoxyflavylium chloride) display quite different reactivities toward catechin. The electronic donating effect of the catechin moiety and the formation of noncovalent dimers in acidic aqueous or methanolic solution should be mainly responsible for the improved stability of the flavylium chromophore in the new pigments. * No J value is given because of superimposition in the spectra. t The order may be reversed.