Direct condensation between flavanols and anthocyanins

F-A pigments are described to result from the direct condensation between anthocyanins and condensed tannins (proanthocyanidins). The presence of these pigments in wines was confirmed recently (Salas et al, 2004a). Their formation mechanism was described to involve two fundamental steps (see Fig. 3.9) first, the acid-catalyzed cleavage of inter-flavan linkage of proanthocyanidins, yielding a carbocation in C-4 and then, the carboca-tion may react on the nucleophilic position C-8, or less likely the C-6, of the hemiacetal form of anthocyanin to form the F-A adduct. 

The F-A+ pigments formation is expected to occur very slowly during wine aging because the acid-catalyzed cleavage of the intervanoid linkage of proanthocyanidins is not straightforward at wine pH. Recently, Salas et al (2003) have demonstrated that the 

Similarly to anthocyanins, the same authors demonstrated that the red color of cate-chin-mv3glc (F-A+) obtained by synthesis fades significantly with the increase in pH from 1 to 4 as a result of the hydration of the chromophore moiety, leading to the formation of the respective colorless hemiacetal form (Salas etal, 2004b). 

Anthocyanin may also dimerize yielding A-A dimers. Such dimers were tentatively identified by ESI-MS in grape skin extracts (Vidal et al, 2004), as well as in red wine fractions obtained by high-speed countercurrent chromatography (Salas et al, 2005) and by fractionation on Toyopearl after bisulfite bleaching (Alcalde-Eon et al, 2007). Two structures have been proposed for A-A+ A-type bond (C4-C8 and C2-0-C7) flavan-fiavylium dimers and B-type bond (C4-C8) flavene-flavylium (Vidal et al, 2004). Oligomers consisting of a flavanol residue directly linked to an anthocyanin dimer (F-A-A ) have also been recently identified in red wine fractions (Alcalde-Eon et al., 2007). 

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