Demonstration of the occurrence of flavanol-anthocyanin adducts in wine and in model solutions

Flavanol-anthocyanin (F-A(+)) adducts were detected in red wine. A mechanism involving acid-catalysed cleavage of flavanol oligomers followed by nucleophilic addition of the anthocyanin moiety (in its hemiketal form) to the resulting carbocation (F+) was postulated. To confirm this mechanism, reactions between malvidin3-O-glucoside (Mv3glc) and procyanidin dimerepicatechin-(4-8)-epicatechin 3-O-gallate (B2-3'OG) were studied in a model solution system at pH 2. A new pigment with a UV-Vis spectrum similar to that of Mv3glc and a signal at m/z = 781 in the positive ion mode was detected and was attributed to Ec-Mv3glc, in agreement with the proposed reaction pathway. A protocol adapted from the synthesis of procyanidin dimers, in which the terminal flavanol units was replaced with Mv3glc, was tested. Two new pigments were formed with a signal at m/z = 78 1, in the positive ion mode. These ions were attributed to catechin-Mv3glc. Both adducts obtained by hemisynthesis showed exactly the same fragmentation pattern as that present in wine. In particular, the loss of a 126 amu fragment corresponding to the unsubstituted A-ring of the flavanol unit indicated that all of them were (epi)catechin-Mv3glc adducts. These results prove that reactions between the carbocations resulting from cleavage of tannin interflavanic bonds and anthocyanins occur in wine. (C) 2003 Elsevier B.V. All rights reserved.