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 malvidin 3-O-glucoside (Mv3glc) and procyanidin dimer epicatechin-(48)-epicatechin 3-O-gallate (B2-3ʹOG) 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=781, 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.