Incidence of Molecular Structure in Oxidation of Grape Seed Procyanidins

The kinetics of decomposition of the following flavan-3-ol derivatives isolated from grape seeds under oxidative conditions by airing and using metal ion catalysis (iron and copper) are determined: (+)-catechin and (-)-epicatechin; seven natural procyanidin dimers, B1 [(-)-epicatechin-(4-->8)-(+)-catechin], B2 [(-)-epicatechin-(4-->8)-(-)-epicatechin], B3 [(+)-catechin-(4-->8)-(+)-catechin], B4 [(+)-catechin-(4-->8)-(-)-epicatechin], B6 [(+)-catechin-(4-->6)-(+)-catechin], B7 [(-)-epicatechin-(4-->6)-(+)-catechin], and B8 [(-)-epicatechin-(4-->6)-(-)-epicatechin]; trimers, C1 [(-)-epicatechin-(4-->8)-(-)-epicatechin-(4-->8)-(-)-epicatechin], (+)-catechin-(4-->8)-(+)-catechin-(4-->8)-(-)-epicatechin, and (+)-catechin-(4-->8)-(-)-epicatechin-(4-->6)-(+)-catechin, monogallate esters of B2 and B4 and digallate of B2, which were isolated from grape seeds. Kinetic decomposition comparisons were monitored by HPLC. The following order was found for oxidative decomposition for procyanidin dimers: B3 approximately B4 > B7 approximately B6 > B1 approximately B2 > B8. In the conditions of this study, the gallate ester of (-)-epicatechin is more unstable than (-)-epicatechin; inversely, kinetic decompositions of dimeric procyanidins B2 and B4 are much more important than those of their gallate esters. Molecular mechanics (MM2) and (1)H NMR studies of dimeric 3-O-gallate structures show a pi-pi stacking arrangement between the aromatic gallate and catechol rings, absent in analogous dimeric procyanidins, which reduces the total surface accessible to oxidizing agents.