Catechin-(4,8)-malvidin-3-glucoside, a red pigment adduct (at acid pH) found in red wine and resulting from the reaction between malvidin-3-glucoside and flavan-3-ols during wine aging, was synthesized. The thermodynamic and kinetic constants of the network of chemical reactions were fully determined by stopped flow: (i) Direct pH jumps, from thermal equilibrated solutions at pH = 1.0 (flavylium cation, AH(+)), show three kinetic processes. The first one occurs within the mixing time of the stopped flow and leads to the formation of quinoidal bases A and/or A(-) depending on the final pH; the second one takes place with a rate constant equal to 0.075 + 33[H(+)] and was attributed to the hydration reaction that forms the pseudobases (hemiketals), B/B(-). The third process is much slower, 2 × 10(-4) s(-1), and is due to the cis-trans isomerization giving rise to a small fraction of trans-chalcones, Ct/Ct(-). (ii) Reverse pH jumps from the thermally equilibrated solution at moderate to neutral pH values back to a sufficiently acidic medium clearly distinguish three kinetic processes: the first one takes place within the dead time and is due to the protonation of the bases; the second process occurs with the same rate constant of the hydration reaction monitored by direct pH jumps and is attributed to the formation of flavylium cation from the B; the last process occurs with a rate constant of 1.8 s(-1), and results from the formation of AH(+) from Ct through B, reflecting the rate of the ring closure (tautomerization). The separation of the hydration from the tautomerization upon a reverse pH jump is only possible because at pH < 1 the former reaction is faster than the last. An identical situation was observed for malvidin-3-glucoside (oenin) for pH < 2.

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The aim of this study is to investigate interactions possibly taking place in red wine between three flavanols (copigments, CP), i.e., two epimeric vinylcatechin dimers (CP1 and CP2) and catechin dimer B3 (CP3), and a specific pigment resulting from the condensation between the main grape anthocyanin malvidin 3-O-glucoside (oenin) and catechin, catechin-(4→8)-oenin. By comparison with our previous work on oenin itself, the influence of the catechin moiety of the anthocyanin in the binding is established. The thermodynamic parameters show that both vinylcatechin dimers exhibit a higher affinity for catechin-(4→8)-oenin, in comparison with proanthocyanidin B3, as previously observed with oenin. However, the corresponding binding constants are weaker, probably due to steric hindrance in the anthocyanin brought by the flavanol nucleus. Consequently, catechin-(4→8)-oenin should be much less stabilized by copigmentation in hydroalcoholic solution than oenin. Quantum mechanics and molecular dynamics simulations are also performed to interpret the binding data, to specify the relative arrangement of the pigment and copigment molecules within the complexes, and to interpret their absorption properties in the visible range.