The formation mechanism of pinotin A in model wine: Experimental and theoretical investigation

Pinotin A is a crucial pyranoanthocyanin in aged red wine. The formation mechanism of pinotin A was investigated through experimental and theoretical approaches in model wine solution. Using UHPLC-DAD-QTOF, three intermediates were identified in the reaction solution of malvidin-3-O-glucoside and caffeic acid. The pH and oxidative conditions affected the reaction rate by modulating the accumulation of intermediates. Upon preparative HPLC and oxidation experiment, the transformation of intermediate 3 to intermediate 2 was observed through free radical oxidation, p-benzoquinone oxidation and autooxidation. However, the production of pinotin A was only achieved through autooxidation. A three-steps reaction mechanism of reversible addition reaction and decarboxylation, and autooxidation was proposed. The validity of the proposed mechanism was verified through quantum chemistry calculation. The negative variation of Gibbs free energy of the entire reaction was obtained, where the autooxidation step played a crucial role for the spontaneity of the whole reaction.