A comprehensive investigation of guaiacyl-pyranoanthocyanin synthesis by one-/two-dimensional NMR and UPLC–DAD–ESI–MSn
Résumé
In red and rose wines, the grape anthocyanins are progressively converted to more stable pigments, including phenylpyranoanthocyanins. One-/two-dimensional NMR and UPLC-DAD-ESI-MSn measurements were used to monitor the synthesis of guaiacylpyranomalvidin 3-O-glucoside from malvidin 3-O-glucoside and vinylguaiacol in model solutions and identify the products formed during the reaction. The highest conversion rates (30%, determined by H-1 qNMR) were obtained with a small excess of vinylguaiacol in methanol/water (70/30) at pH 3 and 35 degrees C. Two reaction pathways competed with the formation of guaiacylpyranomalvidin 3-O-glucoside. The first one only concerns malvidin 3-O-glucoside and consists in C-ring cleavage with formation of malvone and smaller molecular weight breakdown products. This pathway is favored at higher pH and incubation temperature. At lower pH values or in the presence of large vinylguaiacol excess, faster consumption of malvidin 3-O-glucoside resulted from the formation of more complex pyranoanthocyanins substituted by vinylguaiacol oligomers.