Proposed mechanism for the degradation of pelargonidin 3-glucoside by caffeic acid o-quinone

Caffeic acid o-quinone (CQ) was prepared by oxidation of caffeic acid with o-chloranil in organic media. The reaction between the purified CQ and pelargonidin 3-glucoside (Pg 3-glc, non-o-diphenolic anthocyanin) was monitored by HPLC and spectrophotometric analysis. Quantitative analyses were performed to establish the stoichiometry of the reactions. The results showed that Pg 3-glc reacted with the hydroxycaffeic acid o-quinone formed by dismutation of CQ in aqueous media. This reaction led to the formation of condensation products with a stoichiometry close to 1:1. The degradation products were isolated by gel filtration on Sephadex G-25. The UV-vis spectra and chemical analysis of the degradation products suggested that they contained both caffeic acid (or secondary products of oxidation) and anthocyanin moieties. HPLC analysis showed that the purified fractions contained several degradation products.