Degradation of cyanidin 3-glucoside by caffeic acid o-quinone. Determination of the stoichiometry and characterization of the degradation products

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 cyan idin 3-glucoside (Cy 3-glc, o-diphenolic anthocyanin) was monitored by HPLC , and quantitative analyses were performed to establish the stoichiometry o f the reaction. The results indicate that Cy 3-glc is degraded by a coupled oxidation mechanism with integration of CQ into the degradation products. The ratio of degraded Cy 3-glc to CQ incorporated into the condensation pro ducts was similar to 2.0. No brown products could be detected, only a sligh t orange color. Moreover, the addition of purified polyphenol oxidase to th e slightly colored media resulted in the disappearance of the caffeic acid formed from the reaction of coupled oxidation (Cy 3-glc/CQ) and the formati on of brown polymers. The degradation products were isolated by gel filtrat ion on Sephadex G-25. The UV-vis spectra and chemical analysis (acidic hydr olysis) of the degradation products suggest that they resulted from the con densation of caffeic acid and Cy 3-glc. HPLC analysis showed that the parti al purified fraction contained a mixture of complex condensation products.