One-electron oxidation of quercetin and quercetin derivatives in protic and non protic media

Quercetin (3,3',4',5,7-pentahydroxyflavone) and quercetin derivatives (3-methylquercetin, rutin) are strong flavonoid antioxidants abundant in plants and in human diet. Their oxidation by DPPH, CAN or dioxygen (autoxidation) is studied in protic and non protic solvents. From kinetic investigations by UV-visible spectroscopy, oxidation rate constants are estimated. Fast disproportionation of flavonoid radicals is shown to give quinones which can be identified by their adducts with methanol (quercetin quinone) or sodium benzenesulfinate (rutin quinone). In strongly alkaline non aqueous conditions, the quercetin quinone can also be evidenced by strong charge transfer absorption bands in the range 700-800 nm. The consequences of these observations for the antioxidant properties of quercetin and quercetin derivatives are discussed.