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-me thylquercetin, 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 b y UV-visible spectroscopy, oxidation rate constants are estimated. Fast dis proportionation of flavonoid radicals is shown to give quinones which can b e identified by their adducts with methanol (quercetin quinone) or sodium b enzenesulfinate (rutin quinone). In strongly alkaline non aqueous condition s, the quercetin quinone can also be evidenced by strong charge transfer ab sorption bands in the range 700-800 nm. The consequences of these observations for the antioxidant properties of qu ercetin and quercetin derivatives are discussed.