Oxidation of Quercetin and Its Structural Analogues Differentially Affects Its Antioxidant Properties

Abstract

We addressed whether the oxidation of quercetin (Q) necessarily leads to the loss of its antioxidant properties. Towards that end, the reactive oxygen species (ROS)-scavenging (ORAC), the Folin-Ciocalteau(FC)- and Fe-reducing capacities of quercetin, and that of thirteen other structurally-related flavonoids, was assessed following their exposure to oxidative conditions. Oxidation was chemically-induced by direct dissolution of each flavonoid in an alkaline medium (pH12, 0-300 min). HPLC-DAD analyses revealed that the flavonoids exhibiting a HO group in the C3-ringC position in conjugation with a HO in C3’-4’-ringB were the fastest in disappearing. Those lacking a HO in C3-ringC, and those containing no catechol in the B-ring underwent either a slower and/or an only partial disappearance within 300 min. After its total disappearance, the mixture containing the Q-derived oxidation products was found to retain 95%, 77% and 77% of its original ORAC, FC- and Fe-reducing capacities. Analogues containing a flavonol structure conserve at least 82% of their ORAC values

excepting for catechins and eriodictyol (which exhibited near 50%), all other tested flavonoids retained at least 69% of their ORAC. Between 51% and 95% of the original FC-reducing capacity and between 12% and 68% of the original Fe-reducing capacity was retained by all tested flavonoids. Present results are first in reporting that the mixtures resulting from the chemical-induced oxidation of either quercetin or some of its structurally-related analogues, are able to largely conserve their ROS-scavenging and redox properties. Ongoing studies are being conducted to further establish the identity of the some of the oxidized metabolites which account for retained antioxidant activity. Fondecyt #1150090/3150359