Substituted p-hydroquinones as inhibitors of lipid peroxidation

Abstract

The technique based on monitoring oxygen consumption was applied to study 12 alkyl- and methoxy-substituted p-hydroquinones (QH 2) as a chain-breaking antioxidant during the oxidation of styrene and methyl linoleate (ML) in bulk as well as ML oxidation in micellar solution of sodium dodecyl sulfate (SDS) at 37 °C. The antioxidant activities of QH 2 were characterized by two parameters: the rate constant k 1 for reaction of QH 2 with the peroxy radical LO 2 : QH 2+LO 2 →QH +LOOH and the stoichiometric factor of inhibition, f, which shows how many kinetic chains may be terminated by one molecule of QH 2. In the case of styrene and ML oxidation in bulk, f values never exceed two; for the majority of QH 2, f was found to be significantly less than two due to the interaction of QH with molecular oxygen. In the absence of superoxide dismutase (SOD), all the studied QH 2 displayed a very moderate if any antioxidant capability during ML oxidation in SDS micelles. When 20 U/ml SOD was added, the majority of QH 2 showed a pronounced ability to inhibit ML oxidation, f parameter being ca. one. The features of QH 2 as an antioxidant in aqueous environment are suggested to associate with the reactivity of semiquinone (Q −). Q − reacts readily with molecular oxygen with formation of superoxide (O 2 −); further reactions of O 2 − result in fast depleting QH 2 and chain propagation. The addition of SOD results in purging a reaction mixture from O 2 − and, as a corollary, in depressing undesirable reactions with the participation of O 2 −. With all the oxidation models, QH 2 were found to be very reactive to LO 2 . The rate constants k 1 decreased progressively when going from the oxidation of styrene to ML oxidation in bulk and further to ML oxidation in SDS micelles.