Kinetic study of the prooxidant effect of tocopherol. Hydrogen abstraction from lipid hydroperoxides by tocopheroxyls in solution

Abstract

AbstractA kinetic study of the prooxidant effect of vitamin E (tocopherol, TocH) has been carried out. The rates of hydrogen abstraction (k−1) from methyl linoleate hydroperoxide (ML‐OOH) by α‐tocopheroxyl (α‐Toc.) (1) and eight types of alkyl substituted Toc. radicals, (2–9) in benzene solution have been determined spectrophotometrically. The results show that the rate constants decrease as the total electron‐donating capacity of the alkyl substituents on the aromatic ring of Toc. increases. The k−1 value (5.0×10−1M−1s−1) obtained for α‐Toc. (1) was found to be about seven orders of magnitude lower than the k1 value (3.2×106M−1s−1) for the reaction of α‐TocH with peroxyl radical, which is well known as the usual radical‐scavenging reaction of α‐TocH. The above reaction rates (k−1) obtained were compared with those (k3) of methyl linoleate with Toc. (1–9) in benzene solution. The rates (k−1) were found to be about six times larger than those (k3) of the corresponding Toc.. The results suggest that both reactions may relate, to the prooxidant effect of α‐TocH at high concentrations in foods and oils. The effect of the phytyl side chain on the reaction rate, of Toc. in micellar dispersions has also been studied. We have measured the rate constant, k−1, for the reaction of phosphatidylcholine hydroperoxide with a Toc. radical in benzene,tert‐butanol and in Triton X‐100 micellar dispersions, and compared the observed k−1 values with the corresponding values for ML‐OOH.