Reactions of biological antioxidants: I. Fe(III)-catalyzed reactions of lipid hydroperoxides with α-tocopherol

Rates of α-tocopherol oxidation were measured for free-radical reactions produced by Fe(III)-catalyzed dissociations of hydroperoxides. The kinetics were treated as first-order in α-tocopherol. The hydroperoxides were preformed from methyl linoleate, methyl linolenate, ethyl arachidonate, methyl eicosapentaenoate, and a fraction of polyunsaturated fatty esters of menhaden oil. The degree of unsaturation of the lipid hydroperoxides had little effect on the rates of α-tocopherol oxidation. The rates of oxidation decrease with the concentration of water and increase with the acidity of the media. The pH data suggest a transition from one predominant mechanism to another, which may involve principally acid catalysis. A mechanism for α-tocopherol oxidation is suggested.