Oxidative Dimerization of Tocopherols during the Course of Autoxidation of Methyl Esters of Saturated and Unsaturated Fatty Acids. III.

Abstract

Theeffects of phospholipids on the oxidative decompositionand dimerization of d-γ-tocopherol (γ-T) or d-δ-tocopherol (δ-T) during the autoxidation of methyl linoleate (Me-Li) or methyl laurate (Me-La) were investigated. The fatty acid methyl esters, which contained tocopherol (0.5 %) and one of following additives (0.01-0.5 %) were oxidized by air under A.O.M. conditions. The additives used in the experiments are as follows : phosphatidylcholine (PC) and phosphatidylethanolamine (PE) as phospholipids, tri-n-butylphosphate (TBP) as a model component of phosphate ester, and tri-n-octylamine (TOA) as that of a nitrogenous base. The amounts of tocopherol and its dimers formed in a reaction mixture were determined by HPLC. PC and PE improved the stability of the tocopherol monomer in Me-Li (Figs.-1 and 7) and suppressed the formation of its dimers (Figs.-2, 3, 8 and 9). The total amount of the tocopherol monomer and its dimers was constant regardless of the presence of phospholipids (Figs.-4 and 10). It is thus evident that phospholipids suppress the initial stage of the oxidation of tocopherol, in which tocopherol changes to its dimers, and that they also contribute to the regeneration of the tocopherol monomer from its dimers. TBP, by which the stability of tocopherol under the condition of the thermal oxidation was improved, promoted the oxidation of tocopherol (Figs.-1 and 7) and suppressed the dimerization of tocopherol (Figs.-2, 3, 8 and 9) under the autoxidative conditions of Me-Li. The oxidation of tocopherol in Me-Li was promoted by TOA at low concentration (0.01 %). The oxidative decomposition of tocopherol decreased with the concentration of TOA. However, TOA eventually suppressed the oxidation of tocopherol (Figs.-1 and 7) and formed mainly diphenylether type dimed (Figs.-2 and 8).