Formation of trimers of α‐tocopherol and its model compound, 2,2,5,7,8‐pentamethylchroman‐6‐ol, in autoxidizing methyl linoleate

Abstract

The reaction products of alpha-tocopherol and its model compound, 2,2,5,7,8-pentamethylchroman-6-ol, during the autoxidation of methyl linoleate at 37 C were investigated. Two isomeric trimers were obtained as the major reaction products of alpha-tocopherol, and a trimer was obtained as that of its model chroman. The structure of each trimer has been characterized by IR, UV, 1H and 13C NMR, and mass spectroscopy. The 13C NMR spectra showed the presence of a quaternary carbon atom and a carbon atom bearing two oxy substituents in the molecule. On methanolysis of each trimer, equimolar amounts of the 5-methoxymethyl compound and the dihydroxy dimer were formed, indicating the presence of a ketal group in the molecule. From these results, new trimeric structures are proposed. The reaction products of alpha-tocopherol could be well-separated by reverse-phase high performance liquid chromatography. When methyl linoleate was autoxidized in the presence of 1 mol% alpha-tocopherol, spirodiene dimer was the initial reaction product of alpha-tocopherol, and trimers were formed with the decrease of alpha-tocopherol.

Photo- and Free Radical-Mediated Oxidation of Lipid Components During the Senescence of Phototrophic Organisms

“…In order to check this hypothesis, we searched for the presence of 5a-methoxytocopherol (30) after methanolysis of NaBH 4 -reduced and non-reduced lipid extracts obtained from cells of Emiliania huxleyi s t r a i n T W P 1 a n d Chrysotila lamellosa strain HAP17. The detection of significant amounts of this methanolysis product of trimers (Yamauchi et al, 1988) in these extracts (Nassiry et al, 2009) well supported the presence of such trimeric oxidation products of vitamin E in these algae.…”

“…However, methanolysis of the residues obtained resulted to the formation of high amounts of 5a-methoxytocopherol (30) arising from the methanolysis of the ketal group of trimers (Fig. 12) (Yamauchi et al, 1988). ESI-TOF MS analyses of oxidation products were also carried out in order to confirm the presence of high proportions of trimers (Nassiry et al, 2009).…”

“…These authors detected these trimers in seeds of Euryale ferox containing extraordinarily high content of tocopherols. It is interesting to note that trimers were previously obtained as the major reaction products of vitamin E autoxidized under mild conditions in solution (1%) in methyl linoleate (Yamauchi et al, 1988). In plastoglobules, which are lipid monolayer subcompartments of the thylakoid membranes of chloroplasts (Maeda and Dellapenna, 2007), the concentration of tocopherols can reach 10% of the total fatty acids (Vidi et al, 2006).…”

Vitamin E: Mechanism of Its Antioxidant Activity.

“…It has been determined that c!-tocopherol is the most efficient chain-breaking antioxidant among the four isomers. cY-Tocopherol also can react with alkoxyl radicals (Gardner et al, 1972;Kaneko & Matsuo, 1985) or undergo self-coupling to form dimers and trimers (Csallany et at, 1970;Yamauchi et al, 1988). When oxygen is present in trace amounts, c!-tocopherol can react directly with alkyl radicals (Evans et al, 1992;Yamauchi et al, 1993).…”

“…Figure 2 shows the possible pathways for the reactions of c!-tocopherol with lipid free radicals. It has been reported that the products of a-tocopherol (1) formed under mild conditions of autoxidation are a-tocopherylquinone (4), c!-tocopherol spirodiene dimer (6), and c!-tocopherol trimer (7) (Csallany et al, 1970;Yamauchi et al, 1988). However, the other reaction products might also be formed during the autoxidation of unsaturated lipids.…”