Relative autoxidative and photolytic stabilities of tocols and tocotrienols

Abstract

The relative stabilities of selected individual tocols and tocotrienols and of equimolar mixtures of either alpha- plus gamma- or alpha- plus delta- tocopherols were determined in methyl myristate and methyl linoleate during autoxidation and photolysis. Solutions containing 0.05% of the appropriate tocopherol(s) or tocotrienols were subjected to UV light (254 nm) or to a flow of 4.3 ml/min of oxygen, both at 70 C. Tocopherols (T) and tocotrienols (T-3) were determined by gas chromatography without preliminary separation or purification. Under photolytic conditions, stabilities in increasing order in methyl myristate were gamma-T-3 less than alpha-T-3 less than delta-T less than alpha-T less than gamma-T less than 5,7-T less than beta-T and in methyl linoleate were alpha-T less than alpha-T-3 less than or equal to gamma-T-3 less than or equal to beta-T less than or equal to 5,7-T less than gamma-T less than delta-T. A solvent effect on the initial rate of photolysis was observed for 5-methyl substituted tocols but not for the tocols with an unsubstituted 5-position or for the tocotrienols. Under autoxidative conditions, stabilities in increasing order in methyl myristate were alpha-T = alpha-T-3 less than beta-T-3 less than gamma-T-3 less than delta-T-3 less than gamma-T less than delta-T = beta-T and in methyl linoleate were alpha-T less than alpha-T-3 less than gamma-T-3 less than beta-T less than gamma-T less than delta-T. Tocopherols were much more stable during autoxidation in methyl myristate than they were in methyl linoleate. In mixtures, there was no significant protection of alpha-tocopherol by either gamma- or delta-tocopherol under any of the conditions used. However, alpha-tocopherol was highly effective in protecting gamma- and delta-tocopherols in methyl myristate during both photolysis and autoxidation and in methyl linoleate during photolysis. During autoxidation in methyl linoleate, alpha-tocopherol protection of gamma- and delta-tocopherols after 24 hr was slight tough measurable.