In vitro test for the effectiveness of antioxidants as inhibitors of thiyl radical-induced reactions with unsaturated fatty acids.

Attacks of thiyl radicals on the double bonds of unsaturated fatty acids lead to stereomutation (cis-, trans-isomerization without double-bond migration) and addition reactions (thioether formation). On the basis of these findings, an in vitro test system has been developed which allows the study of the effectiveness of specific antioxidants in preventing thiyl radical-induced attacks on unsaturated fatty acids. The test involves thermal treatment of a mixture of oleic (cis-9-octadecenoic) acid and 1-tetradecanethiol with the antioxidant, followed by measurement of the extent of formation of the products of stereomutation and addition (i.e., elaidic (trans-9-octadecenoic) acid and isomeric 9(10)-S-tetradecylstearic acids, respectively) by gas chromatography of their methyl esters as a function of antioxidant concentration. Antioxidants such as octyl gallate, ascorbic acid 6-O-palmitate, ubiquinone 50 (coenzyme Q(10)), rac-alpha-tocopherol, and 2,6-di-tert-butyl-4-methylphenol (BHT) were tested for their ability to protect the >C=C< double bond of oleic acid against attacks of thiyl radicals generated from 1-tetradecanethiol by heating. The results show that octyl gallate, ascorbic acid 6-O-palmitate and, to some extent, ubiquinone 50 (coenzyme Q(10)) were highly effective in preventing reactions of free thiyl radicals with oleic acid, whereas rac-alpha-tocopherol and BHT were moderately effective.