A model study using 2,2,5,7,8-pentamethyl-6-hydroxychroman (a-COH) and 2~2,7,8-tetramethyl-6-hydroxychroman (1-COH) as antioxidants, and linoleic acid and its methyl ester (both in bulk phase) as the lipids was performed. After having demonstrated that the antioxidative activities of the model substances did agree with those of the corresponding tocopherols (a-T, y-T), the stability of a-COH and y-COH was determined, and the products arising from a-COH, y-COH, linoleic acid and its methyl ester were identified. a-COH did oxidize to different major products (quinone, trimer) depending on the reaction temperature (37 'C and 47 "C) and the antioxidant concentration, whereas the products obtained by oxidation of y-COH (diphenyl ether dimer, biphenyl dimer) did not seem to he affected by differences in the reaction conditions. It was concluded that y-T was superior to a-T as antioxidant because it appears more stable and, also, being oxidized to compounds which are still effective as antioxidants.
I n t r o d u c t i o n
The antioxidant activity of the tocopherols (T's) occurring in fats and oils has been compared by many investigatorsl-lO. Most of the experiments performed at higher temperatures (2 6OOC) resulted in the relative antioxidant activity order of 6-T > y-T > p-T > a-l3 7-9.10. At lower temperatures the results obtained were quite different, e.g. Elegdi-Kovats and Berndo fer-Krasmer found an order of a-T > y-T > p-T > 6-T for the protection of lard at 20' C and 4OoC, whereas Okott and Kin der Ken5 observed that y-T and 6-T were superior antioxidants to a-T in the case of menhaden oil stored at 37°C. These results support the conclusions drawn by Lea and Ward3 as well as by Kauf mann et al.4 that the order of antioxidant activity of the T's varies depending on the temperature, on the composition of the fat and on its state (bulk phase, emulsion), and on the concentration of the T's.
Model experiments with linoleic acid (LH), or its methyl ester (ML) as defined lipid have been conducted by several authors in order to clarify the causes for the different reactivity of the T's. Under various conditions (temperature, dispersion of the lipid) y-T and 6-T were better antioxidants than a-Tl1-l4 and were, also, more stable than a-Tl49 15. a-T inhibited most effectively the autoxidation of LH at lower concentrations (up to 0.38 ' Yo a-T by weight of the lipid)I4. At higher concentrations (3.8%) a-T exhibited a pro-oxidant activity, observed also in fat+, whereas 6-T and y-T were even antioxidants'*.
According to the mechanism suggested by Mahoney17, the autoxidation of LH or ML is inhibited by the reaction of the peroxyl radical of LH or ML with the a-T.
a-Tocopherolquinone, dimers and trimers of a-T were identified as stable products of the chromanoxyl radical resulting from a-T as an intermediateI3, 19, and two types of dimers were obtained from the corresponding chromanoxyl radicals of yT and 6-T19.20.
The antioxidant activity of a-T is not influenced by the isoprenoid side chain21, 22. 6-Hydroxychroman derivatives, in which the isoprenoid side chain is replaced by a methyl