Tocopherol reaction with peroxy radical

The antioxidant activity of tocopherols increases from a 5. It is the reverse of the vitamin E activity (cf. 6.2.3) and of the rate of reaction with peroxy radicals. Table 3.39 demonstrates that a-tocopherol reacts with peroxy radicals faster than the other tocopherols and the synthetic antioxidants DBHA and BHT. 

Lipid hydroperoxide caused the oxidation of a-tocopherol to a-tocopherolquinone through an unidentified intermediate (40). Porter et al. (41) proposed a mechanism of oxidation that includes an intermediate from combination of a-tocopherol semiquinone with a peroxy radical (Reaction 0). 

The free radical reaction may be accelerated and propagated via chain branching or homolytical fission of hydroperoxides formed to generate more free radicals (equations (11.4), (11.5)). Free radicals formed can initiate or promote fatty acid oxidation at a faster rate. Thus, once initiated, the free radical reaction is self-sustaining and capable of oxidizing large amounts of lipids. On the other hand, the free radical chain reaction may be terminated by antioxidants (AH) such as vitamin E (tocopherols) that competitively react with a peroxy radical and remove a free radical from the system (equation (11.6)). Also, the chain reaction may be terminated by self-quenching or pol)rmerization of free radicals to form non-radical dimers, trimers and polymers (equation (11.7)). 

Recent studies indicate that a-tocopherol can also directly react with alkyl macroradicals, this reaction being more likely due to the higher mobility of the alkyl macroradicals compared to that of peroxy radicals [70]. The alkyl macroradicals react with a-tocopherol, giving a polymeric saturated molecule and a stabilized phenoxy radical (Scheme 13, Part B). The phenoxy radical can in turn interact with another alkyl macroradical leading to the formation of a,(3-unsaturated ketones [69, 70]. This mechanism indicates a double efficiency of stabiUzation of vitamin E one vitamin E molecule can convert up to two alkyl macroradicals to umeactive species. 

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