The theory of initiation in lipid autoxidation, which deals with the supply of radicals to the chain reaction, has not been substantively advanced for several decades. Most researchers have long assumed a mechanism of initiation in which main-product hydroperoxide is centrally responsible for autocatalytic radical generation. However, this paper, in which we investigate autoxidizing methyl linoleate, presents decisive evidence against such an assumption: Autoxidation-accelerating activity under mild conditions was not found in the chromatographically separated main-product hydroperoxide fraction but was found in other fractions; and highly active substances with structures containing a peroxide-linked dimer with two hydroperoxy groups were actually obtained.

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The formation of hydroxy radicals, hexanal, and 2,4-decadienal was demonstrated from the autocatalytic dimer peroxide which had been reported by us in autoxidizing linoleate (Morita and Tokita in Lipids 41:91-95, 2006). Then, autoxidizing linoleate containing eleostearate was investigated for new autocatalytic substances. The substances obtained were identified as peroxide-linked polymers consisting of both linoleate- and eleostearate-origin units with one hydroperoxy group, and also revealed activity of hydroxy-radical generation. The background of this study is as follows: the above paper reported this autocatalytic dimer peroxide as one of the real radical generators in linoleate autoxidation; this is a peroxide-linked dimer consisting of two linoleate moieties with two hydroperoxy groups, and was much more important than the main-product hydroperoxide in autocatalytic radical supply; its proposed decomposition mechanism has suggested the generation of hydroxy radicals, hexanal, and 2,4-decadienal; on the other hand, analogy to the formation mechanism of this dimer peroxide has predicted the formation of similar polymeric products from conjugated polyene components in lipids. In this study, these two predictions were successfully verified and a discussion is presented in connection with them.

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