Autoxidation of methyl linoleate: Identification of the bis-allylic 11-hydroperoxide

Based on the understanding of lipid peroxidation as a free radical chain reaction, over 50 yr ago the three primary products of linoleic acid autoxidation were predicted to be the 9-, 11-, and 13-hydroperoxides. The 9- and 13-hydroperoxides were found at the time, but formation of 11-hydroperoxylinoleate or any other bis-allylic fatty acid hydroperoxide has not been reported heretofore as a product of lipid peroxidation reactions. In vitamin E-controlled autoxidation of methyl linoleate, the 11-hydroperoxy derivative was identified as the next most prominent primary peroxidation product after the 9- and 13-hydroperoxides. It was present in approximately 5-10% of the abundance of the 9- or 13-hydroperoxide. The structures of 11-hydroperoxylinoleate and its 11-hydroxy derivative were established by high-pressure liquid chromatography, ultraviolet spectroscopy, gas chromatography-mass spectroscopy, and H-1 nuclear magnetic resonance spectroscopy. The 11-hydroperoxide was not detectable in the absence of alpha-tocopherol, indicating that efficient trapping of the 11-peroxyl radical as the hydroperoxide is critical to permitting its accumulation.