Kenneth E. Peers

Controlled synthesis of monohydroperoxides by α-tocopherol inhibited autoxidation of polyunsaturated lipids

Abstract α-Tocopherol influences product formation during the autoxidation of unsaturated lipids and, if present in sufficient quantity (5%; 0.1M), only cis,trans -conjugated diene monohydroperoxides are formed: the formation of trans,trans -isomers and secondary oxidation products (e.g. hydroperoxyepidioxides) is suppressed. At this high concentration α-tocopherol does not exert its recognised antioxidant effect but allows autoxidation to proceed smoothly and quite rapidly to produce good yields of monohydroperoxides. This property, coupled with preparative high performance liquid chromatography, has been utilised to prepare and isolate the isomeric monohydroperoxides of methyl arachidonate and 2-linoleoyl-1,3-dipalmitoylglycerol.

Oxidative reactions of unsaturated lipids

Abstract Monohydroperoxides, which are the products of free radical autoxidation of unsaturated fatty acids, can under go further free radical chain reactions initiated by abstraction of hydrogen to reform peroxy radicals. In the autoxidation of methyl linolenate, the peroxy radicals corresponding to ‘inner’ hydroperoxides may cyclise and, after further oxygenation, yield diperoxides. The mixtures of the two peroxidic species, monohydroperoxides and diperoxides, obtained from linolenate can be identified and determined chromatographically. Mechanistic studies showed that the oxygenation of pentadienyl radicals to form peroxy radicals—an important propagation step in autoxidation—is reversible. H-donors can be used to control the relative rates of the forward and reverse reactions and so determine the relative proportions of different peroxidic species formed. α-Tocopherol was shown to be an effective H-donor and drastically altered the distribution of products formed in linolenate autoxidation. Such control of peroxide formation may be a factor determining the manifestations that are specific to the oxidative deterioration of a food product.

Thermal decomposition of individual positional isomers of methyl linolenate hydroperoxides, hydroperoxy cyclic peroxides and dihydroperoxides.

The methyl esters of 4 individual positional isomers of hydroperoxides, 2 positional isomers of hydroperoxy cyclic peroxides and a 9,16-dihydroperoxide were prepared by autoxidation of methyl linolenate and separated by preparative high pressure liquid chromatography. Isolated hydroperoxide isomers were thermally decomposed and the resulting volatile components analyzed. Each hydroperoxide or hydroperoxy cyclic peroxide isomer yielded characteristic volatile products. The major volatiles from each acyclic hydroperoxide corresponded with those predicted to arise by carboncarbon scission on either side of the corresponding alkoxy radical intermediate and little evidence was found of isomerization between the various positional isomers occurring during the process. A similar mechanism would account for the volatile products obtained from the cyclic peroxides. 2,3-Pentanedione was a significant odor contributor arising from the 13,15-epidioxy-16-hydroperoxide isomer although it was only a minor decomposition product.

Selective formation of dihydroperoxides in the α-tocopherol inhibited autoxidation of methyl linolenate

Autoxidation of methyl linolenate in the presence of α-tocopherol leads to the selective formation of 9,16-dihydroperoxides in addition to the four previously reported isomeric monohydroperoxides.