Photo-sensitized oxidation of unsaturated fatty acid methyl esters. The identification of different pathways |
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Authors: | Henry W -S Chan |
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Affiliation: | (1) Agricultural Research Council Food Research Institute, Colney Lane, NR4 7UA Norwich, UK |
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Abstract: | The photo-sensitized oxidation of methyl linolenate and methyl oleate was studied using erythrosine and riboflavin as sensitizers.
The complex mixtures of hydroperoxides obtained were analyzed for the proportion of conjugated products and, after reduction
to the corresponding mixtures of hydroxystearates, for the distribution of positional isomers. By comparing the mixtures with
that obtained from autoxidation, it was shown that the riboflavin reaction involved the “Type 1” mechanism of photosensitized
oxidation which proceded via the formation of diene-radicals and yielded the same positional isomers of hydroperoxides as
autoxidation. Thus, mixtures of the 8, 9, 10, and 11 positional isomers of allylic hydroperoxides were formed from oleate
and the 9, 12, 13, and 16 isomers of conjugated diene-hydroperoxides from linolenate oxidation. The erythrosine reaction,
on the other hand, proceded via the “Type 2”. mechanism which involved singlet oxygen as the oxygenating species. The mixtures
of isomers resulting from oxidation involving singlet oxygen were different from those obtained by autoxidation. Oleate oxidation
gave rise to a mixture of only the 9 and 10 positional isomers while the mixture obtained from oxidation of methyl linolenate
contained non-conjugated hydroperoxide isomers (with the hydroperoxide group at positions 10 and 15) as well as the conjugated—9,
12, 13, and 16—isomers. |
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