Radicals induced by illumination of a mixture of unsaturated fatty acids with ultraviolet light at 77 K

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Abstract
Electron paramagnetic resonance has been used to identify the radicals produced by photoirradiation of a mixture of polyunsaturated fatty acids in ethanol. One solvent and one solute radical were formed during irradiation under anaerobic conditions at 77 K. The radicals were formed in hydrogen transfer reactions belonging to Type I photosensitization processes, and were produced from the same photosensitizer. The yield of the solvent radical varied linearly with the mixing ratio of the fatty acids in ethanol whereas the solute radical, as expected, varied with the concentration of the solute to the second power. The solute radical decayed by secondary thermal reactions at 77 K into a third radical with no hyperfine interaction structure. When oxygen was present in the sample the superoxide radical or, possibly, a peroxy radical was also produced. At 77 K the effect of the antioxidant spermine was to increase the yield of radical formation by preventing the recombinations of solvated electrons with the ionized sensitizer. Spermine is hence acting as an electron scavenger in this system. These results indicated that photoirradiation will cause radical formation by hydrogen transfer reactions and hence contribute to initiation processes in the oxidation of polyunsaturated fatty acids, and that spermine is not operative as an antioxidant in the initiation phase of the total oxidation.