Ethylene formation from methional mediated by lipid hydroperoxides

Allsop, Keith John (1977) Ethylene formation from methional mediated by lipid hydroperoxides. Masters thesis, Memorial University of Newfoundland.

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Abstract

Ethylene could be formed by a model system containing lipoxygenase, linolenate, sulphite and methional. The system had an optimal pH of 7.8. Free radicals formed during the lipoxygenase-catalyzed oxidation of linolenate were thought to initiate sulphite oxidation. Hydroxyl radicals formed during sulphite oxidation reacted with methional. Ethylene was one of the products of the reaction. -- Ethylene could also be formed from a model system containing linoleic acid hydroperoxide, sulphite and methional. The pH optimum was 5.0. It is thought that two species were responsible for converting the methional to ethylene, the hydroxyl radical, formed during sulphite oxidation and singlet oxygen which probably arose from the collision of two sec-peroxy radicals. -- Studies of oxygen uptake by a LAHPO/sulphite system showed that the hydroxyl radical was formed during sulphite oxidation and may be a chain carrying species. Because both hydroxyl radicals and methional inhibited sulphite oxidation initiated by LAHPO it was thought that methional was reacting with hydroxyl radicals in our system. Because inhibitors of sulphite oxidation only partially inhibited ethylene formation it was thought that another species, not formed during sulphite oxidation could also be responsible for the conversion of methional to ethylene. -- The evidence for singlet oxygen production by linoleic acid hydroperoxide and sulphite came from three techniques - thin layer analysis of the products of diphenylfuran oxidation, inhibitor studies on diphenylfuran exudation using fluorescence spectrophotometry and chemiluminescence. Diphenylfuran was converted by linoleic acid hydroperoxide and sulphite to cis-dibenzoylethylene, the product formed when diphenylfuran reacts with singlet oxygen. Both diphenylfuran oxidation and chemiluminescence were inhibited by singlet oxygen quenchers and antioxidants but not inhibitors of sulphite oxidation. -- Several simple experiments were performed to show that singlet oxygen could react with methional to produce ethylene. Light, methylene blue, and methional produce ethylene by a singlet oxygen reaction. Other singlet oxygen producing systems, H₂O₂ and OCl⁻, peroxidase, H₂O₂ and Cl⁻ converted methional to ethylene.

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