The effect of molecular environment on the peroxidation of linoleic acid (LA), a polyunsaturated fatty acid (PUFA), initiated by ferrous ions was investigated in acidic and neutral pH conditions. Mixed nonionic surfactants TWEEN®-20/LA micelles were established as a model system to obtain a surfactant-in-lipid aqueous system at high acidity level. The peroxidation of LA was induced by ferrous ions and the kinetics of the produced conjugated dienes was followed by UV measurements and the ferric thiocynate method. Ferrous ions were oxidized only by the preformed LA hydroperoxides, which under established conditions produced lipid alkoxyl and peroxyl radicals as the sole initiators of propagation. The results revealed the LA peroxidation process remained mainly unaffected within the 2.5<pH<5.5 range, while highly pH sensitive around pH 7. The propagation process prevailed at optimal concentrations of 500 µM of LA and 280 µM TWEEN®-20, and at the ferrous ion concentration up to 75 µM, irrespective of the buffer used.
Practical applications: A simple model system in water, suitable for the selective study of the lipid peroxidation propagation phase induced by ferrous ion is presented here. Fatty acids serve as model compounds susceptible to processes associated with oxidative radical initiated-modifications of lipids. The obtained results contribute to a better understanding of the oxidative behavior of lipids, particularly those soluble in nonionic surfactant micelles in acidic medium. The oxidative stability of the PUFA in model systems containing TWEEN®-20 and ferrous ion at low pH could be predicted and controlled by measuring the lipid hydroperoxide formation. The experimental conditions presented may also provide a suitable system for the study of the termination phase of lipid peroxidation.