Linoleic Acid Oxidation Catalysed by Various Amino Acids and Cupric Ions in Aqueous Media

AbstractModel systems were designed to study the linoleic acid oxidation in the presence and absence of various amino acids and with or without cupric ions. The tested amino acids have shown to possess potential pro‐oxidant capacity in linoleic acid dispersed in aqueous media. The effectiveness of various amino acids on linoleic acid oxidation decreased in the following order: cysteine > serine > tryptophan > phenylalanine > histidine > alanine. The addition of alanine, serine, phenylalanine, histidine or tryptophan to linoleic acid have shown features of an autocatalytic oxidation chain reaction. With cysteine, there was a linear relation between concentration of hydroperoxides and time during the early stages of oxidation. The pro‐oxidative activity of the tested amino acids in general could be attributed to the presence of the α‐amino group in the form H3‐N‐R. The apparent difference in the pro‐oxidative activity is mainly due to the functional groups attached to β‐carbon in the amino acid molecules. The addition of cupric ions at concentration of 10–5 M to linoleic acid catalysed with various α‐amino acids have shown that these amino acids had no significant effect. The increasing copper concentration from 10–5 to 10–1 M shortened the induction period of linoleic acid catalysed by amino acids having aromatic side chain, had no effect on the induction period but increased the oxidation rate during the propagation step in the model systems catalysed by alanine and serine and in the model system containing cysteine increased the linoleic acid oxidation linearly from the start of the reaction.