engleski

Decomposition of lipid hydroperoxides by ferrous ion

The small pool of non-bound ferrous iron in cells is known to provide one of the active species involved in probably the most important route for free radical formation leading to increased levels of lipid peroxidation. One of the pathways for excessive production of free radicals is decomposition of hydroperoxides of unsaturated fatty acids catalyzed by iron. Ferrous ions-chelate ligand systems in the presence of LOOH serve as models to establish the reactions that result in the formation of reactive oxyradicals. Rate constants of these reactions are essential to determine which reaction will dominate under selected conditions. However, the literature is scarce and inconsistent with regard to the oxidation of Fe2+ with LOOH in the absence of macromolecular ligands or in the presence of simple unidentate ligands. All potentially occurring subsequent radical reactions are much faster than the rate of the reaction. Thus, the overall reaction rate should be determined only by this reaction. The aim of this work was determination of the overall rate constants of oxidation of Fe2+ with various LOOH and with several non-lipidic hydroperoxides in CH2Cl2:CH3OH solvent mixture at 8 × 10–4 mol dm–3 HCl. The course of the reactions was followed by spectrophotometric determination of ferric ion complexed with thiocyanate. Stoichiometry and the rate constant of the oxidation of Fe2+ with ROOH depended on their structure. The average second-order rate constant of the oxidation of Fe2+ with tertiary ROOH was 250 ± 40 M-1 s-1, while with secondary lipid LOOH was 1390 ± 340 M-1 s-1 ; the rate constants of the oxidation of Fe2+ with LOOH from different biological materials were in the same range. There was no difference between activation enthalpies of geometrical isomers, methyl esters of OlH, and the rate constant of saturated fatty acid hydroperoxides (CeOOH) was very comparable with others. However, discreet differences among the rate constants of the oxidation of Fe2+ with mono- (920 ± 160 M-1 s-1) and poly-unsaturated fatty acids (1420 ± 90 M-1 s-1) were observed. The activation energies for these reactions average 30 kJ mol-1, the value which was relatively small. All these values obtained were comparable. The small activation enthalpy of 28.5 kJ mol–1 indicated the entropy controlled reaction. We assume that the oxidation proceeds by an inner-sphere mechanism considering that the breakdown of the successor inner-sphere complex forming reactive alkoxyl radicals probably is the rate-limiting step.

ferrous iron; lipid hydroperoxide; rate constant

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