engleski

Oxidation Kinetics of Fe(II) Ion with Alkyl Hydroperoxide

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. Further, it is important to consider this mechanism under conditions other than normal physiological ones since, in comparison with normal tissue, tumors have relatively low pH levels. This consideration could also be relevant to a direct involvement of extracellular and intracellular acidosis on the onset of acidosis-associated ischemic disorders. According to the literature, assuming that in the presence of excess Fe2+ the reaction (2) successfully competes to other possible alkoxyl radical reactions in solution (if alkoxyl radicals are stable), the oxidation of Fe2+ with LOOH would proceed in two steps: Fe2+ + LOOH     Fe3+ + LO• + OH– (1) Fe2+ + LO• Fe3+ + LOH (2) 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 dichloromethane:methanol 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, and these results were compared with the ones obtained in the presence of chloride ions only. The rate constants of the oxidation of Fe2+ ion by LOOHs obtained in non-aqueous medium and their overall energetics with a special attention given to the rate constants and activation energies as affected by the medium composition, have been considered in an effort to attain an insight into the apparent reaction mechanism.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 LOOH determined via [FeNCS]2+ complex was 1390 ± 340 dm3 mol– 1 s– 1 ; the rate constants of LOOHs from different biological materials were in the same range. Further, the rates of formation of  FeNCS 2+ complex were mainly twice as high as the rates of formation of  FeCl 2+ complex, 640  140 dm3 mol– 1 s– 1. The activation energies for these reactions average 30 kJ mol– 1. 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.

iron; kinetic; alkyl hydroperxide

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