title: Chelated iron-catalyzed OH. formation from paraquat radicals and H2O2: mechanism of formate oxidation.
abstract: Traces of iron, when complexed with either EDTA or diethylenetriaminepentaacetic acid (DTPA), catalyze an OH.-producing reaction between H2O2 and paraquat radical (PQ+.): H2O2 + PQ+.----PQ++ + OH. + OH-.[1]. Kinetic studies show that oxidation of formate induced by this reaction occurs by a Fenton-type mechanism, analagous to that assumed in the metal-catalyzed Haber-Weiss reaction, in which the rate determining step is H2O2 + Fe2+ (chelator)----Fe3+(chelator) + OH. + OH-,[7]; with k7 = 7 X 10(3) M-1 s-1 for EDTA and 8 X 10(2) M-1 s-1 for DTPA at pH 7.4. PQ+. rapidly reduces both Fe3+ (EDTA) and Fe3+ (DTPA), and hence allows both agents to catalyze [1] with comparable efficiency, in contrast to the much lower efficiency reported for the latter as a catalyst for the Haber-Weiss reaction. The catalytic properties of these chelating agents is attributed to their lowering of E0 (Fe3+/Fe2+) by 0.65 V, thus making [7] thermodynamically possible at pH 7. Approximately 2.5% of the OH. produced is consumed by internal or "cage" reactions, which decompose the chelator and produce CO2; however, the majority (97%) diffuses into the bulk solution and participates in competitive reactions with OH. scavengers.
mesh_terms: D055598:Chemical Phenomena; D002621:Chemistry; D005561:Formates / Q000378:metabolism*; D006861:Hydrogen Peroxide / Q000378:metabolism*; D006878:Hydroxides / Q000378:metabolism*; D017665:Hydroxyl Radical; D007501:Iron / Q000378:metabolism*; D010269:Paraquat / Q000378:metabolism*
chemical_list: D005561:Formates; D006878:Hydroxides; C030544:formic acid; D017665:Hydroxyl Radical; D006861:Hydrogen Peroxide; D007501:Iron; D010269:Paraquat