Efficiency of chelated iron compounds as catalysts for the Haber-Weiss reaction.

The oxidation of formate to CO2 has been used to quantify . OH yields produced in oxygenated solutions by chelated iron salts reacting either directly with H2O2 in the Fenton reaction, or as catalysts in the Haber-Weiss reaction between H2O2 and radiolytically generated superoxide. This system involves a chain sequence since . OH regenerates O2- when producing CO2. Kinetic studies have been employed to show that catalysis by Fe-EDTA occurs by reduction of Fe3+-EDTA by O2- followed by its reoxidation by H2O2, and to show how O2- is ultimately consumed. At pH 7.3 more than 50 . OH radicals can be produced per molecule of Fe-EDTA and CO2 yields can exceed five per molecule of radiolytically generated O2-. Iron chelated with pyrophosphate, DTPA, citrate, ATP or ADP in phosphate or Tris buffer at pH 7.3 has less than 7% of the catalytic ability of Fe-EDTA (considerably less in most cases) even though all these ferrous chelates give appreciable yields of . OH in the Fenton reaction. Unchelated iron has no catalytic ability. Catalysis of the Haber-Weiss reaction in homogenous solution by iron salts, either free or chelated with nucleotides or citrate, is evidently a very inefficient process, and its possible role in superoxide toxicity must be viewed with these reservations.