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 $\text{O}{}^{\mathrm{⨪}}{}_2$ when producing CO₂. Kinetic studies have been employed to show that catalysis by FE-EDTA occurs by reduction of Fe³⁺-EDTA by $\text{O}{}^{\mathrm{⨪}}{}_2$. followed by its reaoxidation by H₂O₂, and to show how $\text{O}{}^{\mathrm{⨪}}{}_2$ 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 $\text{O}{}^{\mathrm{⨪}}{}_2$. 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.