MECHANISM OF CATALASE ACTIVITY IN AQUEOUS-SOLUTIONS OF DIMANGANESE(III,IV) ETHYLENEDIAMINE-N,N'-DIACETATE

Manganous ions, ligated by ethylenediamine-N,N′-diacetate (edda = L) decompose hydrogen peroxide with a rate law-d[H2O2]/dr = k17[Mn(edda)] [H2O2] where k17 = 5.4 M−1 s−1 at pH 7. The reduction of peroxide to water is initiated by the reaction of MnIIL with a dinuclear MnIII,IVL2. A subsequent fast reaction between the transient product of this reaction and hydrogen peroxide or tert-butyl hydroperoxide effectively oxidizes Mn(II) to Mn(IV) in a concerted step without formation of the hydroxyl radical. The green mixed-valence complex, which is probably a bis(μ-oxo)-bridged structure, is stable in neutral aqueous solution and exhibits a 16-line ESR signal in frozen solution. The basis of catalase activity is the autocatalytic formation of this complex when hydrogen peroxide is reduced by manganese(II) according to the overall reaction {formula-omited}. The catalase cycle is independent of the formation of oxy radicals. Mononuclear MnIIIedda and MnIIedda react with superoxide radicals, but the decomposition of peroxide is virtually independent of these reactions. In unbuffered solutions, with a moderate excess of hydrogen peroxide, an oscillation in the concentration of the dinuclear complex is detected. © 1990, American Chemical Society. All rights reserved.