The kinetics of the reaction of aquo Fe(III) myoglobin with hydrogen peroxide at pH 8

The kinetics of the reaction of aquo Fe(III) myoglobin (Mb(III)) with H₂O₂ at pH 8.0 and 20° have been investigated quantitatively under non-linear second order conditions. The data obtained at five concentrations of H₂O₂ (molar ratio [H₂O₂]/[Mb]_total in the range 0.3 to 4) were analysed simultaneously by means of non-linear numerical integration and optimization techniques. The only mechanism that satisfied the triple requirement of a standard deviation within the standard error of the experimental data, good determination of the optimized parameters and a random distribution of residuals is as follows: Mb(III) reacts with H₂O₂ to form species X with a second order rate constant k₊₁ = 3.53 × 10² M^-1 s^-1; species X in turn reacts with H₂O₂ to form species R with a second order rate constant k₊₂ = 3.40 × 10³ M^-1s-1; species R is then converted back into Mb(III) with a first order rate constant k₌₃ = 9.60 × 10^-5 s^-1. At 408 nm Δε{lunate}_Mb(III)-R = 71.9 mM^-1 cm^-1 and δε{lunate}_Mb(III)-X = 104 mM^-1 cm^-1. Species R is spectrally identical to the Fe(IV) myoglobin species described by George and Irvine [Biochem. J., 52, 511-517 (1952)]. The chemical nature of species X and R is discussed on the basis of the data in the present paper and data in the literature from EPR, Mössbauer, resonance Raman and magnetic susceptibility studies.