Kinetics and mechanism of reduction of sperm-whale metmyoglobin by dithionite ion

Abstract

The kinetics of reduction of sperm-whale aquometmyoglobin by excess dithionite ion was studied at 25°C, pH 6.9–9.8, and ionic strength 0.50 M (adjusted with potassium nitrate). The dependence of the pseudo first-order rate constant for the disappearance of metmyoglobin on hydrogen and dithionite ion concentrations is (k₁[S₂O₄²⁻] + k₂[S₂O₄²⁻]^{$\text{1}\text{2}$})[H⁺]/(K_a + [H⁺]), where k₁ = 52 ± 11 M⁻¹ s⁻¹, k₂ = 40.9 ± 2.3 M^{$\text{1}\text{2}$} s⁻¹, and K_a (the ionization constant of the water coordinated to the iron) = (5.79 ± 0.49) × 10⁻¹⁰ M. The rate law is interpreted on the basis of parallel pathways for the reaction of aquometmyoglobin with S₂O₄²⁻ and SO₂⁻, the hydroxometmyoglobin in equilibrium with the aquo form being unreactive. From a comparison of the rate constants for anation of aquometmyoglobin and the rate constant for reduction by SO₂⁻, it is inferred that an outer-sphere redox mechanism is operative. It is postulated that the activation process for reduction of aquometmyoglobin requires considerable stretching of the Fe-OH₂ bond, and this model is utilized to assign an outer-sphere mechanism to the reduction by S₂O₄²⁻. The dithionite reduction of cyanometmyoglobin proceeds in two stages. The first stage proceeds at a rate that is dependent on dithionite concentration and corresponds to the outer-sphere reduction of cyanometmyoglobin. The second stage proceeds at a rate that is independent of dithionite concentration and corresponds to the dissociation of the transient cyanodeoxymyoglobin intermediate produced in the first stage. The results of the present investigation are compared with those obtained in three independent, previous studies.