The Reduction by Dithionite of Fe(III) Myoglobin Derivatives with Different Ligands Attached to the Iron Atom

Abstract

1 The reductions of a number of sperm whale Fe(III) myoglobin-ligand complexes by sodium dithionite in a phosphate buffer pH 6.4, were investigated by using rapid-wavelength-scanning stopped-flow spectrophotometry. The ligands were azide, cyanide, fluoride, imidazole, thiocyanate and water. 2 The reduction of Fe(III) myoglobin cyanide led to the transient formation of Fe(II) myoglobin cyanide but no intermediate species were observable during the reductions of the other derivatives. The final product of the reaction in all cases was unliganded Fe(II) myoglobin. 3 Investigation of the effect of dithionite concentration on the rate of reduction indicated that the SO₂ radical ion was the active species in reducing the azide, cyanide, fluoride and thiocyanate derivatives. 4 Comparison of the observed rates of reduction at different ligand concentrations with those predicted for a pathway of reduction involving prior dissociation of the ligand, allowed us to estimate the rate of reduction with the ligand in position (outer-sphere reduction). There was a large variation in the relative rates of outer-sphere reduction in the order imidazole ≫ CN⁻ > SCN⁻≫ N₃⁻≫ F⁻. The fluoride derivative was so resistant to outer-sphere reduction that the reaction with SO₂⁻ proceeded only by a pathway involving dissociation of F⁻ before reduction. It was calculated that any direct reduction of this complex was at least 100 times slower than that of the azide derivative. 5 The results are discussed in terms of the possible rôle of the axial ligands in haem proteins and it is suggested that the pathway of the electron to the Fe(III) centre may be via the pπ orbitals of these ligands.