Oxidation of hemoglobin by arenediazonium salts. The influence of dioxygen

The reactions of human hemoglobin with p-nitro- and p-chlorobenzenediazonium tetrafluoroborates in the presence and absence of molecular oxygen have been investigated in kinetic detail. The oxidation of iron(II) occurs with first order rate dependence on both the hemoglobin and diazonium salt concentrations, but inverse first order dependence on the concentration of molecular oxygen characterizes reactions performed in the presence of O₂. In the absence of O₂, nitrobenzene is the only product observed from hemoglobin oxidation by p-NO₂C₆H₄N₂⁺BF₄^?, and a 1:1 stoichiometry exists between nitrobenzene produced and Fe(II) oxidized. In the presence of O₂, p-nitrophenol is the dominant product, but product yield is dependent on the ratio of reactants. Electron transfer to the diazonium salt rather than its corresponding diazohydroxide or diazoate is inferred from the relative absence of pH dependence on the rate of oxidation. The composite results are consistent with a mechanism for hemoglobin oxidation that requires molecular oxygen dissociation from oxyhemoglobin prior to oxidation by the diazonium salt. Implications of this investigation for the mechanism of arylhydrazine reactions with hemoglobin are discussed.