THE ELECTROCHEMICAL REDUCTION OF IRON PORPHYRIN NITROSYLS IN THE PRESENCE OF WEAK ACIDS

The reduction of coordinated nitrosyls to ammonia in assimilatory nitrite reductases consists of a series of electron transfer/protonation steps. Very little detail is known about these steps, either from the enzyme itself or model complexes. In this work, the mechanism for the reduction of iron nitrosyl complexes, which are nitrite reductase models, was examined in solutions with limited proton availability. In this way, the reduction mechanism can be investigated in a system which has been shown quantitatively to produce ammonia. The reduction mechanism was investigated for the reduction of Fe(P)(NO), where P is porphyrins, hydroporphyrins or oxoporphyrins, in the presence of substituted phenols using pulse polarography. In THF, the limiting current for the first wave (wave 1) was unaffected by the presence of phenol, while the half-wave potential shifted to positive potentials at high concentrations of phenol. The shift in E1/2 of the first wave, E1/2,I, was consistent with a mechanism which involved the protonation of Fe(TPP)(NO)- with two protons to form Fe(TPP)(NH2O+). From the variation in the half-wave potential, the equilibrium constant for the formation of Fe(TPP)(NH2O+) from Fe(TPP)(NO)- was measured for a series of phenols. The chemical and electrochemical reversibility of the first wave in the presence of phenol was verified by the Nernstian shape of the waves, by square-wave voltammetry, and by shifts caused by the presence of phenolate ion. The reduction in the presence of 2,6-dichlorophenol was an exception in that only one proton was transferred. In addition to the changes in the first wave, a new wave was observed between the first and second waves of Fe(TPP)(NO). This new wave corresponded to a three-electron reduction of Fe(TPPXNO)-. The kinetics of this reduction were monitored using normal pulse polarography. The behavior of the new wave in the presence of phenol was consistent with a CE mechanism which involved two phenol molecules in equilibrium with Fe(TPP)(NO) - to form Fe(TPP)(NH2O+) prior to the electron transfer. The rate constant for the reduction was measured for all the phenols for which the equilibrium constant for the formation of Fe(TPP)(NH2O+) could be measured. Changes in the macrocycle from porphyrin to isobacteriochlorin led to a significant increase in the rate of nitrosyl reduction, while the rate decreased substantially for the oxoporphyrins. Most of the changes in the rate of nitrosyl reduction were consistent with changes in the basicity of the Fe(P)(NO)-complex. It is interesting to note that Fe(P)(NH2O+), the reactive intermediate, was formed most readily for the assimilatory nitrite reductase models (iron isobacteriochlorins), and least readily for the dissimilatory nitrite reductase models (iron dioxo-isobacteriochlorin).