The Electrochemistry and Spectroelectrochemistry of E. coli Sulfite Reductase Hemoprotein and Model Iron Porphyrin Complexes
The aim of this work was (1) to characterize the reactions of ferric porphyrins with sulfur-oxy anions using electrochemical and spectroscopic techniques, and (2) to characterize the spectroelectrochemical reduction of the E. coli sulfite reductase hemoprotein. The reduction of iron(III) to iron(II) for (Fe(P))2SO4 (P = TPP, OEP) in methylene chloride occurred in two well-separated reduction waves. The first wave corresponds to the reduction of (Fe(P))2SO4 to Fe(P) and (Fe(P)(OSO3)) -, and the second corresponds to the reduction of (Fe(P)(OSO3)) - to Fe(P). In coordinating solvents, (Fe(P))2SO4 dissociated and a single iron(III)/iron(II) wave was observed. The addition of excess sulfate to (Fe(P))2SO4 or Fe(P)Cl in all solvents gave rise to the complex, (Fe(P)(OSO3)) -, which exhibited only one iron(III)/iron(II) wave. The bisulfate complex of ferric OEP was also characterized. The reactions of thiosulfate and bisulfite with ferric porphyrins were studied. The addition of excess thiosulfate ion to Fe(P)Cl in dimethylsulfoxide and dimethylformamide produced (Fe(P)(SSO3)) -. The electrochemical reduction of (Fe(P)(SSO3)) - was similar to that of (Fe(P)(OSO3)) -. The kinetics of the chemical reduction of (Fe(TPP)(SSO3)) - by free thiosulfate ion in solution were also examined. The reaction of excess bisulfite ion with Fe(TPP)(OClO3) in acetonitrile formed an intermediate spin bisulfite monomer complex. During the electrochemical reduction of this complex, the bisulfite catalytically reduced at the iron(II)/iron(I) wave. The spectroelectrochemistry of the E. coli sulfite reductase hemoprotein in pH 7.7 phosphate buffer was also examined. The mediated spectropotentiometric titration of SiR-HP yielded E° values of -576 and -651 mV (vs Ag/AgCl), respectively, for the two sequential 1-electron reductions of siroheme and the 4Fe-4S cluster. The unmediated thin-layer derivative cyclic voltabsorptammetry of SiR-HP was characteristic of an electrochemically irreversible electron transfer system. The formal heterogeneous rate constants calculated for the two 1-electron transfers were 2.0 ⨯ 10-5 and 6.1 ⨯ 10-6 cm/s, respectively.