Spectroelectrochemical elucidation of the kinetics of two closely spaced electron transfers

The use of spectroelectrochemistry to facilitate the analysis of an EE mechanism was reported in this work. Using a set of spectra as a function of potential, the spectra of all three oxidation states were determined using evolving window factor analysis. From these spectra, the concentration of each species in solution was determined for each potential. Using these data, the current was calculated. Unlike the direct measurement of current, the current due to each redox process was determined, allowing one to analyze each redox process separate from the other. With the use of the Butler–Volmer equation, the redox potential and the heterogeneous electron transfer parameters were measured. The spectrally determined current has the advantage of determining the current due to each redox process which is not generally possible with voltammetric data when the redox potentials are close together. This method was applied to the spectroelectrochemical reduction of Escherichia coli sulfite reductase hemoprotein (SiR-HP) in a phosphate buffer and in the presence of cyanide. The electrochemical parameters (E°’s, k°’s and α’s) for each electron transfer were calculated for both the uncoordinated and cyanide coordinated species. The rates of electron transfer for the siroheme and iron–sulfur cluster were slower than the rates observed for other heme proteins. This is probably due to the fact that this protein is significantly larger than most of the heme protein previously studied. This approach is a powerful tool for two-electron transfers when the E° values are close together.