Circular dichroism and magnetic circular dichroism of iron-sulfur proteins

Abstract

Circular dichroism (CD) and magnetic circular dichroism (MCD) spectra are reported for the 2-Fe ferredoxins from Pseudomonas putida and Spirulina maxima. Chromatium HIPP, the 4-Fe ferredoxin from Bacillus stearothermophilus and the 8-Fe ferredoxin from Clostridium Pasteurianum. The spectral range spans the near IR; visible and near UV. In all cases except oxidized 2-Fe ferredoxins, electronic absorption is observed continuously from less than 5000 cm-1 to above 30,000 cm-1. The CD spectra of the 2 2-Fe ferredoxins are similar. The CD of the 4-Fe and 8-Fe proteins, for a given 4-Fe cluster oxidation level, varies considerably with protein. MCD is less sensitive to protein environment than is CD. In the 2-Fe proteins, MCD at 5 T is appreciably smaller than the CD; in the 4-Fe and 8-Fe proteins, MCD and CD are comparable in magnitude. Both CD and MCD are more highly structured than the corresponding absorption spectra. The CD and MCD spectra reported provide a broader base than heretofore available for the characterization of Fe-S proteins containing 2-Fe and 4-Fe clusters and for the evaluation of electronic structural models for these clusters.