Circular dichroism and magnetic circular dichroism of iron-sulfur proteins

Circular dichroism (CD) and magnetic circular dichroism (MCD) spectra are reported for the 2-Fe ferredoxins from Pseudomonas putida and Spirulina maxima, Chromatium HIPIP, the 4-Fe ferredoxin from Bacillus stearothermophilus, and the 8-Fe ferredoxin from Clostridium pasteurianum. The spectral range spans the near-infrared, visible, and near ultraviolet. 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 two 2-Fe ferredoxins are similar. In contrast, the CD of the 4-Fe and 8-Fe proteins, for a given 4-Fe cluster oxidation level, varies considerable 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 iron-sulfur proteins containing 2-Fe and 4-Fe clusters and for the evaluation of electronic structural models for these clusters.