| Abstract
| - Sulfur K-edge X-ray absorption spectroscopy (XAS) is reported for [Fe4S4]1+,2+,3+ clusters. Theresults are quantitatively and qualitatively compared with DFT calculations. The change in covalency uponredox in both the [Fe4S4]1+/2+ (ferredoxin) and the [Fe4S4]2+/3+ (HiPIP) couple are much larger than thatexpected from just the change in number of 3d holes. Moreover, the change in the HiPIP couple is higherthan that of the ferredoxin couple. These changes in electronic structure are analyzed using DFT calculationsin terms of contributions from the nature of the redox active molecular orbital (RAMO) and electronicrelaxation. The results indicate that the RAMO of HiPIP has 50% ligand character, and hence, the HiPIPredox couple involves limited electronic relaxation. Alternatively, the RAMO of the ferredoxin couple ismetal-based, and the ferredoxin redox couple involves extensive electronic relaxation. The contributionsof these RAMO differences to ET processes in the different proteins are discussed.
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