Investigation of iron-sulfur covalency in rubredoxins and a model system using sulfur K-edge X-ray absorption spectroscopy

X-ray absorption spectroscopy at the sulfur K-edge at ~2470 eV has been applied to a series of mononuclear iron-sulfur complexes to determine the covalency and its distribution over the ligand field split d-orbitals. A comparison is made between the S K-edges of a model and three different rubredoxin proteins to define the changes in covalency upon incorporation of the site into protein. It is found that the covalency decreases in the proteins relative to the model. The thiolate-Fe(III) bond in these systems is highly covalent, and a modulation of this covalency in the proteins can contribute to the redox properties of the active site. It is determined that, while the hydrogen bonding effects seem to influence covalency, there is not a direct correlation between the change in covalency, the number of hydrogen bonds, and the redox potentials of these sites.