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 similar to 2470 eV has been applied to a series of mononuclear iron-sulfur complexes t o determine the covalency and its distribution over the ligand field s plit d-orbitals. A comparison is made between the S K-edges of a model and three different rubredoxin proteins to define the changes in cova lency upon incorporation of the site into the protein. It is found tha t the covalency decreases in the proteins relative to the model. The t hiolate-Fe(III) bond in these systems is highly covalent, and a modula tion of this covalency in the proteins can contribute to the redox pro perties of the active site. It is determined that, while the hydrogen bonding effects seem to influence covalency, there is not a direct cor relation between the change in covalency, the number of hydrogen bonds , and the redox potentials of these sites.