Investigation of the electronic structure of 2Fe-2S model complexes and the Rieske protein using ligand K-edge X-ray absorption spectroscopy

X-ray absorption spectroscopy at the sulfur K-edge (similar to 2470 eV) has been applied to a series of 2Fe-2S model complexes to obtain insight into their electronic structures. Since these 2Fe-2S complexes contain both term inal thiolates and bridging sulfides, contributions to covalency from both sets of ligands can be evaluated. Importantly, the pre-edge feature of sulf ide can be resolved from that of thiolate due to differences in effective n uclear charge. In our previous studies, the covalency of the metal-thiolate bond in [Fe(SR)(4)](-) was determined. In this study, sulfide covalency is quantified for the first time on the basis of an analysis of previous X-ra y photoelectron and X-ray absorption spectroscopic studies of [FeCl4](-) wh ich are then applied to the bis-mu(2)-sulfide compound KFeS2. With referenc es for both sulfide and thiolate covalencies thus established for open d-sh ell systems, comparisons are made between thiolate and sulfide bonding. Sul fide-Fe covalency in the [Fe2S2(SR)(4)](2-) complexes is higher than thiola te-Fe covalency, indicating extensive charge donation of the bridging sulfi des. Finally, this investigation of model complexes is extended to the oxid ized and reduced 2Fe-2S cluster of the Rieske protein of Paracoccus denitri ficans which has terminal thiolates on one Fe center, and histidines on the other Fe center. It is determined that thiolate covalency of the Fe(III) c enter is the same in both the oxidized and reduced Rieske clusters and simi lar to that of the [Fe2S2(SR)(4)](2-) model complexes. Further, in the full y oxidized Rieske cluster, the sulfide covalency of the ferric center conta ining terminal histidine ligation is similar to 18% higher than the Fe(III) containing terminal thiolate ligation. This is consistent with the fact th at the histidine ligands are poorer donors and supports the suggestion that the terminal histidine ligation makes a significant contribution to the hi gher reduction potential of the Rieske protein.