L-edge x-ray absorption spectroscopy of Pyrococcus furiosus rubredoxin

In this communication we present new experiments and theoretical simulations, using iron L-edge X-ray absorption spectroscopy, to study the metalloprotein Pyrococcus furiosus rubredoxin. The 3d transition metal L-edges are found between 400 and 1100 eV, in the soft X-ray region. Rubredoxins are small proteins which contain single iron atoms coordinated by a distorted tetrahedron of cysteinyl sulfur ligands. This being the simplest iron-sulfur protein, an understanding of the rubredoxin electronic structure and redox mechanism is necessary for progress on biological electron transfer and more complex iron-sulfur proteins. Comparison of our experimental data with theoretical simulations reveals, as expected, an approximately tetrahedral symmetry for the Fe site. The analysis also finds similar covalency for the oxidized and reduced rubredoxin sites. Transition metal L-edge spectroscopy thus seems to be a promising new technique for bioinorganic systems. In order to obtain the spectra, several difficulties in measuring soft X-ray absorption of metalloproteins were overcome. Because absorption cross sections in the soft X-ray region are very high, work in this region requires ultrathin windows or vacuum conditions.