ELECTRONIC-STRUCTURE AND SYMMETRY IN NICKEL-L EDGE X-RAY-ABSORPTION SPECTROSCOPY - APPLICATION TO A NICKEL PROTEIN

We have studied the effects of electronic structure and symmetry on Ni L(2),(3) edge X-ray absorption spectra by measuring the L(2),(3) edges of several nickel compounds with different structural symmetries. Using ligand field atomic multiplet calculations, we find that there is a close relationship between the Ni L(2,3) edge spectral features and the electronic structure at the nickel site. The L(2),(3) absorption edge is very sensitive to the spin state and the oxidation state of the nickel site, even for the formally trivalent nickel oxidation state. The Ni L(2),3 edge is also sensitive to different structural nickel site symmetries, but less sensitive to changes in individual ligands. In the protein system investigated, the Ni-substituted Pyrococcus furiosus rubredoxin, we find a strongly distorted T-d symmetry and a large zero field splitting, comparable to that observed in an optical MCD study. Because of the chemical sensitivity and specificity for only the nickel site, NiL(2,3) edges are a strong spectroscopic tool for investigating the nickel sites in large metalloproteins. The information obtained at the Ni L(2,3) edge complements information from EXAFS measurements at the nickel K edge.