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) edge s of several nickel compounds with different structural symmetries. Us ing 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 e dge 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 si te symmetries, but less sensitive to changes in individual ligands. In the protein system investigated, the Ni-substituted Pyrococcus furios us rubredoxin, we find a strongly distorted T-d symmetry and a large z ero field splitting, comparable to that observed in an optical MCD stu dy. Because of the chemical sensitivity and specificity for only the n ickel site, NiL(2,3) edges are a strong spectroscopic tool for investi gating the nickel sites in large metalloproteins. The information obta ined at the Ni L(2,3) edge complements information from EXAFS measurem ents at the nickel K edge.