Electronic Structure and Symmetry in Nickel L Edge X-ray Absorption Spectroscopy: Application to a Nickel Protein

Abstract

We have studied the effects of electronic structure and symmetry on Ni L[sub 2.3] edge X-ray absorption spectra by measuring the L[sub 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[sub 2,3] edge spectral features and the electronic structure at the nickel site. The L[sub 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[sub 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[sub 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, Ni L[sub 2,3] edges are a strong spectroscopic tool for investigating the nickel sites in large metalloproteins. The information obtained at the Ni L[sub 2,3] edge complements information from EXAFS measurements at the nickel K edge. 39 refs., 5 figs.