L-edge X-ray absorption spectroscopy of some Ni enzymes: probe of Ni electronic structure

L-edge X-ray absorption spectroscopy has been used to study the electronic structure of Ni in the Ni-Fe hydrogenases and CO-dehydrogenases under a variety of conditions. The L-edge spectra are interpreted by comparison with the spectra of Ni model complexes and by ligand field multiplet simulations to examine the Ni oxidation and electronic spin states. The spectra for Ni in oxidized Desulfovibrio gigas and Pyrococcus furiosus enzymes are consistent with a covalent Ni-III species. All of the reduced hydrogenases in this study exhibit a high spin Ni-II spectrum, and no Ni-I has been observed. In contrast to hydrogenases, the native Clostridium thermoaceticum CO-dehydrogenase has a low spin Ni-II and exhibits a clearly different spectral multiplet. Spectroscopy of Ni enzymes using a 15-eV resolution STJ detector and using the new ALS beamline 4.0.2 with a 0.2 eV energy resolution show great promises for future biological L-edge spectroscopy. (C) 2001 Elsevier Science B.V. All rights reserved.