L-edge X-ray absorption spectroscopy has been used to study, under a variety of conditions, the electronic structure of Ni in the Ni-Fe hydrogenases from DesulfoVibrio gigas, DesulfoVibrio baculatus, and Pyrococcus furiosus. The status of the enzyme films used for these measurements was monitored by FT-IR spectroscopy. The L-edge spectra were interpreted by ligand field multiplet simulations and by comparison with data for Ni model complexes. The spectrum for Ni in D. gigas enzyme "form A" is consistent with a covalent Ni(III) species. In contrast, all of the reduced enzyme samples exhibited high spin Ni(II) spectra. The significance of the Ni(II) spin state for the structure of the hydrogenase active site is discussed.

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