| Abstract
| - L-edge X-ray absorption spectroscopy has been used to study, under a variety of conditions, theelectronic structure of Ni in the Ni−Fe hydrogenases from Desulfovibriogigas, Desulfovibriobaculatus, andPyrococcusfuriosus. The status of the enzyme films used for these measurements was monitored by FT-IRspectroscopy. The L-edge spectra were interpreted by ligand field multiplet simulations and by comparisonwith data for Ni model complexes. The spectrum for Ni in D.gigas enzyme “form A” is consistent with acovalent Ni(III) species. In contrast, all of the reduced enzyme samples exhibited high spin Ni(II) spectra. Thesignificance of the Ni(II) spin state for the structure of the hydrogenase active site is discussed.
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