NICKEL L-EDGE X-RAY-ABSORPTION SPECTROSCOPY OF PYROCOCCUS-FURIOSUS HYDROGENASE

We have investigated the reduced, thionine-treated at 20 degrees C, an d thionine-treated at 80 degrees C forms of Pyrococcus furiosus [NiFe] hydrogenase using L-edge X-ray absorption spectroscopy. At 20 degrees C, the Ni site is apparently not redox active, since the reduced and 20 degrees C thionine-treated forms exhibit the same spectra. Results of theoretical simulations as well as comparison with the spectra of m odel compounds indicate the presence of high-spin Ni(II) in these form s. On the basis of a comparison with the spectral features of model ni ckel complexes, the nickel site in the hydrogenase appears to be 5- or 6-coordinate. The 80 degrees C thionine-treated form has a broader Mi L-edge centered at a higher energy, consistent with a charge distribu tion of at feast two holes on the nickel and at least one hole signifi cantly delocalized onto the ligand framework.