L-edge X-ray magnetic circular dichroism of Ni enzymes: direct probe of Ni spin states

X-ray magnetic circular dichroism (XMCD) measures the inner shell absorption difference between left and right circularly polarized X-rays in the presence of magnetic field, and provides us a direct probe of the spin values localized in the specific metal sites. In this study, using Desulfovibrio desulfuricans and Desulfovibrio gigas hydrogenases as examples, we have measured the L-edge XMCD of Ni enzymes for the first time and analyzed them in comparison with a doped high spin Ni-II model complex. The reduced hydrogenases have a non-zero XMCD effect, which is consistent with a 'high spin' Ni-II site. The magneto-optical sum rules have also been used to derive the orbital and spin angular momentum. (C) 2001 Elsevier Science B.V. All rights reserved.