Characterization of Heterogeneous Nickel Sites in CO Dehydrogenases fromClostridium thermoaceticumandRhodospirillum rubrumby Nickel L-Edge X-ray Spectroscopy

Abstract

Carbon monoxide dehydrogenase from Clostridium thermoaceticum (Ct-CODH) is a nickel-containing enzyme that catalyzes acetyl-CoA synthesis and CO oxidation at two separate Ni sites, the A-cluster and C-cluster, respectively. Carbon monoxide dehydrogenase from Rhodospirillum rubrum (Rr-CODH) contains only a C-type cluster and catalyzes only CO oxidation. We have used L-edge X-ray absorption spectroscopy to study the Ni electronic structure of these two enzymes. The spectra indicate that most of the Ni in asisolated Ct-CODH is low-spin Ni(II). Upon CO treatment, a fraction of the nickel is converted either to highspin Ni(II) and/or to Ni(I). Ni in dithionite-reduced Rr-CODH also exhibits a clear low spin Ni(II) component, again mixed with either high-spin Ni(II) or Ni(I). The spectrum of Rr-CODH shifts to higher energy upon indigo carmine oxidation, suggesting either that most of the high-spin Ni(II) is converted to low-spin Ni(II) and/or that some Ni is oxidized between these two forms. These results are discussed and compared with recent L-edge spectra for the Ni site in hydrogenase.

The Geometric and Electronic Structure of a One‐Electron‐Oxidized Nickel(II) Bis(salicylidene)diamine Complex

“…Ni L‐edge X‐ray absorption spectroscopy is a direct probe of the charge and ligand field of the Ni center in the complex and assesses the relative oxidation level of the Ni center 20. The normalized L‐edge spectra of 1 and 1 + (http://www.wiley-vch.de/contents/jc_2001/2007/z701194_s.pdf) are within the expected range for Ni II compounds 21. The similarity between the energies of the L 3 edges for 1 (854.1 eV) and 1 + (854.4 eV) indicates that the Ni oxidation state remains as Ni II in the one‐electron‐oxidized complex 22…”

Molybdenum X-ray absorption edges from 200 to 20,000eV: The benefits of soft X-ray spectroscopy for chemical speciation

“…L-edge measurements were prepared as finely ground powders mixed with graphite and dusted onto Mylar tape. The soft x-ray M-edge samples were prepared as finely ground powders spread on carbon adhesive tape and then sealed inside a capped sample-holder for transfer into the vacuum chamber as described elsewhere [18]. …”

XANES, EXAFS and Kbeta spectroscopic studies of the oxygen-evolving complex in Photosystem II

“…307 This is a rich amount of information about oxidation states relative to what is available from K-edge XANES spectra. L -edge spectra have already been obtained for a number of Fe, Cu, and Ni metalloproteins, [309][310][311][312][313][314][315][316][317][318] and have been fit using multiplet calculations to provide quantitative conclusions about the oxidation states of the metals involved. However, as detailed above, extending these experiments to Mn in PS II is not straightforward.…”