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Acetyl-CoA synthase (ACS) is involved in the bacterial carbon oxide conversion pathway. The binuclear nickel sites in ACS enzyme and two biomimetic synthetic compounds containing a Ni(II)Ni(II) unit (1 and 2) were compared using XAS/XES. EXAFS analysis of ACS proteins revealed similar Ni-N/O/S bond lengths and Ni-Ni/Fe distances as in the crystal structure in oxidized ACS, but elongated Ni-ligand bonds in reduced ACS, suggesting more reduced nickel species. The XANES spectra of ACS and the dinickel complexes showed overall similar shapes, but less resolved pre-edge and edge features in ACS, attributed to more distorted square-planar nickel sites in particular in reduced ACS. DFT calculation of pre-edge absorption and Kβ2,5 emission features reproduced the experimental spectra of the synthetic complexes, was sensitive even to the small geometry differences in 1 and 2, and indicated low-spin Ni(II) sites. Comparison of nickel sites in proteins and biomimetic compounds is valuable for deducing structural and electronic differences in response to ligation and redox changes.

A major enterprise for scientists worldwide is the search for alternative fuels and molecular hydrogen (H2) is a promising candidate. Its large-scale technical production needs to involve water-oxidation catalysts from inexpensive and abundant materials. Here, a water-oxidizing Co-based catalyst film (CoCF) is investigated. We review and extend our previous X-ray absorption spectroscopy (XAS) measurements (at Helmholtz-Zentrum Berlin/BESSY) by comparison to LiCoO2, a CoIII compound of similar structure and composition. Further evidence is presented that the bulk oxidation state of cobalt in the CoCF is 3+. We propose that the catalyst film is composed of interconnected complete and/or incomplete Co-oxo cubanes possibly forming a disordered network of the basic Co3/4(μ-O)4 units.

Advanced X-ray spectroscopy experiments can contribute to elucidation of the mechanism of water oxidation in biological (tetra-manganese complex of Photosystem II) and artificial systems. Although the electronic structure of the catalytic metal site is of high interest, it is experimentally not easily accessible. Therefore, we and other researchers are working towards a comprehensive approach involving a combination of methods, namely (1) quantitative analysis of X-ray absorption near-edge structure (XANES) spectra collected at the K-edge and, in the long run, at the L-edge of manganese; (2) high-resolution X-ray emission spectroscopy (XES) of Kα and Kβ lines, (3) two-dimensional resonant inelastic X-ray scattering (RIXS) spectra. Collection of these spectroscopic data sets requires state-of-the-art synchrotron radiation facilities as well as experimental strategies to minimize the radiation-induced modifications of the samples. Data analysis requires the use and development of appropriate theoretical tools. Here, we present exemplary data collected for three multi-nuclear synthetic Mn complexes with the Mn ions in the oxidation states II, III, and IV, and for MnVII of the permanganate ion. Emission spectra are calculated for the MnVII ion using both multiple-scattering (MS) approach and time-dependent density functional theory (TDDFT).

Ni-Fe hydrogenases are proteins catalyzing the oxidative cleavage of dihydrogen (H2) and proton reduction to H2 at high turnover rates. Their active site is a heterobimetallic center comprising one Ni and one Fe atom. To understand the function of the site, well resolved structural and electronic information is required. Such information is expected to become accessible by high resolution X-ray absorption and emission techniques, which are rapidly developing at third generation synchrotron radiation sources. We studied a number of synthetic Ni compounds, which mimic relevant features of the Ni site in hydrogenases, and the Ni site in the soluble, NAD-reducing hydrogenase (SH) from the bacterium Ralstonia eutropha by resonant inelastic X-ray scattering (RIXS) using a Rowland-type spectrometer at the ESRF. The SH is particularly interesting because its H2-cleavage reaction is highly resistant against inhibition by O2. Kα-fluorescence detected RIXS planes in the 1s→3d region of the X-ray absorption spectrum were recorded on the protein which allow to extract L3-edge type spectra Spectral features of the protein are compared to those of the model compounds.

The tetra-manganese complex of photosystem II is the site of the production of most of the atmospheric dioxygen. To understand its function, an atomic-resolution structural model is required. X-ray absorption spectroscopy and protein crystallography yielded models for the arrangement of the four Mn atoms. The location of the functionally essential calcium cofactor within the complex, however, is still debated. To dwell on this point, XAS was performed at the Ca K-edge. Simulations of Ca EXAFS spectra reveal that at least two Mn atoms are located at a distance of ~3.3 A to one Ca ion. Ca bound to the complex seems not to be fully hydrated. On basis of the complementary structural information derived from XAS at the Mn and Ca K-edge, a model of the Mn4/Ca complex in its S1-state is proposed which predicts intimate links between the Ca ion and Mn, possibly by bridging substrate-water molecules.

For protein-bound transition metals, the structure of the metal site (geometric arrangement of the nuclei) and the electronic structure (oxidations state and orbital occupancy) can be investigated by x-ray absorption spectroscopy. While the extended x-ray absorption fine-structure (EXAFS) is theoretically well understood and the advantages of EXAFS spectroscopy on protein-bound metal centers have been repeatedly demonstrated, quantitative analysis of the near-edge structure XANES is more demanding. However, by now XANES simulations, for which a number of codes are available, may be useful to understand geometric and electronic structure effects. For a simplified model of the manganese complex in photosystem II we simulate polarization-dependent XANES spectra using the FEFF 8.2 code [Ankudinov et al, Phys. Rev. B 12 (1998)], which includes self-consistent muffin-tin potentials and polarization-dependent full multiple-scattering calculations. We derive a scheme to analyze the polarization dependence of XANES data that facilitates a treatment of partially uni-directionally oriented samples. Reasonably good agreement between simulations and experiments is obtained for the chosen model of the photosystem II manganese complex and its orientation with respect to the protein-carrying membrane.

Photosynthetic water oxidation proceeds at a tetra-manganese complex bound to a cofactor-protein complex denoted as Photosystem II (PSII). To understand this process of fundamental importance for life on Earth, the crucial changes of the active-site occurring upon the individual transitions in the catalytic S-state cycle have been investigated by x-ray absorption spectroscopy at the Mn K-edge. For the S2-S3 transition, the changes in position and shape of the x-ray edge have been interpreted to be either associated with oxidation of Mn(III) to Mn(IV) or to formation of a ligand radical without manganese oxidation. Here we simulate XANES spectra of simple Mn-complexes using modern multiple-scattering code. Features of the XANES spectra are assigned to molecular orbitals by means of comparative semi-empirical calculations. It is concluded that the changes in position and shape of the edge spectra observed for the S2-S3 transition are explainable by the transformation of five-coordinated Mn(III) in a square-pyramidal geometry to six-coordinated Mn(IV).