X-ray absorption spectroscopy to analyze nuclear geometry and electronic structure of biological metal centers?potential and questions examined with special focus on the tetra-nuclear manganese complex of oxygenic photosynthesis

Abstract

X-ray absorption spectroscopy (XAS) has become a prominent tool for the element-specific analysis of transition metals at the catalytic center of metalloenzymes. In the present study the information content of X-ray spectra with respect to the nuclear geometry and, in particular, to the electronic structure of the protein-bound metal ions is explored using the manganese complex of photosystem II (PSIII) as a model system. The EXAFS range carries direct information on the number and distances of ligands as well as on the chemical type of the ligand donor function. For first-sphere ligands and second-sphere metals (in multinuclear complexes), the determination of precise distances is mostly straightforward, whereas the determination of coordination numbers clearly requires more effort. The EXAFS section starts with an exemplifying discussion of a PSII spectrum data set with focus on the coordination number problem. Subsequently, the method of linear dichroism EXAFS spectroscopy is introduced and it is shown how the EXAFS data leads to an atomic resolution model for the tetra-manganese complex of PSII. In the XANES section the following aspects are considered: (1) Alternative approaches are evaluated for determination of the metal-oxidation state by comparison with a series of model compounds. (2) The interpretation of XANES spectra in terms of molecular orbitals (MOs) is approached by comparative multiple-scattering calculations and MO calculations. (3) The underlying reasons for the oxidation-state dependence of the XANES spectra are explored. Furthermore, the potential of modern XANES theory is demonstrated by presenting first simulations of the dichroism in the XANES spectra of the PSII manganese complex.

Double perovskites as a family of highly active catalysts for oxygen evolution in alkaline solution

“…The energy scale was calibrated by simultaneously measuring BSCF with known inflection points 12 . The Co oxidation states were calculated by relating the edge position obtained by the integral method 36 with the known oxidation states of singlephase Co 2 þ O, Co 2 þ (OH) 2 , Co 2.6 þ 3 O 4 and LiCoO 2 using linear regression. The errors in the edge position are estimated and the errors in the oxidation state were calculated by taking the root of the squared s.e.…”

Electrosynthesis, functional, and structural characterization of a water-oxidizing manganese oxide

“…[49][50][51] The inset in Fig. 6 shows the X-ray absorption near-edge structure (XANES) spectrum of active and inactive Mn oxides, both frozen using the same protocol (after application of 1.35 V for 2 min in Mn-free phosphate buffer at pH 7; for details see ESI, S-1E †).…”

“…Edge-rise energies are indicative of the mean oxidation state of manganese. 51 On the basis of a calibration employing simple Mn oxides, the average oxidation state of manganese in the active Mn film was estimated to be +3.8, while in the inactive film it was +4.0 (see ESI, Fig. S12a and S12b †).…”

[NiFe] and [FeS] Cofactors in the Membrane-Bound Hydrogenase of Ralstonia eutropha Investigated by X-ray Absorption Spectroscopy: Insights into O2-Tolerant H2 Cleavage

“…Unfiltered k 3 -weighted spectra were used for least-squares curve-fitting and Fourier-transform (FT) calculation with the in-house program SimX (52). In EXAFS simulations, phase-functions from FEFF7 (62) and amplitude reduction factors (S 0 2 ) of 0.9 (Ni) and 0.85 (Fe) were used.…”

“…In the present study, the metal centers of the MBH were characterized by an elementspecific technique, namely X-ray absorption spectroscopy (XAS) (51)(52), by which the coordination environment and redox changes of the protein-bound metal atoms were determined. The spectroscopic features of the MBH are compared to those of the PH standard H 2 ase, for which crystal structures are available (27,53).…”