Employed X-ray Absorption Spectroscopy Research Articles

Structural and Oxidation-State Changes at Its Nonstandard Ni−Fe Site during Activation of the NAD-Reducing Hydrogenase from Ralstonia eutropha Detected by X-ray Absorption, EPR, and FTIR Spectroscopy

Structure and oxidation state of the Ni-Fe cofactor of the NAD-reducing soluble hydrogenase (SH) from Ralstonia eutropha were studied employing X-ray absorption spectroscopy (XAS) at the Ni K-edge, EPR, and FTIR spectroscopy. The SH comprises a nonstandard (CN)Ni-Fe(CN)(3)(CO) site; its hydrogen-cleavage reaction is resistant against inhibition by dioxygen and carbon monoxide. Simulations of the XANES and EXAFS regions of XAS spectra revealed that, in the oxidized SH, the Ni(II) is six-coordinated ((CN)O(3)S(2)); only two of the four conserved cysteines, which bind the Ni in standard Ni-Fe hydrogenases, provide thiol ligands to the Ni. Upon the exceptionally rapid reductive activation of the SH by NADH, an oxygen species is detached from the Ni; hydrogen may subsequently bind to the vacant coordination site. Prolonged reducing conditions cause the two thiols that are remote from the Ni in the native SH to become direct Ni ligands, creating a standardlike Ni(II)(CysS)(4) site, which could be further reduced to form the Ni-C (Ni(III)-H(-)) state. The Ni-C state does not seem to be involved in hydrogen cleavage. Two site-directed mutants (HoxH-I64A, HoxH-L118F) revealed structural changes at their Ni sites and were employed to further dissect the role of the extra CN ligand at the Ni. It is proposed that the predominant coordination by (CN),O ligands stabilizes the Ni(II) oxidation state throughout the catalytic cycle and is a prerequisite for the rapid activation of the SH in the presence of oxygen.

Stepwise Transition of the Tetra-Manganese Complex of Photosystem II to a Binuclear Mn 2( μ-O) 2 Complex in Response to a Temperature Jump: A Time-Resolved Structural Investigation Employing X-Ray Absorption Spectroscopy

In oxygenic photosynthesis, water is oxidized at a protein-cofactor complex comprising four Mn atoms and, presumably, one calcium. Using multilayers of Photosystem II membrane particles, we investigated the time course of the disassembly of the Mn complex initiated by a temperature jump from 25°C to 47°C and terminated by rapid cooling after distinct heating periods. We monitored polarographically the oxygen-evolution activity, the amount of the Y D ox radical and of released Mn 2+ by EPR spectroscopy, and the structure of the Mn complex by x-ray absorption spectroscopy (XAS, EXAFS). Using a novel approach to analyze time-resolved EXAFS data, we identify three distinct phases of the disassembly process: ( 1) Loss of the oxygen-evolution activity and reduction of Y D ox occur simultaneously ( k 1 = 1.0 min −1). EXAFS spectra reveal the concomitant loss of an absorber-backscatterer interaction between heavy atoms separated by ∼3.3 Å, possibly related to Ca release. ( 2) Subsequently, two Mn(III) or Mn(IV) ions seemingly separated by ∼2.7 Å in the native complex are reduced to Mn(II) and released ( k 2 = 0.18 min −1). The x-ray absorption spectroscopy data is highly suggestive that the two unreleased Mn ions form a di- μ-oxo bridged Mn(III) 2 complex. ( 3) Finally, the tightly-bound Mn 2( μ-O) 2 unit is slowly reduced and released ( k 3 = 0.014 min −1).

The zinc environment in Langmuir-Blodgett phospholipid multi-layers.

The Zinc environment of a system formed by Langmuir-Blodgett phospholipid multi-layers is investigated by employing X-ray absorption spectroscopy. A comparative analysis of the Extended X-ray Absorption Fine Structure and Near Edge regions of the X-ray absorption spectra at the Zinc K-edge, in presence and in absence of the Myelin Basic Protein, clearly indicates that Zinc ions are bound to the heads of the phospholipidic molecules, while the presence of Myelin Basic Protein induces a visible distorsion of the geometry of the Zinc environment. These findings represent a first important step in understanding the interplay among the lipids of the myelin sheath, Myelin Basic Protein and Zinc, as Langmuir-Blodgett phospholipid multi-layers represent a valuable model for the multilamellar structure of the membrane surrounding the nerve axon.

Order in CdS nanoparticles in glass

We have employed X-ray absorption spectroscopy to study the local bonding of Cd atoms in borosilicate glasses doped with Cd and S after heat treatment at temperatures from 550 to 750°C. Varying temperature and time can form CdS particles with average size from a few to several nanometers. In crystals grown below ∼700°C, the apparent number of second neighbors found from analysis of X-ray absorption fine structure is only a small fraction of that expected for well-ordered crystallites of these sizes. We attribute this apparent coordination deficit to substantial short-range structural disorder.

X-ray absorption spectroscopy of the [2Fe-2S] Rieske cluster in Pseudomonas cepacia phthalate dioxygenase. Determination of core dimensions and iron ligation.

We have employed X-ray absorption spectroscopy to obtain structural information about the Rieske Fe/S center in the phthalate dioxygenase (PDO) from Pseudomonas cepacia. Native PDO contains a dinuclear Rieske Fe/S center and an additional mononuclear Fe site. In order to study selectively the Fe/S cluster, we measured data for samples in which the mononuclear site was either depleted of metal or reconstituted with Co or Zn. Our results demonstrate that the iron environment in the Rieske cluster is structurally indistinguishable from that found in other Fe/S clusters, thus strongly supporting the suggestion that the unusually high reduction potentials for Rieske clusters are due to electrostatic rather than structural effects. The average Fe-Fe distance is 2.68 (3) A for both oxidized and reduced Rieske clusters. The average Fe-S distance is 2.24 (2) A in the oxidized cluster and 2.28 (2) A in the reduced cluster. Careful analysis of the EXAFS Debye-Waller factors suggests that the bridging and terminal Fe-S distances for the oxidized cluster are 2.20 and 2.31 A, respectively. Taken together with recent ENDOR results, these studies provide a detailed structural model for the Rieske [2Fe-2S] centers.

Structural studies of electrochemical interfaces with x rays

We have employed x-ray absorption spectroscopy, including surface EXAFS (extended x-ray absorption fine structure) and x-ray standing waves (XSW) in the in-situ structural study of electrochemical interfaces. The SEXAFS technique was used to study the structure of half a monolayer of copper underpotentially deposited on a Pt(111) electrode. From analysis of the data a Cu–Cu distance of 2.85 Å and a coordination number of six were determined, strongly suggesting that the deposit is largely composed of clustered sites. The electrosorption isotherm of iodide on a Pt(111) electrode surface was measured in situ using x-ray absorption spectroscopy, and it exhibited two plateaus which we ascribe to changes in packing density and surface structure. Using x-ray standing waves, we have studied the potential dependence of the adsorption of iodide onto a Pt/C LSM and have interpreted the results in terms of distributional changes in the vicinity of the electrode surface.