Copper, K-edge EXAFS

Fig. 5, Nature of the binuclear copper center in oxy- and deoxyhemocyanin proposed on the basis of copper K-edge EXAFS (93) (nitrogen atoms derive from imidazole groups of histidine residues X is an oxygen or nitrogen atom).

The copper EXAFS of the ruthenium-copper clusters might be expected to differ substantially from the copper EXAFS of a copper on silica catalyst, since the copper atoms have very different environments. This expectation is indeed borne out by experiment, as shown in Figure 2 by the plots of the function K x(K) vs. K at 100 K for the extended fine structure beyond the copper K edge for the ruthenium-copper catalyst and a copper on silica reference catalyst ( ). The difference is also evident from the Fourier transforms and first coordination shell inverse transforms in the middle and right-hand sections of Figure 2. The inverse transforms were taken over the range of distances 1.7 to 3.1A to isolate the contribution to EXAFS arising from the first coordination shell of metal atoms about a copper absorber atom. This shell consists of copper atoms alone in the copper catalyst and of both copper and ruthenium atoms in the ruthenium-copper catalyst. 

Comparison of the Cu K-edge EXAFS signals for the monometallic Cu/Si02 and the bimetallic Ru-Cu/Si02 catalyst, on the other hand, provides clear evidence for the proximity of ruthenium to copper atoms in the latter. This is seen in the different shape of the measured EXAFS signal and the distorted inverse transform of the first coordination shell. Note that the intensity of the latter is weaker for the bimetallic catalyst, while the region between k=8 and k=15 A-1 has become more important, which points to the presence of a scattering atom heavier than copper in the first coordination shell. The reduced intensity in the Cu Fourier transform of the bimetallic catalyst is indicative of a lower coordination of the copper, which is characteristic of surface atoms. 

Most recently, they have developed a cell configuration for the study of modified electrodes that employs, as a working electrode, colloidal graphite deposited onto kapton tape (typical window material). Such an arrangement minimizes attenuation due to the electrolyte solution. They coated the working electrode with a thin film of Nafion (a perfluoro sulfonate ionomer from E. I. DuPont de Nemours, Inc.) and incorporated [Cu(2,9-dimethy-1,10-phenanthroline)2] by ion exchange. They were able to obtain the EXAFS spectra around the copper K edge for the complex in both the Cu(I) and Cu(II) oxidation states. 

M(II) = Mn and Cd M (I) = Cu or Ag monocations were supposed to be distributed in an ordered manner over the interlamellar sites. In the particular case of Mn g,— Cug 26P 3> suitable crystals (0.30 0.25 0.05 mm) were grown and a complete XRD analysis could be carried out at room temperature. Also low and room temperature EXAFS spectra were recorded at both manganese and copper K edges The apparent discrepancies between the results of the two studies gave rise to an interesting discussion which led to a deeper insight in the structure of the compound. We report the arguments hereunder. 

Rh2(At-02CC H2 i)4] for n = 4-24 behave very similarly to the copper complexes and form, on heating, columnar meso-phases. Rh-K-edge EXAFS measurements showed no change in the core structure passing from the solid state to the mesophase [48]. 

Figure 2. Normalized EXAFS data (copper K absorption edge) at 100°K, with associated Fourier transforms and inverse transforms, for silica supported copper and ruthenium-copper catalysts. Reproduced with permission from Ref. 8. Copyright 1980, American Institute of Physics.

X-ray diffraction data were obtained using Rigaku RAD-C with a copper X-ray tube in air atmosphere. X-ray absorption measurements of Cu K-edge were performed with laboratory EXAFS equipment (Technos EXAC-820). The X-ray source with a rotating Mo target and a LaBe filament was operated at 17 kV, 100 mA (EXAFS) and 20 kV 150 mA (XANES). The samples were pressed into wafers with methyl cellulose as a binder. The measurements were carried out in air atmosphere at room temperature. EXAFS Fourier transformations were carried out over the ranges of photoelectron wave vector, k, of 2.5 -10.0 A 1. 

One recombinant FetSp mutant is unique among multicopper oxidase species and has been particularly informative about the structure of the type 3 binuclear cluster in these species. This is the T1D/T2D double mutant that contains only this type 3 site (Blackburn et al., 2000). EXAFS analysis of this protein contains contributions from electron ejection and scattering from only the type 3 copper atoms and thus provides direct structural information about this cluster. The K-edge XAS spectrum for this mutant in its oxidized and reduced states is shown in Fig. 21. The oxidized sample has a nearly featureless edge with a midpoint energy of 8990 eV typical of tetragonally distorted type 2 Cu(ll) centers, i.e., those with predominantly histidine imidazole coordination. The reduced type 3 cluster exhibited a pronounced shoulder at 8984 eV just below the... 

Mercury(ll) is readily replaced by copper(ll) ion at rate faster than direct reaction of copper(ll) with the free base porphyrin H2tpps. Nine solutions were prepared and EXAFS was measured at both Hg Lm and Cu K edges. 

Figure 4,17 Contributions of nearest neighbor copper and osmium backscattering atoms (points in fields b and c, respectively) to the EXAFS associated with the copper K absorption edge of a silica-supported osmium-copper catalyst containing 2 wt% Os and 0.66 wt% Cu (32). (Points in field a show how the individual contributions combine to describe the experimental EXAFS represented by the solid line.) (Reprinted with permission from the American Institute of Physics.)...

Figure 4.35 X-ray Absorption Spectrum of Cu,Zn-metallothionein. The copper and zinc K-edges are observed and the XANES and EXAFS regions are shown for the zinc K-edge.

Extended X-ray-absorption fine structures (EXAFS) on copper and zinc K-edge were measured in aqueous solution After reduction of copper, it is co-ordinated to... 

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