Copper EXAFS studies

The results of the EXAFS studies on osmium-copper clusters lead to conclusions similar to those derived for ruthenium-copper clusters. That is, an osmium-copper cluster Is viewed as a central core of osmium atoms with the copper present at the surface. The results of the EXAFS investigations have provided excellent support for the conclusions deduced earlier (21,23,24) from studies of the chemisorption and catalytic properties of the clusters. Although copper is immiscible with both ruthenium and osmium in the bulk, it exhibits significant interaction with either metal at an interface. 

EXAFS results, Cu atoms locate on the top of Pt atoms. Experimental and theoretical EXAFS studies were carried out for Cu underpotential deposition on Pt(lll) in a 0.1 M H2SO4 -1- SQ/iM Cu " solution, giving the position of copper underpotential deposition at three hollow sites of Pt(lll), which is partially positively charged. ... 

Blue copper proteins, 36 323, 377-378, see also Azurin Plastocyanin active site protonations, 36 396-398 charge, 36 398-401 classification, 36 378-379 comparison with rubredoxin, 36 404 coordinated amino acid spacing, 36 399 cucumber basic protein, 36 390 electron transfer routes, 36 403-404 electron transport, 36 378 EXAFS studies, 36 390-391 functional role, 36 382-383 occurrence, 36 379-382 properties, 36 380 pseudoazurin, 36 389-390 reduction potentials, 36 393-396 self-exchange rate constants, 36 401-403 UV-VIS spectra, 36 391-393 Blue species... 

Intriguingly, the blue copper sites, especiaUy those with a carbonyl oxygen at the axial coordination position, display high affinity for Zn + ions. Mutants in which the Met is replaced by Gin or Glu preferentiaUy bind Zn + when expressed in heterologous systems, e.g., Escherichia coli. Examples include azurin, amicyanin, nitrite reductase, and possibly also plastocyanin (Diederix et al., 2000 Hibino et al., 1995 Murphy et al., 1995 Nar et al., 1992a Romero et al., 1993). In the case of azurin it has been shown that both wild-type and the Met—Gin mutant have the same affinity for both Zn +and Cu + (Romero ci a/., 1993). In addition, EXAFS studies showed that some preparations of blue copper proteins purihed from their natural sources also contain small fractions of Zn derivatives (DeBeer George, personal communication). 

The Copper Site. In a crystal form of ECAO shown to contain catalytically-active protein (Parsons et al., 1995), the eopper is penta-coordinated in approximate square pyramidal eonfiguration by four basal (equatorial) ligands (His 524, His 526, His 689 and a water [We]) and an apical (axial) water (Wa). The presence of equatorial and axial waters had been first reported by Barker et al. (1979) from EPR, water proton relaxation and kinetic studies on pig plasma amine oxidase and the prediction of histidines and waters as the copper ligands came from EXAFS studies by Scott and Dooley (1985). The equatorial water (We) is labile and not always present. In the HP AO structure (Li et al., 1998) it is present in some, but not all, of the six independent subunits in the same erystal. A comprehensive discussion of the spectroscopic properties of the copper site in amine oxidases, including the exchange rates for the equatorial and axial waters, is given in the review by Knowles and Dooley (1994). 

Deoxy-form. The deoxy-forms of He s and Ty s are colorless and EPR silent. In agreement with XAS studies, both coppers are present in oxidation state - -I. The X-ray structure of P. interruptus deoxy-Hc showed that the two Cu ions are separated by <3.6 A, which is in agreement with EXAFS studies of deoxy-Hc indicating most probably a 3,3-coordination of the coppers, which is also in agreement with... 

IV EXAFS Study of Reaction of Hg2(tpps)2- with Copper(II)15,16... 

Rusticyanin has a high reduction potential (680 mV), which is similar to that for the Type 1 Cu center in fungal as opposed to tree laccase (785 mV) (73). This trend is so far unexplained. From the sequence and EXAFS studies, His-Cys-His-Met coordination is a reasonable possibility for rusticyanin (55). It may well be that the reduction potential is determined by effects of a polypeptide backbone on Cu—S(Cys) and Cu—S(Met) bond distances and the Cu ligand field (74). If this is the case, however, rusticyanin would be expected to have one or both Cu—S distances shorter than in other blue copper proteins, which is not borne out by information from EXAFS (Table IV). A further possibility that the Cu(I) form is three-coordinate, as in the case of plasto-cyanin at low pH (Fig. 2), has no strong support at present (75). 

EXAFS Studies of Ruthenium-Copper Clusters (31). An X-ray absorption spectrum at 100°K showing the extended fine structure beyond the K absorption edge of ruthenium is given in Figure 4.6 for a catalyst containing 1.0 wt% ruthenium and 0.63 wt% copper in the form of small metal clusters... 

The discussion of EXAFS on ruthenium-copper clusters in the previous paragraphs emphasizes the qualitative aspects of the data analysis. A quantitative data analysis, yielding information on the various structural parameters of interest, has also been made and published (31). Of particular interest was the finding that the average composition of the first coordination shell of ruthenium and copper atoms about a ruthenium atom was about 90% ruthenium, while that about a copper atom was about 50% ruthenium. Details of the methods involved in the quantitative analysis of EXAFS data on ruthenium-copper clusters are not considered here, since the technique is described in the following discussion of EXAFS studies on osmium-copper clusters. 

EXAFS Studies of Osmium-Copper Clusters (32). Results of EXAFS studies on osmium-copper clusters dispersed on silica lead to conclusions similar to those derived for ruthenium-copper clusters. The discussion here is concerned with a catalyst with a 1 1 atomic ratio of copper to osmium. The clusters constituted 2.66 wt% of the total catalyst mass (2% Os, 0.66% Cu). The average diameter of the clusters was estimated to be about 15 A. 

In summary, the results of the EXAFS studies on osmium-copper clusters indicate that the osmium atoms in the clusters are coordinated predominantly to other osmium atoms, while the copper atoms are extensively coordinated to both copper and osmium atoms. The results are thus very similar to those obtained for ruthenium-copper clusters. Since the copper atoms appear to have essentially equal numbers of copper and osmium atoms as nearest neighbors, it seems reasonable to conclude that the osmium-copper clusters consist of small patches or multiplets of copper atoms located on the surface of the osmium. 

Another Cu(II)Ni(II) chain, of formula CuNi(ox)2 4 H20 with ox = oxalato, has also been described1391. The available structural information does not come from X-ray diffraction but from an EXAFS study which is consistent with ordered Cu(II)Ni(II) chains and oxalato bridges between the metal centers. The chains, however, are poorly isolated from each others. As a matter of fact, each copper atom of a chain sees two other copper atoms at 4 A, belonging to other chains. The poor one-dimension character likely explains why the XmT versus T plot does not display the expected minimum. In fact XmT continuously decreases upon cooling. Since the low temperature data are severely affected by the relatively strong interchain interaction, the intrachain interaction parameter J has been deduced from the magnetic data above 50 K and found equal to... 

The first EXAFS study of copper oxalate is based upon room temperature data only >. The comparison of CuCjO EXAFS spectrum with those of model compounds... 

Figure 4.9 TEM micrographs recorded on a platinum shadowed replica prepared by FFET. (a) Sample prepared on 5 % dispersion of overbased calcium didodecylbenzene sulfonate in dodecane. Spherical nanoparticles of 10 nm diameter are easily visible, (b) Extractive replica prepared on the same dispersion. (c) X-ray analysis of the selected area (black circle) reveals the composition of the particles (Ca, S, O) corresponding to the sulfonate. The presence of Pt is due to the shadowing layer and copper to the copper support grid, (d) Radial distribution functions (from EXAFS studies) uncorrected from phase shifts obtained on caldte, and two OCABS dispersions in oil. The presence of a single peak corresponding to the first Ca-0 distance points out the amorphous structure of the mineral part of the OCABS particles...

Blum L, Abruna FI D, White J, Gordon J G, Borges G L, Samant M G and Melroy 1986 Study of underpotentially deposited copper on gold by fluorescence detected surface EXAFS J. Chem. Phys. 85 6732-8... 

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