Applications of combined EXAFS and powder diffraction analysis in solid state chemistry.

Recent applications of combined EXAFS/powder neutron and X-diffraction analysis are reviewed and provisional results for three additional compounds are presented. Criteria for successful refinements are suggested. The new results relate to the materials: CoAl2O4, La(6.4)Ca(1.6)Cu6Ni2O20 and Pr(0.5)Sr(0.5)FeO(2.75).

State-of-the-Art Analysis of Whole X-ray Absorption Spectra.

Fitting an entire X-ray spectrum rather than its components, EXAFS and XANES, has been an aim of the practitioners of these techniques. Recent developments have made the calculations of both the scattering and atomic components practicable. We present the analysis of four representative model compounds using the EXCURVE package, which was modified to undertake this. The details of these modifications are also given. A comparison of matrix-inversion and finite-path-sum methods is made which shows that the latter method is more promising for fitting the edge region. A number of enhancements are required before this approach can be used for accurate structure determination. These include improvement in atomic contribution, better potentials/phase shifts, and the ability to calculate and refine multiple-scattering terms to at least fifth order.

Constrained and restrained refinement in EXAFS data analysis with curved wave theory.

This paper describes methods of constrained and restrained refinement of EXAFS data which provide a means of substantially reducing the number of independent parameters compared to conventional least-squares methods commonly used. Constrained refinement allows a major reduction in the number of free parameters for a refinement of a structural model. In restrained refinement, additional structural information from well-characterized small molecules is used to provide additional observations in the data analysis. Even though these methods are of general application to the majority of complex systems, they are particularly valuable for biological molecules. The methods are of major advantage for ligands where significant multiple scattering is present, e.g., histidine, tyrosine, CO, CN, etc. The bases of these methods are described, and applications to some complex chemical and biological systems are given.

The local environment of metal sites in intracellular granules investigated by using X-ray-absorption spectroscopy.

We report the use of X-ray-absorption spectroscopy (x.a.s.) to study the local atomic environment of cations in intracellular granules from the hepatopancreas of Helix aspersa. Both the calcium K-edge in these concretions and the manganese K-edge in doped specimens were measured. Electron-microprobe measurements confirm that the introduced Mn2+ is concentrated in irregular growths on the surfaces of the granules. The near-edge structure (x.a.n.e.s.) of calcium is similar to that of manganese, indicating that the oxygen-co-ordination spheres of both cations share a similar symmetry. From the extended structure (e.x.a.f.s.) the metal-oxygen bond lengths of 0.230 nm (2.30A) for Ca-O and 0.218 nm (2.18A) for Mn-O [+/- 0.004 nm (0.04A)] were determined, reference being made to a variety of model compounds. The low density of the granules (2.07 g/cm3), together with the local atomic distribution, suggest an open hydrated structure for these phosphate deposits. Detailed analysis of the distribution of nearest-neighbour oxygen atoms demonstrates that this is asymmetric and considerably broader for Ca2+ than for Mn2+. Compared with the model compounds, the Ca2+ environment in the granules is similar to that observed in Ca2P2O7. I.r. spectra indicate the presence of condensed phosphate groups in the granules, with the strong possibility these are pyrophosphate (P2O7(4-) groups.

Direct structural information for the copper site of dopamine beta-mono-oxygenase obtained by using extended X-ray-absorption fine structure.

Copper K-edge e.x.a.f.s (extended X-ray-absorption fine structure) was measured for dopamine beta-mono-oxygenase in aqueous solution. Comparison with the Cu K-edge e.x.a.f.s. of bovine erythrocyte superoxide dismutase shows a close resemblance. Detailed analysis of the e.x.a.f.s. indicates that the copper atom is bound to four imidazole groups at 0.201 nm with one or two oxygen atoms at 0.23 nm.

An extended-X-ray-absorption-fine-structure study of bovine erythrocyte superoxide dismutase in aqueous solution. Direct evidence for three-co-ordinate Cu(I) in reduced enzyme.

Copper and zinc K-edge e.x.a.f.s. (extended X-ray-absorption fine structures) were measured for the metal sites of oxidized and reduced bovine superoxide dismutase in aqueous solution. Detailed analysis of the spectra indicates that the copper site of the enzyme changes on reduction and is most probably co-ordinated to three imidazole groups at a shorter distance Cu-N(alpha) = 0.194 nm (1.94 A) in the reduced form compared with a co-ordination of four imidazole groups at 0.199 nm (1.99 A) and an oxygen atom from solvent water at 0.224 nm (2.24 A) in the oxidized form. Examination of the edge, near-edge structure and e.x.a.f.s. of the zinc sites indicates that the stereochemical changes at copper that accompany reduction introduce minimal perturbation on the stereochemistry at zinc.

An extended X-ray-absorption-fine-structure study of the copper and zinc sites of freeze-dried bovine superoxide dismutase.

Copper and zinc K-edge-extended X-ray-absorption fine structures were measured for the metal sites of freeze-dried bovine superoxide dismutase and the model compounds tetrakis(imidazole)cupric nitrate and tetrakis(imidazole)zinc perchlorate. Detailed simulation of the spectra indicates that the copper site of the enzyme is best fit by co-ordination of four imidazole groups with Cu-N(alpha) distances of 0.198 nm (1.98 A). The zinc site is best fit by three imidazole groups at 0.201 nm (2.01 A) and an oxygen (from aspartate) at 0.203 nm (2.03 A).