XAFS Analysis using EXCURV90

Elucidating Structural Mechanisms for Cordierite Ceramic Formation using Synchrotron Radiation

Cr2O3 is a common nucleating agent useful for forming ceramics from oxide glasses. In this study we have used a variety of synchrotron radiation techniques to examine the atomic structure, crystallinity and microstructure of a magnesium alumino-silicate glass ceramic whose composition is close to that of the mineral cordierite, Mg2Al4Si5O13. X-ray Absorption spectra on the Cr K-edge have been performed with samples that were heat treated at different temperatures and times to examine the metamorphosis of the nucleating site. This study reveals that Cr3+ is always octahedrally coordinated with oxygen. In addition, the second nearest neighbour environment changes from an amorphous, single shell of Al/Si, but upon crystallisation, develops into a well ordered Al/Mg shell indicative of the formation of a dilute Cr spinel phase. Powder X-ray Diffraction (XRD) patterns, however, reveal that the major phase initially precipitated (˜900°C) is a stuffed quartz and from the lattice parameters that the composition is SiO2 -rich. With prolonged heat treatment a small quantity of a spinel phase emerges whose composition from its lattice parameter is close to MgAl2O4. Combined Small Angle X-ray Scattering (SAXS) and XRD establish that devitrification at these temperatures is associated with particles about 250 Å in size, on average. Energy dispersive powder diffraction patterns were collected in situ to observe changes in crystallinity with temperature and time. From these measurements the stuffed quartz phase identified at 900°C is found to be intermediate, being eventually replaced at higher temperatures by cordierite with a small quantity of spinel.

Structural investigations of the CuA centre of nitrous oxide reductase from Pseudomonas stutzeri by site-directed mutagenesis and X-ray absorption spectroscopy

Nitrous oxide reductase is the terminal component of a respiratory chain that utilizes N2O in lieu of oxygen. It is a homodimer carrying in each subunit the electron transfer site, CuA, and the substrate-reducing catalytic centre, CuZ. Spectroscopic data have provided robust evidence for CuA as a binuclear, mixed-valence metal site. To provide further structural information on the CuA centre of N2O reductase, site directed mutagenesis and Cu K-edge X-ray absorption spectroscopic investigation have been undertaken. Candidate amino acids as ligands for the CuA centre of the enzyme from Pseudomonas stutzeri ATCC14405 were substituted by evolutionary conserved residues or amino acids similar to the wild-type residues. The mutations identified the amino acids His583, Cys618, Cys622 and Met629 as ligands of Cu1, and Cys618, Cys622 and His626 as the minimal set of ligands for Cu2 of the CuA centre. Other amino acid substitutions indicated His494 as a likely ligand of CuZ, and an indirect role for Asp580, compatible with a docking function for the electron donor. Cu binding and spectroscopic properties of recombinant N2O reductase proteins point at intersubunit or interdomain interaction of CuA and CuZ. Cu K-edge X-ray absorption spectra have been recorded to investigate the local environment of the Cu centres in N2O reductase. Cu K-edge Extended X-ray Absorption Fine Structure (EXAFS) for binuclear Cu chemical systems show clear evidence for Cu backscattering at approximately 2.5 A. The Cu K-edge EXAFS of the CuA centre of N2O reductase is very similar to that of the CuA centre of cytochrome c oxidase and the optimum simulation of the experimental data involves backscattering from a histidine group with Cu-N of 1.92 A, two sulfur atoms at 2.24 A and a Cu atom at 2. 43 A, and allows for the presence of a further light atom (oxygen or nitrogen) at 2.05 A. The interpretation of the CuA EXAFS is in line with ligands assigned by site-directed mutagenesis. By a difference spectrum approach, using the Cu K-edge EXAFS of the holoenzyme and that of the CuA-only form, histidine was identified as a major contributor to the backscattering. A structural model for the CuA centre of N2O reductase has been generated on the basis of the atomic coordinates for the homologous domain of cytochrome c oxidase and incorporating our current results and previous spectroscopic data.