Towards a black-box for biological EXAFS data analysis. II. Automatic BioXAS Refinement and Analysis (ABRA)

Structural Characterization of Titanium-Silica Oxide Using Synchrotron Radiation X-ray Absorption Spectroscopy

In this study, titania−silica oxides (TixSiy oxides) were successfully prepared via the sol−gel technique. The Ti and Si precursors were titanium (IV), isopropoxide (TTIP), and tetraethylorthosilicate (TEOS), respectively. In this work, the effects of pH and the Ti/Si atomic ratio of titanium−silicon binary oxide (TixSiy) on the structural characteristics of TixSiy oxide are reported. 29Si solid-state NMR and FTIR were used to validate the chemical structure of TixSiy oxide. The structural characteristics of TixSiy oxide were investigated using X-ray diffraction, XRF, Fe-SEM, diffraction particle size analysis, and nitrogen adsorption measurements. By applying X-ray absorption spectroscopy (XAS) obtained from synchrotron light sources, the qualitative characterization of the Ti−O−Si and Ti−O−Ti bonds in Ti−Si oxides was proposed. Some Si atoms in the SiO2 network were replaced by Ti atoms, suggesting that Si−O−Ti bonds were formed as a result of the synthesis accomplished using the sol−gel technique described in this article. Upon increasing the pH to alkaline conditions (pH 9.0 and 10.0), the nanoparticles acquired a more spherical shape, and their size distribution became more uniform, resulting in an acceptable nanostructure. TixSiy oxide nanoparticles were largely spherical in shape, and agglomeration was minimized. However, the Ti50Si50 oxide particles at pH 10.0 become nano-sized and agglomerated. The presence of a significant pre-edge feature in the spectra of Ti50Si50 oxide samples implied that a higher fraction of Ti atoms occupied tetrahedral symmetry locations, as predicted in samples where Ti directly substituted Si. The proportion of Ti atoms in a tetrahedral environment agreed with the value of 1.83 given for the Ti−O bond distance in TixSiy oxides produced at pH 9.0 using extended X-ray absorption fine structure (EXAFS) analysis. Photocatalysis was improved by adding 3% wt TiO2, SiO2, and TixSiy oxide to the PLA film matrix. TiO2 was more effective than Ti50Si50 pH 9.0, Ti50Si50 pH 10.0, Ti50Si50 pH 8.0, and SiO2 in degrading methylene blue (MB). The most effective method to degrade MB was TiO2 > Ti70Si30 > Ti50Si50 > Ti40Si60 > SiO2. Under these conditions, PLA/Ti70Si30 improved the effectiveness of the photocatalytic activity of PLA.

Protein and metal cluster structure of the wheat metallothionein domain γ-E(c)-1: the second part of the puzzle

Metallothioneins (MTs) are small cysteine-rich proteins coordinating various transition metal ions, including Zn(II), Cd(II), and Cu(I). MTs are ubiquitously present in all phyla, indicating a successful molecular concept for metal ion binding in all organisms. The plant MT E(c)-1 from Triticum aestivum, common bread wheat, is a Zn(II)-binding protein that comprises two domains and binds up to six metal ions. The structure of the C-terminal four metal ion binding β(E) domain was recently described. Here we present the structure of the N-terminal second domain, γ-E(c)-1, determined by NMR spectroscopy. The γ-E(c)-1 domain enfolds an M (2) (II) Cys(6) cluster and was characterized as part of the full-length Zn(6)E(c)-1 protein as well as in the form of the separately expressed domain, both in the Zn(II)-containing isoform and the Cd(II)-containing isoform. Extended X-ray absorption fine structure analysis of Zn(2)γ-E(c)-1 clearly shows the presence of a ZnS(4) coordination sphere with average Zn-S distances of 2.33 Å. (113)Cd NMR experiments were used to identify the M(II)-Cys connectivity pattern, and revealed two putative metal cluster conformations. In addition, the general metal ion coordination abilities of γ-E(c)-1 were probed with Cd(II) binding experiments as well as by pH titrations of the Zn(II) and Cd(II) forms, the latter suggesting an interaction of the γ domain and the β(E) domain within the full-length protein.