Results 1 - 10 of 616
Results 1 - 10 of 616. Search took: 0.02 seconds
|Sort by: date | relevance|
[en] Following the seminal work of Von Dreele, powder X-ray diffraction studies on proteins are being established as a valuable complementary technique to single-crystal measurements. A wide range of small proteins have been found to give synchrotron powder diffraction profiles where the peak widths are essentially limited only by the instrumental resolution. The rich information contained in these profiles, combined with developments in data analysis, has stimulated research and development to apply the powder technique to microcrystalline protein samples. In the present work, progress in using powder diffraction for macromolecular crystallography is reported.
[en] A preliminary neutron crystallographic study of the proteolytic enzyme proteinase K is presented. Large hydrogenated crystals were prepared in deuterated crystallization buffer using the vapor-diffusion method. Data were collected to a resolution of 2.3 on the LADI-III diffractometer at the Institut Laue Langevin (ILL) in 2.5 days. The results demonstrate the feasibility of a full neutron crystallographic analysis of this structure aimed at providing relevant information on the location of H atoms, particularly at the active site. This information will contribute to further understanding of the molecular mechanisms underlying proteinase K's catalytic activity and to an enriched understanding of the subtilisin clan of serine proteases
[en] A non-diffracting surface layer exists at any boundary of a crystal and can comprise a mass fraction of several percent in a finely divided solid. This has led to the long-standing issue of amorphous content in standards for quantitative phase analysis (QPA). NIST standard reference material (SRM) 676a is a corundum (α-Al2O3) powder, certified with respect to phase purity for use as an internal standard in powder diffraction QPA. The amorphous content of SRM 676a is determined by comparing diffraction data from mixtures with samples of silicon powders that were engineered to vary their specific surface area. Under the (supported) assumption that the thickness of an amorphous surface layer on Si was invariant, this provided a method to control the crystalline/amorphous ratio of the silicon components of 50/50 weight mixtures of SRM 676a with silicon. Powder diffraction experiments utilizing neutron time-of-flight and 25 keV and 67 keV X-ray energies quantified the crystalline phase fractions from a series of specimens. Results from Rietveld analyses, which included a model for extinction effects in the silicon, of these data were extrapolated to the limit of zero amorphous content of the Si powder. The certified phase purity of SRM 676a is 99.02% ± 1.11% (95% confidence interval). This novel certification method permits quantification of amorphous content for any sample of interest, by spiking with SRM 676a.
[en] Transient molecular structures along chemical reaction pathways are important for predicting molecular reactivity, understanding reaction mechanisms, as well as controlling reaction pathways. During the past decade, X-ray transient absorption spectroscopy (XTA, or LITR-XAS, laser-initiated X-ray absorption spectroscopy), analogous to the commonly used optical transient absorption spectroscopy, has been developed. XTA uses a laser pulse to trigger a fundamental chemical process, and an X-ray pulse(s) to probe transient structures as a function of the time delay between the pump and probe pulses. Using X-ray pulses with high photon flux from synchrotron sources, transient electronic and molecular structures of metal complexes have been studied in disordered media from homogeneous solutions to heterogeneous solution-solid interfaces. Several examples from the studies at the Advanced Photon Source in Argonne National Laboratory are summarized, including excited-state metalloporphyrins, metal-to-ligand charge transfer (MLCT) states of transition metal complexes, and charge transfer states of metal complexes at the interface with semiconductor nanoparticles. Recent developments of the method are briefly described followed by a future prospective of XTA. It is envisioned that concurrent developments in X-ray free-electron lasers and synchrotron X-ray facilities as well as other table-top laser-driven femtosecond X-ray sources will make many breakthroughs and realise dreams of visualizing molecular movies and snapshots, which ultimately enable chemical reaction pathways to be controlled.
[en] A procedure is described for constructing a series of progressively stronger restrictions on the magnitudes and phases of individual structure factors in terms of sets of other structure factors. The existence of the Cholesky factors of Hauptman-Karle matrices is used to ensure that the electron density is everywhere positive. (orig.)
[en] It has long been known that there exists an infinite number of types of tile-transitive periodic three-dimensional tilings. Here, it is shown that, by contrast, the number of types of face-transitive periodic three-dimensional tilings is finite. The method of Delaney symbols and the properties of the 219 isomorphism classes of crystallographic space groups are used to find exactly 88 equivariant types that fall into seven topological families. (orig.)
[en] The structures of α-Ba3RAl2O7.5 and β-Ba3RM2O7.5 complex oxides (R = rare-earth elements, M=Al,Ga) have been studied by a combination of X-ray diffraction, electron diffraction (ED) and high-resolution electron microscopy (HREM). The α and β forms have cell parameters related to the perovskite subcell: a = 2aper, b = aper(2)1/2, c = 3aper(2)1/2, however, the α form has an orthorhombic unit cell whereas the β form adopts monoclinic symmetry. The crystal structure of monoclinic Ba3ErGa2O7.5 was refined from X-ray powder data (space group P2/c, a = 7.93617 (9), b = 5.96390 (7), c = 18.4416 (2) A, β = 91.325 (1) , RI = 0.023, RP = 0.053), the structure of the α form (space group Cmc2I) was deduced from ED and HREM data. The important feature of the α and β structures is the presence of slabs containing strings of vertex-sharing tetrahedral Al2O7 pairs. Two almost equivalent oxygen positions within the strings can be occupied either in an ordered manner leading to the low-temperature β phase or randomly resulting in the high-temperature α structure. The critical temperature of this order-disorder phase transition was determined by high-temperature X-ray diffraction and by differential thermal analysis (DTA). In situ ED and HREM observations of the second-order phase transition confirmed the symmetry changes and revealed numerous defects (twins and antiphase boundaries) formed during the phase transformation. (orig.)