K Absorption Edge (k + absorption_edge)

Distribution by Scientific Domains

Selected Abstracts

Precursory microstructures in Zr,Cu,Al,Ni bulk metallic glasses examined by anomalous small-angle scattering at the Zr K edge

Isao Murase
Anomalous small-angle X-ray scattering measurements of Zr,Cu,Al,Ni quaternary alloys have been made at the Zr K absorption edge. In melt-quenched samples, small cluster components without crystallization were found. The contrast change at the edge suggested that compositional fluctuation of Al is incorporated. [source]

Spin-polarized XANES: theoretical analysis of the Ni K-edge of NiF2

G. Smolentsev
Abstract Theoretical interpretations of spin-dependent X-ray absorption near edge structure (XANES) spectra measured by selectively monitoring of the Ni K-beta emission while scanning the excitation energy through the Ni K absorption edge have been performed. Analysis is based on a combination of self-consistent spin-polarized calculation of muffin-tin potential and a full multiple scattering theory of X-ray absorption. This approach allows us to separate the influence of dipole transition matrix elements and the density of empty electronic states on spin-dependent XANES. It is found that the matrix elements affect splitting between spin-up and spin-down spectra only near the absorption threshold, while differences in densities of states slightly shift the spectra in the region 25,35 eV above the main edge. The effects of the multielectron-term-dependent broadening of spin-dependent XANES and mixing of purely spin-polarized spectra were taken into account. ( 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

The endoproteinase furin contains two essential Ca2+ ions stabilizing its N-terminus and the unique S1 specificity pocket

Manuel E. Than
The mammalian prohormone/proprotein convertase (PC) furin is responsible for the maturation of a great variety of homeostatic but also many pathogenic proteins within the secretory pathway and the endosomal pathway and at the cell surface. Similar to other members of the PC family, furin requires calcium for catalytic activity. In a previous paper, the structural association of the catalytic and the P-domain of furin was shown and data were presented indicating two or three calcium-binding sites. The exact number and the three-dimensional localization of the essential calcium sites within furin have now been determined by collecting X-ray diffraction data on either side of the Ca,K absorption edge and by calculating a novel type of double difference map from these anomalous scattering data. Two calcium ions were unambiguously identified: the purely structural Ca-1 also conserved in the bacterial digestive subtilisins and the Ca-2 site specific to PCs and essential for the formation of the P1 specificity-determining S1-binding pocket. In addition, these anomalous diffraction data show that no tightly bound K+ sites exist in furin. [source]

High-phasing-power lanthanide derivatives: taking advantage of ytterbium and lutetium for optimized anomalous diffraction experiments using synchrotron radiation

. Girard
Ytterbium and lutetium are well suited for optimized anomalous diffraction experiments using synchrotron radiation. Therefore, two lanthanide complexes Yb-HPDO3A and Lu-HPDO3A have been produced that are similar to the Gd-HPDO3A complex already known to give good derivative crystals. Derivative crystals of hen egg-white lysozyme were obtained by co-crystallization using 100,mM solutions of each lanthanide complex. De novo phasing has been carried out using single-wavelength anomalous diffraction on data sets collected on each derivative crystal at the LIII absorption edge of the corresponding lanthanide ( = 28,e,). A third data set was collected on a Lu-HPDO3A derivative crystal at the Se,K absorption edge with = 10,e,. The structures were refined and compared with the known structure of the Gd-HPDO3A lysozyme derivative. The quality of the experimental electron-density maps allows easy model building. With LIII absorption edges at shorter wavelengths than the gadolinium absorption edge, lutetium and ytterbium, when chelated by a ligand such as HPDO3A, form lanthanide complexes that are especially interesting for synchrotron-radiation experiments in structural biology. [source]

Phasing power at the K absorption edge of organic arsenic

Pascal Retailleau
Single/multiple-wavelength anomalous dispersion (SAD/MAD) experiments were performed on a crystal of an organic arsenic derivative of hen egg-white lysozyme. A para -arsanilate compound used as a crystallizing reagent was incorporated into the ordered solvent region of the lysozyme molecule. Diffraction data were collected to high resolution (,2.0,) at three wavelengths around the K edge (1.04,) of arsenic at beamline BM30A, ESRF synchrotron. Anomalous Patterson maps clearly showed the main arsanilate site to be between three symmetry-related lysozyme molecules, at a location previously occupied by a para -toluenesulfonate anion. MAD phases at 2, derived using the program SHARP led to an electron-density map of sufficient quality to start manual building of the protein model. Amplitudes from a second crystal measured to a resolution of 1.8, at the peak wavelength revealed two additional heavy-atom sites, which reinforced the anomalous subset model and therefore dramatically improved the phasing power of the arsenic derivative. The subsequent solvent-flattened map was of such high accuracy that the program ARP/wARP was able to build a nearly complete model automatically. This work emphasizes the great potential of arsenic for de novo structure determination using anomalous dispersion methods. [source]

X-ray-induced debromination of nucleic acids at the Br,K absorption edge and implications for MAD phasing

E. Ennifar
Multi-wavelength anomalous dispersion (MAD) using brominated derivatives is considered a common and convenient technique for solving chemically synthesized nucleic acid structures. Here, it is shown that a relatively moderate X-ray dose (of the order of 5 1015,photons,mm,2) can induce sufficient debromination to prevent structure determination. The decrease in bromine occupancy with radiation dose can be accounted for by a simple exponential, with an estimated rate constant at the absorption-peak wavelength, 7.4,(0.8),MGy, that is not significantly different from its value at the absorption-edge wavelength, 9.2,(2.6),MGy (the given e.s.d.s assess the relative closeness of the two values, not their absolute accuracy, which is probably worse). Chemically, these results (and others) are consistent with bromine cleavage resulting from direct photodissociation and/or from the action of free electrons, rather than from the action of hydroxyl radicals originating from water dissociation. The free bromine species (Br,) diffuse too quickly, even in amorphous ice around 100,K, to allow the determination of a diffusion coefficient. From a practical point of view, it is suggested that a single data collection with a crystal consisting of iodinated instead of brominated derivatives could provide both anomalous scattering and SIR phase information by the progressive cleavage of iodine. [source]