Recently, Imaging Plate and X-ray CCD detectors have been used widely in X-ray diffraction experiments with laboratory X-ray sources. Principle and Performance of Imaging Plate and X-ray CCD detectors are compared together with those of scintillation counters. Especially, the efficiencies of the detectors are compared in details in terms of DQE (Detective Quantum Efficiency) . Also, items to be kept in mind when one uses X-ray area detectors are described.
Recently, 2D detectors, such as IP (imaging plate) or CCD (chargecoupled device), become popular in single crystal data acquisition. As a practical guide, basic information is discussed on diffraction geometry and data processing of 2D detectors, in comparison with those of four-circle diffractometers. Main topics are a typical flow in data collection, indexing, evaluation of spot positions, data collection strategy, evaluation of integrated intensities, Lorentz factor, and determination of precise cell parameters.
The crystal structure of catalytic subunit of the protein phosphatase 1 (PP1), PP1γ, in complex with a marine toxin, calyculin A, was determined at 2.0 Å resolution. Calyculin A is located in two of the three major grooves, namely, in the hydrophobic groove and the acidic groove on the molecular surface. This is the first case that the inhibitor does not adopt a pseudocyclic conformation but an extended one in order to form a complex with the protein.
Although extensive studies have been carried out on structural analyses of amorphous silicon carbide (a-SiC), there are still controversial pictures on short-range order (SRO) of a-SiC. In this study, we examined the SRO structure of a-SiC as well as its relaxation process using transmission electron microscopy in combination with imaging plates. Atomic pair-distribution functions showed that not only heteronuclear Si-C bonds but also homonuclear Si-Si and C-C bonds exist in a-SiC networks. The ratio of heteronuclear to homonuclear bonds was altered upon annealing ; chemical order developed as annealing progresses. This result suggests that structural relaxation of a-SiC occurred. The validity of the experimental results was confirmed by molecular-dynamics simulations.
We report the crystal structures of human hematopoietic prostaglandin (PG) D synthase bound to glutathione (GSH) and Ca2+ or Mg2+. Using GSH as a cofactor, prostaglandin D synthase catalyzes the isomerization of PGH2 to PGD2, a mediator for allergy response. The enzyme is a homodimer, and Ca2+ or Mg2+ increases its activity to - 150 % of the basal level, with half maximum effective concentrations of 400 μM for Ca2+ and 50 μM for Mg2+. In the Mg2+-bound form, the ion is octahedrally coordinated by six water molecules at the dimer interface. The water molecules are surrounded by pairs of Asp93, Asp96 and Asp97 from each subunit. Ca2+ is coordinated by five water molecules and an Asp96 from one subunit. The Asp96 residue in the Ca2+-bound form makes hydrogen bonds with two guanidium nitrogen atoms of Arg 14 in the GSH-binding pocket. Mg2+ alters the coordinating water structure and reduces one hydrogen bond between Asp96 and Arg14, thereby changing the interaction between Arg 14 and GSH. This effect explains a four-fold reduction in the Km of the enzyme for GSH. The structure provides insights into how Ca2+ or Mg2+ binding activates human hematopoietic PGD Synthase.
A system on X-ray dark-field imaging under development and its application is reported. That comprises an asymmetric monochromator and a Laue case analyzer that has a specified thickness for a given X-ray photon energy or wavelength and a sample locating inbetween these. This system uses Si 4, 4, 0 diffraction for both X-ray optics element in a parallel arrangement. In order to achieve the dark-field imaging condition the Si Laue analyzer should be 1.075 mm in thickness for the X-ray energy of 35 keV. Since this system is very simple one can expect a variety of applications including material science, biology, palaeontology and clinical medicine where a large view area with size of 100 mm × 100 mm is needed.
The employment of the X-ray anomalous dispersion effect allows us to detect the change in structure of catalytic converters with the environment exposed. Here we show that palladium atoms in a perovskite crystal move into and out of the crystal by anomalous X-ray diffraction and absorption techniques. This movement of the precious metal plays an important role to keep the catalytic activity long-lived.