The relationship between stress and strain in polycrystalline materials with fibre texture is examined on the basis of the strain analysis in the constituent crystallites within the Reuss approximation. By introducing the symmetry of reciprocal lattices for the constituent crystallites, the physical meaning of taking an average of the strains observed by X-ray diffraction (XRD) is made clear. By using formulae obtained by the present treatment for the stress-strain relation in cubic specimens with <111> fibre texture in the Laue classes m3m hkl Bragg reflections with h≠k≠l split into doublets owing to the existence of crystallites with two different orientations under the stress field. This technique was confirmed by the profile analysis in XRD data observed for reflections of 222 and 420 in a cubic TiN thin film sputtered on a polyimide film. The technique of the stress analysis and its confirmation are introduced.
d-Serine is an endogenous coagonist for the N-methyl-d-aspartate receptor and is involved in excitatory neurotransmission in the brain. Mammalian pyridoxal 5’-phosphate-dependent serine racemase, which is localized in the mammalian brain, catalyzes the racemization of l-serine to yield d-serine and vice versa. We have determined the structures of three forms of the mammalian enzyme homolog from Schizosaccharomyces pombe. Lys57 and Ser82 located on the protein and solvent sides, respectively, with respect to the cofactor plane, are acid-base catalysts that shuttle protons to the substrate. The modified enzyme, which has a unique lysino-d-alanyl residue at the active site, also binds the substrate serine in the active site, suggesting that the lysino-d-alanyl residue acts as a catalytic base in the same manner as Lys57 of the wild type enzyme.
The mechanism that ATP-pyrophosphate exchange reaction catalyzed by Arg-tRNA, Gln-tRNA and Glu-tRNA synthetases requires assistance of the cognate tRNA, remained to be established. On the basis of crystal structure of Arg-tRNA synthetase from Pyrococcus horikoshii complexed with tRNAArgCCU and ATP analog, we constructed a structural model for a mechanism in which formation of a hydrogen bond between 2’-OH group of A76 of tRNA and carboxyl group of Arg induces both formation reaction of Arg-AMP from Arg and ATP and the reverse reaction pyrophosphorolysis at low pH. The hydrogen bond assists also editing of misacylated Thr-tRNAVAl or Thr-AMP bound Val-tRNA synthetase.
To help resolve long-standing questions regarding the catalytic activity of the serine proteases the structure of porcine pancreatic elastase has been analyzed by high-resolution neutron (1.65 Å resolution) and X-ray (0.94 Å resolution) crystallography. In order to mimic the tetrahedral transition intermediate, a peptidic inhibitor was used. The neutron and X-ray data show that the hydrogen bond between His57 and Asp102 is not consistent with a low-barrier hydrogen which is predicted to have the hydrogen midway between the donor and acceptor atom. The neutron analysis also shows that the oxygen of the oxopropyl group of the inhibitor is present as an oxygen anion rather than a hydroxyl group, supporting the role of the “oxyanion hole” in stabilizing the tetrahedral intermediate in catalysis.
We developed single-bent and double-bent graphite X-ray condenser devices by subjecting multiple layers of polyimide films to high-temperature graphitization. The mosaic spread of the diffraction lines of graphite using this condenser device was in the order of 0.4 degrees, which enabled X-ray condensation by up to a factor of about 240 times for a toroidal type condenser device.