The first order phase transitions of nucleoside and nucleotide hydrates are governed by relative humidity. Transformations associated with the reconstruction of hydrogen-bonding networks and stabilization of electrostatic interactions are induced by increase or decrease of the number of crystal water molecules. The transitions are characterized by hysteresis and presence of metastable states.
Crystal structures and electron density distributions of some manganese and cobalt oxide materials have been studied by the maximum entropy method using single-crystal X-ray diffraction data measured at room temperature. The nature of chemical bonding in these oxides is understood from the present structural and electron density analyses. The features of electron density distribution obtained theoretically by FLAPW calculations for these oxides were in good agreement with the present experimental observations.
The Rad52 protein and its homologs play critical roles in eukaryotic homologous recombination. In the present study, we have determined the crystal structure of the N-terminal, homologouspairing domain of Rad52. The structure revealed an undecameric ring that has an exposed groove essential for DNA binding. Amino acid residues inside the groove are directly involved in DNA binding as evidenced by mutational analyses.
New composite crystals with two structure types, monoclinic and orthorhombic forms, were found on pseudobinary systems InAO3- In2Ti2O7 (A : Fe, Ga, Al, Cr, Mn) . Conventional X-ray diffraction analyses for samples of A = Fe clarified that both structures are constructed by the alternate stacking of the InO6 octahedral sheet and the plane consisting of metal (Fe, Ti) and oxygen atoms, although the stacking modes are different between the two structures. Furthermore, the modulated structure of the orthorhombic form was refined on the basis of a 4-dimensional superspace group.
New synthetic halogen-bridged mixed-valence diplatinum (II, III) compounds with linear chain structures, Pt2 (EtCS2) 4I (1) and Pt2 (n-BuCS2) 4I (2), show high electrical conductivity at room temperature and undergo metal-semiconductor transitions at TM-S = 205 K for (1) and 325 K for (2) . It has been revealed by the X-ray diffraction studies for (1) at SPring-8 that diffuse scattering with line shapes corresponding to 2-D periodical valence ordering are observed above TM-S and converted to diffuse planes near TM-S, and finally to superlattice reflections below 130 K. Alternate valence ordering structures -Pt2+-Pt3+-I-Pt3+-Pt2+-I- with lattice distortion have been determined by the crystal structure analyses for (1) at 48 K including superlattice reflections and for (2) at 167 K. The lattice parameter of (1) along the chain direction exhibit significant temperature dependence with drastic change in slope at 205-240 K. In variable-temperature crystal structure analyses for (1), a steep increase in U22 of the bridging I atom near TM-S is observed. These results suggest remarkable thermal vibration with large amplitude for the I atom at room temperature phase, which should be coupled with dynamic valence ordering modes.
A layered sodium cobalt oxide with a composition of NaxCoO2·yH2O (x≈0.35 and y≈1.3) was synthesized from Na0.7CoO2 via a soft chemical route, i.e., partial deintercalation of Na+ ions and intercalation of H2O molecules. Its structural analysis showed that its adjacent CoO2 layers were separated by a thick insulating layer consisting of Na+ ions and H2O molecules. Its magnetization and resistivity data provided clear evidence that the material is the first superconductor in cobalt oxide systems with a superconducting transition temperature of 5 K.