Endohedral metallofullerenes are novel forms of fullerene-related materials, which encage metal atom inside a carbon cage. The recent X-ray structural studies of metallofullerenes, Sc@C82 and Sc2@C84, by the synchrotron powder diffraction experiment are presented to illustrate the usefulness of the MEM/Rietveld analysis, which is a self-consistent iterative way in combination with the MEM (Maximum Entropy Method) and Rietveld analyses. The obtained MEM charge densities clearly reveal the distinct features of encapsulated metals inside the fullerene cages as well as endohedral cage structures for Sc@C82 and Sc2@C84.
Crystal structures of turkey and human lysozymes were refined at atomic resolution by full-matrix least-squares method with anisotropic temperature factors. The refinement converged at the R-value of 0.104 for turkey lysozyme and 0.115 for human lysozyme at 1.12 Å and 1.15 Å resolution, respectively, and the estimated r.m.s. coordinate errors for respective pro-teins were 0.031 Å and 0.034 Å. The magnitude and the degree of anisotropy of the atomic thermal motion have strong positive correlation with the square of distance from the molecular centroid. The statistical analysis suggested that such characteristics of anisotropic thermal motion are ascribed to the rigid-body rotation and local motions rather than the breathing motion.
The crystal structure of Eschelichia coli asparagine synthetase has been determined by X-ray diffraction analysis. The overall structure of the enzyme is remarkably similar to that of the catalytic domain of yeast aspartyl-tRNA synthetase despite low sequence similarity (11%) . These enzymes have a commom reaction mechanism that implies the formation of an aminoacyl-adenylate intermediate. The active site architecture and most of the catalytic residues are also conserved in both enzymes. These proteins have probably evolved from a common ancester even though their sequence similarities are small.
We review preparations and crystal structures of various kinds of new niobium oxides. Ilmenite-type ANbO3 (A=Li, Na), which were prepared by low temperature hydrothermal reaction and ion-exchange reaction, are the first examples of ilmenite-type niobium oxides. These niobium oxides are metastable phases and transform into the stable phases at elavated temperatures. A new reduced niobium oxide, Rb1.51Nb10O17 has a Nb6O12 cluster characteristic of strongly reduced niobium oxides. Weakly reduced niobium oxides, K2YNb5O15-δ and K6Nb10.9O30 have NaNb6O15F-type and tetragonal tungsten bronze type strcutures, respective-ly. In the A2O-MO- Nb2O5 (A=K, Rb, Cs; M=Mg, Fe) system, two types of compounds K2M2Nb4O13 and A (M, Nb) 2O5, which are isostructural with K2Ti6O13 and KTiNbO5 respectively, were obtained.
We have synthesized a twelvefold quasicrystal Ta62Te38 by non-equilibrium solid state reactions. A transmission electron microscopy study reveals that the so-called twelvefold quasicrystal Ta62Te38 is a crystal subjected to the structure modulation. It is composed of two incommensurate layers rotated by 30° (or 90°) to each other about their normal. High resolution electron microscope images show the formation of commensurate domains and their discommensuration. The modulation is considered to be due to the rearrangement of atomic vacancies as a response to the occurrence of charge density waves.
Rare earth monophosphides REP (RE=La, Ce, Pr, Nd, Sm, Gd, Tb, Tm and Yb) crystallize in a NaCl-type structure at ambient pressure. Using synchrotron radiation X-ray diffractions of REP have been studied up to about 60 GPa at room temperature. All phosphides are found to undergo structural phase transitions at high pressures. The high pressure phases of LaP, PrP and NdP can be assigned to be a tetragonal structure, which can be seen as the distorted CsCl-type structure. The pressure-induced phase transitions of SmP, GdP, TbP, TmP and YbP occur at around 35, 40, 38, 53 and 51 GPa, respectively. The structure of the high pressure phases is unknown. X-ray diffraction patterns of the compounds with many f-elecrons become more complex at high pressure. It is expected that 4f-electrons in rare earth atoms influence the structure of the high pressure phases.