issue contents

ISSN: 2053-2733

July 2008 issue

Highlighted illustration

Cover illustration: An octagonal mosaic from the Kasbah de Telouet, High Atlas, Morocco, based on a modified quasilattice. The underlying quasilattice can best be seen in the blue-and-black portions of the pattern where the `standing' lozenges define bars of unit width and the diagonally positioned lozenges define bars with a width equal to [square root]2. Four orientations of the bar scheme, 45° apart, and phason shifts (alternation of bars in the sequence) are present. Picture taken by Emil Makovicky during the 24th European Crystallographic Meeting 2007.

research papers

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The quantum-mechanical and crystallographic framework and details of the X-ray atomic orbital analysis including the basis functions, the forms of atomic orbitals in the crystal fields of all the point-group symmetries, limitations and the advantages of the method are introduced.

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The upgraded Schrödinger-equation algorithm is tested for many aspects of convergence, an essential prerequisite for practical applications. Structure factors and close to ab initio structure determination for three crystal structures (11, 41 and 110 non-H atoms in the asymmetric unit) are finally obtained by combining the Schrödinger-equation constraint and a least-squares procedure that minimizes the Rmoduli factor.

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The Neumann principle is used to derive the restrictions on the phason and phonon–phason coupling parts of the elastic tensor for quasicrystals with crystallographic point groups.

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Recent procedures for estimating anisotropic displacement parameters for H atoms are shown to be capable of giving excellent results for several benchmark systems. A revised and extended library of internal H-atom mean-square displacements is derived for use with Madsen's SHADE web server, resulting in improved agreement with benchmark results. The SHADE web server is recommended as a routine procedure for deriving estimates of H-atom anisotropic displacement parameters suitable for use in charge-density studies on molecular crystals.

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It is shown that the average intensity and mean-square intensity difference of Friedel opposites are the same for all the non-centrosymmetric space groups, under certain assumptions. The corresponding intensity average multiples are derived and exemplified for general as well as for special reflections.

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Estimates of the standard uncertainty of the Flack parameter are calculable from a knowledge of chemical composition and X-ray wavelength.

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The architecture of four protein cages (bacterio ferritin, human mitochondrial ferritin, sulfur oxygenase reductase and small heat-shock protein) are compared top-to-bottom, starting from polyhedra with vertices at cubic lattice points enclosing the cage down to indexed polyhedral forms of single monomers.

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The structural elements of four octahedral holoenzymes (bacterio and mitochondrial ferritins, small heat-shock protein and sulfur oxygenase reductase) are considered in a bottom-to-top approach. The vertices of the monomeric enclosing forms at points of a cubic lattice, ordered according to the primary structure, appear to delimit the secondary structural elements and play a role in the ternary structure. Connectivity models of these indexed forms suggest possible self-assembly processes in the formation of the quaternary structure.

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