issue contents

ISSN: 2053-2733

March 2007 issue

Highlighted illustration

Cover illustration: The molecular form of the guanosine-5'-phosphate tetramer [Zimmerman (1976). J. Mol. Biol. 106, 663-672] has vertices at points of a cubic lattice with parameter a defined by the distance of O6 from the centre. The height is H = 2a and the basis a square scaled by a factor 6 from the central hole. Another square, scaled by 3[square root]2 and turned by 45°, separates phosphates and bases. The indices of the molecular forms are indicated. Courtesy of A. Janner.

lead articles

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Techniques for coherent X-ray scattering measurements are detailed. Applications in the study of the dynamics of fluctuations and in lensless high-resolution imaging are described.

research papers

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A library describing the precise charge density of all chemical groups present in proteins has been developed from the charge-density analysis of several peptide crystals.

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A more economic distortion-free FFT sampling of two-dimensional sections of functions defined on the rotation domain is presented.

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A new phase refinement procedure maximizing the origin-free modulus sum function with the FFT algorithm is described so that no explicit use is made of the triple phase relationships. It is general for all space groups and especially effective for low-symmetry ones.

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Theoretical structure factors for a range of small molecules are calculated with and without dynamic electron correlation, to establish the size of these effects, and if they can be detected in experimental measurements.

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The Gumbel–Fisher–Tippett (GFT) extreme-value analysis is applied to evaluate the distribution, expectation value and standard deviation of the intensity Ja of the strongest reflection in a resolution shell.

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It is demonstrated that the dual-space charge flipping algorithm is able to solve structures using neutron diffraction data alone. For this purpose, its band flipping variant was introduced which reverses the sign of scattering density only in a zero-centred band. While band flipping is absolutely necessary for developing negative densities, it is the `basic + band' combination of algorithm variants that works most efficiently.

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A reliable integer minimal principle model for phasing in all centrosymmetric space groups is presented. It dramatically reduces the number of cycles required to phase a selection of structures in the SnB framework.

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In icosahedral carbon nanoparticles, the diamond-like core can undergo a reversible topological transition into and coexist coherently with the onion shells.

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Hybrid quasiperiodic-periodic structures (octagonal, dodecagonal and pentagonal/decagonal) are constructed by cut and projection in two stages.

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Tangled cubes contain cycles of edges that form links and knots, yet retain the topology of the edge-net of the cube. The simplest tangled cubes are derived, constrained to have cycles of maximal length eight. Five left-handed and right-handed pairs result. All these examples wrap on the genus-one torus, forming a {6,3} tiling with four hexagonal faces in the torus.

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