Crystallographic resources

European Synchrotron Radiation Facility

In the first non-resonant magnetic X-ray diffraction experiments conducted at the European Synchrotron Radiation Facility by R. Bateson (ESRF) and S. T. Bramwell (Inst. Laue Langevin), magnetic charge interference scattering from a single crystal of iron was measured. Significant enhancement of the magnetic interference signal was observed and the crystal appeared to scatter dynamically. Also at ESRF, recent high pressure small angle X-ray scattering studies of biological  macromolecules, included examination of solid polymers and a solution of aspartase transcarbamylase held in a specially designed high pressure cell.

Recent experiments on the surface diffraction beamline include studies of the atomic structure of a semiconductor face and of a chemisorbed system. If Si or Ge crystals are cooled, their 001 face exhibits a peculiar surface structure, a periodic arrangement of branched dimers with alternated configuration of the tilt angles in agreement with predictions made 16 years ago by D. J. Chadi. The study of CO adsorbed on the surface of N, (110) done in collaboration with external users in the lab ofI. K. Robinson, revealed that the most intense reflections arising exclusively from the CO layer had peak intensities of around 2000 counts per second and no evidence of damage to the Co layer by X-rays was detected.

Diffraction Anomalous Fine Structure (DAFS) experiments combine the element selectivity of XAFS and the site selectivity of X-ray diffraction. By measuring energy dependent {anomalous) intensity variations of diffraction peaks while scanning an absorption edge information about the electronic structure and local atomic environment can be obtained that is comparable to that obtained by XAFS techniques.

The production of coherent hard X-ray beams several orders of magnitude more intense than previously available are facilitating the use of coherent X-rays in the study of disordered systems. When coherent light is scattered from a disordered system, it gives rise to a speckle pattern. The speckle pattern is related to the spatial arrangement of the disorder and its changes with time are a measure of dynamics in systems such as synthetic multilayers.

High energy X-ray diffraction can permit simultaneous measurement of texture and grain morphology in alloys via analysis of Debye rings recorded with a 2D detector. Analysis of Debye rings identified the n three-dimensional position of particular grains in a sample, details of lattice strain and internal stress and the crystallographic texture of the material. These properties can be altered by thermomechanical treatment, and monitored by diffraction of high energy X-rays in order to optimize the properties in the final product.

Software Sought

Assistance is requested in the assembly of a crystallographic library of public domain shareware and demonstration software for PC (running under MS-DOS or MS-Windows) in Cuba. Presently, internet links to the outside world are not of high enough quality to allow the retrieval of crystallographic software over the global scientific research networks. A list of software presently in the Cuban crystallography software library is located at: http://www.unige.ch/crystal/stxnews/cuba/library/software.htm.

If you can contribute public domain, shareware and demonstration software to the library, please send programs on 3.5" (1.44 Meg) DOS formatted floppy disks to: A. Gomez, X-ray Diffraction Lab., Chemistry Division, CNIC (National Center for Scientific Research), Ave 25 y 158, Rpto Cubanacan, Playa, PO Box 6990, Ciudad de La Habana (Havana), Cuba; Tel.: (53-7)21-01-35, FAX: (53-7)21-01-35 (not reliable), e-mail: xray@infomed.sld.cu (contact Ariel in advance to avoid duplication).

If colleagues know of crystallography software available on the internet and not mentioned on the above WWW page, contact L. M. D. Cranswick, e-mail: lachlan@melbpc.org.au.

Errors in Protein Structures

A search for anomalies in 3,442 structures from the Brookhaven Protein Data Bank (PDB) using the program WHAT_CHECK was conducted by R. W. W. Hooft, Gert Vriend (EMBL Heidelberg, Germany), C. Sander (EMBL-EBI, Cambridge, UK), and E. E. Abola (Brookhaven PDB, New York, USA).

The analyses revealed a larger number of 4σ deviations than one would expect, wrong atom names, and a surprising number of errors such as bond angles that are more than 10° off, tryptophans with a 90° angle between the two rings, and proteins that are solved in the wrong spacegroup.

For purposes of detailed biological studies of large surveys, it is important that these irregularities be identified. T ogether with European colleagues working in this field the PDB is making validation tools part of its production process. The BIOTECH structure validation server provides structure verification tools via the World Wide Web. WHAT-CHECK is part of this server. Hooft et al. hope that structure factor deposition will soon be as much an accepted practice as coordinate deposition, so that in the future more problems can be detected and corrected. WWW addresses are: http://biolech.emlbl-heidelberg.de:8400/, http://biotech.embl.ebi.ac.uk:8400/ and http://biotech.pdb.bnl.gov:8400/.

Ionic Crystals

John Bollinger and Dave Wineland of the National Inst. of Standards and Technology (NIST) in Colorado, have used Bragg Scattering of visible light to study the internal structure of ionic crystals. Ionic crystals are created by magnetic and electrical fields that are used to trap ions that are then laser cooled to near absolute zero. Cooling causes them to take up a configuration of lowest possible energy similar to a crystallized solid. William Phillips of NIST in Maryland and Theodor Hansch of the U. of Munich have used Bragg scattering to establish the long range periodicity of optical lattices in which laser cooled atoms are trapped in the nodes of periodic interference patterns.

Parasites

The results of X-ray microscopic images of individual human blood cells containing malarial parasites produced at the Lawrence Berkeley National Laboratory by Cathie Magowan revealed membrane structure and organelles in the parasites.

Desktop Crystallography

Oxford Cryosystems announces the launch of CRYSTALLOGRAPHICA, a new software toolkit for research and teaching involving crystallography. It has a specially extended Pascal interpreter, which gives it flexibility and makes it useful for non-standard research problems. Its flexibility, friendly user interface and built-in crystallographic knowledge makes CRYSTALLOGRAPHICA a fine teaching aid. CRYSTALLOGRAPHICA runs on IBM PC compatible machines running Windows 3.1 or 95. Its main features are: an extensive library of crystallographic routines, crystal structure drawing, powder diffraction pattern simulation, powerful (x,y) graphing, databases of physical properties of elements, import/export of Crystallographic Information Files, and comprehensive on-line help including a tutorial guide. A down-loadable demonstration version is available at http://www.demon.co.uk/oxcryo/cgraph.html.

Agrochemical Design

Molecular Structure Corp. (MSC) of the Woodlands, TX, has signed a multi-year agreement with DowElanco of Indianapolis, IN, for structure-based design of new agrochemicals which are effective at lower concentrations and environmentally safe. MSC, a subsidiary of Rigaku/USA, was the first company to offer commercial X-ray crystallographic structure services beginning in 1973. DowElanco, a joint venture of chemical andpharmaceutical giants Dow Chemical Company and Eli Lilly and Company, is one of the world's leading producers of agrochemicals.