British Crystallographic Association 2001 Spring meetingBragg Lecture. The first Bragg lecture was given by P.P. Ewald in 1962, the centenary of the birth of W. Lawrence Bragg’s father, W.H. Bragg and the jubilee of the discovery of X-ray diffraction by crystals. The presentation of the 2001 Bragg lecture by David Stuart in the midst of the worst Foot and Mouth Disease outbreak since 1967 was remarkably timely. Determination of the structure of Foot and Mouth disease virus was published by Stuart in 1989. The structure of the virus was beautiful, but initially did not reveal the details of a surface loop, crucial for immune recognition of the virus. Further work including cryo-electron microscopy confirmed that the loop has multiple conformations, which could potentially be harnessed in vaccine design.
Stuart’s lecture illustrated how far crystallography has travelled since Lawrence Bragg described a method to determine the structure of NaCl (28 Da) in 1912. Bragg can little have imagined that within a century it would be possible to use the same methods to visualize the atomic structure of a virus particle of 108 Daltons in the act of transcription.
Other Awards. The Alun Bowen Industrial Lecture, 'Crystallography in the Aerospace industry', was presented by C. Small (Rolls Royce, Derby). When a titanium fan blade failed a bird test despite no apparent structural changes, the texture was checked and a pole figure revealed dramatic differences between the texture of the failed blade and that of a normal blade. A particular forging process had caused the texture change. The lecture highlighted just how important it is for a crystallographer to be able to translate his/her analytical results into meaningful information for the engineer.
C. Wilson, U. of Nottingham, received the Cambridge Crystallographic Data Centre Prize for a series of variable temperature single crystal X-ray and neutron studies of di-copper complexes. The key structural change was a 3° variation in the angle between the magnetic planes over a 35 to 295 K temperature range. The David Blow poster prize (a piece of Blue John crystal) was presented to A. Carter from the LMB, Cambridge for his poster 'Crystal Structure of an Initiation Factor Bound to the 30S Ribosomal Subunit'. The 2001 Philips Award was given to J. Kreisel of ENS de Physique de Grenoble, for his work on phase transitions in perovskite and related phases. Poster Prizes were awarded to P. McGregor (Edinburgh) for 'The High Pressure Crystal Structure of Ethylene Glycol'; M. Gutmann (ISIS, RAL) for 'Charge Inhomogeneities in La2-xSrxCuO4 Evidenced from Pulsed Neutron Diffraction'; and H. Thompson (Heriot-Watt) for 'The Dehydration of α-Lactose Monohydrate'.
P. Rizkallah (Daresbury) described using longer wavelength radiation from second generation sources to collect anomalous signals from xenon and sulphur. The xenons were located with Shake’n’Bake, and the resulting phases used to locate the sulphur atoms. T. Schneider (Göttingen) described the latest software for solving the anomalous atom substructures, focusing on SHELXD and SnB (Shake’n’Bake) with which substructures of up to 60 Se atoms can be solved routinely. Z. Dauter discussed soaking crystals in high concentrations of 1M NaBr or 0.5M KI to incorporate large numbers of bromine or iodine atoms. Bromine substitution provided the best MAD data while iodine was best suited for SIRAS or SAD phasing. J. Löwe (LMB, Cambridge) illustrated that MAD can be successful even when the crystals diffract poorly (3.1 Å), have a high mosaicity and the resulting data have a high Rmerge (10%). In spite of these problems, a 3 wavelength Se MAD data set collected using inverse beam geometry on ID29 at ESRF gave a readily interpretable map in which sites were correctly located with SnB.
Atomic Resolution. E. Garman (Oxford) explained how data collection to resolutions around 1.0 Å allows refinement of atomic anisotropic displacement parameters, dual conformations of disordered residues, hydrogen atom positions and much improved water structure.
K. Wilson (York), described the computer program ACORN (Jia-Xing Yao) for ab initio protein structure determination using atomic resolution data starting with seed structures derived either from anomalous data or by molecular replacement. A dynamic density modification algorithm obviates the need for manual rebuilding of the molecule and has been successful in solving a number of structures where other direct methods packages failed.
Detection of electron density for catalytic hydrogen atoms on an active site histidine of human aldose reductase was reported from 0.66 Å data (A. Podjarny, Strasbourg).
The 0.8 Å resolution studies of pentaerythritol tetranitrate reductase uncovered significant errors in a previous 1.5 Å structure report (P. Moody, Leicester).
Hot and Cold Structures. J.A.K. Howard (Durham) described low temperature studies of electron densities, phase transitions and unstable samples. Below 100K, disorders can often be resolved, and different ordering at low temperatures can manifest itself in phase transitions. Howard described the variety of devices available at neutron sources including a high pressure device that uses nitrogen gas at 5 kBar.
R. Morris (St. Andrews) described 'Unusual Properties of Zeolites Studied by Single Crystal Synchrotron X-ray Diffraction'. Using 20 x 20 x 5 micron crystals, the structure determination of STT-SSZ-23 showed that it is essentially pure silica, with 9 atoms in one ring and 7 in the other, rather than the even-numbered rings expected.
P. Coppens (SUNY, Buffalo) combined crystallography, photochemistry, and laser irradiation and diffraction in the study of 'Light Induced Metastable States and Transient Species'. At liquid helium temperatures metastable species have essentially infinite lifetimes, and different species can be isolated by changing the wavelength of the laser or the temperature, and the population of excited states can be raised to a pseudo steady state.
S. Parsons (Edinburgh) described low temperature and high pressure crystallization of samples which are gases or liquids at ambient temperatures. Crystal growth is achieved by establishing a stable solid-liquid equilibrium in a capillary, and cooling slowly to induce crystal growth.
Diffraction at the Edge. G. Rosair (Heriot-Watt) outlined how absorption edges contain information on the local electronic and magnetic structures of crystalline, amorphous or liquid materials. The use of EXAFS and XANES to elucidate the number and type of atoms surrounding metals was expanded. Examples of studies of excited states, covalency in metal-ligand bonds, and the enhancement of the catalytic activity of Pt by Ge were presented. M. Helliwell (Manchester) described applications of anomalous dispersion in chemical crystallography and the essential features of phosphate-based molecular sieves. (Sandy Blake)
Why Industry Uses Crystallography. R. Copley (GlaxoSmithKline Pharmaceutical, Harlow, Essex) described how single crystal studies are used at all stages of the Drug Discovery Process (molecular conformation and absolute configuration studies of proteins, drugs and their complexes) in conjunction with powder diffraction, solid state NMR, IR and Raman spectroscopy.
X-ray diffraction analysis being used in the minerals industry from extraction, separation, blending, storage and transport was described by N. Elton (Exeter Advanced Technologies). Typical challenges to quantitative XRD analysis are: (a) complex mixtures, (b) solid solution polymorphs, (c) poor crystallinity, (d) low concentration, and (e) preferred orientation. In the cement industry these problems are compounded by lack of standards and peak overlaps in XRD patterns. Some European plants are using on-line XRD for clinker analysis. In the aluminum industry too, bauxite processing suffers from lack of good standards, variable crystallinity and preferred orientation, requiring the XRD/XRF mass balance approach. (David Rendle)
Future Powder Diffraction. Recent developments in the Powder Diffraction File (PDF) (J. Faber, ICDD), Line Broadening Analysis (R. Delhez, Delft U.), and Rietveld Refinement including dynamic studies and protein application were reported (J. Cockcroft, Birkbeck College). Technical presentations from Philips and Bruker AXS wound up the session. (Stephen J. Maginn)
Coherence Workshop. D. Laundy (CCLRC Daresbury) defined lateral and temporal coherence and described how both could be measured via synchrotron radiation. B. Tanner (Durham) discussed measuring coherence length using grazing incident surface scattering to determine near surface density. For the technique to be successful the coherence level needs to be larger than the grain size of the sample. Because the ‘wobble’ of the synchrotron electron beam decreases, the coherence of the x-ray beam experiments that are carried out over large time scales have a reduced coherence length. M. Hart stated that coherence length is a function of sample, set-up, and apparatus, and recommended Optical Physics by Lipson & Lipson as the best reference.
Basic Powder Diffraction Workshop. The Basic Powder Diffraction Workshop consisted of three half-day sessions for people new to the field of powder diffraction. The sessions included a well-balanced introduction to the principles of crystallography, powder sample preparation, and sample identification. (Jo Jutson)
CRYSTALS Workshop. A CRYSTALS workshop was held at which R. Cooper explained the design aims behind the current release and gave a real-time demonstration of a structure analysis. Software aims to provide tools which will enable a synthetic chemist to make a reasonable attempt at performing a structure analysis on data that they have either collected themselves, or which has been provided for them. D. Watkin presented an overview of some of the crystallographic model building, manipulation, and analysis tools in CRYSTALS. Horst Puschman gave a demonstration of a structure archiving and tracking suite that assists in record keeping (Richard Cooper)