XXII IUCr Congress and General Assembly
Madrid, Spain, August 22-30, 2011
The congress brought together nearly 2800 people (1880 standard participants, 360 students, 350 grantees, 65 exhibitors and 113 accompanying persons). 73 countries were represented by 2040 abstracts that were distributed across 98 Microsymposia with 490 oral and 1550 poster presentations. The following reports are grouped by subject matter.
Polymer and gel structure: arriving at a structural model (MS03)
Chairs: Parthasarathi Dastidar, B. Escuder Gil
This session focused on understanding the (macro)molecular organization found in soft matter - polymers and molecular gels. I. Hamley (U. of Reading, UK) reported on self-assembly studies of peptides related to amyloids, peptide sequences believed to be involved in neurodegenerative diseases such as Alzheimers. S. Raghavan (U. of Maryland, USA) focused on the structural differences between crystalline and amorphous molecular gels and made an analogy with related self-assembling biological systems. J. van Esch (Technical U. of Delft, The Netherlands) reported on controlling soft-matter structure by mixing gelators able to self-assemble orthogonally and on the study of self-assembly in non-equilibrium systems. G. Lloyd (Cambridge U., UK) discussed the relationship between crystallization and gel formation and the use of molecular gels as media for the crystallization of different polymorphs of drugs. The session ended with B. Bagautdinov (SPring-8, Japan) reporting on structural studies of polyacetylenes.
Crystal structure prediction: practice and applications (MS06)
Chairs: J. Kendrick, S. L. Price
A. Oganov showed how searching for potential structures of simple elements has predicted new high-pressure phases of elements, emphasising the importance of the search algorithm. The microsymposium then shifted to organic molecules, particularly the results of the 2010 blind test of crystal structure prediction, a joint collaborative test of advances in methodology organised by the CCDC. The results of the test were reported by A. Cruz-Cabeza (Málaga U., Spain). Prediction of the structures of some fairly simple small molecules had been successfully demonstrated previously, but this test included more challenging systems: a molecular salt, a large flexible pharmaceutical-like molecule and a novel polymorph of gallic acid monohydrate. At least one correct prediction had been made for each structure, although no method was universally successful. This could be attributed to the problem of accurately evaluating the relative energies. J. van de Streek (Avant-garde Materials Simulation, France) presented post-blind test work on developing and evaluating improved dispersion-corrected density functional periodic quantum mechanical calculations. It has become clear that many organic molecules have a large number of crystal structures that are very close in energy. M. Habgood (University College London, UK) described a method for evaluating structures likely to have static disorder due to configurational entropy outweighing the variation in lattice energy, and presented criteria for assessing this from an examination of the near-symmetry of intermolecular interactions from the low-energy calculated structures. Finally, R. de Gelder (Radboud U., The Netherlands) discussed a method of picking out which crystal structure might approximate that which corresponded to a powder X-ray diffraction diagram. The microsymposium emphasised the advances which have taken place in searching and energy evaluation, and demonstrated applications of the methodology, but still showed the challenges remaining in understanding the kinetics of crystallisation sufficiently to determine which new solid phases might be found.
Instrumentation, data collection, processing, analysis, databases
Dynamical structural science (MS45)
Chairs: Pance Naumov, Eric Collet
Recent advances in pulsed X-ray and electron sources make it possible to track structural reorganization in real time, with a resolution of 100 femtoseconds. Optical pumps coupled to X-ray or electron probe experiments allow the tracking of atomic or molecular motions on their intrinsic time-scale, i.e. the phonon frequency. Dynamical structural science was the subject of a special issue of Acta Cryst. (Vol. A66, March 2010). In this symposium, state-of-the-art dynamical structural science was presented, dealing with development of X-FEL (D. Fritz, SLAC Nat'l Accelerator Lab., USA) and new electron guns (D. Miller, U. of Hamburg, Germany). New applications of synchrotron pulsed sources to study dynamics of excited states of molecular solids (P. Coppens, SUNY Buffalo, USA), biomolecular systems (S. Techert, GWDG, Germany) and of molecules in solution (S. Nozawa, KEK, Japan) were described.
Data reduction of area-detector measurements (MS68)
Chairs: G. J. Mcintyre, E. Litvinenko
The focus of talks in the symposia included strategies of data collection with position-sensitive area detectors, methods for correction of distortion and inhomogeneity, subtraction of background, visualization of data containing both Bragg reflections on discrete grids and diffuse scattering, acceleration of and higher accuracy in the extraction of integrated intensities, and application of area detectors to fibre diffraction, texture measurement, and powder diffraction.
D. Chateigner (ENSICAEN, France) gave a lively lecture about the qualitative advances that area detectors have brought to quantitative texture analysis with X-rays and neutrons. Most of the other talks, and the four poster presentations, were devoted to reduction of discrete Bragg data, including Laue diffraction at both reactor and spallation neutron sources, simultaneous measurement of magnetic flipping ratios using area detectors, the latest developments at J-PARC, and methods to get the best from the newest CMOS detectors. Of further note was the talk by R. Piltz (ANSTO, Australia), who described improved methods of reduction of neutron Laue data from the instruments KOALA (ANSTO) and VIVALDI (ILL), which use image-plate detectors, to give significantly better quality in structure refinement.
Automated data processing and structural solution for powder crystallography (MS95)
Chairs: Rosanna Rizzi and Anton Meden
Modern X-ray powder diffractometers have the potential to collect data sets in a matter of minutes. Beamlines are equipped with robots and all aspects of data collection and analysis can be executed in a completely automatic way. B. Toby (Argonne Nat'l Lab., USA) explained how, at the Argonne Advanced Photon Source, they achieve high-throughput remote access by a large number of users via automation of all the aspects of the workflow (i.e. requests for sample-mounting kits, programming of the data-collection parameters, post-collection data reduction, and sample storage). This uniform sample management is linked to computer programs for data analysis that carry out, automatically and quickly, the complete structure determination process. A. Altomare (CNR, Italy) discussed new algorithms introduced in the EXPO program for automatic and efficient ab initio crystal structure determination. L. McCusker (ETH Zurich, Switzerland) illustrated the application of a charge-flipping algorithm to determine complex zeolite structures. The combination of the algorithm with HRTEM and PED techniques was also explained. R. Oishi-Tomiyasu (Inst. of Materials Structure Science, Japan) discussed the main functions of the Conograph program for primitive unit-cell determination, lattice symmetry determination and parameter refinement. M. Milanesio (U. del Piemonte Orientale, Italy) showed how structural information on a subset of atoms can be selectively obtained when the sample is subject to a periodically varying external stimulus (temperature, pressure, concentration etc.).
Chirality in the solid state, from organic molecules to chiral MOF and helical materials (MS87)
Chairs: Susan Bourne and Berta Gomez Lor
The significant role played by chirality in chemistry and biology was highlighted in talks that ran the gamut from inorganic coordination complexes to large and small supramolecular systems, utilizing a range of bonding and intermolecular intractions.
M. Miyata (U. of Osaka, Japan) began the session with a somewhat provocative suggestion that it is possible to assign handedness to all helices. After demonstrating the assignment of handedness to two-fold helices by invoking a tilt-chirality, he illustrated his talk with examples of chinchona alkaloid structures that commonly crystallize in either P21 or P212121. After assigning the helices as either right- or left-handed, he suggested a modification be made to their space group representations. The suggestion generated animated discussion from the floor.
I. Justyniak (Polish Acad. of Sciences, Poland) used the chinchona alkaloids, specifically chinchonine and chinchonidine, as ditopic ligands in coordination polymers. Using these ligands to connect Aluminium ions gave rise to homochiral helical channel compounds, with either coordination or supramolecular interactions holding the helices in place. The channels have a chiral nature and can be used to effect enantiomeric separation of liquid guests. They are also able to absorb a range of gases under relatively mild conditions.
N. Báthori (Cape Peninsula U. of Tech., South Africa) uses organic host compounds with imperfect selectivity for guests to probe the mechanism by which chiral discrimination can be effected through crystallization. Combining NMR or HPLC measurements with crystal structure analysis in compounds with mixed guests, she elucidated principles guiding the inclusion of one enantiomer over another. Hirshfeld surface analysis applied to the supramolecular units proved particularly valuable in probing the intermolecular interactions at play in related enantiomeric structures and allowed her to draw some conclusions about the driving forces in the separation crystallization of these compounds.
G. Terraneo (Politecnico de Milano, Italy) has used halogen bonding as an intermolecular interaction to produce chiral compounds from achiral moieties. After providing an overview of halogen bonding, a hot topic in crystal engineering, he presented results for a range of cocrystals produced with long-chain diiodoperfluorocarbons (PFCs). The CSD contains only 45 structures of PFCs with 8 or more carbons, while the cocrystals reported in this talk utilise I(CF2)12I as coformer, taking these crystals to the crystalline/liquid interface. Through these examples Terraneo illustrated how the halogen bond can be used to overcome the low affinity of PFCs and hydrocarbons and, furthermore, how long-chain PFCs without chiral centers can be induced to crystallize in chiral space groups. S. Parsons (U. of Edinburgh, UK) presented some new approaches to refining the Flack parameter which gives a measure of the correct assignment of absolute configuration.
Automation of data collection and remote control of experiments (MS13)
Chairs: Olof Svensson, Masaki Yamamoto
This microsymposium focused on how automation and remote access can enhance user-friendliness and efficient use of often limited beamtime available at synchrotrons. After an introduction by M. Yamamoto, A. Popov presented the current status of the BEST program. The presentations given by A. Gonzales (SSRL, USA), K. Hasegawa (Spring8, Japan) and S. Wasserman (APS, USA) described their use of BEST and gave an overview of the state-of-the-art of automation and remote data collection on MX beamlines at three major synchrotron radiation sites. In the last talk of the session R. Cooper presented the theory and implementation of improving experimental results by selectively re-measuring the most relevant reflections for the particular study.
Developments and directions for crystallographic databases (MS27)
Chairs: S. Gražulis, J. Westbrook
Crystallographic data resources face a variety of challenges. The availability of high-bandwidth internet connectivity offers new possibilities for data access and sharing; at the same time crystallographers expect deeper annotation and more robust data quality indicators from modern internet-based resources. These challenges were discussed by the three invited speakers representing the Crystallography Open Database (COD), the Cambridge Crystallographic Data Center (CCDC) and the Worldwide Protein Data Bank (wwPDB).
Maintaining high-quality data resources inevitably requires careful and detailed data curation to ensure database consistency, homogeneity and reliability. All of the speakers in this session reported how their resources address the technical, social and resource requirements associated with this data curation. The participants were also sensitive to the need to provide data consistency while faithfully preserving the author-provided primary data content. While the infrastructure for data curation has been well established in the PDB and CCDC archival resources, this is a area of active development for initiatives such as COD.
The availability of high-quality structural data also makes possible new approaches for data analysis, mining and prediction. New graphical query interfaces for inorganic data require advanced mathematical concepts to retrieve different types of structures efficiently, and application of fractal geometry may be necessary to analyze trends hidden in raw data files.
Electron diffraction and crystallography (MS79)
Chairs: P. N. H. Nakashima, C. T. Koch
The talks and posters in this session covered electron diffraction and imaging techniques for the solution of crystal structures (including structures with unit-cell volumes as large as 38.3 nm3) and characterization of nanomaterials. A prominent theme was the extension of electron diffraction beyond 2D patterns to explore the 3rd dimension. This was presented in several different flavors, including classical convergent-beam electron diffraction (CBED), precession electron diffraction (PED), automated diffraction tomography (ADT) and the very recently established large-angle rocking-beam electron diffraction (LARBED) and rotation methods.
Biological neutron scattering and deuteration (MS82)
Chairs: Trevor Forsyth, Joanna Krueger
This microsymposium focused on the advantages of using neutron scattering in combination with sophisticated deuteration approaches to enhance structural studies of macromolecular systems in crystals, solutions and partially ordered systems. The presentations included studies that have provided new information on the location of protons and structured water in proteins (information of central importance in optimising ligand interactions), solution scattering studies where contrast variation has been used to characterize different regions of a macromolecule in relation to function, and reflection work relevant to biological interfaces. A. Podjarny (IGBMC, France) described his crystal structure determination at 293 K of a perdeuterated type-III antifreeze protein (AFP). The water structure was clearly visible from the neutron diffraction study, demonstrating an interesting tetrahedral geometry that explains the specificity of these AFPs for the ice versus liquid structure of water. O. Byron (U. of Glasgow, UK) described a neutron scattering contrast-variation study of the pyruvate dehydrogenase (PDH) complex. Solution techniques have been used to model specific parts of the complex and SANS has been used to study the stoichiometry of subunit associations by titrating a specifically deuterated subunit (E2) with a second subunit E3BP. These results, used in combination with other biophysical techniques, resolved a conflict between current models of the PDC core and allowed clearer visualization of the subunit content and organization. M. Cuypers (Keele U., UK) discussed the difficulty of reliable X-ray data acquisition from the reduced (Fe2+) form of the perdeuterated rubredoxin given the effects of beam-induced oxidative degradation. This problem was resolved in a high-resolution neutron crystallographic analysis of both Fe2+ and Fe3+ forms of the protein. The study correlated redox properties with associated protonation shifts and detected H3O+ ions in the structure. L. Coates (ORNL, USA) described the protonation states and H-bonding network in the ground state active site of β-lactamase. T. Darwish (ANSTO, Australia) described collaborating with structural chemists to design deuterated compounds that increase the scope of neutron diffraction, scattering and reflectometry experiments.
Industrial applications of SAXS and SANS (MS04)
Chairs: Robert Knott, Aldo Craievich
M. Shibayama (Tokyo U., Japan) presented examples of the use of contrast-variation SANS. He demonstrated that catalyst inks consist of dendritic clusters of carbon nanoparticles surrounded by ionomers, with platinum located in the ionomer layers. This structure makes platinum more accessible and therefore more efficient.
J. Doucet (Novitom, France) described the use of micro-focused SAXS to determine the asymmetric distribution of proteins in hair fibers. Among other things, such information is valuable in developing products that make curly hair straight and straight hair curly. D. Londono (DuPont, USA) presented SAXS studies of the interaction between silica particles and polymers. A key finding was that, during nanocomposite formation, high molecular weight polymers freeze a monodispered population of silica particles to produce a well-dispersed material.
A strategy for developing novel materials-by-design was outlined by A. Allen (NIST, USA). He described how the contrast-variation SANS technique is used to determine the composition and density of calcium silicate hydrates in cured cement. H. Brand (CSIRO/ANSTO, Australia) presented a study of a series of jarosite compounds that form crystalline structures having an impact in mineral and environmental industries.
The posters included information on instrumentation development, structural investigations of polymers under shear, and thin films and lignins related to fuel production from biomass.
Hybrid methods: the EM-crystallography interface (MS02)
Chairs: Eva Nogales, Jorge Navaza
The structural characterization of large macromolecular assemblies often requires the use of multiple and complementary techniques in order to obtain the maximum mechanistic insight. Cryo-EM structures in the subnanometer regime, combined with crystallographic atomic models of components, can be a very powerful combination to mechanistically define essential biological processes. J. Johnson (Scripps Research Inst., USA) utilized X-ray crystallographic data, time-resolved SAXS data and cryo-EM viral structures obtained from coexisting populations of viruses to map the structural pathway or reorganization in the viral capsid during the pH-controlled maturation of the virus. While SAXS provided a read out in real time of the overall changes in the capsid volume, the cryo-EM studies described the viral populations that coexist under a finite number of defined conditions at subnanometer resolution. The latter could then be interpreted with pseudo-atomic detail using crystallographic information and molecular dynamics.
Cell signalling and protein-protein interactions (MS22)
Chairs: Michael Parker, Madhusoodan Hosur
This microsymposium focused on the role of PPI's in cell signalling. The talk given by S. Teichmann (MRC Lab, Cambridge, UK) described analysis of PPI's using bioinformatics tools. Her analysis revealed: 1) evolutionary constraints on surface residues forming sticky patches, and 2) the predictability of binding-induced conformational changes through relative accessible surface area calculations. The next two talks focused on the largest family of signal transducers in the cell: protein kinases. J.-W. Wu (Tsinghua U., China) presented her work on AMP kinase, highlighting the importance of obtaining structures that are as complete as possible. Discovery of activators of AMPK could lead to a 'fat' pill! C. Gee (U. of California, Berkeley, USA) described the structure and function of a pseudokinase. The final talks provided fascinating insights into molecular recognition: how the 'small' molecule vitamin B12 is taken up by cells (C. Anderson, Aarhus U., Denmark) and how 'larger' molecules are recognized by an atypical myosin (Y. Hirano, Nara Inst., Japan).
Viruses and viral proteins (MS29)
Chairs: Fasseli Coulibaly, Ying-Fang Liu
The microsymposium 'Viruses and Viral Proteins' featured presentations by M. J. van Raaij, M. Kvansakul and L. Castillo that represented the multiple aspects of structural virology with emphasis on understanding the assembly, evolution and virulence of viruses.
In addition, the structure of human adenovirus presented by Vijay Reddy stood out as an impressive feat allowing visualization of the intricate network of major and minor structural proteins in the virion. The cryoEM structure of adenovirus determined independently (MS02, Hong Zhou) provided a rare opportunity for meaningful comparison of data obtained by X-ray crystallography and cryoEM. A clear point of divergence was the assignment of a four-helix bundle on the capsid exterior to protein IIIa or protein IX, depending on the study.
Finally, Felix Rey introduced the structure of an EFF-1 homotypic fusion protein essential to organogenesis in C. elegans. The presentation ventured outside the virus world...only to show that these cellular proteins are homologous to viral class II fusion proteins. This groundbreaking discovery opens exciting avenues to understanding the function of fusion machineries from a totally new perspective. The ensuing discussion initiated by Jack Johnson and Michael Rossmann examined frameworks that could account for this unanticipated evolutionary link.
Understanding bacterial pathogenesis (MS36)
Chairs: Armando Albert and Dirk Heinz
This microsymposium highlighted the structural basis for infection and immunity. X. Wang (Tsinghua U., China) reported on the structure of the cytokine interleukin-1β (IL-1β) and its interactions with the IL-1 receptor type I (IL-1RI) and IL-1 receptor accessory protein (IL-1RAcP). Using X-ray crystallography, site-directed mutagenesis and surface plasmon resonance (SPR), he was able to decipher the mode of assembly of this cytokine-receptor complex. T. Stehle (U. of Tuebingen, Germany) discussed the high-resolution structure of a staphylococcal autolysin which provided detailed insights into the enzymatic function of this peptidoglycan hydrolase. Small-angle X-ray scattering (SAXS) experiments were used to analyze molecular flexibility and to provide a model for bacterial cell wall recognition.
H. J. Kang (U. of Auckland, New Zealand) presented new results on the structure, assembly and function of pilins from pathogenic gram-positive bacteria. Pilins polymerize on the bacterial surface to from pili, which are essential for bacterial conjugation and inter-species DNA exchange. Using structural biology and mass spectrometry he could identify unusual isopeptide bonds further cross-linking pilins. J. Le Nours ( Monash U., Australia) reported the crystal structures of a Shiga toxigenic E. coli AB5 bacterial toxin and individual subunits that are important virulence factors for several major bacterial pathogens. B. Brown (Brown U., USA) described so-called toxin-antitoxin (TA) complexes that are heavily upregulated in persister cells belonging to bacterial biofilms that are notoriously resistent towards antibiotic treatment. Extensive structural and functional analysis on the MqsA/R TA system showed that it acts as a master regulator of E. coli biofilm formation, thus representing a new target for antibiotic therapy.
Complementary biophysical methods: adding value to protein structures (MS51)
Chairs: Nathan Cowieson, Dorothy Beckett
This session highlighted the power of combining X-ray crystallography with other techniques in elucidating structure-function relationships in biomolecular systems. A recurring theme in the presentations was the integral role that plasticity plays in proteins. R. Loris's (VIB-VUB, Belgium) talk on the toxin:antitoxin system illustrated how application of NMR spectroscopy, SAXS, equilibrium binding measurements and X-ray crystallography enabled determination of the roles of disorder-to-order and protein association in this system. E. Egelman (U. of Virginia, USA) combined high-resolution cryoEM with X-ray crystallography to uncover the range of helical filamentous structures accessible to nearly identical monomeric subunits of actin, flagellin and pilin. J. Cherfils (CNRS, France) presented results of NMR spectroscopy, SAXS and X-ray crystallography in demonstrating the roles of transient unfolding and enhanced dynamic states in dictating alternate specificities of the structurally identical small G-proteins. D. Tomchick (U. Texas Southwestern, USA) discussed the contributions of SAXS, X-ray crystallographic and NMR analysis to determining the roles of structure and internal dynamics in multi-level regulation of the vav1 signaling protein. N. LaRonde LeBlanc (U. of Maryland, USA) demonstrated that the combination of analytical ultracentrifugation, kinetic analysis of enzyme function and X-ray crystallography yields information to enable formulation of a model for control of enzyme function via regulated oligomerization.
Hot structures in biology (MS64)
Chairs: Tsuyoshi Inoue, Se Won Suh
A. Athanasiadis (Gulbenkian Inst. of Science, Portugal) presented the unstable structure of the Z-to-Z junction found in Z-DNA, which was stabilized by Za, the Z-DNA binding domain of the RNA editing enzyme ADAR1. He showed the difference in the stacking interactions between B-to-Z and Z-to-Z junctions. Shigeyuki Yokoyama (U. of Shizuoka, Japan) described the distinctly different structures of two type II aminoacyl-tRNA synthetases of (alanyl- and histidyl-tRNA synthetase) in complex with their cognate tRNAs, respectively, and showed the mechanisms of tRNA recognition. The indirect mechanism of aminoacyl-tRNA synthesis was also described on the basis of complex structures with tRNAs. Both speakers offered insights into the structures of DNA and the translation machinery.
Other speakers described structures involved in cellular signaling. H. Matsumura (Osaka U., Japan) presented the binary complex structure of an intrinsically disordered protein, CP12, and glyceraldehyde-3-phosphate dehydrogenese (GAPDH), the structure of phosphoribulokinase (PRK), and the proposed ternary complex structure of CP12/GAPDH/PRK, regulating the Calvin cycle in plants. M. Lawrence (Walter and Eliza Hall Inst., Australia) described the structure of insulin in complex with Site 1 on the insulin receptor (IR). Site 1 consists of elements of the first leucine-rich repeat domain (L1) of one receptor monomer and the C-terminal segment (αCT) of the α-chain of the second monomer. The αCT helix was repositioned upon binding of the insulin B-chain, following the binding toward Site 2 within an intact IR dimer. H. Wu (Weill Cornnel Medical College, USA) presented the structure of the Myddosome death domain complex in the Toll-like receptor and IL-1 receptor superfamily pathway and also the inhibitor-bound structure of IKKβ which is activated by IRAK in the subsequent gene transcription. She showed the high-order oligomerization in TLR/IL-1R signaling. Structural information on proteins bound with DNA and RNA and complex structures in signal transduction can provide deeper insights into functional mechanisms of systems biology.
Transcription and translation (MS78)
Chair: Anders Liljas
P. Cramer (Gene Center of the U. of Munich, Germany) described eukaryotic gene transcription and three different RNA polymerases that have many common proteins as well as unique ones. The complex goes through a number of conformational changes during the functional cycle, including movement of the RNA and DNA components through tilted and regular conformations while maintaining base-pairing and the binding of a number of factors to a very long C-terminal tail domain that depends upon on site specific phosphorylation.
M. Yusupov (IGBMC, France) described purification of homogenous yeast ribosomes after glucose starvation that produces crystals that diffract to 2.6 Å resolution. More than 5000 nucleotides are placed in the current 3.0 Å map as well as 78 of 80 proteins. The expanded segments of rRNA and the additional proteins compared to bacterial ribosomes are all on the external sides. The asymmetric unit contains two 80S ribosomes in different stages of ratcheting. Here it was observed that the bases of the small subunit that stabilize the cognate interaction between mRNA and tRNA were in very different locations. The most remarkable situation was the binding of the stress response factor Stm1 to part of the binding site for the mRNA.
T. Lavy (Cold Spring Harbor Labs, USA) described the three primary proteins of the GAL regulon in S. cerevisiae, Gal3p (transducer), Gal4p (transcriptional activator) and Gal80p (repressor). Through crystal structures of different complexes much of their interplay could be described in detail. Through mutants, primarily of Gal80p, dramatic conformational and functional effects could be studied.
K. Kihira (Osaka U., Japan) described novel studies of the bacterial release factor 3 (RF-3). Two different forms of the protein could be isolated late in the growth phase, one containing GDP and the other ppGpp. This could be identified first from electron density and subsequently biochemically. The ppGpp is part of the stringent response and probably due to the large extent of uncharged tRNA at the late stage in bacterial growth.
Methods and software for perodic and aperiodic crystals (MS69)
Chairs: Michal Dusek, Iian-Mao Peng
The symposium began with talks on the state of the art of electron microscopy, image simulation and surface structure determination using electron based techniques. In 'Modeling thermal scattering and solving structures using Z-contrast imaging', L. J. Allen (U. of Melbourne, Australia) emphasized the utilization of the thermal diffusely scattered electrons in atomic resolution imaging via Z-contrast mechanism, and the EDX mapping down to a single atom column. In 'SrTiO3 surface structure from diffraction and DFT: homologous series and glasses', L. D. Marks (Northwestern U., USA) emphasized the difficulties of conventional crystallographic methods for the determination of complex oxide surface structures. O. Gourdon (Juelich Center for Neutron Science, USA) described how to use ToF data provided by the Powgen instrument in Oak Ridge to calculate magnetic structures. J. J. Lovelace (Eppley Inst., USA) talked about simulations of modulated protein structures. S. Deloudi (ETH Zurich, Switzerland) discussed periodic average structures associated with n-fold quasiperiodic tilings (n = 7-15).
What is order and how can we measure it? (What kinds of matter diffract?) (MS84)
Chairs: Marjorie Senechal and Uwe Grimm
The diffraction pattern of Penrose tilings (first shown at the 1981 Ottawa IUCr congress) and the discovery of quasicrystals a year later raised a fundamental question: if lattices and Bragg peaks are not synonymous, then what is a crystal? Between 120 and 150 people attended our Microsymposium to discuss the current state of this question. Our two titles - 'What is order and how can we measure it?' and 'What kinds of matter diffract?' reflect thirty years of multi-disciplinary research and debates. As our first speaker, W. Steurer (ETH Zurich, Switzerland), explained, we now understand that order/disorder are not opposites: instead, they encompass a spectrum. The spectrum can be measured by 'complexity', but different complexity measures - such as algorithmic, symbolic, structural and diffraction pattern - do not always agree. Nor do researchers agree on which has priority. R. Mosseri (U. Pierre et Marie Curie, France) and V. Kolosov (Ural State U., Russia) discussed another aspect of disorder - geometric frustration - and its representations in curved spaces, and Shelomo Ben-Abraham (Ben-Gurion U., Israel) returned to the complexity theme. We held an open discussion of the issues raised by the speakers. The debate was intense and illuminating. The Commission on Aperiodic Crystals still has work to do!
Crystal chemistry and physics of modulated crystals (MS88)
Chairs: J. Haderman, A. Schönleber
Modulated and composite crystals are being characterized in inorganic materials, minerals and molecular compounds. K. Friese (U. of Basque Country, Spain) and C. Basilio Pinheiro (U. Federal de Minas Gerais, Brazil) presented overviews of incommensurate modulations occurring in minerals and molecular compounds, respectively, with examples from their work. A. Abakumov (U. of Antwerp, Belgium) described a completely homologous series of incommensurately modulated perovskite structures, tunable in cation as well as anion content. O. Perez (CRISMAT, France) discussed the likelihood that many of the crystals now described as disordered could actually be modulated crystals. N. Van Tri (Hanoi U. of Science and Tech., Vietnam) tried to put aperiodic structures into relation to nanostructures.
Chemical bonds in aluminium (MS41)
Chairs: N. Bouhmaida, C. Jelsch
P. N. H. Nakashima (Monash U., Australia) described the use of quantitative convergent-beam electron diffraction (QCBED) to determine the bonding electron density in aluminium. The results resolved a long-standing disagreement based on several theoretical and experimental investigations as to whether bonding in aluminium is octahedrally or tetrahedrally coordinated.
A density functional theory (DFT) first-principles calculation of the ground-state electron distribution was also performed. The QCBED and DFT determinations show that the bonding electron density is concentrated exclusively in the tetrahedral interstices. The results were correlated with the mechanical anisotropy of aluminium and the morphologies of precipitates that form in aluminium alloys.
Electron momentum and spin densities in correlated electron systems (MS48)
Chairs: Bernardo Barbiellini, Masa-Hisa Ito
Magnetism and superconductivity are two pillars of the physics of correlated electron systems. The techniques presented in this session for probing the spin polarisation in spintronic materials can have an important industrial impact, while the momentum-density studies on the effect of hole doping in high-temperature superconductors can give an indication of the correctness or applicability of many competing models of high-temperature superconductors. S. Dugdale (Bristol U., UK) presented results of recent Compton scattering measurements performed on optimally Co-doped BaFe2As2 pnictide superconductor. These data show that the Fermi surface of this compound has a nontrivial 3-dimensional shape, which can only be explained by ab initio calculations, where the As atomic position can relax. M. Okube (Tokyo Inst. of Technology, Japan) used resonant X-ray magnetic scattering to determine the electronic character of the Verwey transition and help resolve a long-standing debate regarding the nature of this transition. A. Bansil (Northeastern U., USA) used high-resolution Compton scattering to image holes in cuprate high-temperature superconductors and aid in understanding the metal-insulator transition (MIT) in complex materials. A. Baranov (Max-Planck-Inst., Germany) described how insight into the MIT can also be obtained by analyzing electronic motion in real space. He introduced electron localization indices showing the degree of electron-pair exchange inside and between various regions of space. Y. Sakurai (SPring8, Japan) presented a method based on magnetic Compton scattering which provides unique information on the transport and magnetic properties of half-metallic materials. The technique has been applied successfully to ferromagnetic manganites.
Hydrogen bonding: from the solid state to solutions (MS90)
Chairs: Alan Soper, Mark Tuckerman
A. Michaelides (U. College London, UK) gave an introduction to the combined technique of path-integral molecular dynamics with density functional theory. He explored the transition of double-well hydrogen-bond potential that occurs in Ices VIII and VII, with two possible locations of the hydrogen atom, to the single-well potential of Ice X, where the hydrogen has only one site with significant broadening along the O-O axis. C. Bull (U. of Edinburgh, UK) described experimental studies of hydrogen bonding under pressure using diamond anvil and Paris-Edinburgh pressure cells and neutron scattering. C. Jelsch (CNRS, France) described multi-polar atom electron-density calculations of hydrogen bonds, revealing the weak lone pairs in water, hydroxyl and carbonyl groups, which disappear in the oxygen ester group. K. Fucke (Durham U., UK) described the small water clusters encapsulated in p-sulfonatocalixarene. These molecules contain four phenol-like rings with the -OH groups pointing into the centre where they form hydrogen bonds with water. Such molecules are studied as models for drug delivery.
Descriptors of electronic structure and chemical bonding (MS34)
Chairs: Louis J. Farrugia, Angel Pendás
The first two talks in the session had a marked methodological character, showing state-of-the-art descriptors in chemical bonding from real-space analyses. E. Matito (Basque Country U., Spain) provided a detailed account of multicenter bonding as related to aromaticity measures. It has become clear that very good correlations with a large set of aromaticity indices proposed in the literature have been found with the so-called multicenter delocalization descriptors, which involve the n-central moments of the electron population in different real-space basins. Unfortunately, the calculation of these quantities is far from trivial, requiring nth-order reduced density matrices in the n-center case. Matito clearly illustrated these shortcomings, and showed how simplified versions of the indices in terms of the first-order density may be obtained, thus approximating the nth-order matrices in different ways. P. Bultinck (Ghent U., Belgium) introduced the iterative Hirshfeld procedure to the crystallographic community. This is a partitioning method based on the Stockholder prescription that overcomes the severe dependence of traditional Hirshfeld atoms on the chosen atomic reference densities. He clearly showed how this partitioning may be used as an extremely cheap alternative to other widely used Atoms in Molecules, like those coming from the Quantum Theory of Atoms in Molecules of Bader and coworkers. The iterative Hirshfeld atoms were shown to be independent of the starting reference, and very transferable among different systems in similar chemical environments. Bultinck also emphasized the quality of their charged multipolar moments, which make the Hirshfeld-I atoms a good alternative for several crystallographic problems.
The remaining three talks dealt with applications to experimental (and potentially experimental) systems. The talk by G. Eickerling (U. of Augsburg, Germany) emphasised two important points in the accurate modeling of electron densities for heavier elements, (a) the importance of including relativistic corrections in providing accurate densities and (b) the necessity of including core polarizations in multipole modeling of high-resolution theoretical static structure factors. The model systems studied included M(CH3) (M = Sc, Y, La) and they were successfully dealt with by a simple extension of the Hansen-Coppens pseudo-atom multipole model using two or more pseudo atoms per heavy-atom site. As expected, these core polarisation effects become increasingly important as the resolution of the data increases.
A very similar approach was reported by J. Overgaard (Aarhus U., Denmark) in the combined experimental and theoretical study on an N-heterocyclic carbene complex of Ga(I). A standard Hansen-Coppens pseudo-atom modeling of the experimental and theoretical data gave quite unsatisfactory residuals, which were resolved by treatment of the core density as two separate pseudo atoms. This implicit rescaling gave excellent agreement between theory and experiment for the Laplacian properties, which showed intriguing evidence for the presence of a lone pair on the Ga(I) site. The metal-ligand charge distribution was consistent with a good σ-donor ligand with poor π-acceptor properties.
The final talk by S. Mondial (U. of Bayreuth, Germany) dealt with an experimental charge-density study on γ-B28, a fascinating new allotrope of boron reported in recent independent studies by Organov et al. and Zarechnaya et al. This consists of icosahedral B12 units which are linked directly by B-B interactions and also through isolated B2 units. From the topological properties of ρ(r) and ∇2ρ(r), it was concluded that there is a 2c-2e bond between the B atoms of the B2 units and these are linked to the icosahedra by 3c-2e bonds. A Bader partioning gave rise to small charges on the B atomic basins in this formally covalent material.
To be continued in the next issue.
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