Meeting report

[IUCr Congress logo]Symposium reports

Soft matter studied by small-angle scattering

[Speakers] Speakers in the Soft Matter Studied by Small-angle Scattering symposium
The symposium covered aggregates of surfactants, liquid crystals, polyelectrolyte polymers and semicrystalline polymers. B. Demé (ILL) showed how various techniques can be used to study aggregates formed in water-rich, salt-free mixtures of anionic and cationic surfactants. Small-angle neutron scattering was used to obtain quantitative information on icosahedral shells, large stiff discs and lamellar phases. P.S. Clegg (U. of Toronto) presented a study of liquid crystal-aerosil mixtures, where the aerosil works as a random field influencing the ordering of the liquid-crystal molecules. The system was studied by x-ray diffraction and its behavior was found to be in agreement with one of the available theories and in clear disagreement with others. J. van der Maarel (The Netherlands) described a small-angle neutron contrast variation study of micelles formed by block copolymers with a polyelectrolyte block. The scattering was separated into basic contributions and modeling of these revealed complex behavior when the charge density of the polymer and the salt concentration of the solution were varied. J. D. Barnes (Maryland) talked about SAXS and SANS studies of semicrystalline polymers with lamellar structures of amorphous and crystalline regions. In some of the systems, the difference in scattering length density of the amorphous and the crystalline regions vary strongly with temperature and there is even a temperature, where the contrast vanishes. For the samples which had been exposed to various forms of processing, a strong orientation of the lamellar structure was present and a proper analysis of the structure required that 2D scattering data be recorded for all principal orientations of the samples.

Thomas Zemb, Andrew Allen and Jan Skov Pedersen

Nanostructural characterization by small-angle scattering

[Speakers] Speakers in the Nanostructural Characterization by Small-angle Scattering symposia
The talks focused on recent developments, the possibilities these developments foster and new results. B. Chu (USA) described templating mesostructures of spontaneously formed block copolymer micelles of polyoxymolybdates. The work revealed that the formation of a mesoporous structure did not depend on the presence of the micelles, but could be achieved also by homopolymers in solution. O. Paris (Austria) described the application of the scanning-SAXS method to determining the local structure of inhomogeneous biological materials at one micrometer, with synchrotron radiation. A.J. Allen described characterization of multi-component systems of industrial interest using the multiple SANS technique. R. Gilles discussed in situ high-temperature SANS results for Ni-based superalloys that are used in turbines and jet engines. The modeled microstructure provides information needed for improved materials production. N. Ibraimov described the use of Kumakhov capillary optics to increase the flux and reduce the background of an X-ray reflectivity study of the surface structure of samples of relevance for microelectronics. K. Sakurai extended the discussion to include off-specular measurements on multilayers with the same composition as those used in x-ray monochromators. The measurements provide additional information on lateral correlations in the surface layers.

Iris L. Torriani and Jan Skov Pedersen

Molecular and crystal properties from charge and spin densities

[Woolfman, Hauptman] Ewald Prize winner Michael Woolfson with Herb Hauptman
This microsymposium demonstrated the practical impact that theoretical and experimental density studies have on biology, crystal engineering and materials science. C. Lecomte demonstrated the transferability of electron densities in a multipolar refinement of a protein using models derived from previous charge density studies of a series of polypeptides. The application of the MOPRO refinement strategy to ultra high resolution data on Aldose Reductase (dmin = 0.64 Å) revealed for the first time the electrostatic complementarity between the protein and the ligand. The era of electrostatic, rather than of purely geometrical, interpretation of the 'lock and key' mechanism is thus opened.

M. Spackman reviewed Hirshfeld surfaces (HSs) as a way of exploring packing modes and fingerprinting intermolecular interactions in molecular crystals using a very simple and efficient partitioning of crystal space. Since HSs encode information about all intermolecular interactions simultaneously, a 2D mapping of these surfaces was recently devised. The mapping plots the fraction of points on the surface as a function of the closest distances from the point to nuclei inside and outside the surface. In this manner all types of interactions (hydrogen bonding, H..H contacts, C-H..π interactions, π-π stacking, etc.) are readily identifiable as are the similarities and differences of packing modes in related molecular crystals. These visually stunning plots are unique for a given crystal structure and polymorph. Their practical impact on crystal engineering was highlighted. Recent applications showed that partitioning using ionic electron densities yields a remarkably diverse range of shapes, especially for anions. The resulting basins are similar to those obtained through Bader’s partitioning of an ab-initio periodic Hartree-Fock electron density.

Lectures by Cooper and Paolasini revealed how combined studies of the spin and charge densities of inorganic crystals enables one to understand their material properties. M. Cooper discussed magnetization in ferromagnets with polarized x-rays. The crucial role of complementary Compton and diffraction studies to understand physical properties was illustrated by the study of Sm0.98Gd0.02Al2 in which gadolinium doping produces a zero total magnetization at temperatures near the boiling point of liquid nitrogen. The results show that it is the small gadolinium itinerant electron spin contribution that preserves the magnetic order of this material above the magnetization compensation point.

Paolasini described Non-Resonant and Resonant X-ray Scattering (NRXS and RXS) studies at ESRF, of the magnetic and orbital ordering in the pseudocubic perovskite KCuF3. RXS has its origin from processes in which photons are virtually adsorbed by exciting core electrons to empty states and subsequently re-emitted when the excited electrons and the core holes recombine. Structurally forbidden reflections, corresponding to magnetic and 3d-orbital long-range order, have been observed by Paolasini and coworkers. It appears that the orbital and magnetic orderings are closely related (even though the associated energy scales differ by more than a factor of 20) and with a definite interplay between magnetic and orbital order parameters as revealed by an increase in the intensity of orbital Bragg peaks in the temperature range close to the Neél point.

K. Tanaka discussed a multi-temperature measurement of electron density distributions (EDD) in crystals of CeB6. In this rare-earth complex crystal 4f-electrons spontaneously flow from Ce to B6 with a decrease in temperature, which starts above room temperature and ultimately leads to an anomalous resistivity increase below 2K. Research on the EDD in rare-earth compounds is a frontier field in X-ray diffraction.

Carlo Gatti and Malcolm Cooper

Nanocrystalline materials

[Delegates] Delegates Reception
The symposium addressed the design and applications of nanocrystalline materials. R. Nesper (Zurich) described metal oxide nanotubes obtained by controlled scrolling of layered structures and the collapse of nanotubes into nanorods. Applications included cation selectivity, anisotropic NLO materials and potential sensors. M. Remskar (Ljubljana) described the growth of metal disulfide nanotubes, the self-assembly of multi-walled nanotubes, rotation of nanotubes at elevated temperatures, and the rapid migration of Cs+ ions along WS2 nanotubes. Finally, the role of C60 was explored as a templating agent in the nucleation of crystals, which contain molybdenum sulfide nanotubes with iodine between them. M. Green (Oxford) described the growth of inorganic crystals within the confines of single-walled carbon nanotubes and the changes in ion coordination number arising from confinement. Crystals of potassium iodide only two or three ions in diameter were characterized by electron microscopy techniques. A. Eichöfer (Karlsruhe) described the synthesis of cadmium chalcogenide based nanoclusters with sizes ranging form 0.8 - 2.0 nm in diameter and containing from 4 - 32 cadmium atoms that exhibit interesting photoluminescent properties. Z. Somogyvari (Budapest), described neutron diffraction studies of transition metal oxides important as magnetic recording materials. In barium hexaferrite (Co, Ti doped) neutron diffraction was crucial to determining the cation distribution, taking full advantage of the substantial differences in scattering lengths between Fe, Co and Ti. The effect of crystal size (nanocrystals vs. microcrystals) on magnetic properties was also explored.

Lee Brammer

Diffraction and symmetry of aperiodic crystals

[Speakers] Microtomography and High Resolution Imaging Session Speakers
This symposium featured recent theoretical and experimental studies of aperiodic crystals. H. Takakura reported on applications of the low electron density elimination (LDE) method to the phase reconstruction of X-ray single crystal data of quasicrystals (i-Al-Pd-Mn). He indicated that the location, sizes and shapes of the occupation domains in the n-dimensional unit cells are obtained without model structures. M. Honal examined the refined characteristic diffuse scattering observed in the diffraction patterns of decagonal AlCoNi. Features of the scattering are similar to the 'atomic size effect' scattering common in crystalline alloys. Monte Carlo techniques were applied to study the phenomena based on the size effect of the constituent atoms. A. Cervellino compared the structures of the decagonal Al71Co7Ni22 phase and the related decagonal Edagawa superstructure (Al71Co13Ni16). The former is stable over 950°C, has a low density (3.9 Mg/m3) and high structural occupational disorder, the latter is stable between 500-850°C is denser (4.1-4.2 Mg/m3) and presents lower occupational disorder. Orientational disorder was measured by diffuse scattering. G. Chapuis described a molecular dynamics simulation of the incommensurate structure in hexamethylenetetramine solved previously with single crystal X-ray data and a superspace approach. The simulation was carried out in the temperature range from 15 K to 580 K with a time step of one femto-second. A unique compensating pressure tensor was applied in order to get lattice parameters close to the experimental ones. The current MD simulation reproduces the experimental diffraction results well. M. Ohmasa showed that the two-dimensionally modulated structure of Ca2CoSi2O7 is stable below 489K and the component q of the primary modulation wave-vector, varies from 0.286 to 1/3 with temperature change. The variation of the long-range order in the modulated structure can be derived from the structure parameters determined at room temperature.

Masaaki Ohmasa and Marc de Boissieu

Imaging

[People]
This symposium brought together leaders in high resolution X-ray imaging, microtomography and phase-contrast imaging. A variety of techniques were described including those using an objective lens [e.g. Fresnel Zone Plate (FZP) or Compound Refractive Lens (CRL)] to provide 'full-field' imaging and propagation-based techniques where mathematical methods are used to correct for 'defocus' of the imaging system.

P. Cloetens described how quantitative information can be extracted from high resolution absorption and phase-contrast images and micro-CT obtained via multi-distance phase retrieval methods from holographic data. He finds that phase-retrieved images become preferred as X-ray energy increases. Y. Kagoshima reported on 8 - 12 keV imaging from an undulator at SPring-8 using a Zernike-type phase-contrast microscope. Both positive (π/2) and negative (3π/2) phase plates were used to obtain images approaching 60 nm resolution for line pairs. Somewhat inexplicably, the negative phase plate regularly gave considerably better quality images, e.g. of diatoms and conidia even in water. G. Tromba presented images obtained using a PDI/DEI-type technique which includes processing of multiple images to obtain separate absorptive and refractive components.

Other topics addressed included: projection-based methods of phase-contrast imaging including high resolution microscopy and tomography of semiconductor devices (S. Wilkins, CSIRO); high energy full-field X-ray microscope with a current resolution of 0.3 microns (C. Schroer); soft- and hard-tissue images using a Bonse-Hart interferometer with micro-CT and projection-based images obtained at DESY (F. Beckman); and the application of dark-field ('Owl') methods to phase-contrast imaging using monolithic multi-crystal systems (M. Ando).

Steve Wilkins (CSIRO)

Horizons in hydrogen bonding

[Schenk, Larsen] Henk Shenk and Sine Larson
Advances in fundamental and applied aspects of hydrogen bonding in small molecule and macromolecular structures were combined in this session. Lee Brammer (Sheffield U.) discussed weak hydrogen bonds D–H…X–C (D = C, N, O; X = F, Cl, Br, I) and compared them with their stronger cousins D–H…X–M (M = transition or main group metal). The use of directional halogen-mediated interactions in the design of ionic hydrogen bonded network solids was illustrated. Bing Gong (SUNY Buffalo) showed bifurcated three-center N–H…O hydrogen bonds characterized by X-ray diffraction that are stable in solution (NMR). It is possible to tune the cavity size of helical nanotubes (15–30 Å diameter) in folded oligomers through the choice of carbon backbones and the conformation of the hydrogen bonding groups. I. Olovsson (Uppsala U.) discussed the refinement of the hydrogen atom position in a short hydrogen bond in KHdichloromaleate determined by low temperature neutron diffraction. Interestingly, when the heavy atoms are refined in P-1 and the protons are refined in P1, the non-hydrogen structure is still centrosymmetric but the hydrogen atoms are off-centered! The details of this unconventional structure refinement procedure continued to be debated well after the formal session was over. P. Chakrabarti (Bose Inst.) presented results on the significance of aromatic–aromatic and charged polar interactions between Phe, Tyr, Trp and His residues in protein structures. The geometry of recognition motifs (face-to-face, offset-facestacked, edge-to face) and their occurrence in proteins with greater than predicted probablity may be explained through specific interactions (N–H…O, N–H···π, C–H···π). According to P. Auffinger (IBMC, CNRS) hydration in RNA, bifurcated hydrogen bonds, stability of C–H…O H-bonds, and the processes of melting and crystallization may be better understood through the use of molecular dynamics simulations. With a better understanding of the hydrogen bond phenomenon it is possible to design and analyze complex structures, thus providing a continuum from fundamental to applied and biological studies.

Ashwini Nangia (Hyderabad U.)

Detectors

[Speakers]Speakers in the Workshop on Protein Crystal Growth
The microsymposium focused on X-ray area detectors for use with synchrotron radiation. H. Graafsma (ESRF), gave a comprehensive overview of the role and importance of detectors in the field of synchrotron radiation. E. Eikenberry (SLS) described pixel arrayed detectors with extremely high counting rates. The development and application of an imaging plate scanner with a large format (400 mm x 2000 mm) for automated protein crystallography, was discussed by N. Sakabe (Japan). The final talk was on the development of a phosphor screen with improved efficiency based on a new concept.

Yoshiyuki Amemiya (U. of Tokyo) and Heinz Graafsma (ESRF)

MAD, SAD and difficult phasing

The role of selenium in MAD phasing is now well-established but, as G. Leonard demonstrated, other elements and softer X-rays should not be neglected. For instance, if the technical difficulties were overcome, a single uranium atom would give an rms ΔF/F signal of 6.9% for the whole ribosome at a wavelength of 3.5Å. Even at a wavelength of 1.7Å, a SAD experiment exploiting intrinsic sulfur atoms is sufficient to solve many protein structures. SAD experiments have become more popular recently because problems of radiation damage are reduced compared to multi-wavelength approaches. R. Pannu showed that it is possible to get better phases from such experiments by using a new SAD likelihood function. This new target takes full account of correlations ignored by previous methods, such as errors in anomalous scattering predicted for the two members of the Friedel pair. G. Bricogne straddled the theoretical and experimental by describing improvements in the speed and performance of the popular SHARP phasing program, and new ideas for introducing anomalous scatterers by soaking in tri-iodide or combinations of salts. Z. Otwinowski ended with a philosophical discussion of considerations in processing data to preserve small anomalous differences, as a prelude to using such differences to exploit radiation damage for phasing.

Case histories illustrated what can be learned from difficult structures that would have been impossible a few years ago. The structure of KPHMT was solved by Se-Met MAD, setting a record for the size of the anomalous substructure: 160 seleniums. In describing this work, Frank von Delft showed that greater redundancy is not always better. Finally, Gabby Rudenko showed how persistence paid off in solving the extracellular domain of the LDL receptor where iterative improvement of the phases from tungsten clusters led to an interpretable 3.7Å map.

Randy Read and Andy Thompson

Molecular dynamics of crystals and phase transitions

Chairperson, A.Gavezzotti, described the application of modern molecular dynamics simulation techniques, to the study of the properties of crystals and of their transformations. Cochair, L.R.Nassimbeni, commented on experimental problems in phase transitions of inclusion compounds. These preliminary statements elicited a lively discussion. F. Herbstein (Haifa) gave a masterly description to the main thermodynamic features of phase transitions, with examples taken from real-life crystallography. J. Scott (Melbourne) reported on her exciting results in experimental synthetic solid-state chemistry on compounds of interest to agricultural applications. J. van de Streek (Nijmegen) described the dynamics of the complex phase behavior of nalkanes, highlighting the importance of the inclusion of kinetic energies and temperature in theoretical simulations.

A.Gavezzotti