Bookmark and Share

ECM showcases theory and application

Comparison of cellulose structure derived from fiber diffraction analysis (Kroon) and single crystal analysis of 8-D-cellotetraose (Saenger).

The Fifteenth European Crystallographic Meeting was held in Dresden, Germany, from August 28 to September 2, 1994. The meeting featured ten plenary lectures, 21 microsymposia and over 500 poster presentations covering all aspects of crystallography.

Plenary lectures

J. Kroon (Utrecht) presented new observations on the  structure of cellulose brought to light by physical and structural study of its polymorphs. Kroon said that basic differences in the hydrogen bonding of cellulose I and II can be attributed to conformational preferences exhibited by the hydroxymethyl groups. Based on Molecular Dynamics simulations, and NMR studies, J. Kroon concluded that in cellulose I all hydroxymethyl groups assume trans gauche conformations in contrast to the gauche trans conformations of the same residues in form II. In the discussion, W. Saenger (Berlin) reported on his structure determination of a cellulose single  crystal, which corroborates conclusions Kroon inferred from fiber studies. (A. Kalman, Budapest)

Protein Synthesis System. As techniques for crystallization have been developed and perfected, complex assemblies of macromolecules such as photosystems, viruses and ribosomes have become the subject of X-ray analysis. Due to the limited diffraction achievable from crystals of the entire complex, their low resolution studies are being pursued in parallel with high resolution determinations of the structures of individual components of the complexes. In his brilliant plenary lecture, A. Liljas reviewed the results from his laboratory and others concerning components of the 30S and 50S ribosomal particles and placed them in the context of the low resolution structure of the entire ribosome emanating from the laboratory of A. Yonath. Of the 56 proteins that, together with rRNA, comprise the small and large subunits of a procaryotic ribosome, the structures of a dozen have been solved to atomic resolution. Many exhibit similar folding due perhaps to the common feature of interaction with rRNA. Liljas also described the structure of the elongation factor G, a large, extended protein consisting of 5 well separated domains. The progress reported by Liljas and others toward understanding the mechanisms of protein synthesis common to all living organisms is phenomenal. (W. Höhne, Humboldt University of Berlin)

Powder Structure. C. Giacovazzo presented a comprehensive review of Structure Determination from Powder Data including advances in radiation, correction for peak overlap, background and preferred orientation effects, and the development of programs for automatic solution of complex structures. (H. Fuess, Darmstadt)

NMR. In his plenary lecture on NMR, Engelhart stressed that the combined strengths of NMR and X-ray analysis more than double the information derived from a study. He described the most recent advances, including results achievable from the technique of Doubly Oriented Rotation and studies of the temperature dependence of phase transitions. (W. L. Duax, USA)

Microsymposia

The microsymposium Crystal Growth - New Development focused upon recent progress in vapour growth, the effects of external fields on the homogeneity and structural properties of inorganic crystals, defect structures of organic molecular crystals grown from the melt and twinning in epitaxially aligned diamond films grown by chemical vapour deposition. The greatest interest was stirred by the achievements of vapour growth. Using this technique better control of the thermal field has led to growth of  homogeneous single crystals of mercuric iodide (weighing 500 mg), a candidate for new laser frequency generation in the UV spectral region were reported. (C. Paorici, Italy)

In the microsymposium Aperiodic Crystals, new icosahedral phases in Al-Mg-Si and Zn-Mg-rare earth alloys were reported (A. P. Tsai, Japan). The strong diffuse scattering observed in decagonal quasicrystals is beginning to be thoroughly investigated. F. Frey (Germany) and C. Schetelich (Germany) showed that despite strong diffuse scattering, decagonal quasicrystals can exhibit clear dynamical diffraction effects. (J. M. Perez-Mato, Spain)

Participants in the  microsymposium "Liquid crystals and plastic crystals". Front row, E. Eckardt and D. Durand; back row(left to right), S. Diele, V. M. Kaganer, B. I. Ostrovskii and W. Haase (co-chairman).

In the microsymposium on Liquid Crystals and Plastic Crystals, the rich polymorphism of Langmuir monolayers and ultra-thin smectic films was described in the framework of the Landau theory of phase transitions (V. M. Kaganer and E. B. Loginov, Moscow). One of three coupled order parameters in the system appears to govern the collective tilt of the molecules, while the other two describe one-dimensional "weak crystallization" along the bond direction and normal to it respectively. Based on this analysis, the relationship between symmetry and packings of different layered systems of long organic molecules may be understood.

Several liquid crystalline structures were found in highly concentrated DNA solution, including cholesteric and columnar hexagonal phases (D. Durand, Paris). With increased DNA concentration, the coupling between neighboring DNA helices leads to a continuous transition from acolumnar hexagonal phase to a three dimensional crystalline phase and the number of nucleotides per helix turn decreases continuously without apparent change in the molecule conformations. Another highlight of the session was the identification by X-ray and electro-optical measurements of new types of dislocation domains in ferroelectric liquid crystals reported by L. A. Beresnev of Darmstadt and Moscow. (Boris I. Ostrovskii, Germany)

Boris Vainstein, a pioneer in the field of electron diffraction speaks with Gustaaf Van Tendello after the latter's superb plenary lecture on the topic.

With electron crystallography, one can simultaneously obtain information in real space and reciprocal space and obtain local information, rather than average information. One, two or three dimensional defects which in diffraction are only "visible" through deviations from ideally sharp reflection or weak diffuse intensity, can be directly observed and characterized on an atomic scale. Electron diffraction is therefore not in competition with other diffraction techniques, but a complement to them. This was perfectly illustrated in the Electron Microscopy of Crystal Defects microsymposium where L. Potez (Onera-Paris) described the atomic structure of Cu3Au. The fact that antiphase boundaries in cube planes are formed was illustrated by electron microscopy in the fifties. Conventional electron microscopy and high resolution microscopy (HREM) have now revealed detailed information on the nature of the interfaces and the lattice relaxation associated with these interfaces. In situ heating experiments illustrated the different wetting behavior of conservative and non-conservative boundaries. This wetting, as in CuPd alloys, can be quantified as a function of temperature and is shown to follow a log (T-Tc) law. The time when HREM was just a collection of "nice pictures" is definitely over. Structures of oxomolybdates of the type AxByMo4n+2O6n+4 have been determined by X-ray diffraction and beautiful HREM (R. Ramlau, Stuttgart) not only showed the complex intergrowth between different structures, but also identified the different stacking sequences and sub-unit cell twinning, a typical example of the use of electron microscopy to unravel the most complex intergrowth structures or superstructures, once the basic structure is known. Cristobalite (SiO2), simple as it is, is a nightmare for crystallographers. The phase transition in quartz, known for over a century, was only understood in detail after electron microscopy observations revealing the presence of an incommensurate phase over a temperature interval of about 1°. The phase transition in cristobalite seems to be equally complex. The tetragonal low temperature phase, of which M. Czank (Germany) showed high resolution observations, contains a high density of defects, some inherited from the high temperature phase, others being introduced by the phase transition. (Gustaaf Van Tendello, Belgium)

This illustration of the fluorescence from an 8 micron thick Cr fiber as it passes a focal spot of 40*0.8 microns was presented in the microsymposium on "Methods and Instrumentation". The wavelength is 0.8 Å and the measured flux in the focal spot is ~108 ph/s. (Courtesy of A. Snigirev.)

A microsymposium on Methods and Instrumentation organized by J. P. Ihringer (Tubingen) and Å. Kvick (Grenoble) included oral presentations on high pressure (O. Shimomura), micro focussing (A. Snigirev), imaging plate detectors (S. O. Svensson), microcrystal diffraction (R. B. Neder), computational methods for structure solution (K. Wagner) and instrumentation for protein crystallography with soft X-rays (J. Thomas). Developments in instrumentation and methods are being stimulated by new advances in synchrotron radiation sources. The areas of high pressure, microcrystal diffraction, protein crystallography and the use of anomalous dispersion effects are also growing in importance as suggested by reports on new high pressure techniques, the use of novel optics such as Bragg-Fresnel lenses for focusing, and vacuum and other techniques to reduce absorption and background. Rapid development in the detector area focusing on fast and accurate two-dimensional detectors was also evident. Shimomura described high pressure studies of unknown phases of Bi, ZnTe and AgCl and a charge density study of iodine using the Rietveld technique, imaging plate detectors, and the maximum entropy method.

Svensson presented a careful analysis of the precision and reliability of imaging plate detector systems and evaluated the necessity for time decay, spatial distortion and flood field corrections. Snigirev discussed the principles of Bragg-Fresnel optics used for focusing and showed results from submicron experiments in microdiffraction and microimaging. Experimental difficulties in collecting data from crystals only tens of microns in size were discussed by Neder who stressed the necessity for low background and efficient means of obtaining the orientation matrix. (Åke Kvick, France)

Contributions to the microsymposium Surface and Interface Crystallography ranged from traditional surface crystallography of metal semiconductor surfaces over soft condensed matter to the morphology of surfaces. Contributions based on X-ray scattering, Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM) showed that there is room for both real and reciprocal space based techniques. X-ray reflectivity has become a standard technique that can give answers to questions about structures of soft condensed matter (V. Safronov, Russia) and morphologies of deposited films (M. Jergel, Slovakia).

Probability density function for Cs at 90 K (left) and 303 K (center); the unit cell of the structure is shown at right. (Courtesy of H. L. Meyerheim.)

The accuracy of diffraction data on surface structures is now such that it is possible to refine anisotropic thermal vibrations for surface atoms and adsorbates. Very convincing results obtained for Cs on the Ge(100) surface at different temperatures were demonstrated by H. Meyerheim (Germany). In another presentation, J. Zegenhagen (Germany) elegantly discussed how the X-ray standing waves techniques can shine light on incommensurate structures. (R. Feidenhansl, Denmark)

Papers presented in the microsymposium Atomic Mechanisms of Phase Transitions included the first 2D-model of martensitic transformations which achieves a global as well as a microscopic transformation, a model particularly relevant for materials such as duplex steels (P. Donnadieu, France). Competing, biquadratic coupling between order parameters developed in a Landau free-energy model to explain the conformational order/disorder processes in alkali-polyphosphates were discussed by W. Schmahl of Germany. Molecular dynamics simulation of the mechanism of crystallization of liquid Co, mechanisms of epitaxy deposition (O. S. Trushin, Russia), and mechanisms of crystallization of amorphous alloys (N. Mattern, Germany) were also described.

The role of oxygen ordering in the superconducting transition of high Tc materials (the YBaCuO system) and the consequences of the superconducting transition temperature were described by N. H. Andersen (Denmark). Of great current interest are molecular systems which present a change of electronic state under the influence of temperature, pressure or photo irradiation. The study of TTF-p-chloranil (M. Le Cointe, France) provided an excellent comprehensive illustration of such systems. (F. Denoyer, France)

Molecular structure at 243 K. ·Atoms of four- and eight-membered rings are disordered. (Courtesy Y. Ohashi.)

The microsymposium on Time-Resolved Studies included reports of the resolution of a phase transition on a millisecond time scale, achieved with the spallation neutron source ISIS (G. Eckold, Germany) and the hydrothermal synthesis of CoAPO-5 at high temperatures (l65°C) studied by in situ powder diffraction at NSLS on a 30 second time scale (P. Norby, USA). In the same session, Y. Ohashi (Japan) graphically illustrated a solid state reaction in which an eight membered ring is converted to a tricyclic assembly of four-membered rings. (A. N. Christensen, Denmark)

The standing room only microsymposium on Structure and Properties of Fullerenes was dedicated almost exclusively to the different structures and phase transitions of pure C60 and C70 since higher fullerenes are not yet available in microscopic quantities. A very clear picture of the high- and low-temperature phases of C60 has now emerged, based on beautifully consistent X-ray and neutron Bragg and elastic diffuse scattering data. In addition, new high-pressure (5 gPa) forms of C60 were presented: an fcc cubic structure at 300° and a rhombohedral form at 800°. New intercalates of C60 with Yb were also reported.

Pseudo-element "Bucky" presented as a new entry in the Periodic Tables of Elements. (Courtesy of T. Siegrist.)

The structures and phase transitions of pure C70 are less well established, but considerable progress has been made using single crystal X-ray data. The existence of cubic close-packed and hexagonal close-packed structure-families is well established, each with very similar transitions from a free-rotor phase to a phase formed by spinning tops  and then to an ordered phase. But the transitions show large hysteresis, and there are indications of at least one additional phase transition at about 305 K. Two structures of the hexagonal family were presented: the ordered low-temperature phase and the room-temperature spinning-top phase. But many questions remain open: stacking faults are omnipresent and the observations on the cubic phase appear to be somewhat contradictory. (Dieter Schwarzenbach, Switzerland)

The microsymposium Crystallography of Small Molecules included discussions concerning electron deformation densities, the relationship between hydrogen bond and crystal formation (J.C. Barnes) and phase relationships between single enantiomorphs and related conglomerate systems (K. Marthi). (Alajos Kalman, Budapest)

In the microsymposium on Inclusion Compounds and Molecular Recognition, the complementarity of crystallography and chemistry was most apparent since the only source of reliable information on host-guest stoichiometry and chemical composition is often provided by X-ray structure determinations. E. Weber, known for his synthesis of macrocyclic host materials, was the co-author of three of the six oral contributions and two posters which described tailored synthesis of host molecules having desired c1athratogenic ability (i.e. shape and selectivity). Examples of the use of chiral building blocks in designing host/guest systems were provided by M. Czugler (2-ethyl-tiperidine and phenyloxiranes), A. L. Llamas-Saiz (alanine/carbonamide  frameworks) and M. Gdaniec (gossypol). These systematic studies are particularly important in view of the increasing importance of chirality in pharmaceutical and food chemistry.

Studying clathrates with a combination of thermogravimetry (L. Nassimbeni), DSC, diffuse scattering (T. Weber et al.) and X-ray structure analysis revealed details of dynamic properties of non-stoichiometric clathrate structures, including domain structures of the solids and partial orientational disordering. In addition to selectivity in bonding and transport processes, inclusion compounds may be used to stabilize reactive or otherwise unstable guest species. In the poster session a paper on stabilization of BF3 and SiF4 by complexes of 18 crown 6 derivatives were shown together with crystal structures in which not only BF3 or SiF4 but also water plays an important role in complexation. (J. Lipkowski)

One of the highlights of the microsymposium on the Crystallography of Biological Macromolecules was a detailed account on the system of yeast tRNAAsp with its cognate aspartyl-tRNA synthetase (J. Cavarelli, Strasbourg, France), based on the crystal structures of the two macromolecules, their binary complex and the tertiary and quaternary  complexes with the substrates of the aminoacylation reaction. W. D. Schuber (Berlin, Germany) presented the current state of the structure determination of the membrane embedded protein complex Photosystem I, one of the two protein pigment complexes responsible for the primary conversion of visible light into chemical energy in water oxidizing photosynthesis. T. Unge (Uppsala, Sweden) gave an account of the structure of the N-terminal half of human immunodeficiency virus type 1 reverse transcriptase (RT), important for the prediction of new inhibitors of RT. M. Schiltz (Orsay, France) revisited the use of xenon to make heavy atom derivatives which are highly isomorphous; a simple technique has been designed for diffraction studies of protein structures under xenon gas pressure of up to 50 bar. Xenon heavy atom derivatives have already been crucial for the elucidation of two new protein structures by groups in Paris and Strasbourg. Finally, A. Wlodawer (Frederick, USA) showed the interplay of crystallography and modeling to give new results on the mode of binding for interleukin-4. (R. Fourme, France)

Speakers and chairs in the microsymposium on Crystallography Aided Drug Design, including (from left to right): J. Kallen, N. Borkakoti, in front M. Egli, H. P. Webber, I. Sinning, G. Klebe and W. L. Duax.

The microsymposium Crystallography Aided Drug Design focused on drug design based on models of receptor and ligand, the most direct, promising, and popular route to design novel, highly specific drugs. Examples of drug design starting from the crystal structure of a complex of an enzyme with a lead inhibitor, showed how the interaction pattern of enzyme and ligand can be studied, and how one then can design chemical modifications in the inhibitor to increase affinity and specificity for a particular receptor. Typical of the presentations was the description of the design of new drugs against sleeping sickness. A careful study of the crystal structures of the human and the trypanosomal NAD: glyceraldehyde-3-phosphate dehydrogenases revealed some significant binding differences for the adenosine part of NAD cofactor of the enzyme. A "selectivity cleft" was detected and used to propose specific modifications in the adenosine part to achieve higher affinity selectivity of the modified cofactor towards the trypanosomal enzyme. In the final discussion, both strength and limitations of crystallography-aided drug design were pointed out. Clearly, crystal structures of enzyme-inhibitor complexes are by far the best starting point for any drug design. The famous 'lock and key' hypothesis of Emil Fischer, the basic principle of structure based drug design, is one hundred years old and as fresh as ever. (Hans Peter Webber)

Peter Paufler at the Mayor's reception.

With the increase of computer power available from workstations to parallel machines, theoretical work becomes progressively more important as was demonstrated in the microsymposium Computer Simulation of Crystal Structures (K. Schwarz and B. Winkler, Chairmen). V. Milman (Oak Ridge) illustrated the sort of problems that can now be solved by first principles calculations based on density functional theory (DFT) using pseudo-potential techniques, e.g. the interface reconstruction between Si and CoSi2. Such problems require large unit cells which nowadays can be studied using massively parallel computers. M. Catti (Milano, Italy) reported on his use of the Hartree-Fock method and a posteriori corrections for electron correlation based on DFT. His calculation of the enthalpy in a mineral spinel that decomposes under pressure compares favorably with experiments.

Other candidates for computational analysis included the study of crystal packing for which an energy function that is accurate and computationally feasible must be found. In addition an efficient and reliable search algorithm to locate the lower minima of the potential function is required. B. P. van Eijck (Utrecht, The Netherlands) described the few most stable structures among a million trial structures of monosaccharides and F. J. J. Leusen (Cambridge, UK) used a modified Monte Carlo procedure to find the most likely polymorphs of organic crystalline compounds. Once these are limited in number a full geometry optimization can be done. (Karlheinz Schwarz, Austria)

Dresden University of Technology provided a wonderful setting for the scientific sessions. Microsymposia were held in elegant old chemistry class rooms as well as modern new facilities. The 24 exhibits in the trade show filled the hallways along with the colorful posters.

A. Bacchi, C. Puppi, W. Clegg, K. A. Fraser, J. T. Cressey, M. L. Werk-Albers and H. Grimmer on the ship's rail on the Elbe.

Highlights of the social program included a reception given by the Lord Mayor of Dresden, a Saxon Farewell evening in the Rathskeller, a magnificent hike down the Basteir cliffs and a boat ride on the Elbe through the Saxon countryside into the center of Dresden, a city whose rich architecture and cultural heritage is being restored.

The meeting was organized by the European Crystallographic Committee by an international program committee, chaired by M. Schenk, and a local organizing committee, chaired by P. Paufler.

The French Singing Society performing at the Rathskeller.

The support of the following organizations was critical to the success of the meeting: Freistaat Sachsen; International Science Foundation; International Union of Crystallography; the Nederlandse Vereniging voor Kristallografie; Debeka Versicherungen; Hochschulservice Dresden; Degussa, Frankfurt; Deutsche  Forschungsgemeinschaft, Bonn; Deutsche Gesellschaft für Kristallographie, Berlin; Dresdner Bank AG, Dresden; IBM Deutschland, Dresden; Macmillan Magazines Ltd/'Nature', London; Molecular Structure Corp., Dusseldorf; Oldenbourg Verlag, München; Oxford Cryosystems, Oxford; Schering AG, Berlin; Stoe & Cie., Darmstadt; and Tcchniker Krankenkasse, Dresden. Session Chairs and W. L. Duax