The microsymposium Macromolecular Cryocrystallography (1.01) was a very well attended and lively session on Cryocrystallography for Macromolecular studies was introduced by Steve Ealick. He pointed out that an ever increasing proportion of newly published macromolecular structures are determined using the technique. He briefly summarised the steps involved in flash freezing a crystal, as well as the many new possibilities presented by the technique. These arise not only due to the greatly reduced radiation damage suffered by crystals held at 100K during data collection, but because the gentler mounting techniques allow projects which were not previously feasible to be attempted, and also reaction pathways can be investigated by trapping crystals in various states by freezing them. The session consisted of 3 talks on cryocrystallographic techniques interspersed with 4 on exciting applications of it. The techniques and then the applications talks are reported below. Hakon Hope, drawing on his extensive small molecule and protein flash cooling experiences, then described some technical aspects of crystal handling and various tools designed in his laboratory to ensure a high success rate in crystal freezing, storage and retrieval. He strongly recommended plunging crystals into liquid nitrogen cryogen for flash cooling. David Rogers gave an introduction to the practicalities of crystal cooling in the laboratory, encouraging those who had never tried the technique to attempt it. He described the experimental side of cryocrystallography and from his wide experience, he explained in detail how to determine suitable cryoprotectant conditions, emphasising the importance of this step for the success of the technique. He outlined how to mount, transfer, store and transport crystals, and also felt that liquid nitrogen was a very good cryogen. He pointed out that the fibre mounting loops could be used to good effect at room temperature if enclosed in a little dome and also that some radiation damage is still suffered by frozen crystals. T.Y.Teng discussed some of the physics involved in the shock cooling process and how these principles affected the actual cooling rates achieved in real experiments. The rates are notoriously difficult to measure reliably, and depend critically on the volume of the crystal as well as on the initial temperature and on the particular cryoprotectant used. Measurements presented by Teng using different liquid cryogens showed that the cooling rates were from around 50 to 700 degrees/s, depending on the cryogen and the temperature range being considered. An application of cryocrystallography was presented by Pamela Williams, who had flash frozen partially reduced crystals of nitrite reductase in order to follow the reaction pathway, monitoring the reduction state of the haems during the X-ray data collection using a microspectrophotometer mounted around the goniometer. This enzyme converts nitrite to nitric oxide and 6 different states along the pathway were crystallographically characterised, a study made possible by the combination of spectroscopy and X-ray cryocrystallography techniques. Cryocrystallography applied to virus crystals is a relatively new and expanding use of the technique. Brenda Temple reviewed current work and results in this field, pointing out that although it had been widely thought that freezing virus crystals would be extremely difficult, it was possible and in principle should be manageable on all viruses. Some success with freezing various virus crystals has already been achieved by careful choice of cryoprotectant conditions; more components are usually required in the cryobuffer than for protein crystals. Again, radiation damage effects had been observed in data from frozen crystals. However, since virus crystals are usually exquisitely sensitive to radiation damage, the technique holds much promise for virus crystallographers. This reduction in radiation damage and also the freezing out of thermal disorder in crystals held at 100K can enable atomic resolution data to be obtained from some well ordered proteins. Elspeth Garman reported the 1.05A structure of a 42kDa bacterial enzyme: neuraminidase from Salmonella Typhimurium. At this resolution dual conformation side chains, extensive ordered water networks and also hydrogen atom positions can be indentified. Previously such atomic resolution studies have only been possible on large small molecules. Lastly, Martin Walsh outlined a study of the three redox states of flavodoxin and the structural changes associated with mutating some specific amino acids at the binding site. The redox states were isolated by freezing the crystals at the appropriate time during the reduction, and performing all the experiments at low temperature made the whole investigation much easier than it would have been at room temperature.
In the microsymposium Synchrotron Radiation II - Macromolecules. (01.03) M. E. Wall, from Princeton University, reported on the first complete digitization of a three-dimensional map of diffuse scattering from a crystal of a nuclease. He and his coworkers have analyzed this pattern of scattering to study the nature of the internal dynamics of the protein. Z. Dauter gave an overview of high-resolution macromolecular data collection (in the range 0.9-1.3 Ang.) as pioneered at the EMBL Outstation at Hamburg; detailed high resolution refinements are now available for more than twenty protein structures, thus bridging the gap between small and large structures. The phase problem was central in four communications. E. Weckert (Karlrushe) reported on the phasing of tetragonal lysozyme using three-beam interference effects and W. Weis (Stanford U) described the derivation of model-free protein phases by the MAD technique. The combination of the large anomalous scattering effect at the L(III) absorption edge of lanthanide ions and a large partial structure contribution from these ions led to phases of an unprecedented accuracy. M. Schiltz (LURE) discussed the use of xenon and krypton as heavy atoms and anomalous scatterers, and described a test experiment on an elastase crystal combining a SIRAS experiment and solvent flattening. Data were collected at a single wavelength on the same sample at normal pressure and under compressed krypton. The weak anomalous and isomorphous effects resulting from the binding of about 0.5 Kr atom per protein molecule were sufficient to get a very accurate electron density map. G. Privé, (Toronto) described the phasing of a 450-atom peptide structure by the Shake-and-Bake direct method of structure determination. High resolution data obtained with SR at BNL's NSLS beamline X12-C was a prequisite for this successful determination. Finally, C. Nave, (Daresbury) elaborated on the optimisation of data collection with SR: a timely communication indeed, since new instruments for macromolecular crystallography are being installed at several third generation SR facilities.
Roger Fourme & Robert Sweet
Neutron Scattering I: Dynamical Aspects. (01.07) The Neutron Scattering Commission attempted to cover the whole gamut of systems available to neutron investigation from electronic excitations in magnetic systems to the complexity of slow motions in biomolecular aggregates. No casualties were reported! Energy levels from microelectron volts to electron volts can be studied using inelastic neutron scattering and the intensity of the modes is directly related to the amplitude of vibration of the atom involved. After hearing of the latest studies of the dynamics of high Tc's from M. Arai we moved to the rotational dynamics of ammonium ions in a talk from S. Belushkin. J. Eckert showed data on the dynamics of hydrogen in co-ordination complexes and J. Larese demonstrated the sensitivity to quantum tunnelling of molecules on surfaces as you move from two-dimensional to three dimensional behaviour. B. Asmissan reviewed the beautifully systematic work on rotational dynamics of methane in rare gases and its theoretical interpretation. Finally A. Deriu shared latest results on the study of the diffusion of water in bio-gels.
The Protein-DNA Session (04.05) featured researchers tackling the problem of understanding how proteins recognize and modify DNA from many different vantage points. C. Calladine (Cambridge U.) presented results of detailed analyses of DNA conformation in protein DNA complexes and demonstrated the structural changes in DNA caused by binding of the catabolite activator protein. M. Lewis (U. of Pennsylvania) and J. Geiger (Yale U.) both described cocrystal structures in which protein binding causes DNA deformation. The Lac repressor operator complex (Lewis) revealed a pair of alpha helices binding in the minor groove creating a bent DNA stucture that provides a model for binding of Lac repressor to all three operators of a canonical Lac operon. Geiger presented the structure of a triple complex of transcription factor IIA, TATA box-binding protein and a TATA element. In sharp contrast to the Penn work, this structure demonstrates minor groove binding and DNA bending by the anti-parallel beta sheet of the TATA box-binding protein. In the arena of molecules that perform chemistry on DNA, we were treated to four structures presented by D. Vassylyev (Protein Engineering Research Institute), S. Fujii (Osaka U.), A. Mondragon (Northwestern U.), and L. Beese (Duke U.). Vassylyev and Fujii described structures of molecules that participate in DNA repair, with the PERI structure demonstrating base flipping. Mondragon's topoisomerase structure was beautiful in its own right, and suggested an intriguing model for its mechanism of action. The high point of the session was an even more penetrating view of a similarly complicated enzyme. Beese presented the structure of a thermostable DNA polymerase at work! She has been able to record diffraction data from a cocrystal of the polymerase with substrate, add a single nucleotide triphosphate to the crystal and then repeat the crystallographic experiment. The resulting data gave a Fourier synthesis that showed that the polymerase had catalyzed single base addition to the substrate within the confines of the crystal lattice. This striking result and other equally impressive structural studies presented throughout the XVII Congress underscore an important transition that all of protein crystallography is undergoing. Technical advances in computing, X-ray production, position-sensitive X-ray detectors, cryopreservation of the crystal, and multiwavelength anomalous dispersion are allowing us to tackle ever larger and more difficult problems. I can think of no more interesting profession that that of structural biologist.
Stephen K. Burley
In the microsymposium on Charge, Spin and Momentum Density (09.01) four speakers described their algorithms and programs for application of MEM methods in charge density analysis. Contradictory results, including the existence or non existence of non nuclear attractors in Be, led to a very simulating discussion on the need of a non uniform prior probability before anybody is able to use MEM routinely in multipolar analysis. An interesting talk on transferability of the electron density parameters to macromolecules suggested that given a data bank of multipolar parameters of atoms, we may soon be able to estimate the electrostatic properties of small proteins.
High Tc Superconducting Materials. (10.05). New insight leading
to more predictive ability and new materials was the theme of the microsymposium
on High Tc Superconducting Materials. The keynote speaker Catherine Chaillout
(Grenoble) described the need to use simultaneously different techniques
to understand the complex structures exhibited by high temperature superconductors.
Structural features of spin-ladder compounds (M. Takano, Kyoto U.) prepared
by high pressure techniques, a new class of cuprates closely related to
the cuprate superconductors, were described in the context of their novel
physics. Presentations on metal oxide metal-pnictide layered compounds (S.
Kauzlarich, University of California-Davis), new layered cuprates with tin
and titanium in the blocking layer (K. Poeppelmeier, Northwestern U.) and
new compounds derived from substitution in the (HgO
Kenneth R. Poeppelmeier
In his Keynote Address The Crystal Packing of Organic Small Molecules (11.00) A. Gavezzoti gave an overview of the methods employed in the analysis of crystal packing including analysis of packing coefficients (the ratio between the molecular volume and the unit cell volume) and the use of principal component analysis to define independent properties and computational models that link structure and thermodynamics. He discussed the calculation of the packing energy for different polymorphic forms using an empirical force field, and the differences between crystal and solution structures. He described the calculation of the carboxylic acid dimer to catemer jump fluctuations in solution by molecular dynamics that lead to an understanding of crystal growth and the prediction of crystal structure.
Surface II: Thin Films and Multilayers. (12.02) Seven talks were
presented, of which three concerned metal films, one semiconductors, one
oxides, one liquid/solid interfaces and one scattering theories. All talks
described the structure of multilayer interfaces studied by X-ray scattering.
Most of the experimental studies used synchrotron radiation. Discussion
was very active x-ray scattering is becoming increasingly important to investigate
the interface structures of thin layers, crystalline or non-crystalline,
without destroying samples. In particular, correlated roughness structures
in multilayers, which have important relevances to the electronic, optical
and chemical properties of synthetic multilayers and their growths can only
be studied with X-rays. New structures and techniques are coming into the
scene, like quantum dots and speckle techniques.
Open Commission Meeting on International Tables for Crystallography
The editors of the three published (A, B, C) and the four proposed (D, E, A1, A2) volumes of International Tables reported on the present status and the future plans of their volumes. Th. Hahn demonstrated two new features of the Fourth Edition (1995) of Volume A, "Space-Group Symmetry": completion of the space-group diagram project for all 17 plane and 230 space groups, and introduction of the "double glide plane" e by means of new graphical symbols for 17 space-group diagrams and modifications of five orthorhombic space-group symbols. The plans for the Second Edition of Volume B, "Reciprocal Space", were presented by the editor U. Shmueli. In addition to minor corrections and major revisions of several existing chapters the Second Edition will contain five new contributions on the following topics: 1. Space-group representations in reciprocal space. 2. Direct methods in electron crystallography. 3. Diffraction by polymers. 4. Reciprocal-space images of aperiodic crystals. 5. Dynamical theory of neutron diffraction. This edition is scheduled for publication in 1997. The editor of Volume C, "Mathematical, Physical and Chemical Tables", E. Prince, explained that the preparations for the Second Edition of this Volume (scheduled for publication in 1997) were about 85% complete at the time of death of the previous editor, A.J.C. Wilson. The rest is presently being edited. Due to rapid obsolence of much of the tabular material in this volume, an electronic version of Vol. C is presently under active consideration. The new Volume D, "Physical Properties of Crystals", scheduled for 1997, was introduced by its editor A. Authier. It consists of three parts: Part 1 treats Tensorial Aspects of Physical Properties; it contains both the general mathematical background and the individual physical properties. Part 2 is devoted to the Symmetry Aspects of Excitations (phonons, electrons, Raman scattering, Brillouin scattering). Part 3 is devoted to the Symmetry Aspects of Structural Phase Transitions, Twinning and Domain Structures. Relevant tables will be included in an accompanying diskette or CD ROM. The new Volume E, "Subperiodic Groups", is edited by V. Kopsky and D.B. Litvin. Its publication is planned for 1997. The volume is devoted mainly to layer and rod groups, i.e. to groups in three dimensions with lattices of only two or one dimensions. The volume consists of two parts: Part 1: Subperiodic group tables: Frieze, Rod and Layer Groups. Part 2: Scanning of space groups, i.e. layer groups as subgroups of space groups. Volume A1, "Maximal Subgroups of Space and Plane Groups", was introduced by its editor H. Wondratschek. It is a companion volume to Vol. A, containing the maximal subgroups of space and plane groups. Maximal subgroups of indices 2, 3, and 4 will be listed individually. The infinitely many maximal isomorphic subgroups will be presented by series with the index as parameter. Conjugacy relations will be indicated. The final project of the session was presented by T. Janssen: Volume A2, "N-Dimensional Crystallography". This volume consists of a computer programme (CD-ROM) and an accompanying printed volume with a manual and a treatment of the theory of symmetry in arbitrary dimension. This implies that it gives the information on two- and three-dimensional point groups as well. For more than three dimensions these groups are used already by crystallographers dealing with incommensurate phases and quasicrystals. From a data base are calculated the space groups, the symmetry elements, the Wyckoff positions, the general and special extinction rules, in short all that can be found in the existing tables for 2 and 3 dimensions.