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International Union of Crystallography

The IUCr is an International Scientific Union. Its objectives are to promote international cooperation in crystallography and to contribute to all aspects of crystallography, to promote international publication of crystallographic research, to facilitate standardization of methods, units, nomenclatures and symbols, and to form a focus for the relations of crystallography to other sciences.

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The silent partner in macromolecular crystals

jm5003The mother liquor from which a biomolecular crystal is grown will contain water, buffer molecules, native ligands and cofactors, crystallization precipitants and additives, various metal ions, and often small-molecule ligands or inhibitors. On average, about half the volume of a biomolecular crystal consists of this mother liquor, whose components form the disordered bulk solvent.

The solvent is therefore integral and also often an intrinsic part of almost any macromolecular crystal structure. Its disordered bulk components as well as its ordered constituents of varying nature need to be accounted for in modelling and refinement. The improvement of bulk-solvent description from a fundamental perspective is largely driven by methods development. In bulk-solvent refinement, users have limited choice beyond solvent-model selection and therefore not much opportunity for the introduction of bias or specific model errors, with the caveat that suboptimal masking sometimes can introduce density artefacts. In contrast, modelling of distinct solvent electron density almost always requires thoughtful interpretation, and using appropriate tools for (automated) building and validation can greatly improve the quality of structure models.

In a paper by [Weichenberger et al. (2015). Acta Cryst. D71, 1023-1038; doi:10.1107/S1399004715006045] a group of authors examine how to estimate the overall solvent content of a macromolecular crystal, how to account for and model disordered bulk solvent and how to properly identify and model distinct electron (or nuclear, in the case of neutron diffraction) density of ordered solvent molecules. The authors also emphasize that modelling of the biologically important interface region between the protein molecule and solvent is still incomplete, and advanced solvent models of these dynamic regions need to be developed.

Posted 25 Jun 2015 


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XXIV Congress & General Assembly of the International Union of Crystallography (IUCr 2017)

iucr2017logoIndia will host the 24th Congress & General Assembly of the International Union of Crystallography 2017 from August 21-28, 2017. We extend a warm welcome to all crystallographers from across the world!

A cutting edge program with plenaries, keynotes, microsymposia and posters, commercial exhibits, satellite meetings, workshops and official meetings of the IUCr is planned. This will be a state of the art international meeting set in the unique ambience of India.

Hyderabad is a modern city of 9 million people that has gained prominence as an important software and pharmaceutical centre. Its iconic symbol is the Charminar or four minarets, which draws its inspiration from the tetragonal crystal system. It is the central motif in our logo for IUCr 2017.

Hyderabad International Airport has a wide range of connections through hubs in the Gulf, South East Asia and Europe. Of course, one can also fly in through Delhi, Mumbai or just about any major airport in India. The Congress venue, Hyderabad International Convention Centre (HICC) is on par with the best Congress destinations around the world and equipped with the finest technological, communication, digital and audio-visual tools.

We are sure that IUCr 2017 will be an experience to remember for everyone. 

Gautam R. Desiraju
Immediate Past President, IUCr
Posted 23 Jun 2015 

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Aperiodic crystals and beyond

Once a contradiction in terms, aperiodic crystals show instead that “long-range order” has never been defined. Whatever it means, decades of intense research have shown it to be more complex and surprising than anyone suspected [Senechal (2015). Acta Cryst. B71, 250-251; doi: 10.1107/S2052520615009907]

The human brain is very skilled at detecting patterns and recognising order in a structure, and ordered structures permeate cultural achievements of human civilisations, be it in the arts, architecture or music. The ability to detect and describe patterns is also at the basis of all scientific enquiry.

Crystals are paradigms of ordered structures. While order was once seen as synonymous with lattice periodic arrangements, the discoveries of incommensurate crystals and quasicrystals has led to a more general perception of crystalline order, encompassing both periodic and aperiodic crystals. The current definition of crystals rest on their essentially point-like diffraction.

Considering a number of recently investigated model systems, with particular emphasis on non-crystalline ordered structures, the limits of the current definition are explored in a paper [Grimm (2015). Acta Cryst. B71, 258-274; doi:10.1107/S2052520615008409].

The current definition of a crystal is based on the currently known catalogue of periodic and aperiodic crystals. Scientists currently do not know of any materials that have aperiodically ordered structures beyond incommensurate crystals and quasicrystals. The definition of a crystal also reflects the lack of understanding of what constitutes order in matter, and in this sense should be seen as a working definition that may well need to be revised in the future. In crystallography, order is linked to diffraction, which makes sense because diffraction is the method of choice to experimentally determine the structure of a solid. Grimm demonstrates that there are ordered structures which are not captured by the current definition, either because their pure point diffraction fails to be finitely generated, or because they do not have any non-trivial point component in their diffraction.

While we do not know whether such structures are realistic in nature, it should become possible to manufacture materials with purpose-design structure and properties. In this sense, these are structures that are relevant and should be considered to be within the realm of crystallography.

Posted 16 Jun 2015 

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Framework materials yield to pressure

[CoII octahedral packing diagram]Pressure is a powerful thermodynamic variable that enables the structure, bonding and reactivity of matter to be altered. In materials science it has become an indispensable research tool in the quest for novel functional materials.

Materials scientists can exploit the effectiveness of pressure for probing and tuning structural, mechanical, electronic, magnetic and vibrational properties of materials in situ; crystallography plays a crucial role, enabling on the one hand the unravelling of structural phenomena through a better understanding of interactions, and on the other shedding light on the correlation of structure and properties [Fabbiani (2015), Acta Cryst. B71, 247-249; doi: 10.1107/S2052520615009427].

With high pressure promoting effects such as magnetic crossover, spin transitions, negative linear compressibility, changes in proton conductivity, or even phase transitions that generate porous structures, high-pressure crystallographic studies on dense framework materials are on the rise. More generally, coordination compounds are a fascinating class of materials for high-pressure crystallographic studies, compared with purely organic compounds; they have an inherent extra degree of flexibility for responding to moderate applied pressures, as the geometry at the metal centre can undergo marked changes, whereas other primary bond distances and angles remain largely unaffected.

A group of scientists [Yakovenko et al. (2015), Acta Cryst. B71, 252-257; doi:10.1107/S2052520615005867] demonstrate that pressure offers a novel approach for generating new phases and exploring the structure-property relationships of molecular materials.

In their study the researchers present a high-pressure crystallographic study of α-Co(dca)2, including the structural determination of the high-pressure phase γ-Co(dca)2. The pressure dependence of the atomic structure was probed within a diamond-anvil cell using synchrotron-based powder diffraction methods.

Future work from the group based at Argonne National Laboratory will involve investigations of the pressure-dependent structures of further transition metal dicyanamides, including members of the iso-structural α-MII(dca)2 family as well as other polymorphs, to uncover any universality or metal-ion dependence associated with the α→γ transition, and if other new phases can be generated.

Posted 10 Jun 2015 

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On-demand X-rays at synchrotron light sources

Consumers are now in the era of "on-demand" entertainment, in which they have access to the books, music and movies they want thanks to the internet. Likewise, scientists who use synchrotron light sources are welcoming an era of “on-demand” X-rays, in which they have access to the light beams they want thanks to a technique developed at the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab).

Working at Berkeley Lab’s Advanced Light Source (ALS), a DOE Office of Science User Facility, researchers have created an operational mode for synchrotron light sources that provides full control of the timing and repetition rate of single X-ray pulses without affecting beams for other users [Hertlein et al. (2015). J. Synchrotron Rad. 22, 729-735; doi:10.1107/S1600577515001770]. The mode, which is called "pseudo-single-bunch kick-and-cancel", or PSB-KAC for short, works by displacing and routing a single electron bunch – dubbed the "camshaft bunch" – from the multi-bunch train of an electron beam in a synchrotron storage ring so that only X-ray light from this camshaft bunch reaches the experiment.

"With PSB-KAC, synchrotron light source users conducting stroboscopic experiments have full control over which of the X-ray pulses will arrive at their sample out of a pulse train that usually arrives at a fixed frequency of 500 megahertz", says ALS beamline scientist Andreas Scholl. "PSB-KAC allows users to do timed experiments in which the X-ray pulses must be synchronized to a laser, detector, or some other device during normal light source operations because the X-ray pulse pattern can be dynamically controlled for their individual experiment."

Says David Robin, the ALS division deputy for Accelerator Operations and Development, "The on-demand X-rays provided by PSB-KAC greatly reduces radiation damage to samples while improving signal-to-noise and background ratios for sensitive measurements. Until now, the X-ray pulse train in synchrotron light sources has been static and limited by the design and operation of the storage ring. With PSB-KAC, synchrotron light source users can optimize their experimental set-up for single shot, kilo- or mega-hertz repetition rate pulses."

PSB-KAC has been in use at the ALS for nearly a year and the response by users has been enthusiastic. Robin says the technique is applicable to other existing synchrotron light sources and should be advantageous for future light sources as well.

"We have demonstrated the use of PSB-KAC at the ALS in pump–probe measurements on spin crossover complexes, and in a warm dense matter demonstration experiment using a time-integrating single-shot streak camera", Robin says. "I would not be surprised if other synchrotron light source facilities, including next generation storage rings, begin PSB-KAC operations in the near future."

This article is reprinted from material taken from Lawrence Berkeley National Laboratory, with editorial changes made by IUCr.
Posted 28 May 2015 

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IUCr sponsorship and support of meetings

AONSAIn order to fulfill its roles to promote international cooperation in crystallography and to contribute to the advancement of crystallography in all its aspects the IUCr regularly sponsors symposia and workshops on topics relevant to crystallography. There is a well defined procedure that should be followed when applying for sponsorship. The rules can be found here.

The Executive Committee has considered the latest quarterly batch of applications submitted to the Sub-Committee on the Union Calendar and we have pleasure in announcing the following events have been granted sponsorship and financial support for young scientists.

The conditions of sponsorship are as follows: A substantially reduced registration fee should be provided for recipients of the Young Scientist Awards. Preference should be given to those young scientists who have published in IUCr journals. For meetings of a more general nature preference should be given to young scientists with a crystallographic background. Awards for young scientists should be used solely for travel and subsistence expenses and not as a waiver of registration fees. Young scientists are graduate students, post-graduate students or post-doctoral fellows with a maximum age of 30 (exceptionally 35). A report on the uses made of the above money and a general report on the meeting should be sent to execsec@iucr.org

If you are organizing a meeting and wish to be considered for IUCr support please visit http://www.iucr.org/iucr/sponsorship/meetings.html

Posted 26 May 2015