May 252017

The calculated intensities as Ω and X are varied for a detector at 2θ = 60° (a) and 2θ = 110° (b) for 10 µm crystallites

Are we blindly accepting all the interpretations that arise from our present description of X-ray diffraction?  Is it reasonable that all crystals have to be “ideally imperfect” to determine their structure?  Bragg’s law cannot avoid dynamical effects, and therefore the measured intensity is not equal to the square of the structure factor unless the crystal is assumed to be “ideally imperfect”.  If polycrystalline diffraction is formed from crystals satisfying Bragg’s law, why is the background so high compared with single crystal profiles?  Are more crystals required in polycrystalline diffraction to study complex structures with large unit cells to ensure all the peaks are captured?  If the variation of intensity around the diffraction rings from polycrystalline samples is associated with a large range of crystal sizes, why does the data from a standard reference material of similar size crystals still reveal this variation?  Are we not just modifying our sample description and instrument performance so that the current theory fits the data?  After many years of theoretical and experimental work I am convinced that I have a good explanation.

Let us look back more than a hundred years, when the two Braggs interpreted the experiments of Friedrich, Knipping and Laue.  Their interpretation was simple, clever and explained the data giving us Bragg’s law and the Bragg equation.  This equation gives the position of the diffraction peaks and any surrounding scattering is considered as a perturbation, giving information on the crystal size, strain and defects.  This description struggles to answer the questions above.

Suppose Bragg’s law is not necessary to form a diffraction peak as proposed by Fewster[1], then we can start to answer these questions.  This proposal describes how the specular (mirror) reflections from crystals planes and their periodicity give rise to two peaks, one at the mirror angle and the other at twice the Bragg angle, 2θB.  The mirror peak broadens with crystal defects and distortions, with the whole width scattering intensity towards the angle 2θB.  This broadened mirror peak contributes to the background[2].  The intensity from crystals not satisfying Bragg’s law will form a weak contribution at the angle 2θB, explaining the intensity variation in diffraction rings from polycrystalline samples.  Bragg’s law occurs where the mirror reflection and the angle 2θB peak overlap.  Therefore, if the former is broad as in an imperfect crystal Bragg’s law and therefore the dynamical effects can only exist over a small proportion of the intensity profile.

If Bragg’s law is not a requirement to create a diffraction peak, then it is possible for many peaks to be observed simultaneously.  This explains the diffraction patterns observed at X-ray free electron lasers, i.e. the appearance of several diffraction spots and their variable intensity.  Similarly, the diffraction profiles from polycrystalline materials can be explained, i.e. small numbers of crystals and the full set of peaks from a complex sample.  This description accounts for the data but indicates that a typical measurement of intensity close to a diffraction peak is inadequate, because this is only a proportion of the total intensity, and therefore cannot be directly related to the structure factor.  A study[1] on a polycrystalline silicon sample suggests that this new description gives the structural parameters within acceptable bounds whereas the conventional theory does not.

The significant step in this description is that the intensity is enhanced at the angle 2θB regardless of the crystal orientation.  This can be observed experimentally.  So why has it not been knowingly observed before?  If conventional theory is so strongly part of the crystallographer’s thinking this enhancement is easy to overlook as just some artifact.  This proposal suggests that the derived sample models could be faulty.  The magnitude of this error is difficult to assess, but with three components; the diffraction data, a theoretical description and a model of the structure, we require two of these to be correct to reproduce the third.  Suppose we assume our data is reliable, then if the theory is incomplete the structural model will be biased or unreliable.

I strongly believe that we should be questioning and discussing our current theory of X-ray diffraction, because all our structural models determined to date might be faulty or inaccurate.

[1] Fewster.  (2014).  Acta Cryst. A70, 257-282; doi:10.1107/S205327331400117X

[2] Fewster.  (2016).  Acta Cryst. A72, 50-54; doi:10.1107/S2053273315018975

View the on-demand version of our webinar with Paul Fewster the author of this work on the IUCr YouTube channel.

You can view the questions and answers covered during the webinar here.


May 192017

An example of a three-dimensional structure of a macromolecule solved using cryo-electron microscopy

The invention of the electron microscope revolutionized how scientists view small structural details. The technology has undergone considerable evolution and in recent years single-particle cryo-electron microscopy (cryo-EM) has gained importance in structural biology. A topical review on cryo-EM has recently been published in Acta Crystallographica Section F (Vénien-Bryan et al., 2017, Acta Cryst. F73, 174-183). The review discusses the importance of cryo-EM and highlights recent developments. It describes how cryo-EM and other structural biology techniques, especially X-ray crystallography, now complement each other and how cryo-EM has been used in drug discovery.

The synergistic convergence of technological and computational advances now makes cryo-EM a feasible method for determining structures at near-atomic to atomic resolution (~5-2 Å). The latest generation of cryo-electron microscopes are equipped with direct electron detectors and software for the automated collection of images. In combination with the use of advanced image-analysis methods, the performance of this technique has dramatically improved. Less than a decade ago calculating a sub-10 Å resolution structure was an accomplishment but it is now common to generate structures at sub-5 Å resolution and even better. It is becoming possible to obtain high-resolution structures of biological molecules relatively quickly, in particular large ones (>500 kDa) which, in some cases, have resisted more conventional methods such as X-ray crystallography or nuclear magnetic resonance (NMR).

The potential impact of cryo-EM on drug discovery is large. Newly resolved protein structures may provide details of the precise mechanisms that are essential for cellular physiological processes. The ability to attain atomic resolution may support the development of new drugs that target these proteins, allowing medicinal chemists to understand the relationship between their molecules and targets. In addition, recent developments in cryo-EM combined with image analysis can provide unique information on connections between conformational variability and the function of macromolecular complexes.

The authors conclude that although crystallography remains the method of choice to obtain structural information from proteins for use in drug discovery, the arsenal of methods now available increases the range of possibilities, and cryo-EM is one of these methodologies, particularly for investigating changes in conformation. However, what still remains to be improved is the provision of high-quality proteins for study and so developments in purification processes are becoming fashionable once again.


May 032017

C. Richard A. Catlow, Main Editor, IUCrJ

The papers published during the last year in IUCrJ in the fields of materials and computational science illustrate well the challenges posed by structural problems in the science of materials and the key role that computation can play in this and related fields in structural science. As in previous years, they demonstrate the continuing developments in techniques and instrumentation and the increasingly complex structural problems which these developments now make accessible; the role of computation in interpreting and predicting structures is equally clear.

An excellent example of technical developments facilitating new structural science is provided by the article of [Meng, Y. & Zuo, J.-M. (2016). IUCrJ, 3, 300-308], which probes three-dimensional nano-structures using a technique that employs high-resolution and low-dose scanning electron nano-diffraction (SEND) to acquire three-dimensional diffraction patterns. Their work investigates TiN – a material that is widely used in the electronics industry – and Fig. 1 illustrates how they were able to reconstruct grain structures within the material. Detailed knowledge of this microstructure is essential in understanding and optimizing the properties of the material.

Figure 1. Reconstructed grains and their orientations. Meng, Y. & Zuo, J.-M. (2016). IUCrJ, 3, 300-308

Previous editorials have emphasized the key role of diffuse scattering, which is also facilitated by technical advances. The importance of the field in materials science is well illustrated by the article of [Sawa, H. (2016). IUCrJ, 3, 298-299], which highlights the work of [Welberry, T. R. & Goossens, D. J. (2016). IUCrJ, 3, 309-318] on the interpretation of diffuse scattering from the high-temperature superconductor, HgBa2CuO4 + δ. Analysis of the diffuse scattering data reveals fascinating features involving the displacement of metal atoms around oxygen interstitial chains. This article along with several others demonstrates the need to elucidate complex structural features in disordered materials.

Analysis of diffuse scattering is also vital in the particularly exciting challenge of developing detailed models for the atomic arrangements in quasicrystals. The article of [Ishimasa, T. (2016). IUCrJ, 3, 230-231] highlights the study of [Yamada, T., Takakura, H., Euchner, H., Pay Gómez, C., Bosak, A., Fertey, P. & de Boissieu, M. (2016). IUCrJ, 3, 247-258] on the atomic structure and phason modes of the Sc–Zn icosahedral quasicrystal, which employs synchrotron-based diffraction and diffuse scattering to investigate this difficult problem.

Figure 2. A polyhedral representation of the denisovite structure. Rozhdestvenskaya, I. V., Mugnaioli, E., Schowalter, M., Schmidt, M. U., Czank, M., Depmeier, W. & Rosenauer, A. (2017). IUCrJ, 4, XXX-XXX.

The complexity of structural problem that can now be addressed is well illustrated in the paper of [Rozhdestvenskaya, I. V., Mugnaioli, E., Schowalter, M., Schmidt, M. U., Czank, M., Depmeier, W. & Rosenauer, A. (2017). IUCrJ, 4, XXX-XXX], who use a wide range of techniques including several electron crystallographic methods, XRPD and modelling to solve the structure of denisovite, a highly complex, fibrous, polytypical silicate. The structure revealed is shown in Fig. 2. The article is an elegant illustration of the capacity of, and the need for, a multi-technique approach in addressing structural problems in materials science.

A further example of complex structural science is given by the study of SnTe reported by [Sist, M., Jensen Hedegaard, E. M., Christensen, S., Bindzus, N., Fischer, K. F. F., Kasai, H., Sugimoto, K. & Brummerstedt Iversen, B. (2016). IUCrJ, 3, 377-388]. This material is increasingly investigated owing to its potential as a thermoelectric material and as a topological insulator. Their study again reveals the importance of disorder and emphasizes the need to include the effects of disorder in any theoretical investigation of the material.

Several papers illustrate both the growing power of computational methods in structural science and the role of new methodologies and algorithms in investigating structural problems [Genoni, A., Dos Santos, L. H. R., Meyer, B. & Macchi, P. (2017). IUCrJ, 4, 136-146] explore the concept of X-ray-constrained Hartree–Fock wavefunctions (XC–WF) and discuss how the procedure can be used to extract correlation effects. Their careful analysis demonstrates that the single determinant XC–WF only partially captures the effects of correlation. The paper of [Wall, M. E. (2016). IUCrJ, 3, 237-246] on quantum crystallography and the charge density of urea shows, as the authors comment, the benefits and feasibility of integrating fully periodic quantum charge-density calculations into ultra-high-resolution X-ray crystallographic model building and refinement. While the value of force-field-based methods is illustrated by the paper of [Li, X., Neumann, M. A. & van de Streek, J. (2017). IUCrJ, 4, 175-184], who evaluate different force fields in the context of their use in dynamical simulations for the prediction of chemical shifts in solid-state NMR.

The importance of the structural science of materials is, of course, illustrated by many other articles published in other journals. Of particular interest is the way in which multi-technique approaches are pinning down key structural features of catalytic materials under real operating conditions. We have previously highlighted the work of [Lezcano-Gonzalez, I., Oord, R., Rovezzi, M., Glatzel, P., Botchway, S. W., Weckhuysen, B. M. & Beale A. M. (2016) Angew. Chem. Int. Ed., 55, 5215-5219], which combines high-resolution fluorescence-detection X-ray absorption near-edge spectroscopy, X-ray diffraction and X-ray emission spectroscopy under operando conditions to provide detailed new insights into the nature of the Mo species on zeolite ZSM-5 during methane de­hydro­aromatization. Another recent example is the work of [Malta, G., Kondrat, S. A., Freakley, S. J., Davies, C. J., Lu, L., Dawson, S., Thetford, A., Gibson, E. K., Morgan, D. J., Jones, W., Wells, P. P., Johnston, P., Catlow, C. R. A., Kiely, C. J. & Hutchings, G. J. (2017). Science, 355, 1399-1403], who combined XAFS and modelling to show that in an industrially important acetyl­ene hydro­chlorination catalyst, comprising gold on a carbon support, the active sites are not, as previously thought, gold nano-clusters but single gold ions. Catalysis will unquestionably continue to pose fascinating problems for structural science.

It is hoped that this brief survey gives an impression of the range and excitement of the field of the contemporary structural science of materials and the way in which this can be unravelled by a multi-technique approach using experiment and computation. IUCrJ continues to welcome submissions in this growing field.



Apr 202017

Detail of the coordination spheres of the bromide anion with Et3BuN+Br−

While an IUPAC definition of hydrogen bonding was only released in 2011 after decades of discussions in the scientific community, it did not take such a long time to come up with an analogous definition of halogen bonding, following a revival of this interaction in the literature which can be traced back to the early 1990s, Fourmigué, M. (2017). Acta Cryst. B73, 138-139

The halogen-bonding interaction is essentially described as an electrostatic interaction between a charge concentration (Lewis base) and a charge-depleted area, called an σ-hole, that a covalently bound halogen atom exhibits in the extension of this bond.

In a recent paper by Szell et al, (2017). Acta Cryst. B73, 153-162 single-crystal X-ray diffraction structures have been reported for a series of seven halogen-bonded co-crystals featuring 1,3,5-tris(iodoethynyl)-2,4,6-trifluorobenzene as the halogen-bond donor, and bromide ions (as ammonium or phosphonium salts) as the halogen-bond acceptors. Depending on the stoichiometry, the resulting frameworks can form honeycomb structures of variable geometry, but also systems with four or six halogen bonds to the bromide ion. While the counter-cations generally occupy the void spaces in the present work, the construction of halogen-bonded frameworks with potential gas storage applications is an appealing prospect which may be facilitated in the future by ligands enabling directional and multidentate interactions.


Mar 302017

The large-solid-angle X-ray Raman scattering spectrometer at ID20. Photo credit: ESRF/McBride

S. Huotari and co-workers [J. Synchrotron. Rad. (2017), 24, 521-530] describe an end-station for X-ray Raman spectroscopy at beamline ID20 of the European Synchrotron Radiation Facility. The end-station is dedicated to the study of shallow core electronic excitations using non-resonant inelastic X-ray scattering.
X-ray Raman scattering (XRS) spectroscopy is a versatile tool for studying shallow X-ray absorption edges using hard X-rays. It has proven to be an invaluable technique for the study of electronic excitations in a variety of sample systems such as crystals, liquids and gases. Over the past decades, XRS has been applied to solve geoscientific questions by studying shallow core edges under extreme pressure and temperature conditions, follow chemical reactions in situ, and study liquid samples under well defined thermodynamic conditions.
A drawback of XRS is the orders-of-magnitude weaker scattering cross section in comparison with the probability for photoelectric absorption. This can be compensated for by using light sources with a very high brilliance and efficient signal collection; this has been the guiding motive for the design of the spectrometer presented in this paper.
The new end-station provides an unprecedented instrument for X-ray Raman scattering, and will open the door to renewed studies of low-energy X-ray absorption spectra in materials under in situ conditions, such as operando batteries and fuel cells, in situ catalytic reactions, and extreme pressure and temperature conditions.

Mar 282017

Dr Rosemary Wilson @rawilson80 Scientific training and outreach officer, EMBL Hamburg

Long after the lights went on in the PETRA III Max von Laue experimental hall on the DESY campus, and the 14 beamlines became hives of activity, one corner has remained dark and seemingly forgotten. But there have always been plans for this corner of the Max von Laue Hall, and now those plans are taking shape. This hutch, situated directly behind the EMBL crystallography beamline P14, will house P14.EH2, a new experimental endstation which will cater for time-resolved crystallography experiments. In the autumn of 2016, EMBL group leader Thomas Schneider and Professor Arwen Pearson from the Centre of Ultrafast Imaging (CUI) at the University of Hamburg, were successful in securing a grant from the German federal government which in part provides funds for designing and building the endstation. Since then basic designs for the instruments have been decided, the first bits of equipment ordered and positions advertised for postdocs to help construct and run the endstation. The group hope to be able to welcome friendly users by the end of 2018. So how about some details?

Why a new endstation?

PETRA III produces some of the most brilliant and intense X-rays on the planet. The EMBL macromolecular crystallography beamline P14 is optimised to create a tight and parallel beam. With so many photons reaching the sample at the same time, scientists can start to watch the structural changes a protein goes through during a reaction, rather than a time-averaged snapshot as we are familiar with in traditional crystallography. Schneider, Pearson and their teams have already used P14 to do these types of time resolved crystallography experiments, but the set-up is not ideal in the long run.

Briony Yorke in the hutch that will house the new endstation. The X-ray beam will enter the hutch through the hatch in the back wall. Photo: Rosemary Wilson

The hutch for P14.EH2 sits directly behind P14 and the new endstation will be fed by the same X-ray beam. Each time-resolved experiment will need a different instrumental set-up, optimised to the type of results the scientists want to achieve. Having a separate hutch dedicated to the complicated time-resolved set-ups means that the regular operation on P14 doesn’t need to be interrupted and the scientists can take time to plan, design and build their set-up before pressing the button to allow the X-ray beam into the hutch. “The idea is that the set-up will be very flexible and modular” explains Wellcome Postdoctoral Fellow Briony Yorke (@BrionyYorke), who is working with Pearson on the endstation. Due to the tight angle that the X-ray beam leaves the PETRA III storage ring, the other beamline hutches and endstations are narrow, with access to equipment only possible by climbing over and under beamlines and detectors. The nature of the time-resolved experimental set-up means that access has to be easy and comfortable, and this was a major consideration during the design process. (See the project webpage for more details on the hutch layout design and specifications.)

What methods and hardware will be available?

Time-resolved crystallography works by taking a series of snapshots of a molecule such as a protein in quick succession to build up a movie of the protein’s structural changes during a reaction (see also: Taking crystallography to the fourth dimension). In practical terms, the X-ray beam has to be interrupted to create many successive images. “The trick is to turn the X-ray beam on and off faster than the process we are interested in” explains Briony. “The current shutter on P14 is a milli-second shutter, so the only possibility to look at shorter time slices is to get the detector to count for a shorter time period by repeatedly turning it on and off, since we physically cannot open and shut the shutter fast enough.” On the new endstation, a circular metal disc with pattern of holes stamped in it called a chopper will repeatedly ‘chop’ the X-ray beam, breaking up the signal to provide the sought after shorter time slices.

The prototype Hadamard chopper designed by Briony’s master student Anke Puckert and made on the DESY site. Of the three outer ring sequences, the central pattern encodes the Hadamard sequence, the other two are control patterns. Credit: Anke Puckert

Briony’s particular interest is Hadamard time-resolved crystallography, a method she developed during her PhD, which will be an available option in the new set-up. While a chopper for standard time-resolved crystallography experiments may have a single hole in it, creating an interruption every, say second as it rotates through the beam, Hadamard choppers require a much more intricate pattern of holes. This will create many more interruptions, resulting in more data per crystal, and a more detailed ‘movie’ of the protein dynamics.

“The choppers pose quite a challenge” says Briony. “During experiments they will be spinning at very high speeds (several 1000 rpm), so they have to be cooled with water and placed under a vacuum, and shielded in a bullet-proof case something goes wrong!” Another key bit of equipment that will be found in the new hutch will be a laser for initiation of the (bio)chemical process the experiment aims to study. “The laser impulse will be delivered with fibre optics” explains Briony. “This will enable a really precise initiation of the experiments” she adds.

What science will it be used for?

“We are interested in looking at structural changes within proteins that happen during a chemical reaction, for example in an enzyme. The rate determining steps of these reactions happen on the microsecond to millisecond time-scale which P14.EH2 is designed to handle. We want to provide a regular service for people interested in exploring protein dynamics” says Briony. She will be using the time-resolved crystallography endstation, for example, to investigate how cataracts form in our eyes. “Cataracts form when proteins in our eyes start to stick together. But we don’t know what triggers this process. If we can get a picture of how and why this happens, we might be able to design medication to prevent them forming or even reverse the process” she says. P14.EH2 is likely to be interesting for a whole range of applications – from biotechnology to medicine. “For example, we could use the endstation to understand how some bacteria make crude oil, or eat plastic” explains Briony, “or gain a better understanding of how enzymes interact with medicines in our bodies. And all this evidence can be used to help build better computer simulations of molecular motions” she concludes.

What’s next?

Any day now the first instruments should arrive and the instrumentation group at EMBL can start constructing the endstation. Visit the project webpage for regular updates on their progress!

You can see papers published by Dr Thomas R. Schneider in IUCr Journals here

You can see papers published by Professor Arwen R. Pearson in IUCr Journals here


Mar 082017

The structure of biological macromolecules is critical to understanding their function, mode of interaction and relationship with their neighbours, and how physiological processes are altered by mutations or changes in the molecular environment.

Ideally, classical structural biology research should interface more with cellular biology, as it is crucial for the structural data obtained in vitro to be validated within the cellular or tissue context. A true cellular structural biology approach should allow macromolecules to be characterised directly in their native environment. Such an approach would guarantee the high significance of data obtained in vivo or in the cell with the high resolution of a structural technique.

In the Past decade, NMR spectroscopy has been applied to obtain structural and functional information on biological macromolecules inside intact, living cells. The approach, termed “in-cell NMR”, utilises the improved resolution and sensitivity of modern high-field NMR spectrometers and exploits selective enrichment of the molecule(s) of interest with NMR-active isotopes.

Since its inception, in-cell NMR has gradually emerged as a possible link between structural and cellular approaches. Being especially suited to investigate the structure and dynamics of macromolecules at atomic resolution, in-cell NMR can fill a critical gap between in vitro-oriented structural techniques such as NMR spectroscopy, X-ray crystallography and single-particle cryo-EM techniques and ultrahigh-resolution cellular imaging techniques, such as cryo-electron tomography.

In a topical review IUCrJ (2017), 4, 108-118 Lucia Banci and her co-worker Enrico Luchinat, both based at the University of Florence, summarise the major advances of in-cell NMR and report the recent developments in the field, with particular focus on its application for studying proteins in eukaryotic and mammalian cells and on the development of cellular solid-state NMR.


Mar 022017

Crystallography is the study of the crystalline state of matter. The meaning is contained in the etymology of the word: κρυσταλλογραφ[{\acute \iota}]α (krustallograpia) arising from κρ[{\acute \upsilon}]σταλλος (krustallos, `clear ice’) and γραφο (grapo, `I write’). The modern inter­pretation of this term is widely construed as `the study of crystals.’ Crystallography since the `Traité de Crystallographie‘ of René-Just Haüy (Paris, 1822) has been based on a direct space description of crystals. There are many ways to study crystals, for example, by optical, thermal, mechanical or diffraction techniques. The relationship between the crystalline state of matter and its ability to diffract waves of properly matched wavelength has played and will always play a special role in this area of research. Nuclear magnetic resonance (NMR) spectroscopy has, however, from its earliest days, provided structural information on both periodic and amorphous compounds, ranging from specific inter­nuclear distances to complete structural models of complex materials and biomolecules. The term `NMR Crystallography’ presents a broad polysemy. To some, it represents a stand-alone structure elucidation method for single crystal, polycrystalline or amorphous compounds. For others, it is a source of additional structural information when compounds fail to yield crystals of sufficient quality or size suitable for single-crystal diffraction-based structure determination, or when powder diffraction patterns exhibit a too high degree of complexity for structure model elaboration. Some consider NMR, diffraction and modelling as a synergistic complementary set of methods. Others consider that the multiplicity of specific NMR experiments allows for the progressive build-up of topological sub-graphs of the crystal graph, and thus drives the structure model search. These are all established uses of magnetic resonance toward the investigation of the crystalline state.

The Commission on NMR Crystallography and Related Methods of the Inter­national Union of Crystallography was established at the Montreal General Assembly in August 2014. The commission envisions `NMR Crystallography’ in an even more expansive light, as being: the use of the spin degrees of freedom of the magnetic resonance phenomenon to study the crystalline state of matter. As NMR may also be applied to the liquid state, it may also be used to study crystal genesis. The increasing profile of NMR in the crystallographic community is also nicely evidenced by an inspection of the planned lectures and microsymposia scheduled for the 24th Congress & General Assembly of the Inter­national Union of Crystallography, to be held in Hyderabad (August 21–28, 2017).

The contributions to this special issue of Acta Crystallographica Section C serve as an excellent introduction to the power and scope of NMR crystallographic methods and applications. The issue begins with two insightful review articles, one from Stebbins and co-workers on looking at short-range order in paramagnetic samples, and one from Harris and co-workers on in-situ techniques for time-resolved monitoring of crystallization processes.

NMR crystallographic methods have proven to be popular in characterizing organic co-crystals, particularly those with an active pharmaceutical component. Three contributions explore this area. Brown and co-workers describe the application of advanced two-dimensional solid-state NMR experiments to elucidate the role of weak inter­actions in a co-crystal of two fungicides. Kerr et al. have applied related methods to determine the crystal structure of a co-crystal of naproxen with picolinamide. Vigilante and Mehta report their 13C solid-state NMR work on co-crystals of caffeine and theophylline.

It is important to point out that there are many flavours of NMR crystallography, and many applications combine the information available from many techniques to gain structural insights. The information obtained from X-ray diffraction is typically used to its full potential, in combination with the additional information obtained from NMR. Computational chemistry, often in the form of density functional theory (DFT) calculations, is also often incorporated to the refinement protocol to obtain final structural results in best agreement with all available data. For example, Szell et al. report on a combined NMR, X-ray and DFT study of halogen-bonded frameworks featuring nitro­gen-containing heterocycles. DFT also plays a key role, along with 14N NMR, in the report from Alonso and co-workers on inter­molecular inter­actions in AST zeolites. Laurencin and co-workers describe the prospects available via 25Mg and 43Ca NMR spectroscopy in low-coordination-number organo-complexes with the help of DFT methods. Mali reports on the ab initio crystal structure prediction of magnesium (poly)sulfides, and on how the calculated NMR parameters may be used to distinguish between possible structures.

Nishiyama and co-workers describe a beautiful example of the power of combining the information available from electron diffraction with that of NMR spectroscopy to report on crystalline polymorphs of organic microcrystalline samples. Inorganic aluminophos­phates and layered silicates are explored in eloquent and insightful contributions from Ashbrook and co-workers and from Brouwer et al. Finally, an excellent example of the application of NMR crystallography methods to a metal–organic framework is provided by Pourpoint and co-workers, wherein carbon–aluminum distances are measured using advanced techniques.

This collection of papers provides a broad perspective on the diversity of NMR crystallographic methods and applications. It may be useful to keep in mind some of the particular advantages of NMR methods, including, for example, their sensitivity to H-atom positions and weak inter­molecular inter­actions, and their capacity to characterize disordered and noncrystalline samples. The continuing advances in NMR technology, pulse sequence development and robust structure refinement protocols, which include data available from all available sources, all bode well for the future of this exciting field.

David L. Bryce and Francis Taulelle

Guest editors


Feb 102017

Diana Freire, PhD student, EMBL

When I was a student I never dreamed that one day I would be a scientist, but here I am! My name is Diana Freire, I am from Portugal and I am doing my PhD in structural biology at one of the world’s leading research institutes: European Molecular Biology Laboratory (EMBL).

You may not know but 11 February 2017 marks the second United Nations International Day of Women and Girls in Science, and what better time than now to think about women in science and how we can make an effective and sustainable change to the number of women participating in science. You can learn more about the International Day here.

If you want to pursue a career in science, EMBL is a great place to do it. I have learnt so much here and I am definitely growing as a scientist and also as a person. I have been exposed to many different structural biology methods and had the honour of meeting many exceptional people – all of whom encouraged me to use X-ray crystallography in my study of a novel toxin-antitoxin protein complex from Mycobacterium tuberculosis and to unravel its mechanism of function.

Recently our scientific training and outreach officer Rosemary Wilson @rawilson80 encouraged me to take part in a story for Science News for Students ( The article resonated with me immediately: “We need you! Are you a female-identifying person in science, technology, engineering or math (STEM)? We want to see you and hear your voice!”. I knew this was something I wanted to be part of. While brainstorming the article, I thought back to why as a student I had never considered a career as a scientist. I came to the conclusion that I had a problem identifying with the images of scientists that I saw in the media at that time – I do not look like them!

Since I was 19 years old, fashion has played a major part of my life. I did a modelling course, took part in several fashion shows and photo shoots, and also participated in two contests to be Miss Portugal. Meanwhile, however, I was also always focused on my studies. At first I hid my fashion activities from everybody at university, afraid of being judged. I felt I had to prove to my (mostly male) professors that I was not only a pretty face. Then, a few years ago, a treatment I was receiving for cancer meant I lost my hair; it also effected the shape of my body. I overcame the cancer, and the best part of it is that I lost the fear of showing everybody who I really am. I was able to be open about what I do and love. Fashion and science: some might think, two opposite worlds that cannot mix.

These thoughts and experience encouraged me to make a video about myself and other female scientists to be shared with everybody but mostly with other women. I wish I could have included even more of these amazing female scientists in it, together with their inspiring stories.

I hope women outside science and especially female students can watch this video and see we are like any other women. Women in science can still have a life inside and outside the laboratory. It is possible to make hard work and dedication compatible with life and still have success in both. Science needs women. So, please, if you are one, believe in your career, be hopeful and pursue your dreams.

Women are not a minority in science any more but we are still under-represented at the highest levels. The reasons for this do not seem to be related to performance but mostly to psychological barriers, peer pressure or even feelings of self-doubt: it is important to develop our self-esteem. The guidelines of this year’s United Nations International Day of Women and Girls in Science on the topic “Gender, Science and Sustainable Development: The Impact of Media” highlight the importance of raising the profile of women in science by supporting and inspiring each other.

The general message I want to cover with the video is about life: enjoy it as much as you can. Do what makes you happy, and follow your passions. Fight for your personal and professional goals, and do not let others’ opinions put you down. Life is too short and we only live once!


Feb 102017

Michele Zema, IUCr Outreach officer

How do you encourage youngsters to think about crystallography? Start from the beginning and let them grow a crystal!

Crystal growth is such an important subject that it should be considered as a science in itself. It has its own journals and even an international organization, the International Organization for Crystal Growth, which coordinates the activities of researchers involved in the field.

Crystallization is still one of the most powerful purification methods in chemistry, used frequently at both university and industrial levels.

There is a tremendous amount of theory behind the science of crystal growth, and more and more sophisticated methods have been developed to understand the ever increasing numbers of resultant crystal structures. These discoveries can lead to new or better understood materials and properties covering many inorganic, organic and biological substances including drugs, nanostructured phases and many kinds of materials such as semiconductors, superconductors, magnetic materials and so on.

Don’t forget growing a crystal is also a lot of fun!

To coincide with the celebrations for the UN International Year of Crystallography 2014 (IYCr2014) and with the aim of raising awareness about this growing discipline, the IUCr launched a worldwide crystal-growing competition for schoolchildren (categories 15-18, 11-15 and under 11), which has now reached its fourth cycle. It aims to introduce students to the exciting, challenging and sometimes frustrating world of growing crystals. Students are invited to record a video to convey their experience to a panel of experts. The winning contributions in each category receive ‘Young crystal growers’ certificates and medals.

The IUCr also provides schoolchildren and teachers with educational material (brochures, videos, etc.) to explain the basic concepts of crystals and crystal growth and the importance of crystallization techniques in many fields.

The winners of the 2016 edition have just been announced (watch the videos here) and the 2017 edition launched; the deadline for video submission is 19 November 2017.

At the IYCr2014 Closing Ceremony, it was reported that national crystal-growing competitions were taking place in at least 34 countries. This is great news but we would like to boost this number and encourage more schools to participate in the international competition, and for this we need your help. If you want to inspire youngsters in what may become a lifelong love of crystallography, please do not hesitate to contact me at