Category Archives: News and opinion

Research news and comment from IUCr Journals

Women in Crystallography – we’re not just historical

Helen Maynard-Casely, Christine Beavers, Claire Murray and Amber Thompson

Crystallography is a very unusual science in the landscape of gender equality, with strongly diverse beginnings, which are widely lauded. There have been many celebrated female role models, including two Nobel prize winners. Lonsdale, Hodgkin, Franklin, Megaw and Yonath are but a few of the distinguished women crystallographers in the field. It should be noted that some of this gender diversity stems from the very foundations of the subject: eleven of W.H Bragg’s 18 PhD students were women.  However, how are we placed for the future?  Is the IUCr upholding its pledge to provide congresses that are balanced with respect to gender and nationality? Is crystallography as free of gender bias as many of us assume it is? Is gender equality a given in our science?

Unfortunately, it appears that crystallography has just as many issues as other communities. Conferences are the perfect way to get a ‘snapshot’ of a field so the IUCr’s triennial congress is a great place to gather statistics on the worldwide crystallography community. The proportion of female participants at IUCr congresses seemed like the obvious place to start. Given that statistically speaking women are currently less likely to study science than men, as well as the fact there are systematic barriers obstructing the career progression of women, it would be unreasonable to expect attendees to be 50:50. However, according to data from the Montreal 2014(*) and Madrid 2011 congresses, attendees were 36% and 35%(*) women respectively (data from Hyderabad 2017 were not available when writing this article). Data on the career stages of these individuals is not available, but if there were no barriers or obstructions we would expect to see these statistics reflected elsewhere in the congress, for instance in the statistics of keynotes and plenary talks. This is not the case.

[Histogram showing percentage of female keynote speakers in IUCr Congresses]
Figure of the statistics discussed in the article, presenting the % of women keynote speakers and % of women participants in the IUCr congresses. The red dashed line is an average of the two participant data points.
Invited talks are a key indicator of prestige and it would be hoped that these would track the proportion of female participants. However, from the statistics presented in the figure above, it is clear there is a very distinct gap. The proportion of female keynote speakers at IUCr congresses has ranged from 28-17% over the last five meetings. For a scientist who was completely unfamiliar with the field, this disparity would be incongruous with the community that they would meet on the ground. The young scientists who are attending their first conference or the academic who is venturing into crystallography for the first time would get the impression that the science of women crystallographers is not worth shouting about.



The final statistic to highlight is easy to verify. Plenary lectures, which are the highest indicator of esteem, were first established at IUCr congresses in 2008. There have been a total of 13, of which only one has been given by a woman: Ada Yonath.   It is interesting to note that, while only a third of the male plenary lecturers are Nobel Laureates, the lone woman plenary lecturer is also a Nobel Laureate.  Many studies have been done about impediments to advancement present for women in sciences; recently a study was able to quantify the bias present which underrates the competence of women, regardless of their actual qualifications (C. A. Moss-Racusin et al, PNAS, 16474–16479, 2012).

The crystallography community is echoing the concerns which are being noted in other areas; the number of keynote lecturers do not seem to represent the number of women in the community which in turn means that fewer women are able to progress to ‘plenary level’ within their careers. This is clearly a big barrier to retention. Because there was no data available, we have not been able to dig deeper, and examine the challenges facing racial/ethnic minorities, especially women of colour, who are historically further marginalized within scientific communities.  If we want to ensure there are the same opportunities for all then we need to understand why this happens.

So what can be done?

The biggest change is one that is almost too simple to be believed. It just involves a simple question when organising events or meetings. Everyone always considers the ‘Have I selected the best possible people for this session’ part of the plan, but there is a second half to this question that should be considered. ‘Have I selected the best possible people for this field, remembering that I have inherent biases that may skew my first choices?’ It is a subtle but important difference but it actively encourages you to challenge yourself.  Within the setting of a conference, making sure a crystallography conference is inclusive is an important role for the organising committee, which should have a nominated officer who can report back to the IUCr executive on equity and diversity.

In order for our community to progress more generally, the IUCr Executive Committee and future conference organisers need to acknowledge that the gender balance of invited speaker at IUCr congresses does not reflect the community at large, and subject this to their own review. As scientists, we thrive on data, and hence the more statistics on such representation that can be made available, the better for us all.  There are numerous ways that we can look to make future IUCr congresses more inclusive, and knowing the statistics of our community will greatly assist how such schemes can be targeted. We should be absolutely clear: We are not advocating for the favouritism of women over men to make statistics look good. We are advocating for people to challenge their inherent biases.

The authors would like to thank Jenny Martin for her comments on this article.

A note: Since first drafting this post, the speaker policy for the European Crystallographic Meeting has been published, where they have taken an excellent and a forward thinking move to address the issues we have discussed here.  We advise all that are reading this post to also read their policy .

*A note on the data gathered for this post: The gender proportion for the 2014 conference was determined by the authors from delegates names, with 1464 male delegates, 524 female delegates counted and 224 undetermined (the proportion of the 2011 Madrid congress was supplied to us directly by from the organisers).  The gender proportions of keynote speakers for the 2017, 2014, 2011 and 2008 congresses were also determined from delegate names, by the authors.   We acknowledge that there will be human error in the methodology of determining a delegates gender from their name – and of course there are those in the community who will identify as non-gender binary.  A few have questioned the value of this approach, of determining conference gender statistics in this way, but we would make the case that some data are better than none and that what we have gathered is enough to show that statistics should be gathered as a matter of course by the future congress organisers.

Quality assurance in microbeam radiation therapy

Microbeam radiation therapy (MRT) is an innovative preclinical radiotherapy procedure consisting of many micrometre-sized spatially fractionated radiation fields, obtained by collimating a beam of synchrotron radiation with a multi-slit collimator. A typical radiation field of MRT consists of an array of microbeams, each with a width of 50 µm and a centre-to-centre distance of 400 µm.

MRT differs from external beam radiation therapy (EBRT) due to the properties of synchrotron radiation, such as the small angular divergence of the photon beam, the broad spectrum of energies available and the pulsed high-intensity radiation that is produced. The low divergence of the beam ensures that the field does not spread out as it passes through the patient, thus maintaining the spatial fractionation at depth; the high-intensity radiation allows treatment time to be reduced, thus reducing smearing of the microbeam paths in the tissues due to breathing or cardiosynchronous motion.

The most significant advantage of MRT over EBRT is the different radiobiological response of cancerous and healthy tissues to the micrometre-sized MRT field. As the size of the radiation field decreases to the order of micrometres the dose tolerated by normal tissue increases dramatically, whilst maintaining tumour control. This phenomenon, called the dose-volume effect, makes MRT a promising treatment for radioresistant tumours such as osteosarcomas, or tumours located within or near sensitive structures (e.g. glioblastomas in paediatric patients).

Routine dosimetry quality assurance (QA) prior to treatment is necessary to identify any changes in beam condition from the treatment plan, and is undertaken using solid homogeneous phantoms. Solid phantoms are designed for, and routinely used in, megavoltage X-ray beam radiation therapy. These solid phantoms are not necessarily designed to be water-equivalent at low X-ray energies, and therefore may not be suitable for MRT QA.

Cameron et al. (2017). J. Synchrotron Rad. 24, 866-876 simulated dose profiles of various phantom materials and compared them with those calculated in water under the same conditions, so demonstrating quantitatively the most appropriate solid phantom to use in dosimetric MRT QA.

Based on the study, the adoption of virtual water, plastic water DT, RW3 and RM1457 solid water were recommended for MRT QA as water-equivalent solid phantom materials.


Is the X-ray diffraction theory we use correct?

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.


Cryo-EM and X-ray crystallography: complementary approaches

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.


Developments in the structural science of materials

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.



Coordination chemistry of anions through halogen–bonding interactions

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.


A large-solid-angle X-ray Raman scattering spectrometer

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.

In-cell NMR: a topical review

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.


NMR Crystallography

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


New synchrotron powder diffraction facility for long running experiments

Synchrotron beamlines and their instruments are built to harness the photon beam power of synchrotron radiation (SR), which has special properties – ideally suited to providing detailed and accurate structural information that is difficult to obtain from conventional sources. The common modus operandi for such facilities is that users are allocated a short duration of beamtime, typically a few hours to a few days, in which to perform their experiments.

With technological advances in instrumentation, detection, computing power, automation and remote access, SR facilities are developing new modes of access, designed to increase speed, efficiency and throughput of user experiments, such as on the macromolecular beamlines at Stanford Synchrotron Radiation Light Source in the US and at the Diamond Light Source in the UK.

However, there are a class of experiments that are increasingly excluded by these developments, which nevertheless could greatly benefit from the application of SR. For example, some materials undergo very slow transforming reactions, while others take time to exhibit the effects of curing, ageing or repeated use. These processes can be subtle or take weeks to months or even years to either show gross manifestation or run to completion.

At present off-line processing with before and after SR measurements is the norm, but valuable structural information on growth, change and intermediate phases can be missed or indeed lost. There is therefore a clear need for a facility that allows slow processes to be studied.

In a recently published paper [Murray et al. (2017), J. Appl. Cryst. 50. doi:10.1107/S1600576716019750] scientists report on a new purpose built LDE facility, which has been designed to address the needs of a wide and diverse range of scientific investigations. The new facility takes the form of an additional specially constructed end-station to the existing ultra-high-resolution and time-resolved powder diffraction beamline (I11) at Diamond. The new end-station is dedicated to hosting up to 20 long-term experiments (weeks to years), all running in parallel.

To demonstrate the effectiveness of this new facility, commissioning results from two contrasting science cases are presented. In the first, the slow in situ precipitation of the hydrated magnesium sulfate mineral meridianiite from an aqueous solution was followed. The hydrated phase is believed to be widespread on the surface of Mars and was formed inside a specifically designed low-temperature cell. In the second study, the long term stability of the metal-organic framework material NOTT-300 was investigated. This is a potential supramolecular material for greenhouse gas capture. Initial results show that the facility is capable of detecting phase evolution and detailed structural changes and is well suited for many applied systems and functional materials of interest. The emergence of new science from ongoing experiments is expected soon.