ANALYTICAL CHEMISTRY
Perspectives: In crystallography we trust tween cocrystal solid forms, where the drug molecule remains neutral, and salt forms, where the drug molecule is in an ionic form. Under Food & Drug Administration PAUL HODGKINSON AND CORY M. WIDDIFIELD, DURHAM UNIVERSITY guidance, these have significantly different regulatory requirements. Hence time and hemical structure databases, of hydrogen atoms, which have only one cost can hang in the balance of the position such as the Cambridge Structural electron, and difficulty distinguishing beof a single hydrogen atom. Database (CSD) and the Crystween isoelectronic fragments, such as OH Ideally, we would use independent extallography Open Database, are and F, where the difference in electron den- perimental techniques to address questions indispensable repositories of information sity is negligible. An incorrect positioning that are difficult for diffraction techniques for chemical research. These databases of hydrogen atoms may not have a measurto answer. Although NMR chemical shifts provide deep insights into the complex able effect on R factors. are sensitive to atomic positions, solid-state relationship between chemical function It is tempting to respond to the probNMR has often been limited to providing a and crystal structure, and so they are often lem of localizing hydrogens with “But you fingerprint—for example, for different crysstarting points when screening for new can always use neutrons!” Yet, using large talline forms of the same substance, known pharmaceuticals, testing computational national and international neutron source as polymorphs, or to areas where Bragg difmethods, and designing new materials. facilities to resolve such ambiguities via fraction has limited usefulness, such as in Most database structures are determined diffraction is rarely justified, particularly as disordered and amorphous solids. by X-ray diffraction and will have been valimost crystal structures are obtained simply Developments in solid-state NMR experdated by a combination of automated tools, to confirm chemical identity. imental techniques, such as using very fast such as the International Union of CrystalHydrogen positioning is, however, sample spinning to obtain 1H NMR spectra, lography (IUCr) online checkCIF service, central to hydrogen bonding, which often along with density functional theory (DFT) as well as by peer review and curation by plays a crucial role in determining crystal calculations developed for periodically database maintainers. For example, checkstructures. In pharmaceutical chemistry, it repeating structures, have given birth to anCIF issues alerts for potential problems, is particularly important to distinguish beother option: NMR crystallography. such as unusual bond lengths. But The chemical shifts of light deciding whether a feature is unatoms, such as 1H and 13C, can be usual and potentially interesting or This schematic plot displays hydrogen-position differences predicted with sufficient accuracy simply an error requires expertise. between more than 4,000 pairs of repeated structures in using DFT to determine whether And no validation tool is foolproof. the Cambridge Structural Database. Most of these differa given crystal structure is comSo could databases contain strucences are small and disappear when quantum chemistry patible with experimental NMR tures with errors? Of course. The calculations are used to “relax” the atomic coordinates. In data. For example, we have used real question is “How would we about 1% of the cases, mostly in the OH region, the differNMR crystallography to distinknow?” ences are not reconciled. guish between two CSD structures In addition, database users of the drug terbutaline sulfate, are often confronted with more which differed in the placement than one structure for the same of one OH hydrogen atom (Magn. crystal form of a molecule. Is one Reson. Chem. 2010, DOI: 10.1002/ structure “better” than another? mrc.2636). The 13C NMR spectra Although it is tempting for the predicted from calculations were nonexpert or automated tools to clearly a better match for the make that assessment using simstructure where the hydrogen had ple criteria, such as deposit date, been placed by automated tools, study temperature, and, above all, rather than by trying to find the R factor (a measure of how well the hydrogen atoms from the experiproposed crystal structure matchmental X-ray diffraction data. es the original data), these cannot In another example, Jacco van tell the whole story. This is where de Streek and Marcus A. Neumann nuclear magnetic resonance has a used DFT to assess the validity role to play. of an already peer-reviewed set X-ray diffraction relies on the of well-ordered structures (Acta scattering of incoming X-rays by Crystallogr. 2010, DOI: 10.1107/ the electron cloud of atoms. This s0108768110031873). By observing often leads to significant unceratom movements when the crystal tainties in the precise localization structures were DFT-optimized
How reliable are the structures deposited in crystallographic databases?
C
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C&EN | CEN.ACS.ORG | NOVEMBER 7, 2016
CREDIT: PAUL HODGKINSON AND CORY M. WIDDIFIELD
The problem 1%
CREDIT: COURTESY OF CORY M. WIDDIFIELD AND PAUL HODGKINSON
to achieve an energetically stable form, the authors suggested that six out of the 225 crystal structures in the set would benefit from additional verification. In our own recent survey of repeat CSD structural determinations, we found distinct “patterns of difference” between alternative structures. The schematic shown summarizes the difference between more than 4,000 pairs of repeated structures in the database measured in terms of two parameters: the overall difference (rootmean-square difference between nonhydrogen atom positions) and the largest individual difference in atomic positions. Generally, nonhydrogen atom positions are very consistent and most structural differences lie therefore in the placement of the hydrogen atoms. Often, these differences, such as different orientations of methyl groups, are chemically uninteresting or disappear on DFT geometry optimization. In about 1% of cases, however, the structures continue to differ. These cases regularly involve hydrogens in hydrogen bonds, where localization becomes important to resolve. Taking the pharmaceutical furosemide as an example, we used NMR crystallography to validate one of two alternative structures, again differing in a single hydrogen position, based on the sensitivity of 1H NMR chemical shifts in hydrogen bonds (Chem. Commun. 2016, DOI: 10.1039/ c6cc02171a). Others have used 1H NMR spectra to distinguish between alternative solutions for the complete hydrogen bonding network in α-d-galactose
(Chem. Phys. Lett. 2010, DOI: 10.1016/j. cplett.2010.08.077). Do we trust crystallography? Evidence suggests that most deposited crystal structures are free of problems. Where questions of hydrogen placement or disorder are involved, however, discretion is advised. This is where complementary methods such as NMR crystallography can make a difference. DFT calculations can help assess the plausibility of a crystal structure, but only an independent experimental measurement on a partic-
ular sample can confirm that a proposed structure is compatible with that sample. Although crystallography is now widely assumed to be synonymous with diffraction-based techniques, it is encouraging to see the recent creation of an IUCr commission on NMR crystallography and the associated biennial SMARTER conference series on NMR crystallography. We expect that the use of complementary experimental and computational techniques will be increasingly important as chemists continue to address complex structural problems. ◾
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Paul Hodgkinson is head of the Solid-State NMR Group at Durham University and has research interests in studying the structure and dynamics of organic molecular solids, such as pharmaceuticals, using a combination of NMR and computational techniques. Cory M. Widdifield is an EPSRC postdoctoral fellow in the Solid-State NMR Group at Durham University. His interests include developing NMR crystallography metrics for the structure determination and refinement of various materials. NOVEMBER 7, 2016 | CEN.ACS.ORG | C&EN
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