Crystallography by NMR

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  • Published: Aug 15, 2005
  • Author: David Bradley
  • Channels: NMR Knowledge Base
thumbnail image: Crystallography by NMR

While developments in crystallography previously outstripped NMR to some degree in structural studies, researchers in France have now demonstrated what NMR experts have always known - that NMR is no second choice when it comes to crystal structures!

Bénédicte Elena and Lyndon Emsley of the Ecole Normale Supérieure de Lyon have investigated 1H-1H spin-diffusion build-up curves using a rate matrix analysis approach and shown that high-resolution magic angle spinning proton NMR can provide detailed structural information about crystalline powders of organic compounds. In particular, they revealed how experimental 1H data and simulation depend strongly on crystal structure parameters, such as the unit cell parameters or molecular orientation.

X-ray and neutron diffraction methods for studying crystalline powders have advanced apace, but solid-state NMR, the researchers say, has recently allowed the first determination of a three-dimensional protein structure and produced structural models for the amyloid fibrils involved in disorders such as Alzheimer's disease. While proteins seem to be succumbing to NMR's charms, Emsley points out that small molecules have remained, almost paradoxically, resistant to crystal studies. He explains that combined rotation and multiple pulse techniques (CRAMPS) and recent major advances in homonuclear dipolar decoupling can in principle allow researchers to acquire highly resolved proton spectra directly.

Now, Emsley and Elena have developed a powerful new technique that exploits these advances to obtain unit cell parameters from proton NMR of compounds at natural isotopic abundance. They suggest that quantitative analysis of directly detected 1H-1H correlations due to spin diffusion can be carried out using a multispin kinetic rate matrix approach, summed over the structure. They have demonstrated proof of principle by obtaining crystal data for a microcrystalline organic model compound, beta-L-aspartyl-L-alanine. The researchers obtained good agreement with independent experimental results.

Emsley adds that future developments of this model for spin diffusion could be made that take into account perturbing factors that were not considered in this study. Factors such as internal molecular motions or other sources of inaccuracy, for instance, may be incorporated into the methodology.

"In the future, it is conceivable that this approach could be developed to determine ab initio full crystal structures," Emsley told Resonants, "Certainly, this analysis could be combined with proton and carbon chemical shift calculations to provide an 'NMR only' structure determination protocol for powders, but perhaps even more interesting is potential to combine this with powder XRD data."

Lyndon Emsley
Lyndon Emsley

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