NMR protein structures: No deuteration required

Nuclear magnetic resonance spectroscopy is widely used to get structural information about molecules: When placed in a magnetic field, a molecule’s magnetic nuclei precess at frequencies that depend on their chemical environment. Conventional solution-phase NMR—usually based on hydrogen-1 because it has the largest gyromagnetic ratio and highest natural abundance—relies on molecules’ rapid tumbling to average out anisotropic contributions to the signal. But that averaging breaks down for solid samples and large proteins. Researchers can recover the benefits of rapid tumbling by spinning the sample about an axis at a particular “magic” angle with respect to the field B, as shown in the figure. (For more on magic-angle spinning and the other intricacies of solid-state NMR, see the article by Clare Grey and Robert Tycko, Physics Today, September 2009, page 44 .)

Solid-state NMR complements other techniques for finding protein structures. Unlike x-ray crystallography, it doesn’t require a crystalline sample, and it’s well suited to the small and medium-sized membrane proteins currently beyond the reach of cryoelectron microscopy. Even with magic-angle spinning, ¹H NMR protein spectra often have many overlapping, unresolvable peaks that make them too complicated to interpret. The spectra can be simplified by replacing some of the hydrogen atoms with deuterium atoms, which are invisible to ¹H NMR. But partially deuterating a protein is difficult, expensive, and for many proteins not even possible.

Now Guido Pintacuda and colleagues at the European Center for High-Field NMR in Lyon, France, have used ¹H NMR to solve the structures of two nondeuterated proteins. To narrow and separate the spectral peaks, they used magic-angle-spinning at frequencies of 100 kHz and above. For a typical cylindrical sample container several millimeters in diameter, the supersonic spinning of the outer edge would lead to rotor instabilities. Instead, Pintacuda and colleagues used specially designed containers 0.7 mm in diameter, which housed just 0.5 mg of protein. Normally, such small samples would prohibitively limit measurement sensitivity. But through the combination of the high magnetic field, specially designed RF pulse sequences, and rich data set derived from detecting all the proteins’ hydrogen nuclei, the researchers obtained each structure from less than two weeks of data collection. Other NMR protein experiments can take months or even years. (L. B. Andreas et al., Proc. Natl. Acad. Sci. USA 113, 9187, 2016 .)