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ACS News Service Weekly PressPac: November 07, 2018

Powerful method probes small-molecule structures

The CryoEM Method MicroED as a Powerful Tool for Small Molecule Structure Determination
ACS Central Science

Small molecules — from naturally occurring metabolites and hormones to synthetic medicines and pesticides — can have big effects on living things. But for scientists to understand how the molecules work and how to design beneficial ones, they need to know the precise arrangement of atoms and chemical bonds. Now researchers have found a faster, simpler and potentially more reliable way to solve the structures of small molecules. They report their results in ACS Central Science.

Currently, the gold standard for determining small-molecule structures is X-ray crystallography. In this technique, researchers crystallize a small molecule and then bombard the crystal with X-rays, which diffract in complex patterns that reveal the molecule’s 3D structure. However, producing large, high-quality crystals is time-consuming or impossible for many compounds. Brian Stoltz, Jose Rodriguez, Hosea Nelson and Tamir Gonen wondered if they could use a form of cryoelectron microscopy to characterize small molecules. Known as microcrystal-electron diffraction (MicroED), this technique was developed 5 years ago to study protein structures. In this technique, electron beams, instead of X-rays, are diffracted from crystals, which can be much smaller than those required for X-ray crystallography.

The researchers first tested MicroED on a sample of powdered progesterone, which contained thousands of nanocrystals. They rotated a single crystal and collected electron diffraction data from different angles, determining the structure of the hormone at high resolution (1 angstrom) in less than 30 minutes, compared with weeks or months for X-ray crystallography. They went on to successfully characterize 11 other natural, synthetic and pharmaceutical products, including acetaminophen, ibuprofen and several antibiotics. The researchers even identified four different molecules in a mixture by studying individual nanocrystals. Using MicroED, the researchers analyzed crystals that were a billionth of the size typically needed for X-ray crystallography. The rapid, precise method has the potential to greatly accelerate research in the fields of synthetic chemistry, natural product chemistry and drug discovery, the researchers say.

The authors acknowledge funding from the National Science Foundation, the National Institute of General Medical Sciences, the Department of Energy, the Arnold and Mabel Beckman Foundation, the Searle Scholars Program, Pew Charitable Trusts, the David and Lucile Packard Foundation, the Alfred P. Sloan Foundation and the Howard Hughes Medical Institute.

Note: ACS does not conduct research, but publishes and publicizes peer-reviewed scientific studies.

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MicroED can identify compounds in a mixture from their nanocrystals (black shapes) on a cryoEM grid. Holes (grey circles) are 2 μm in diameter.
Credit: American Chemical Society
High-resolution image