Cryogenic electron microscopy
Cryogenic electron microscopy (cryo-EM) is an electron microscopy (EM) technique applied on samples cooled to cryogenic temperatures and embedded in an environment of vitreous water. An aqueous sample solution is applied to a grid-mesh and plunge-frozen in liquid ethane. While development of the technique began in the 1970s, recent advances in detector technology and software algorithms have allowed for the determination of biomolecular structures at near-atomic resolution. This has attracted wide attention to the approach as an alternative to X-ray crystallography or NMR spectroscopy for macromolecular structure determination without the need for crystallization.
In 2017, the Nobel Prize in Chemistry was awarded to Jacques Dubochet, Joachim Frank, and Richard Henderson "for developing cryo-electron microscopy for the high-resolution structure determination of biomolecules in solution." Nature Methods also named cryo-EM as the "Method of the Year" in 2016.
Transmission electron cryomicroscopy
- Electron crystallography, method to determine the arrangement of atoms in solids using a TEM
- MicroED, method to determine the structure of proteins and small molecules using electron diffraction from 3D crystals
- Electron cryotomography (CryoET), a specialized application of where samples are imaged as they are tilted
History of cryogenic electron microscopy
In the 1960s, scientists were faced with the issue of structure determination methods using electron microscopy damaging the specimen due to high energy electron beams, so cryogenic electron microscopy was considered to overcome this issue as it was expected that low temperatures would reduce beam damage. In 1980, Erwin Knapek and Jacques Dubochet published commenting on beam damage at cryogenic temperatures sharing observations that:
Thin crystals mounted on carbon film were found to be from 30 to 300 times more beam-resistant at 4 K than at room temperature... Most of our results can be explained by assuming that cryoprotection in the region of 4 K is strongly dependent on the temperature.
However, these results were not reproducible and amendments were published in the Nature international journal of science just 2 years later informing that the beam resistance was less significant than initially anticipated. The protection gained at 4 K was closer to “tenfold for standard samples of L-valine,” than what was previously stated.
In 2018, chemists realized that electron diffraction can be used to readily determine the structures of small molecules that form needle-like crystals, structures that would otherwise need to be determined from X-ray crystallography, by growing larger crystals of the compound.
Scanning electron cryomicroscopy
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