Cryo-EM

Cryo-electron microscopy (cryo-EM), or electron cryomicroscopy, is a form of transmission electron microscopy (TEM) where the sample is studied at cryogenic temperatures (generally liquid-nitrogen temperatures). Cryo-EM is gaining popularity in structural biology.

The utility of cryoelectron microscopy stems from the fact that it allows the observation of specimens that have not been stained or fixed in any way, showing them in their native environment. This is in contrast to X-ray crystallography, which requires crystallizing the specimen, which can be difficult, and placing them in non-physiological environments, which can occasionally lead to functionally irrelevant conformational changes.

The resolution of cryo-EM maps is improving steadily, and in 2014 some structures at near-atomic resolution had been obtained, including those of viruses, ribosomes, , ion channels, and enzyme complexes as small as 170 kD at a resolution of 4.5 Å.Bridget Carragher and colleagues at the Scripps National Resource for Automated Molecular Microscopy used techniques she and Clint Potter developed to create the first cryo-electron microscopy structural biology image with a resolution finer than 3 Ångströms, thereby elevating cryo-EM as a tool comparable to and potentially superior to traditional x-ray crystallography techniques. A 2.2 Å map of a bacterial enzyme beta-galactosidase was published in June 2015. A version of electron cryomicroscopy is cryo-electron tomography (CET), where a 3D reconstruction of a sample is created from tilted 2D images.

The original rationale for cryoelectron microscopy was as a means to fight radiation damage for biological specimens. The amount of radiation required to collect an image of a specimen in the electron microscope is high enough to be a potential source of specimen damage for delicate structures. In addition, the high vacuum required on the column of an electron microscope makes the environment for the sample quite harsh.

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