Microcrystal electron diffraction, or MicroED,[1][2] is a CryoEM method that was developed by the Gonen laboratory in late 2013 at the Janelia Research Campus of the Howard Hughes Medical Institute. MicroED is a form of electron crystallography where thin 3D crystals are used for structure determination by electron diffraction. Prior to this demonstration, macromolecular (protein) electron crystallography was mainly used on 2D crystals, for example.[3][4] The method is one of several modern versions of approaches to determine atomic structures using electron diffraction first demonstrated for the positions of hydrogen atoms in NH4Cl crystals by W. E. Laschkarew and I. D. Usykin in 1933,[5] which has since been used for surfaces,[6] via precession electron diffraction,[7] with much of the early work described in the work of Boris Vainshtein[8] and Douglas L. Dorset.[9]
The method was developed for structure determination of proteins from nanocrystals that are typically not suitable for X-ray diffraction because of their size.[10] Crystals that are one billionth the size needed for X-ray crystallography can yield high quality data.[11] The samples are frozen hydrated as for all other CryoEM modalities but instead of using the transmission electron microscope (TEM) in imaging mode one uses it in diffraction mode with a low electron exposure (typically < 0.01 e−/Å2/s). The nano crystal is exposed to the diffracting beam and continuously rotated[2] while diffraction is collected on a fast camera as a movie.[2] MicroED data is then processed using software for X-ray crystallography for structure analysis and refinement.[12] The hardware and software used in a MicroED experiment are standard and broadly available.[13][14]