Everything about Electron Crystallography totally explained
Electron crystallography is a method to determine the arrangement of atoms in solids using an
electron microscope. It can complement
X-ray crystallography on
proteins, such as
membrane proteins, that can't easily form the large 3-dimensional
crystals required for that process. Structures are usually determined from either 2-dimensional crystals (sheets or
helices),
polyhedrons such as
viral capsids, or dispersed individual proteins. Electrons can be used in these situations, whereas
X-rays cannot, because electrons interact more strongly with atoms than X-rays do. Thus, X-rays will travel through a thin 2-dimensional crystal without diffracting significantly, whereas electrons can be used to form an image. Conversely, the strong interaction between electrons and proteins makes thick (for example 3-dimensional) crystals impervious to electrons, which only penetrate short distances.
One of the main difficulties in X-ray crystallography is determining
phases in the
diffraction pattern. Because no X-ray
lens exists, X-rays can't be used to form an image of the crystal being diffracted, and hence phase information is lost. Fortunately, electron microscopes contain
electron lenses, and phase information tends to be much more reliable in electron crystallography.
A common problem to X-ray crystallography and electron crystallography is
radiation damage, by which proteins are damaged as they're being imaged, limiting the resolution that can be obtained. This is especially troublesome in the setting of electron crystallography, where that radiation damage is focused on far fewer atoms. One technique used to limit radiation damage is
electron cryomicroscopy, in which the samples undergo
cryofixation and imaging takes place at
liquid nitrogen or even
liquid helium temperatures. Because of this problem, X-ray crystallography has been much more successful in determining the structure of proteins that are especially vulnerable to radiation damage.
The first electron crystallographic protein structure to achieve atomic resolution was
bacteriorhodopsin, determined by
Richard Henderson and coworkers at the
Medical Research Council Laboratory of Molecular Biology in
1990. Since then, several other high-resolution structures have been determined by electron crystallography, including the
light-harvesting complex, the
nicotinic acetylcholine receptor, and the bacterial
flagellum.
Further Information
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