| Electron Crystallography |
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| crystallography | |
| protein structure | |
| electron | |
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Electron crystallography is a method to determine Protein Structure s using Electron Diffraction . It is conducted with an Electron Microscope , usually on Protein s (such as Membrane Protein s), that cannot easily form the large 3-dimensional Crystal s required for X-ray Crystallography . Rather, structures are usually determined from either 2-dimensional crystals (sheets or Helices ), Polyhedron s such as Viral Capsid s, or dispersed individual proteins. Electrons can be used in these situations, whereas X-ray s cannot, because electrons interact more strongly with proteins 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 (e.g. 3-dimensional) crystals impervious to electrons, which only penetrate short distances. One of the main difficulties in X-ray crystallography is determining Phase s in the Diffraction Pattern . Because no X-ray Lens exists, X-rays cannot 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. Conversely, because X-ray crystallography uses 3-dimensional crystals, one is able to simultaneously gather diffraction patterns from orders of magnitude more proteins than what can be achieved in electron crystallography, enhancing signal-to-noise ratios. For this reason, X-ray crystallography has been much more successful in determining large numbers of protein structures. A common problem to X-ray crystallography and electron crystallography is Radiation Damage , by which proteins are damaged as they are 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 proteins. 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. 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 . |
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