Electron crystallography 1822961 216324335 2008-06-01T03:16:49Z SmackBot 433328 Date the maintenance tags or general fixes '''Electron crystallography''' is a method to determine the arrangement of atoms in solids using an [[electron microscope]]. It can complement [[X-ray crystallography]] on [[protein]]s, such as [[membrane protein]]s, that cannot easily form the large 3-dimensional [[crystal]]s required for that process. Structures are usually determined from either 2-dimensional crystals (sheets or [[helix|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 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 (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 (waves)|phase]]s in the [[diffraction pattern]]. Because no X-ray [[lens (optics)|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 [[Electrostatic lens|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 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 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 (molecular biologist)|Richard Henderson]] and coworkers at the [[Medical Research Council (UK)|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]] {{Fact|date=May 2008}}. ==External links== *[http://www.vega.org.uk/video/programme/122 Interview with Aaron Klug Nobel Laureate for work on crystallograph electron microscopy] Freeview video by the Vega Science Trust. {{Protein structure determination}} [[Category:Crystallography]] [[Category:Protein structure]] [[Category:Electron|Crystallography]]