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Cryo-electron microscopy (cryo-EM), or electron cryomicroscopy, is a form of transmission electron microscopy (EM) where the sample is studied at cryogenic temperatures (generally liquid nitrogen temperatures).〔 Cryo-EM is gaining popularity in structural biology. The popularity 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 using cryoelectron microscopy, including those of viruses, ribosomes, mitochondria, ion channels, and enzyme complexes as small as 170 kD at a resolution of 4.5 Å. 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. == Development == 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. The problem of the vacuum was partially solved by the introduction of negative stains but even with negative stains biological samples are prone to structural collapse upon dehydration of the specimen. Embedding the samples in ice below the sublimation temperature was a possibility that was contemplated early on, but water tends to arrange into a crystalline lattice of lower density upon freezing and this can destroy the structure of anything that is embedded in it. In the early '80s, several groups studying solid state physics were attempting to produce vitreous ice by different means, such as high pressure freezing or flash freezing. In a seminal paper in 1984, the group led by Jacques Dubochet at the European Molecular Biology Laboratory showed images of adenovirus embedded in a vitrified layer of water. This paper is generally considered to mark the origin of cryoelectron microscopy, and the technique has been developed to the point of becoming routine at several laboratories throughout the world. The energy of the electrons used for imaging (120-300 kV) is high enough that covalent bonds can be broken. It is therefore necessary to limit the electron exposure used to acquire the image. These low exposures require that the images of thousands or even millions of identical frozen molecules be selected, aligned, and averaged to obtain high-resolution maps, using specialized software. A significant improvement in structural features was achieved in 2012 by the introduction of direct electron detectors and better computational algorithms.〔〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Cryo-electron microscopy」の詳細全文を読む スポンサード リンク
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