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Homochirality describes a geometric property of some materials that are composed of chiral units. Chiral refers to nonsuperimposable 3D forms that are mirror images of one another, as are left and right hands. A substance is said to be ''homochiral'' if all the constituent units are molecules of the same chiral form (enantiomer). In biology, homochirality is a common property of amino acids and sugars. The origin of this phenomenon is not clearly understood. It is unclear if homochirality has a purpose; however it appears to be a form of information storage. 〔(2009), ''A New Definition of Life'', Carroll, J. D. Chirality, 21: 354–358, 2009. 〕 One suggestion is that it reduces entropy barriers in the formation of large organized molecules.〔''Do Homochiral Aggregates Have an Entropic Advantage?'' Julian, R. R.; Myung, S.; Clemmer, D. E. J. Phys. Chem. B.; (Article); 2005; 109(1); 440-444. 〕 It has been experimentally verified that amino acids form large aggregates in larger abundance from enantiopure substrates than from racemic ones. Homochirality is said to evolve in three distinct steps: mirror-symmetry breaking creates a minute enantiomeric imbalance and is key to homochirality, chiral amplification is a process of enantiomeric enrichment and chiral transmission allows the transfer of chirality of one set of molecules to another. It is also entirely possible that homochirality is simply a result of the natural autoamplification process of life —that either the formation of life as preferring one chirality or the other was a chance rare event which happened to occur with the chiralities we observe, or that all chiralities of life emerged rapidly but due to catastrophic events and strong competition, the other unobserved chiral preferences were wiped out by the preponderance and metabolic, enantiomeric enrichment from the 'winning' chirality choices . The emergence of chirality consensus as a natural autoamplification process has been associated with the 2nd law of thermodynamics. ==Mirror-symmetry breaking== Known mechanisms for the production of non-racemic mixtures from racemic starting materials include: asymmetric physical laws, such as the electroweak interaction; asymmetric environments, such as those caused by circularly polarized light, quartz crystals, or the Earth's rotation; and statistical fluctuations during racemic synthesis.〔 "Special Issue: Proceedings from the Eighteenth International Symposium on Chirality (ISCD-18), Busan, Korea, 2006"〕 Once established, chirality would be selected for. A small enantiomeric excess can be amplified into a large one by asymmetric autocatalysis, such as in the Soai reaction. In asymmetric autocatalysis, the catalyst is a chiral molecule, which means that a chiral molecule is catalysing its own production. An initial enantiomeric excess, such as can be produced by polarized light, then allows the more abundant enantiomer to outcompete the other. One supposition is that the discovery of an enantiomeric imbalance in molecules in the Murchison meteorite supports an extraterrestrial origin of homochirality: there is evidence for the existence of circularly polarized light originating from Mie scattering on aligned interstellar dust particles which may trigger the formation of an enantiomeric excess within chiral material in space.〔Uwe Meierhenrich. ''Amino Acids and the Asymmetry of Life''; (Book) Springer-Verlag; 2008. ISBN 978-3-540-76885-2〕 Another speculation (the Vester-Ulbricht hypothesis) suggests that fundamental chirality of physical processes such as that of the beta decay (see Parity violation) leads to slightly different half-lives of biologically relevant molecules. Homochirality may also result from spontaneous absolute asymmetric synthesis. 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「homochirality」の詳細全文を読む スポンサード リンク
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