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|Section2= |Section7= |Section8= }} Uracil (U) is one of the four nucleobases in the nucleic acid of RNA that are represented by the letters A, G, C and U. The others are adenine (A), cytosine (C), and guanine (G). In RNA, uracil binds to adenine via two hydrogen bonds. In DNA, the uracil nucleobase is replaced by thymine. Uracil is a demethylated form of thymine. Uracil is a common and naturally occurring pyrimidine derivative.〔Garrett, Reginald H.; Grisham, Charles M. ''Principals of Biochemistry with a Human Focus''. United States: Brooks/Cole Thomson Learning, 1997.〕 The name "uracil" was coined in 1885 by the German chemist Robert Behrend, who was attempting to synthesize derivatives of uric acid.〔Robert Behrend (1885) ("Versuche zur Synthese von Körpern der Harnsäurereihe" ) (Experiments on the synthesis of substances in the uric acid series), ''Annalen der Chemie'', 229 : 1-44. From page 11: ''"Dasselbe stellt sich sonach als Methylderivat der Verbindung: (structure of uracil ) welche ich willkürlich mit dem Namen Uracil belege, dar."'' (The same () is therefore represented as the methyl derivative of the compound (of uracil's structure ), which I will arbitrarily endow with the name "uracil".)〕 Originally discovered in 1900 by Alberto Ascoli, it was isolated by hydrolysis of yeast nuclein;〔Alberto Ascoli (1900) ("Ueber ein neues Spaltungsprodukt des Hefenucleins" ) (On a new cleavage product of nucleic acid from yeast), ''Zeitschrift für Physiologische Chemie'', 31 : 161-164.〕 it was also found in bovine thymus and spleen, herring sperm, and wheat germ.〔 It is a planar, unsaturated compound that has the ability to absorb light.〔 Based on 12C/13C isotopic ratios of organic compounds found in the Murchison meteorite, it is believed that uracil, xanthine and related molecules can also be formed extraterrestrially. In 2012, an analysis of data from the Cassini mission orbiting in the Saturn system showed that Titan's surface composition may include uracil. ==Properties== In RNA, uracil base-pairs with adenine and replaces thymine during DNA transcription. Methylation of uracil produces thymine.〔http://www.madsci.org〕 In DNA, the evolutionary substitution of thymine for uracil may have increased DNA stability and improved the efficiency of DNA replication. Uracil pairs with adenine through hydrogen bonding. When base pairing with adenine, uracil acts as both a hydrogen bond acceptor and a hydrogen bond donor. In RNA, uracil binds with a ribose sugar to form the ribonucleoside uridine. When a phosphate attaches to uridine, uridine 5'-monophosphate is produced.〔Horton, Robert H.; ''et al.''Principles of Biochemistry. 3rd ed. Upper Saddle River, NJ: Prentice Hall, 2002.〕 Uracil undergoes amide-imidic acid tautomeric shifts because any nuclear instability the molecule may have from the lack of formal aromaticity is compensated by the cyclic-amidic stability.〔 The amide tautomer is referred to as the lactam structure, while the imidic acid tautomer is referred to as the lactim structure. These tautomeric forms are predominant at pH 7. The lactam structure is the most common form of uracil. : Uracil also recycles itself to form nucleotides by undergoing a series of phosphoribosyltransferase reactions.〔 Degradation of uracil produces the substrates aspartate, carbon dioxide, and ammonia.〔 :C4H4N2O2 → H3NCH2CH2COO− + NH4+ + CO2 Oxidative degradation of uracil produces urea and maleic acid in the presence of H2O2 and Fe2+ or in the presence of diatomic oxygen and Fe2+. Uracil is a weak acid. The first site of ionization of uracil is not known.〔Zorbach, W.W. Synthetic Procedures in Nucleic Acid Chemistry: Physical and Physicochemical Aids in Determination of Structure. Vol 2. New York: Wiley-Interscience, 1973.〕 The negative charge is placed on the oxygen anion and produces a pKa of less than or equal to 12. The basic pKa = -3.4, while the acidic pKa = 9.389. In the gas phase, uracil has 4 sites that are more acidic than water.〔Lee, J.K.; Kurinovich, Ma. ''J Am Soc Mass Spectrom.''13(8), 2005, 985-95.〕 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「Uracil」の詳細全文を読む スポンサード リンク
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