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|Section2= |Section3= }} Uric acid is a heterocyclic compound of carbon, nitrogen, oxygen, and hydrogen with the formula C5H4N4O3. It forms ions and salts known as urates and acid urates, such as ammonium acid urate. Uric acid is a product of the metabolic breakdown of purine nucleotides. High blood concentrations of uric acid can lead to gout. The chemical is associated with other medical conditions including diabetes and the formation of ammonium acid urate kidney stones. ==Chemistry== Uric acid is a diprotic acid with pKa1=5.4 and pKa2=10.3. Thus in strong alkali at high pH, it forms the dually charged full urate ion, but at biological pH or in the presence of carbonic acid or carbonate ions, it forms the singly charged hydrogen or acid urate ion, as its pKa1 is lower than the pKa1 of carbonic acid. As its second ionization is so weak, the full urate salts tend to hydrolyze back to hydrogen urate salts and free base at pH values around neutral. It is aromatic because of the purine functional group. As a bicyclic, heterocyclic purine derivative, uric acid does not protonate as an oxygen () like carboxylic acids do. X-ray diffraction studies on the hydrogen urate ion in crystals of ammomium hydrogen urate, formed ''in vivo'' as gouty deposits, reveal the keto-oxygen in the 2 position of a tautomer of the purine structure exists as a hydroxyl group and the two flanking nitrogen atoms at the 1 and 3 positions share the ionic charge in the six-membered pi-resonance-stabilized ring.〔European Powder Diffraction Conference, EPDIC-9〕 Thus, while most organic acids are deprotonated by the ionization of a polar hydrogen-to-oxygen bond, usually accompanied by some form of resonance stabilization (resulting in a carboxylate ion), uric acid is deprotonated at a nitrogen atom and uses a tautomeric keto/hydroxy group as an electron-withdrawing group to increase the pK1 value. The five-membered ring also possesses a keto group (in the 8 position), flanked by two secondary amino groups (in the 7 and 9 positions), and deprotonation of one of these at high pH could explain the pK2 and behavior as a diprotic acid. Similar tautomeric rearrangement and pi-resonance stabilization would then give the ion some degree of stability. (On the structure shown at the upper-right, the NH at the upper-right on the six-membered ring is "1", counting clockwise around the six-membered ring to "6" for the keto carbon at the top of the six-membered ring. The uppermost NH on the five-membered ring is "7", counting counter-clockwise around this ring to the lower NH, which is "9".) Uric acid was first isolated from kidney stones in 1776 by Scheele.〔Scheele, V. Q. Examen Chemicum Calculi Urinari, Opuscula, 1776, 2, 73.〕 As far as laboratory synthesis is concerned, in 1882, Ivan Horbaczewski claimed to have prepared uric acid by melting urea hydrogen peroxide with glycine, trichlorolactic acid, and its amide. Soon after, repetition by Eduard Hoffmann shows that this preparation with glycine gives no trace of uric acid, but trichlorolacetamide produces some uric acid. Thus, Hoffmann was the first to synthesize uric acid.〔.〕 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「Uric acid」の詳細全文を読む スポンサード リンク
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