Cystine is the oxidized derivative of the amino acid cysteine and has the formula (SCH2CH(NH2)CO2H)2. It is a white solid that is poorly soluble in water. As a residue in proteins, cystine serves two functions: a site of redox reactions and a mechanical linkage that allows proteins to retain their three-dimensional structure.[1] Formation and reactions Structure Cystine is the disulfide derived from the amino acid cysteine. The conversion can be viewed as an oxidation: 2 HO2CCH(NH2)CH2SH + 0.5 O2 → (HO2CCH(NH2)CH2S)2 + H2O Cystine contains a disulfide bond, two amine groups, and two carboxylic acid groups. As for other amino acids, the amine and carboxylic acid groups exist in rapid equilibrium with the ammonium-carboxylate tautomer. The great majority of the literature concerns the l, l-cystine, derived from l-cysteine. Other isomers include d, d-cystine and the meso isomer d, l-cystine, neither of which is biologically significant. Occurrence Cystine is common in many foods such as eggs, meat, dairy products, and whole grains as well as skin, horns and hair. It was not recognized as being derived of proteins until it was isolated from the horn of a cow in 1899.[2] Human hair and skin contain approximately 10–14% cystine by mass.[3] History Cystine was discovered in 1810 by the English chemist William Hyde Wollaston, who called it "cystic oxide".[4][5] In 1833, the Swedish chemist Jöns Jacob Berzelius named the amino acid "cystine".[6] The Norwegian chemist Christian J. Thaulow determined, in 1838, the empirical formula of cystine.[7] In 1884, the German chemist Eugen Baumann found that when cystine was treated with a reducing agent, cystine revealed itself to be a dimer of a monomer which he named "cysteïne".[8][5] In 1899, cystine was first isolated from protein (horn tissue) by the Swedish chemist Karl A. H. Mörner (1855-1917).[9] The chemical structure of cystine was determined by synthesis in 1903 by the German chemist Emil Erlenmeyer.[10][11][12] The history of cystine and cysteine is complicated by the dimer-monomer relationship of the two.[5] The cysteine monomer was proposed as the actual unit by Embden in 1901. The sulfur within the structure of cysteine and cystine has been subject of historical interest.[5] In 1902, Osborne partially succeeded in analysing cystine content via lead compounds. An improved colorimetric method was developed in 1922 by Folin and Looney. An iodometric analysis method was developed by Okuda in 1925.