Cystine is a dimeric amino acid formed by the oxidation of two cysteine residues which covalently link to make a disulfide bond. This organosulfur compound has the formula (SCH2CH(NH2)CO2H)2. It is a white solid, and melts at 247-249 °C. It was discovered in 1810 by William Hyde Wollaston but was not recognized as being derived of proteins until it was isolated from the horn of a cow in 1899. Through formation of disulfide bonds within and between protein molecules, cystine is a significant determinant of the tertiary structure of most proteins. Disulfide bonding, along with hydrogen bonding and hydrophobic interactions is partially responsible for the formation of the gluten matrix in bread. Human hair contains approximately 5% cystine by mass.
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