Cystine and cysteine are both determined as cysteic acid in hydrolysates of oxidised sample, but calculated as cystine (C6H12N2O4S2, M 240,30 g/mol) by using M 120,15 g/mol (= 0,5 x 240,30 g/mol).
Cystine, the oxidized form of cysteine, plays a special role in protein structure since its disulfide group serves as a covalent cross-link between two polypeptide chains or between two points in a single chain. Disulfide cross-links can be cleaved by the action of reducing agents, e.g., mercaptoethanol, which yields two molecules of cysteine from cystine. Cystine can also be oxidized by such agents as performic acid, to yield two molecules of cysteic acid.
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