Cystin
Cystin, commonly written cystine in chemical usage, is the disulfide-linked dimer formed by two cysteine residues. Its chemical formula is C6H12N2O4S2, and its molecular mass is about 240.3 g/mol. Cystin arises when two thiol groups of cysteine are oxidized to form a disulfide bond (S–S), releasing two hydrogen ions in the process. In proteins, cystin cross-links help stabilize three-dimensional structure and contribute to the rigidity of keratin-rich tissues such as hair and nails. Within cells, these bonds can be reduced to regenerate free cysteine residues.
Natural cystin is the diastereomer formed from two L-cysteines, often referred to as L,L-cystine. The cystin/cysteine
In human health, defective renal reabsorption of dibasic amino acids, including cystine, causes the inherited disorder