Question

Draw a balanced equation of cysteine to cystine.

cysteine hydrochloride monohydrate with 2.5 ml of freshly made 10% aqueous sodium hydroxide solution and 10 ml of 3% aqueous hydrogen peroxide solution.

Answer 1

Cystine is the dimeric form of cysteine with a disulfide linkage. In the given reaction, the disulfide linkage is generated by oxidizing cysteine under basic conditions to facilitate deprotonation of the thiol group with hydrogen peroxide as the oxidant. In the balanced equation of this reaction, since the overall reaction is carried out in an aqueous medium, water is present in large excess during the course of the reaction and so can be neglected while this process except the water formed from the neutralization of HCl in Cys with NaOH and that formed from the decomposition of hydrogen peroxide to water and oxygen after oxidizing cysteine.

2C₃H₈NO₂SCl + 2NaOH + H₂O₂ -----> C₆H₁₂N₂O₄S₂ + 2NaCl + 4H₂O