Disulfide bond

When two amino acids bond to each other through their side chains, they normally do so through a disulfide bond. The particular side chain involved is the thiol group[?] (-SH). Oxidation of the thiol group yields a disulfide (S-S) bond. The presence of S-S then helps to maintain the tertiary structure of the protein. An amino acid that commonly forms S-S bonds in proteins in cysteine. When two cysteines are bonded by an S-S bond, the resulting molecule between the two protein chains is called cystine. The figure below shows the formation of a disulfide bond. The R on each side group represents the remainder of the amino acid.

     R                R
     |                |
     SH               S
         oxidation->  |  +  2H    
     SH               S    
     |                |
     R                R 

Disulfide bonds play an imporant protective role for bacteria as a reversible switch that turns a protein on or off when bacterial cells are exposed to oxidation reactions. Hydrogen peroxide (H₂O₂) in particular can severely damage DNA and kill the bacterium at low concentrations if it weren't for the protective action of the SS-bond.

Disulfide bonds also play a significant role in the vulcanization of rubber.