<P> Many specialized organic reactions have been developed for disulfides, again mainly associated with the scission of the S − S bond, which is usually the weakest bond in a molecule . In the Zincke disulfide cleavage reactions, disulfides are cleaved to give a sulfenyl halide by reaction with bromine or chlorine . </P> <P> Disulfide bonds play an important role in the folding and stability of some proteins, usually proteins secreted to the extracellular medium . Since most cellular compartments are reducing environments, in general, disulfide bonds are unstable in the cytosol, with some exceptions as noted below, unless a sulfhydryl oxidase is present . </P> <P> Disulfide bonds in proteins are formed between the thiol groups of cysteine residues by the process of oxidative folding . The other sulfur - containing amino acid, methionine, cannot form disulfide bonds . A disulfide bond is typically denoted by hyphenating the abbreviations for cysteine, e.g., when referring to ribonuclease A the "Cys26--Cys84 disulfide bond", or the "26--84 disulfide bond", or most simply as "C26--C84" where the disulfide bond is understood and does not need to be mentioned . The prototype of a protein disulfide bond is the two - amino - acid peptide cystine, which is composed of two cysteine amino acids joined by a disulfide bond (shown in Figure 3 in its unionized form). The structure of a disulfide bond can be described by its χ dihedral angle between the C − S − S − C atoms, which is usually close to ± 90 ° . </P> <P> The disulfide bond stabilizes the folded form of a protein in several ways: </P>

Disulphide bond in a protein is generated between