Biochemistry - The Chemical Reactions of Living Cells, Volume 1 - D. Metzler 1980

The Molecules We Are Made Of
Proteins
Disulfide Bonds

A characteristic structural feature of the Lysozyme molecule (Fig. 2-9) is the presence of four cross-linking Disulfide Bonds (disulfide bridges) between different segments of the chain. These bridges form spontaneously when the —SH groups of two Cysteine side chains come into close proximity and are oxidized in the presence of O₂ or certain Other Reagents [Eq. (2-8)]. Disulfide bonds are quite common in Proteins secreted by Cells, but are formed much less frequently in intracellular Enzymes. Presumably, intracellular enzymes are protected from various external influences and do not require additional stabilization.

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One of the proteins with the highest concentration of cross-links is found in the so-called keratin "matrix." The Cleavage of disulfide Bonds in this protein is the underlying principle of a chemical Hair perm (permanent wave). This is achieved using a thiol compound, which reductively cleaves the cross-linking disulfide bonds. After the hair is set, exposure to air oxidation leads to The formation of new cross-links.

The geometry of disulfide bridges is also of considerable importance. The torsional angle about the S—S bond is typically ~90°, although quite significant deviations from this value do occur.