Disulfide bonds

In chemistry and biology a disulfide refers to a functional group with the general structure R–S–S–R'. The linkage is also called an SS-bond or a disulfide bridge and is usually derived by the coupling of two thiol groups. In formal terms, the connection is a persulfide, in analogy to its congener, peroxide (R–O–O–R'), but this terminology is rarely used, except in reference to hydrodisulfides (R–S–S–H or H–S–S–H compounds).

In inorganic chemistry disulfide usually refers to the corresponding anion S2−
2 (⁻S–S⁻), for example in disulfur dichloride.

The disulfide bonds are strong, with a typical bond dissociation energy of 60 kcal/mol (251 kJ mol⁻¹). However, being about 40% weaker than C–C and C–H bonds, the disulfide bond is often the "weak link" in many molecules. Furthermore, reflecting the polarizability of divalent sulfur, the S–S bond is susceptible to scission by polar reagents, both electrophiles and especially nucleophiles:

The disulfide bond is about 2.05 Å in length, about 0.5 Å longer than a C–C bond. Rotation about the S–S axis is subject to a low barrier. Disulfides show a distinct preference for dihedral angles approaching 90°. When the angle approaches 0° or 180°, then the disulfide is a significantly better oxidant.

Disulfides where the two R groups are the same are called symmetric, examples being diphenyl disulfide and dimethyl disulfide. When the two R groups are not identical, the compound is said to be an asymmetric or mixed disulfide.

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