Disulfide bonding
In chemistry, a disulfide refers to a functional group with the structure R−S−S−R′. The linkage is also called an SS-bond or sometimes a disulfide bridge and is usually derived by the coupling of two thiol groups. 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 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 (Nu):
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.
...
Wikipedia