In a protein in the oxidative environment, a covalent bond is often formed between a sulfur atom of two Cys residues, when the sulfur atoms exist in a short distance. The bond is called disulfide bond or S-S bond in short. A disulfide bond plays an important role in protein folding and stability.
In proteins with the known three-dimensional structure, the distances of two bonded sulfur atoms is known to be distributed between 1.8 - 2.0Å with a highest frequency at about 2.0 Å and dihedral angles of C-S-S-C around 90º.
Disulfide bond is defined as follows in this site;