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).