Disulfure

In biochemistry, a disulfide (or disulphide in British English) refers to a functional group with the structure . The linkage is also called an SS-bond or sometimes a disulfide bridge and is usually derived by the coupling of two thiol groups. In biology, disulfide bridges formed between thiol groups in two cysteine residues are an important component of the secondary and tertiary structure of proteins. Persulfide usually refers to compounds. In inorganic chemistry disulfide usually refers to the corresponding anion S22− (−S−S−). Symmetrical disulfides are compounds of the formula . Most disulfides encountered in organo sulfur chemistry are symmetrical disulfides. Unsymmetrical disulfides (also called heterodisulfides) are compounds of the formula . They are less common in organic chemistry, but most disulfides in nature are unsymmetrical. 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 and bonds, the disulfide bond is often the "weak link" in many molecules. Furthermore, reflecting the polarizability of divalent sulfur, the bond is susceptible to scission by polar reagents, both electrophiles and especially nucleophiles (Nu): RS-SR + Nu- -> RS-Nu + RS- The disulfide bond is about 2.05 Å in length, about 0.5 Å longer than a bond. Rotation about the 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. Although the hydrogenation of disulfides is usually not practical, the equilibrium constant for the reaction provides a measure of the standard redox potential for disulfides: RSSR + H2 -> 2 RSH This value is about −250 mV versus the standard hydrogen electrode (pH = 7).