Disulfide bond

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. In biology, disulfide bridges formed between thiol groups in two cysteine residues are an important component of the secondary and tertiary structure of proteins. 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 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. In biology, disulfide bridges formed between thiol groups in two cysteine residues are an important component of the secondary and tertiary structure of proteins. 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. Symmetrical disulfides are compounds of the formula R2S2. Most disulfides encountered in organo sulfur chemistry are symmetrical disulfides. Unsymmetrical disulfides (also called heterodisulfides) are compounds of the formula RSSR'. 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 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. 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: This value is about −250 mV versus the standard hydrogen electrode (pH = 7). By comparison, the standard reduction potential for ferrodoxins is about −430 mV. Disulfide bonds are usually formed from the oxidation of sulfhydryl (−SH) groups, especially in biological contexts. The transformation is depicted as follows: