Disulfide Bond

A disulfide bond is a covalent sulphur-sulphur link between the thiols of two cysteine residues, formed by oxidation with loss of two hydrogen atoms. The resulting unit is a cystine, and each bond lowers molecular mass by about 2.02 daltons relative to the reduced chain. The bond is roughly two angstroms long and, unlike a hydrogen bond, is a genuine covalent constraint, stapling distant parts of a sequence together and defining loops the linear order does not imply.

Many peptide hormones depend on one. Oxytocin and vasopressin each close a six-residue ring with a single disulfide, somatostatin and octreotide do the same, and insulin is held together by three, two joining the A and B chains and one internal to the A chain. Disulfides are common in secreted peptides and rare inside cells, because the cytosol is kept reducing by glutathione at millimolar concentration.

Connectivity, not merely the presence of bonds, is part of identity. A peptide with four cysteines has three possible pairings and usually only one is correct, yet every isomer shares a formula and a mass. Establishing that the right pairs are joined requires non-reducing peptide mapping, in which the molecule is cleaved between the cysteines and the linked fragments identified, or comparison against an authentic standard.

The practical failure is trusting a mass to prove folding. A certificate showing the expected molecular weight with a clean chromatogram is equally consistent with correctly folded material, with scrambled isomers and with mixtures of both, and scrambling is a real degradation pathway promoted by alkaline pH, heat and free thiol contamination. Reduction is the same problem in the other direction, converting an active cyclic peptide into an inactive linear one for a mass change too small to notice.