A chemical link between two specific amino acids (cysteines) that acts like a molecular staple, holding a peptide in a stable three-dimensional shape.Formed by oxidation of cysteine thiol groups. Disulfide bonds stabilize peptide structure and reduce susceptibility to proteolytic degradation. Many endogenous peptide hormones contain disulfide bonds — insulin has two disulfide bonds that are essential to its activity. Breaking disulfide bonds (by reducing agents) typically abolishes activity. Relevant when considering storage conditions and reconstitution — oxidizing or reducing environments can disrupt disulfide-bonded peptides.