Protein disulfide bond ($\mathrm{S}-\mathrm{S}$)

A protein disulfide bond is a disulfide bond linking two sulfur chemical atoms carried by suitable amino acid residues. It can connect distant portions of a polypeptide chain or different chains. See Cooper: sulfur-containing residues and disulfide crosslinks. The four internal protein disulfide bonds of ribonuclease A are central to the ribonuclease A refolding experiment; breaking these crosslinks need not break the peptide bonds along its protein backbone.