Disulfide Bridge

A disulfide bridge forms when the thiol groups of two cysteine residues are oxidised into a covalent sulfur–sulfur bond. It is the only widespread covalent crosslink found in natural peptides and proteins, and because two hydrogen atoms are lost in the process, the bridged peptide weighs 2 Da less than the reduced version — a gap that mass spectrometry can see.

Why the bridge is so important

A bridge turns a flexible chain into a constrained loop, and that constraint is frequently what enables receptor binding in the first place. Oxytocin and vasopressin each have one bridge closing a ring of six residues; somatostatin, insulin and numerous toxin-derived peptides rely on them completely. A synthetic peptide delivered in its reduced, unbridged state can have the correct sequence and still show no activity in an assay.

Watch for two ways things go wrong. Scrambling happens when a peptide with several cysteines pairs them incorrectly during oxidation, yielding an isomer of the same mass that elutes at a different HPLC retention time. Thiol-containing buffer additives can also reduce and reopen a bridge that had formed properly. A helpful certificate of analysis reports which cysteines are connected, not just the mass. Related: cyclic peptide and somatostatin.

Related terms

oxidation · linear peptide · reading peptide sequences