Mapping Disulfide Bonds in Insulin with the Route 66 Method: Selective Cleavage of SC Bonds Using Alkali and Alkaline Earth Metal Enolate Complexes

Simple and fast identification of disulfide linkages in insulin is demonstrated with a peptic digest using the Route 66 method. This is accomplished by collisional activation of singly and doubly charged cationic Na+ and Ca2+ complexes generated using electrospray ionization mass spectrometry (ESI-MS). Collisional activation of doubly charged metal complexes of peptides with intermolecular disulfide linkages yields two sets of singly charged paired products separated by 66 mass units resulting from selective SC bond cleavages. Highly selective elimination of 66 mass units, which corresponds to the molecular weight of hydrogen disulfide (H2S2), is observed from singly charged metal complexes of peptides with disulfide linkages. The mechanism proposed for these processes is initiated by formation of a metal-stabilized enolate at Cys, followed by cleavage of the SC bond. Further activation of the products yields sequence information that facilitates locating the position of the disulfide linkages in the peptic digest fragments. For example, the doubly charged Ca2+ complex of the peptic digest product GIVEQCCASVCSL/FVNQHLCGSHL yields paired products separated by 66 mass units resulting from selective SC bond cleavages at an intermolecular disulfide linkage under low-energy collision-induced dissociation. Further activation of the product comprising the A chain reveals the presence of a second disulfide bridge, an intramolecular linkage. Experimental and theoretical studies of the disulfide linked model peptides provide mechanistic details for the selective cleavage of the SC bond.

Graphical AbstractThe Route 66 method, which derives its name from selective cleavage of CS bonds and elimination of H2S2 (66 Da), maps disulfide linkages in peptides and proteins.Figure optionsDownload full-size imageDownload high-quality image (169 K)Download as PowerPoint slide