Identification of intermolecular disulfide linkages in underivatised peptides using negative ion electrospray mass spectrometry. A joint experimental and theoretical study

The [M--H](-) ion of a symmetrical peptide containing one intermolecular disulfide linkage cleaves through the disulfide link to produce up to four fragment anions. Two of these characteristic fragments are formed by a cleavage initiated from the Cys enolate anion on the peptide backbone. The other fragment anions are formed by a cleavage directed from an anion site on the disulfide side chain. In the case of an unsymmetrical peptide containing one intermolecular disulfide, the [M--H](-) anion may cleave through the disulfide unit to give a maximum of eight cleavage anions. These fragmentations are low-energy processes as determined by theoretical calculations carried out at the HF/6-31G(d)//AM1 level of theory. Collision-induced mass spectra of the fragment anions may provide the sequence of the peptide.