Preferential cleavage of SS and CS bonds in electron detachment dissociation and infrared multiphoton dissociation of disulfide-linked peptide anions

Disulfide bonds generally show only limited cleavage in positive ion mode collision activated dissociation (CAD). However, it has been demonstrated that a reverse situation exists in negative ion mode in which preferential SS and CS bond cleavage occurs. Here, we show that electron detachment dissociation (EDD) and infrared multiphoton dissociation (IRMPD) of peptide anions containing disulfide linkages also result in preferential cleavage of SS and CS bonds. Resulting products are mainly radical ions in EDD whereas IRMPD produces even-electron product ions, as expected, thereby supporting different disulfide cleavage mechanisms for these two fragmentation processes. We also show that, in EDD, the presence of tryptophan can result in abundant side chain loss (129 Da), which effectively can compete with disulfide bond cleavage.