Even‐electron [M–H] + ions generated by loss of AgH from argentinated peptides with N‐terminal imine groups

Rationale Experiments were performed to probe the creation of apparent even‐electron, [M–H] + ions by CID of Ag‐cationized peptides with N‐terminal imine groups (Schiff bases). Methods Imine‐modified peptides were prepared using condensation reactions with aldehydes. Ag + ‐cationized precursors were generated by electrospray ionization (ESI). Tandem mass spectrometry (MS n ) and collision‐induced dissociation (CID) were performed using a linear ion trap mass spectrometer. Results Loss of AgH from peptide [M + Ag] + ions, at the MS/MS stage, creates closed‐shell [M–H] + ions from imine‐modified peptides. Isotope labeling unambiguously identifies the imine C‐H group as the source of H eliminated in AgH. Subsequent CID of the [M–H] + ions generated sequence ions that are analogous to those produced from [M + H] + ions of the imine‐modified peptides. Conclusions Experiments show (a) formation of novel even‐electron peptide cations by CID and (b) the extent to which sequence ions (conventional b , a and y ions) are generated from peptides with fixed charge site and thus lacking a conventional mobile proton. Copyright © 2015 John Wiley & Sons, Ltd.