A mechanism for the loss of 60 u from peptides containing an arginine residue at the C-terminus

The loss of 60 u from protonated peptide ions containing an arginine residue at the C-terminus has been investigated by means of low energy tandem mass spectrometry. The lowest energy conformation of singly charged bradykinin is thought to involve a salt-bridge structure, which may lead to the formation of two isomeric forms. It is thought that one isomer retains the ionizing proton at the C-terminal end of the peptide, leading to the formation of the b _n?1 + H + OH]⁺ fragment ion, and the other isomer retains the charge at the N-terminus, leading to the formation of the M + H ? 60]⁺ fragment ion. It was found that the formation of the M + H ? 60]⁺ ion occurs only from singly charged precursor ions. In addition, the loss of 60 u occurs from peptides in which the charge is localized at the N-terminus. These results indicate that the mechanism of formation of the M + H ? 60]⁺ ion may be driven by a charge-remote process.