Collision-induced dissociation and post-source decay of model dodecapeptide ions containing lysine and glycine

Abstract

Collision-induced dissociation (CID) and post-source decay (PSD) of a series of model dodecapeptide ions have been studied by ESI-FT-ICR and MALDI-TOF, respectively. Low-energy CID of quadruply protonated peptide ions, [M + 4H] 4+, where M = (KGG) 4, (K 2G 4) 2, and K 4G 8, was conducted under both on-resonance irradiation and sustained off-resonance irradiation (SORI) conditions. The fragmentation patterns observed were very sequence dependent. For (KGG) 4, the major product ions involved y-cleavages occurring on the C-terminal side of the lysine residues. These dissociation sites and the corresponding charge states of the product ions can be explained on the basis of the four protons being primarily localized on lysine residues. For (K 2G 4) 2, both y and b series ions were observed as dissociation products. The observation of b ions in the vicinity of the C-terminus is not consistent with the model of protons being localized on lysine residues. The data suggest that, during the CID process, protons may migrate to the less basic C-terminal glycine residue in order to minimize Coulomb repulsion. This is also supported by CID studies of K 4G 8 where the major dissociation products are b type ions formed from cleavages near the C-terminus. In contrast with the CID results, PSD of [M + H] + from all three peptides leads to formation of almost complete series of y and b type ions, with cleavages at the lysine residues often being most prominent. In summary, it was found that low-energy CID of [M + 4H] 4+ in an ESI-FT-ICR yielded pronounced cleavages at the basic lysine residues, which may provide information about the number and locations of basic residues; however, PSD of [M + H] + in a MALDI-TOF provided a wider range of fragment ions that should prove useful in peptide sequencing.