Probing the fragmentation reactions of protonated glycine oligomers via multistage mass spectrometry and gas phase ion molecule hydrogen/deuterium exchange

Abstract

An ion trap mass spectrometer equipped with electrospray ionization has been modified to study the structure of protonated polyglycyl peptides G n (where n = 2–5 glycine residues) and their product ions formed by collision induced dissociation tandem mass spectrometry (CID MS/MS) via the novel application of gas phase ion–molecule hydrogen/deuterium (H/D) exchange reactions. In particular, the structures of the b 2, b 3, b 4, and b 5 ions formed via CID MS/MS from various protonated glycine oligomer precursors have been examined. The b 2 ions, formed from the protonated G 2 and G 3 precursor ions, the b 3 ion from the protonated G 3 precursor, and the b 4 ion from the protonated G 5 ion all undergo CID and gas phase H/D exchange consistent with formation of protonated oxazolone structures previously proposed for b n -type ions. However, CID MS/MS, MS 3, and H/D exchange of the putative b 4 and b 5 arising from the protonated G 4 and G 5 precursor ions, respectively, as well as experiments with various methylated derivatives of G 4, suggest that the major portion of these ions are not b n ions, but are instead formed via backbone–backbone neighboring group participation reactions remote to the C-terminal amino acid. Efforts to elucidate the mechanisms behind this loss of H 2O are described.