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

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, MS3, 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 H2O are described. (Int J Mass Spectrom 190/191 (1999) 209-230) (C) 1999 Elsevier Science B.V.