Investigation of Gas Phase Fragmentation Reaction Mechanisms of Doubly-Protonated Bn (n=7-9) Ions by Ion Trap Mass Spectrometry

Abstract

In this dissertation, the gas-phase fragmentation mechanisms of doubly-charged bn (n=7, 8, and 9) ions from basic amino acid containing model peptides were investigated by means of collision-induced dissociation (CID) coupled tandem mass spectrometry (MS/MS). The study utilized two sets of C-terminal amidated model peptides that HYAGFLV-NH2, YHAGFLV-NH2, YAHGFLV-NH2, YAGHFLV-NH2, YAGFHLV-NH2, YAGFLHV-NH2, YAGFLVH-NH2 and HAAAAAA-NH2, AHAAAAA-NH2, AAHAAAA-NH2, AAAHAAA-NH2, AAAAHAA-NH2, AAAAAHA-NH2, AAAAAAH-NH2 where the position of the histidine (His) residue is varied from N-to-C-terminal. Both positional effect and peptide sequence effect were examined for the fragmentation reactions of doubly-protonated b7 ions for these heptapeptides. The CID-MS3 mass spectra of doubly-protonated b7 ions have internal amino acid losses which provide an evidence for macrocyclization reaction. The proposed reaction mechanism involves charge-separation reaction of doubly-protonated b ions in the gas-phase which generates a protonated iminium ion of the N-terminal residue and protonated internal b ion with C-terminal oxazolone group. In addition to these model peptides, the C-terminal acid free forms of peptides such as SVEHAGVIL, SHIGDAVVI, EHAGVISVL, and GRIDKPILK were also used for to understand the extent of macrocyclization of doubly-charged b8 ion and very similar results were obtained. Moreover, the C-terminal acid free forms of peptide KRNGVIIAGY were investigated for the behavior of doubly-protonated larger b9 ion and doubly-protonated internal eliminations were obtained meanwhile singly-protonated amino acid eliminations and the mechanism was adjusted according to the results.