Fragmentation reactions of an (n=4–8) ions containing only alanine residues

Gas‐phase fragmentation reactions of a7 ions containing a glutamine residue

“…In the present work, the gas‐phase fragmentation reactions of the a 7 ions derived from permuted isomers of the Q‐containing model peptides have been investigated via multi‐stage mass spectrometry experiments. The obtained results demonstrated that the a 7 ions undergo cyclization/rearrangement reactions to form rearranged imine‐amide isomers in line with the previous reports 34,36,43,47,48 . In addition to these isomers, a minor population of the classical C‐terminal imine structures was also observed based on detection of ithe immonium ion losses.…”

“…The structure of the a n ions was initially proposed as a linear peptide with an imine‐protonated (HN+═CHR) at the C‐terminal end 37–39 . However, recent extensive studies have shown that a n ( n = 2–5) ions can adopt acyclic and/or a linear rearranged imine‐amide structure(s) prior to fragmentation 13,35,36,40–48 . For instance, IRMPD and DFT results have revealed that the a 2 ions of GGG (triglycine) adopt a cyclic N1‐protonated 4‐imidazolidinone structure in the gas phase in a lower energy state than its linear imine‐protonated isomer 34,40,42 .…”

“…Moreover, Yalcin et al 7 and Harrison and Young 55 have reported that the a n ions with C‐terminal amine groups can dissociate to form next‐lower b n ions (viz., b n‐1 ). More recently, Harrison 36 has examined the fragmentation reactions of a n ( n = 4–8) ion‐containing alanine residues solely and named the fragmentation reactions of ions as either “amide pathway” or “imine pathway.”…”

“…Recently, much more interest has been devoted to the gas‐phase structures and fragmentation reactions of peptide ions 34–36 . a n ions are formed either via CO loss (−28 Da) from their corresponding b n ions 6,7 or directly from protonated peptides 37,38 .…”