Gas‐phase fragmentation reactions of a 7 ions containing a glutamine residue

The gas‐phase fragmentation reactions of the a 7 ions derived from glutamine (Q) containing model heptapeptides have been studied in detail with low‐energy collision‐induced dissociation (CID) tandem mass spectrometry (MS/MS). Specifically, the positional effect of the Q residue has been investigated on the fragmentation reactions of a 7 ions. The study involves two sets of permuted isomers of the Q containing model heptapeptides. The first set contains the Q sequence, and the second set involves of QYAGFLV sequence, where the position of the Q residue is changed from N‐ to C‐terminal gradually for both peptide series. An intense loss of ammonia from the a 7 ions followed by internal amino acid eliminations strongly supports forming the imine‐amides structure via cyclization/rearrangement reaction for all studied a 7 ions. This is in agreement with the pioneering study reported by Bythell et al. (2010, 10.1021/ja101556g). A novel rearrangement reaction is detected upon fragmentation of imine‐amide structure, which yields a protonated C‐terminal amidated hexapeptide excluding the Q residue. A possible fragmentation mechanism was proposed to form the protonated C‐terminal amidated hexapeptide, assisted via nucleophilic attack of the side chain amide nitrogen of the Q residue on its N‐protonated imine carbon atom of the rearranged imine‐amide structure. Highlights The gas‐phase fragmentation reactions of a 7 ions obtained from protonated model peptides containing glutamine residue were studied by ESI‐MS/MS. A rearranged imine‐amide structure is the predominant even for a 7 ions. Novel rearrangement reaction is observed which forms a protonated C‐terminal amidated hexapeptide excluding Q residue upon fragmentation of the imine‐amide structure.