Pathways to Immonium Ions in the Fragmentation of Protonated Peptides

Abstract

The pathways leading to the formation of immonium (An) ions in the fragmentation of protonated peptides were investigated using metastable ion studies, including kinetic energy release measurements, and low‐energy collision‐induced dissociation studies. In addition to the established pathway Bn→An+CO, it is shown that B2 ions, in suitable circumstances, fragment directly to A1 ions. In addition, metastable ion studies show that A1 ions can be formed directly from protonated di‐ and tripeptides most likely by concerted elimination of CO and an amino acid or smaller peptide. A2 ions can be formed directly from protonated dipeptides in part through the sequential loss of H2O+CO, although kinetic energy release measurements suggest direct elimination of HCOOH also may be occurring. Internal immonium ions are shown to originate by further fragmentation of An ions and by further fragmentation of Yn″ ions. © 1997 by John Wiley & Sons, Ltd.

Fragmentation of doubly-protonated peptide ion populations labeled by H/D exchange with CD3OD

“…2, fragmentation of the doubly-protonated precursor ion results in the formation of the ion pairs b 1 /y 8 and b 2 /y 7 . An a 8 ion, which is presumably formed from the b 8 ion [39][40][41][42][43], is also observed. Because the y 8 (Fig.…”

“…A much poorer match to theoretical is found for the pseudoion pair y 1 /a 8 , with the sum of the deuterium incorporations consistently slightly lower than the theoretical for most ion populations. One of the mechanisms thought to occur for the formation of an a n ion is loss of CO from the b n ion [39][40][41][42][43]. However, this mechanism does not involve hydrogen.…”

Computational investigation and hydrogen/deuterium exchange of the fixed charge derivative tris(2,4,6-Trimethoxyphenyl) phosphonium: Implications for the aspartic acid cleavage mechanism

“…Statistical [19 -24] and mechanistic [25][26][27][28][29][30][31][32] analyses have led to a better understanding of peptide fragmentation behavior. Specifically, these studies have revealed residuespecific preferential cleavage N-terminal to proline (Pro), C-terminal to aspartic acid (Asp) and glutamic acid (Glu), and C-terminal to oxidized cysteine (e.g., cysteine sulfinic acid and cysteine sulfonic acid) [19,20,[33][34][35][36][37][38][39][40][41][42][43].…”

Fragmentation of protonated oligoalanines: Amide bond cleavage and beyond

“…Although these b n ions were initially considered to be acylium ions [8,10,11], extensive experimental and theoretical studies [14 -19] have shown that, in many cases, the b n ions have protonated oxazolone structure as shown in Scheme 2 formed by cyclization involving the carbonyl function next-nearest to the amide bond being ruptured. In several cases where there are more reactive side-chain groups in the peptide, alternative cyclic structures are formed by interaction with sidechain functionality as the amide bond cleavage occurs [20,21].Although the fragmentation reactions of b 2 ions have been studied in some detail [14,18,22,23], the fragmentation reactions of larger b ions have seen less study [15] and the fragmentation of a n ions have seen even less study. A related question is whether, for larger protonated peptides, the lower mass b ions are formed directly by fragmentation of the protonated peptide or are secondary fragmentation products.…”

“…Although the fragmentation reactions of b 2 ions have been studied in some detail [14,18,22,23], the fragmentation reactions of larger b ions have seen less study [15] and the fragmentation of a n ions have seen even less study. A related question is whether, for larger protonated peptides, the lower mass b ions are formed directly by fragmentation of the protonated peptide or are secondary fragmentation products.…”