Pathways to Immonium Ions in the Fragmentation of Protonated Peptides

Abstract

The pathways leading to the formation of immonium ions in the fragmentation of protonated peptides were (A n ) investigated using metastable ion studies, including kinetic energy release measurements, and low-energy collisioninduced dissociation studies. In addition to the established pathway it is shown that ions, in B n Ç A n + CO, B 2 suitable circumstances, fragment directly to ions. In addition, metastable ion studies show that ions can be A 1A 1 formed directly from protonated di-and tripeptides most likely by concerted elimination of CO and an amino acid or smaller peptide.ions can be formed directly from protonated dipeptides in part through the sequential loss of A 2 although kinetic energy release measurements suggest direct elimination of HCOOH also may be H 2

Sequencing of the thirteen structurally isomeric quartets of N‐terminal dipeptide motifs in peptides by collision‐induced dissociation

“…Immonium ion formation from the N-terminal amino acid can proceed either directly from the protonated molecular ion or via a secondary fragmentation reaction from the b 2 ion [19][20][21]. Formation via b 2 ions was identified as the major pathway for immonium ion formation from small peptides [19]. The immonium ion intensities in a large number of Q-TOF CID spectra have recently been documented [22], confirming the eminent contribution of the N-terminal residue to immonium ion formation.…”

“…The a 1 /x max-1 fragment ion pairs are observed in about 20% of the spectra summarized in Table 2, indicating a direct formation of the immonium ions from the molecular ions. Immonium ion formation from the N-terminal amino acid can proceed either directly from the protonated molecular ion or via a secondary fragmentation reaction from the b 2 ion [19][20][21]. Formation via b 2 ions was identified as the major pathway for immonium ion formation from small peptides [19].…”

Five-membered ring formation in unimolecular reactions of peptides: a key structural element controlling low-energy collision-induced dissociation of peptides

“…This is in accordance with the view that immonium ion formation proceeds mainly via b-and a-ions as intermediates, although alternative mechanisms have also been described. 16 Regarding the effect of proline on peptide fragmentation, this cyclic N-alkylated amino acid preferentially promotes cleavage at its N-terminal side. 17 The fact that N-methylation of other amino acids favors backbone cleavage at the C-terminal side, whereas proline promotes cleavage at its N-terminal side, has been explained by steric factors since cleavage at the C-terminal side of proline leads to a strained bicyclic oxazolone ion.…”