Pathways for Water Loss from Doubly Protonated Peptides Containing Serine or Threonine

Abstract

The doubly-protonated peptides Ala-Ala-Xaa-Ala-Ala-Ala-Arg show extensive loss of H 2 O when Xaa=Ser or Thr. Using quasi-MS 3 techniques the fragmentation reactions of the [M+2H -H 2 O] +2 ions have been studied in detail. For both Ser and Thr, the [M+2H -H 2 O] +2 ions show three primary fragmentation reactions, elimination of CH 3 CH=NH, elimination of one Ala residue, and elimination of two Ala residues, in all cases forming doubly-charged products. From a study of the further fragmentation of these products, it is concluded that elimination of two Ala residues results in formation of a three-membered aziridine ring by interaction with the adjacent amide function as H 2 O is lost. The elimination of one Ala residue results in formation of a five-membered oxazoline ring through interaction with the N-terminal adjacent carbonyl function as H 2 O is lost. The elimination of CH 3 CH=NH appears to involve formation of an eight-membered ring by interaction with the remote N-terminal carbonyl function as H 2 O is lost. However, this initial structure undergoes rearrangement through interaction with the adjacent C-terminal carbonyl function prior to further fragmentation. The [MH -H 2 O] + ion of Ala-Ala-Ser-Ala-Ala-Ala also shows elimination of CH 3 CH=NH, one Ala residue and two Ala residues.

Large-Scale Examination of Factors Influencing Phosphopeptide Neutral Loss during Collision Induced Dissociation

“…Further evidence for charge directed, ring-forming mechanisms was found by several groups that independently identified neutral-loss-dependent cleavage of the backbone [19–21]. This diagnostic cleavage occurs between the α-carbon and amide carbon, one bond N-terminal to the ring that was formed by neutral loss.…”

“…This diagnostic cleavage occurs between the α-carbon and amide carbon, one bond N-terminal to the ring that was formed by neutral loss. The resulting ion has been variously described as an “x-type ion” [19], a “y+10 ion” [20] and a neutral loss [21]. The diagnostic ion is seen both in neutral loss from pSer and pThr [19] and during neutral loss of water from the non-phosphorylated cognates [20].…”

“…The diagnostic ion is seen both in neutral loss from pSer and pThr [19] and during neutral loss of water from the non-phosphorylated cognates [20]. Harrison also observed an additional ion attributed to a neutral loss mechanism analogous to oxazoline formation, but leading to a larger cyclized product incorporating one or more residues [21]. Although the influence of surrounding amino acid side chains on dissociation events was not systematically explored, these studies point to previously unappreciated mechanisms underlying neutral loss in phosphopeptides that are currently not accounted for by current peptide identification algorithms.…”

Gas Phase Chemistry of Li+with Amides: the Observation of LiOH Loss in Mass Spectrometry

“…In the past few decades, continuous efforts have been devoted to the investigation of the dissociation reactions of charged compounds derived from protonation [4][5][6][7][8] or deprotonation [9][10][11][12][13]. On the other hand, the mass spectrometric fragmentations of metal cationized, especially lithiated biomolecules, have attracted much attention because of the desire to measure intrinsic metal ion affinity and identify the binding sites [14,15], and because the interesting dissociation pathways exhibited by these compounds are usually different from the protonated and deprotonated analogues [16,17].…”

Protonated Dipeptide Losses from b5 and b4 Ions of Side Chain Hydroxyl Group Containing Pentapeptides

“…The water losses from the side chain of hydroxyl group containing amino acid residues have been extensively investigated by several groups [29][30][31][32][33][34][35][36]. In line with these works, a possible mechanism for the formation of m/z 279 fragment ion from protonated b 5 ion of SGGFL-NH 2 is shown in Scheme 1.…”