Pathways for Water Loss from Doubly Protonated Peptides Containing Serine or Threonine |
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Authors: | Alex G Harrison |
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Institution: | (1) Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, ON, M5S 3H6, Canada |
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Abstract: | The doubly-protonated peptides Ala-Ala-Xaa-Ala-Ala-Ala-Arg show extensive loss of H2O when Xaa = Ser or Thr. Using quasi-MS3 techniques the fragmentation reactions of the M + 2H – H2O]+2 ions have been studied in detail. For both Ser and Thr, the M + 2H – H2O]+2 ions show three primary fragmentation reactions, elimination of CH3CH = 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 H2O 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 H2O is lost. The elimination of CH3CH = NH appears to involve formation of an eight-membered ring by interaction with the remote N-terminal carbonyl function
as H2O is lost. However, this initial structure undergoes rearrangement through interaction with the adjacent C-terminal carbonyl
function prior to further fragmentation. The MH – H2O]+ ion of Ala-Ala-Ser-Ala-Ala-Ala also shows elimination of CH3CH = NH, one Ala residue and two Ala residues. |
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