Fragmentation of doubly-protonated Pro-His-Xaa tripeptides: Formation of b
2
2+
ions |
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Authors: | Michaela Knapp-Mohammady Alex B Young Béla Paizs Alex G Harrison |
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Institution: | aDepartment of Chemistry, University of Toronto, Toronto, Canada;bDepartment of Molecular Biophysics, German Cancer Research Center (DKFZ), Heidelberg, Germany |
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Abstract: | When ionized by electrospray from acidic solutions, the tripeptides Pro-His-Xaa (Xaa=Gly, Ala, Leu) form abundant doubly-protonated
ions, M+2H]2+. Collision-induced dissociation (CID) of these doubly-protonated species results, in part, in formation of b
2
2+ ions, which fragment further by loss of CO to form a
2
2+ ions; the latter fragment by loss of CO to form the Pro and His iminium immonium is commonly used in peptide MS work] ions.
Although larger doubly-charged b ions are known, this represents the first detailed study of b
2
2+ ions in CID of small doubly protonated peptides. The most abundant CID products of the studied doubly-protonated peptides
arise mainly in charge separation involving two primary fragmentation channels, formation of the b
2
/y
1
pair and formation of the a
1
/y
2
pair. Combined molecular dynamics and density functional theory calculations are used to gain insight into the structures
and fragmentation pathways of doubly-protonated Pro-His-Gly including the energetics of potential protonation sites, backbone
cleavages, post-cleavage charge-separation reactions and the isomeric structures of b
2
2+ ions. Three possible structures are considered for the b
2
2+ ions: the oxazolone, diketopiperazine, and fused ring isomers. The last is formed by cleavage of the His-Gly amide bond on
a pathway that is initiated by nucleophilic attack of one of the His side-chain imidazole nitrogens. Our calculations indicate
the b
2
2+ ion population is dominated by the oxazolone and/or fused ring isomers. |
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Keywords: | |
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