title: Fragmentation of doubly-protonated Pro-His-Xaa tripeptides: formation of b(2)(2+) ions.
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 b2/y1 pair and formation of the a1/y2 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.
authors.affiliation: Department of Molecular Biophysics, German Cancer Research Center (DKFZ), Heidelberg, Germany.
authors.forename: Michaela
authors.initials: M
authors.lastname: Knapp-Mohammady
authors.forename: Alex B
authors.initials: AB
authors.lastname: Young
authors.forename: Béla
authors.initials: B
authors.lastname: Paizs
authors.forename: Alex G
authors.initials: AG
authors.lastname: Harrison
mesh_terms: D000577:Amides / Q000737:chemistry; D000595:Amino Acid Sequence; D054659:Diketopiperazines / Q000737:chemistry; D019281:Dimerization; D004151:Dipeptides / Q000737:chemistry; D007477:Ions; D007536:Isomerism; D008956:Models, Chemical; D008958:Models, Molecular; D008969:Molecular Sequence Data; D015394:Molecular Structure; D009587:Nitrogen Isotopes; D009842:Oligopeptides / Q000737:chemistry*; D011392:Proline / Q000737:chemistry*; D017433:Protein Structure, Secondary; D011522:Protons; D021241:Spectrometry, Mass, Electrospray Ionization; D013816:Thermodynamics
chemical_list: D000577:Amides; D054659:Diketopiperazines; D004151:Dipeptides; D007477:Ions; D009587:Nitrogen Isotopes; D009842:Oligopeptides; D011522:Protons; C034124:histidinoalanine; C061516:histidylglycine; C020357:histidylleucine; D011392:Proline
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