Electron-induced dissociation of protonated peptides yields backbone fragmentation consistent with a hydrogen-deficient radical

Rapid Communications in Mass Spectrometry (Impact Factor: 2.25). 07/2009; 23(13):2099-101. DOI: 10.1002/rcm.4117
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    ABSTRACT: A combination of electrospray ionization (ESI), multistage, and high-resolution mass spectrometry experiments are used to examine the gas-phase fragmentation reactions of radical cations of cysteine containing di- and tripeptides. Two different chemical methods were used to form initial populations of radical cations in which the radical sites were located at different positions: (1) sulfur-centered cysteinyl radicals via bond homolysis of protonated S-nitrosocysteine containing peptides; and (2) α-carbon backbone-centered radicals via Siu’s sequence of reactions (J. Am. Chem. Soc. 2008, 130, 7862). Comparison of the fragmentation reactions of these regiospecifically generated radicals suggests that hydrogen atom transfer (HAT) between the α C-H of adjacent residues and the cysteinyl radical can occur. In addition, using accurate mass measurements, deuterium labeling, and comparison with an authentic sample, a novel loss of part of the N-terminal cysteine residue was shown to give rise to the protonated, truncated N-formyl peptide (an even-electron xn ion). DFT calculations were performed on the radical cation [GCG].+ to examine: the relative stabilities of isomers with different radical and protonation sites; the barriers associated with radical migration between four possible radical sites, [G.CG]+, [GC.G]+, [GCG.]+, and [GC(S.)G]+; and for dissociation from these sites to yield b2-type ions.
    Full-text · Article · Feb 2010 · Journal of the American Society for Mass Spectrometry
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    ABSTRACT: Secondary fragmentations of three synthetic peptides (human alphaA crystallin peptide 1-11, the deamidated form of human betaB2 crystallin peptide 4-14, and amyloid beta peptide 25-35) were studied in both electron capture dissociation (ECD) and electron-transfer dissociation (ETD) mode. In ECD, in addition to c and z. ion formations, charge remote fragmentations (CRF) of z. ions were abundant, resulting in internal fragment formation or partial/entire side-chain losses from amino acids, sometimes several residues away from the backbone cleavage site, and to some extent multiple side-chain losses. The internal fragments were observed in peptides with basic residues located in the middle of the sequences, which was different from most tryptic peptides with basic residues located at the C-terminus. These secondary cleavages were initiated by hydrogen abstraction at the alpha-, beta-, or gamma-position of the amino acid side chain. In comparison, ETD generates fewer CRF fragments than ECD. This secondary cleavage study will facilitate ECD/ETD spectra interpretation, and help de novo sequencing and database searching.
    Preview · Article · Apr 2010 · Journal of the American Society for Mass Spectrometry
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    ABSTRACT: The fragmentation of tryptophan (Trp) – metal complexes [Trp+M]+, where M = Cs, K, Na, Li and Ag, induced by 22 eV energy electrons was compared to [Trp+H]+. Additional insights were obtained through the study of collision-induced dissociation (CID) of [Trp+M]+ and through deuterium labelling. The electron-induced dissociation (EID) of [Trp+M]+ resulted in the formation of radical cations via the following pathways: (i) loss of M to form Trp+•, (ii) loss of an H atom to form [(Trp-H)+M]+•, and (iii) bond homolysis to form C2H4NO2M+•. Deuterium labelling suggests that H atom loss can occur from heteroatom and/or C–H positions. Other types of fragment ions observed include: C9H7NM+, C9H8N+, M+, C2H3NO2M+, CO2M+, C10H11N2M+, C10H9NOM+. Formation of C2H4NO2M+• and C9H7NM+ cations suggests that the metal interacts with both the backbone and aromatic side chain, thus implicating π-interactions for all M. CID of [Trp+M]+ resulted in: loss of metal cation (for M = Cs and K); successive loss of NH3 and CO as the dominant channel for M = Na, Li and Ag; formation of C2H3NO2M+. Preliminary DFT calculations were carried out on [Trp+Na]+ and [(Trp-H)+Na]+• which reveal that: the most stable conformation involves chelation by the backbone together with a $\pi $\pi -interaction with the indole side chain; loss of H atom from a\alpha -CH of the side chain is thermodynamically favoured over losses from other positions, with the resultant radical cation maintaining a (N, O, ring) chelated structure which is stabilized by conjugation.
    No preview · Article · Oct 2010 · The European Physical Journal D
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