title: Electron capture dissociation of hydrogen-deficient peptide radical cations.
abstract: Hydrogen-deficient peptide radical cations exhibit fascinating gas phase chemistry, which is governed by radical driven dissociation and, in many cases, by a combination of radical and charge driven fragmentation. Here we examine electron capture dissociation (ECD) of doubly, [M + H](2+•), and triply, [M + 2H](3+•), charged hydrogen-deficient species, aiming to investigate the effect of a hydrogen-deficient radical site on the ECD outcome and characterize the dissociation pathways of hydrogen-deficient species in ECD. ECD of [M + H](2+•) and [M + 2H](3+•) precursor ions resulted in efficient electron capture by the hydrogen-deficient species. However, the intensities of c- and z-type product ions were reduced, compared with those observed for the even electron species, indicating suppression of N-C(α) backbone bond cleavages. We postulate that radical recombination occurs after the initial electron capture event leading to a stable even electron intermediate, which does not trigger N-C(α) bond dissociations. Although the intensities of c- and z-type product ions were reduced, the number of backbone bond cleavages remained largely unaffected between the ECD spectra of the even electron and hydrogen-deficient species. We hypothesize that a small ion population exist as a biradical, which can trigger N-C(α) bond cleavages. Alternatively, radical recombination and N-C(α) bond cleavages can be in competition, with radical recombination being the dominant pathway and N-C(α) cleavages occurring to a lesser degree. Formation of b- and y-type ions observed for two of the hydrogen-deficient peptides examined is also discussed.
authors.affiliation: Proteome Exploration Laboratory, Division of Biology, Beckman Institute, California Institute of Technology, Pasadena, 91125, USA.
authors.forename: Anastasia
authors.initials: A
authors.lastname: Kalli
authors.forename: Sonja
authors.initials: S
authors.lastname: Hess
mesh_terms: D000595:Amino Acid Sequence; D002412:Cations / Q000737:chemistry; D004583:Electrons; D005609:Free Radicals / Q000737:chemistry; D005740:Gases / Q000737:chemistry; D006859:Hydrogen / Q000737:chemistry*; D013058:Mass Spectrometry / Q000379:methods*; D008969:Molecular Sequence Data; D010455:Peptides / Q000737:chemistry*
chemical_list: D002412:Cations; D005609:Free Radicals; D005740:Gases; D010455:Peptides; D006859:Hydrogen