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J Am Soc Mass Spectrom. 2012 Oct;23(10):1729-40. Epub 2012 Aug 2.

Electron capture dissociation of hydrogen-deficient peptide radical cations.

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  • 1Proteome Exploration Laboratory, Division of Biology, Beckman Institute, California Institute of Technology, Pasadena, 91125, USA.


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.

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