title: Radical a-ions in electron capture dissociation: on the origin of species.
abstract: Radical a* ions appear in electron capture dissociation mass spectra sporadically, but sometimes with high intensity. Mechanistically, radical a ions are hypothesized to arise due to thermodynamically disadvantaged charge solvation on the backbone nitrogen (instead of carbonyl), which upon neutralization produces a hypervalent group instantly fragmenting into a radical b* and conventional y' ion. The former species is unstable and, after releasing a CO molecule, decays to an a* ion. Here we validate this scenario by direct observation of the complementarity of a* and y' ions by interrogation of an ECD MS/MS database of >10,000 doubly and >5,000 triply charged tryptic peptides. Intriguingly, the most abundant a*/y' pairs are found to come from the cleavage of the same backbone link as the most abundant c' and z* complementary ions. This result gives strong support to the "local" N-Cα bond cleavage mechanism, in which the dissociation occurs at the site of charge solvation. However, a second strong peak is observed in the c'/z* fragment distribution four residues away from the a*/y' cleavage, which supports the indirect N-Cα bond cleavage mechanism. The size distribution of a ions from doubly (but not triply!) charged precursors shows deficit of a3 ions, and possibly a6 ions.
authors.affiliation: Division of Physiological Chemistry I, Department of Medical Biochemistry and Biophysics, Karolinska Institutet, Scheeles väg 2, 171 77, Stockholm, Sweden. Roman.Zubarev@ki.se
authors.forename: Roman A
authors.initials: RA
authors.lastname: Zubarev
authors.forename: David M
authors.initials: DM
authors.lastname: Good
authors.forename: Mikhail M
authors.initials: MM
authors.lastname: Savitski
mesh_terms: D016208:Databases, Factual; D007477:Ions / Q000737:chemistry*; D010446:Peptide Fragments / Q000737:chemistry*; D053719:Tandem Mass Spectrometry / Q000379:methods*
chemical_list: D007477:Ions; D010446:Peptide Fragments