Long-lived electron capture dissociation product ions experience radical migration via hydrogen abstraction

Abstract

To explore the mechanism of electron capture dissociation (ECD) of linear peptides, a set of 16-mer peptides were synthesized with deuterium labeled on the ␣-carbon position of four glycines. The ECD spectra of these peptides showed that such peptides exhibit a preference for the radical to migrate to the ␣-carbon position on glycine via hydrogen (or deuterium) abstraction before the final cleavage and generation of the detected product ions. The data show c-type fragment ions, ions corresponding to the radical cation of the c-type fragments, c·, and they also show c·-1 peaks in the deuterated peptides only. The presence of the c·-1 peaks is best explained by radical-mediated scrambling of the deuterium atoms in the long-lived, metastable, radical intermediate complex formed by initial electron capture, followed by dissociation of the complex. These data suggest the presence of at least two mechanisms, one slow, one fast. The abundance of H· and ϪCO losses from the precursor ion changed upon deuterium labeling indicating the presence of a kinetic isotope effect, which suggests that the values reported here represent an underestimation of radical migration and H/D scrambling in the observed fragments. (J Am Soc Mass Spectrom 2006, 17, 576 -585)

Electron detachment dissociation of neutral and sialylated oligosaccharides

“…We believe there are several lines of evidence to support that these ions observed following EDD are Z ϩ 2H species. Hydrogen addition and abstraction following ECD of peptides have been well documented [60,59], and have also been reported for EDD of peptides [45]. Following EDD of GAGs, hydrogen abstraction from Band C-type ions has also been noted, and observed as B Ϫ H, C Ϫ H, and C Ϫ 2H type ions [32,33].…”

Radical-driven dissociation of odd-electron peptide radical ions produced in 157 nm photodissociation

“…This picture is consistent with what is known about radical migration during ECD. In that case, the unpaired electron in an ␣-radical transfers stepwise to various other ␣-carbons, inducing a "radical cascade" of backbone fragmentations [33,36]. However, one cannot rule out the possibility that there is a similar distance dependence to the rate that radical ions fold.…”

“…Although O'Connor and coworkers have demonstrated that backbone ␣-hydrogens can be easily abstracted [36], it is still unclear whether backbone amidehydrogens are involved in the radical migration process. To address this issue, amide-hydrogens were replaced with deuterium by H/D exchange.…”

“…This differs from collisional dissociation of even-electron peptide ions, which favors the formation of b-and y-type ions. The unusual fragmentation of chargereduced radical ions has been attributed to radical-driven chemistry that can be described by a mobile hydrogen radical [35] or free radical reaction cascade [33,36]. Theoretical studies have also shown that a radical is responsible for these unusual fragmentation processes [9].…”

Electron capture and transfer dissociation: Peptide structure analysis at different ion internal energy levels

“…Particular advances have been achieved as a result of Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS)-based electron capture dissociation (ECD) [4] complementarity to slow heating fragmentation methods [5], such as collision-induced dissociation (CID) [6] and infrared multiphoton dissociation (IRMPD) [7]. In addition to mainly product ion mass-based MS/ MS, product ion abundance (PIA) in ECD is increasingly considered as a new source of information to improve peptide and protein sequencing [8,9], quantitative modification analysis [10,11], higher-order structure characterization [8,[12][13][14][15], providing new insights into ECD mechanism [16,17], suggesting charge location in peptides and proteins [18,19], and indicating routes toward developing a quantitative model of ECD/ETD [15]. Double-resonance (DR) ECD, with and without ion preactivation, is used to estimate the radical intermediate lifetimes and differentiate between short-lived and long-lived intermediates by monitoring PIA variation attributed to radical intermediates ejection from the ICR trap immediately upon formation [20,21].…”