Hydrogen Atom Loss in Electron-Capture Dissociation: A Fourier Transform-Ion Cyclotron Resonance Study with Single Isotopomeric Ubiquitin Ions

In electron-capture dissociation (ECD), a multiply-protonated protein ion, trapped in a Fourier transform-ion cyclotron resonance (FT-ICR) cell, captures a low-energy electron at a protonated site. In a major reaction pathway, the resulting hydrogen atom attacks a backbone carbonyl oxygen to form a hypervalent species that immediately dissociates into a complementary c, z^• ion pair. For larger proteins, the reduced odd-electron ion (M + nH)^{(n −1)+•} is a major product, as shown here using isotopically isolated precursors. In addition, a hydrogen atom can be lost without further reaction, yielding the [M + (n −1)H]^{(n −1)+} even-electron ions. The large effect of charge state on the yield of these ions suggests that the 9+ to 11+ charge states have novel charge-solvated secondary structures.