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.