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Do b-ions occur from vibrational excitation upon H-desorption in electron capture dissociation?

Electron capture dissociation (ECD) has recently been shown in some cases to produce abundant N-terminal b-ion peptide fragments. These product ions are usually only observed when activation occurs via vibrational excitation as in collision-induced dissociation (CID). Here, we show that occurrence of b-ions in the ECD spectra of synthetic peptides are correlated with low gas-phase basicity and that the observed b-ion fragments are N-terminal products. Furthermore, all ECD spectra containing b-ions also had abundant losses of hydrogen and ammonia from the charge-reduced species.