Salt-Bridged Peptide Anion Gaseous Structures Enable Efficient Negative Ion Electron Capture Dissociation

We previously discovered that electron attachment to gaseous peptide anions can occur within a relatively narrow electron energy range. The resulting charge-increased radical ions undergo dissociation analogous to conventional cation electron capture/transfer dissociation (ECD/ETD), thus enabling a novel tandem mass spectrometry (MS/MS) technique that we termed negative ion electron capture dissociation (niECD). We proposed that gaseous zwitterionic structures are required for niECD with electron capture either occurring at or being directed by a positively charged site. Here, we further evaluate this zwitterion mechanism by performing niECD of peptides derivatized to alter their ability to form zwitterionic gaseous structures. Introduction of a fixed positive charge tag, a highly basic guanidino group, or a highly acidic sulfonate group to promote zwitterionic structures in singly charged anions, rescued the niECD ability of a peptide refractory to niECD in its unmodified form. We also performed a systematic study of five sets of synthetic peptides with decreasing zwitterion propensity and found that niECD efficiency decreased accordingly, further supporting the zwitterion mechanism. However, traveling-wave ion mobility-mass spectrometry experiments, performed to gain further insight into the gas-phase structures of peptides showing high niECD efficiency, exhibited an inverse correlation between the orientationally averaged collision cross sections and niECD efficiency. These results indicate that compact salt-bridged structures are also a requirement for effective niECD.