Extraction anode lens effects in gas phase peptide cation-electron reactions

Abstract

Gas phase cation-electron reactions, from electron capture dissociation (ECD; <1 eV electrons) to electron ionization dissociation (>∼26 eV electrons), are highly beneficial for biomolecular structural characterization. These techniques offer high sequence coverage, labile posttranslational modification retention, and sidechain loss fragments which can differentiate isomeric residues. For optimum performance, careful tuning of electron energy, flux, and irradiation time is required to reach efficiency at a particular energy regime. The cathode bias voltage (CBV) is the primary determinant of electron energy, while several parameters including CBV, extraction anode lens voltage (LV), and cathode heating current determine electron flux. We present an in-depth examination of how the interplay of these parameters at variable irradiation times results in differing peptide cation-electron reaction regimes. A particularly interesting finding was the prominent high energy fragmentation pathways observed at low (∼−1.0 V) CBV and high (>50 V) LV, as compared with conventional (∼5 V) LV for peptide ECD. Specifically, high LV resulted in tandem ionization, observed for both singly- and doubly protonated peptides, alongside increased sequence coverage for both charge states from complex spectra containing a multitude of a/b/c′/d/w/x/y′/z•-type terminal fragments as well as internal fragments and a large number of neutral losses. Electron flux and energy measurements as well as electron irradiation at constant flux showed that an increased number of higher energy electrons are present at high vs. low LV, i.e., the observed “lens effect” is likely due to the presence of high energy electrons under such conditions. This extraction anode lens effect may explain previous observations of unexpected internal fragments from ECD.