Electron Capture Dissociation for Structural Characterization of Multiply Charged Protein Cations

Abstract

For proteins of 3. Thus, in an FTMS ion cell with added electron trapping electrodes, capture appears to be achieved best at the boundary between the potential wells that trap the electrons and ions, now providing 80 ± 15% precursor ion conversion efficiency. Capture cross section is dependent on the ionic charge squared (z²), minimizing the secondary dissociation of lower charge fragment ions. Electron capture is postulated to occur initially at a protonated site to release an energetic (∼6 eV) H^• atom that is captured at a high-affinity site such as −S−S− or backbone amide to cause nonergodic (before energy randomization) dissociation. Cleavages between every pair of amino acids in mellitin (2.8 kDa) and ubiquitin (8.6 kDa) are represented in their ECD and CAD spectra, providing complete data for their de novo sequencing. Because posttranslational modifications such as carboxylation, glycosylation, and sulfation are less easily lost in ECD than in CAD, ECD assignments of their sequence positions are far more specific.