Electron Capture and Transfer Dissociation: Peptide Structure Analysis at Different Ion Internal Energy Levels

We decoupled electron-transfer dissociation (ETD) and collision-induced dissociation of charge-reduced species (CRCID) events to probe the lifetimes of intermediate radical species in ETD-based ion trap tandem mass spectrometry of peptides. Short-lived intermediates formed upon electron transfer require less energy for product ion formation and appear in regular ETD mass spectra, whereas long-lived intermediates require additional vibrational energy and yield product ions as a function of CRCID amplitude. The observed dependencies complement the results obtained by double-resonance electron-capture dissociation (ECD) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and ECD in a cryogenic ICR trap. Compared with ECD FT-ICR MS, ion trap MS offers lower precursor ion internal energy conditions, leading to more abundant charge-reduced radical intermediates and larger variation of product ion abundance as a function of vibrational post-activation amplitude. In many cases decoupled CRCID after ETD exhibits abundant radical c-type and even-electron z-type ions, in striking contrast to predominantly even-electron c-type and radical z-type ions in ECD FT-ICR MS and especially activated ion-ECD, thus providing a new insight into the fundamentals of ECD/ETD. Radical and even-electron product ion yield in ion trap-based ETD differs from ECD FT-ICR MS. ETD and CRCID distinguish short and long-lifetime radical intermediate complexes.