On the formation, stability, and dissociation of peptide radicals after femtosecond electron transfer from alkali metal atoms

Abstract

In femtosecond encounters between peptide cations and alkali metal atoms, electron transfer occurs from the latter to the former. The peptide radicals are born in the same structure as their precursors and undergo subsequent dissociation on a microsecond to millisecond time scale. The fragmentation pattern is very similar to that observed from other electron-induced dissociation techniques: electron capture dissociation (ECD) where free electrons are captured and electron transfer dissociation (ETD) where the electron donor is an atomic or molecular anion. Dissociation channels involve either (i) loss of a neutral fragment (hydrogen or ammonia) and in some cases consecutive dissociation or (ii) breakage of the peptide backbone, i.e., NCα bond dissociation. We have dubbed the collisional electron transfer technique Electron Capture Induced Dissociation (ECID) to distinguish it from its relatives, ECD and ETD. Here we review ECID results with focus on the competition between dissociation channels and dependence on peptide size and conformation, crown-ether tagging and methylation of ammonium groups, microsolvation, phosphorylation, and the electronic state from which dissociation takes place. While ECID cannot compete with ECD or ETD as an analytical technique, it has provided a wealth of fundamental knowledge on the fragmentation of peptide radicals from work on simple model systems. Its combination with MALDI time-of-flight instruments may, however, show some potential use in the future as ECID does not rely on precursor ions being doubly charged (or higher charged) as neutral fragments can efficiently be converted to anions in secondary collisions.