Mechanism of protonation of oligopeptides and their interaction with alkali cations

Electron-ion collisions have been studied for various protonated and alkali-metal adducted peptide monocations, using an electrostatic storage ring. Neutral particle production rates vs. collision energy show typical electron-capture-dissociation profiles for all of the protonated peptides and some of alkali-metal adducted peptides, which consist of a rise towards zero-energy and a bump at high energies. The rate is often enhanced by adducting lithium or sodium cation to peptides compared to the protonated ones. For the alkali-metal adducted cations, the rate correlates with the strength of the non-covalent π–cation interaction. Molecular mechanics and quantum-chemical computations performed here help explain these results.