Electron interactions with positively and negatively multiply charged biomolecular clusters

Abstract

Interactions of positively and negatively multiply charged biomolecular clusters with low-energy electrons, from ∼ 0 up to 50 eV of electron energy, were investigated in a high resolution Fourier-Transform Ion Cyclotron Resonance mass spectrometer equipped with an electrospray ionisation source. Electron-induced dissociation reactions of these clusters depend on the energy of the electrons, the size and the charge state of the cluster. The positively charged clusters [Mn+2H]2+ of zwitterionic betaines, M = (CH3)2XCH2CO2 (X = NCH3 and S), do capture an electron in the low electron energy region (< 10 eV). At higher electron energies neutral evaporation from the cluster becomes competitive with Coulomb explosion. In addition, a series of singly charged fragments arise from bond cleavage reactions, including decarboxylation and CH3 group transfer, due to the access of electronic excited states of the precursor ions. These fragmentation reactions depend on the type of betaine (X = NCH3 or S). For the negative dianionic clusters of tryptophan [Trp9-2H]2−, the important channel at low electron energies is loss of a neutral. Coulomb explosion competes from 19.8 eV and dominates at high electron energies. A small amount of [Trp2–H–NH3]− is observed at 21.8 eV.