Low energy photoelectron resonance capture ionization aerosol mass spectrometry of small peptides with cysteine residues: Cys-Gly, γ-Glu-Cys, and glutathione (γ-Glu-Cys-Gly)

The photoelectron resonance capture ionization (PERCI) of cysteine (Cys) and small gas-phase neutral peptides that contain the Cys residue (Cys-Gly, γ-Glu-Cys, and glutathione (γ-Glu-Cys-Gly)) is reported. At an ionization energy less than 1 eV two types of dissociative electron attachment ionization were observed for Cys: hydrogen atom loss, resulting in formation of the ion [Cys-H]−, and dissociation of the CH2-SH bond, resulting in formation of the ion [SH]−. The presence of these ions suggests that both the π*(-CO2H) and σ*(C-S) orbitals can act as low energy electrophores on Cys. This ionization trend was observed for the dipeptides Cys-Gly and γ-Glu-Cys as well as glutathione, with evidence that dissociation of the CH2-SH bond in these peptides can also result in ions of the form [M-SH]−. Also measured were ions resulting from bond dissociation of the amide linkage as well as for the amide N-Cα bond. In both of these cases the charge is retained on the fragment containing the nitrogen of the amide bond, indicating that ion formation by the PERCI process is directed by the electron affinity (EA) of the fragments. The backbone fragmentation of the PERCI process appears distinct from other low energy processes, including electron detachment dissociation and electron capture dissociation, as evidenced by the lack of amide Cα-C cleavage; the dependence of ion formation on the EA of the fragments, not their H affinity; and the observation that PERCI is not a directionally restricted mechanism.