Low-energy (3-24 eV) electron damage to the peptide backbone

We report the mass spectrometric measurement of anions desorbed by 3-24 eV electron impact on thin films of formamide-1-d (DCONH2) and on the self-assembled monolayer (SAM) of two different Lys amide molecules used as a molecular model of the peptide backbone. In the present SAM configuration, the amides are elevated from a gold substrate by hydrocarbon chains to remove the effects of the metal substrate. Electron irradiation produces H- and D- from the formamide-1-d film and H-, CH3-, O-, and OH- from the SAM Lys amides. Below 13 eV, the dependence of the anion yields on the incident electron energy exhibits structures indicative of the dissociative electron attachment process, which is responsible for molecular fragmentation via the initial formation of core-excited anions. Above 13 eV, anion desorption is dominated principally by non-resonant dipolar dissociation. Our results suggest that the sensitivity of the peptide backbone to secondary electrons produced by ionizing radiation depends on the chemical environment (i.e., the amino acids sequence).