Alteration of protein constituents induced by low-energy (<40 eV) electrons.: III.: The aliphatic amino acids

We report mass spectrometric measurements of anions desorbed by 1-40 eV electron impact on thin films of glycine (Gly), alanine (Ala) and cysteine (Cys) physisorbed on a Pt substrate. (H-, CH-, CH2-, CH3-, O-, OH-, and CN-), (H-, CH2-, CH3-, O-, and OH-), and (H-, O-, OH-, and CN-, S-, and SH-) anions are observed to desorb from these irradiated molecular solids, respectively. The anion yield functions exhibit structures at energies below 15 eV, indicating that molecular dissociation operates via dissociative electron attachment (DEA). Above 15 eV, anion desorption is dominated principally by nonresonant dipolar dissociation (DD). However, an additional structure observed at 20 eV in the yield function of H- denotes the presence of a resonant process: DEA or decay of a transient molecular anion into the DD continuum or both. The presence of the sulfur group in cysteine enhances dissociation by 2 orders of magnitude relative to Gly and Ala.