Conversion of amino-acids by electrons at subexcitation energies

Electron attachment to gas phase cysteine is investigated in a crossed electron/molecular beam experiment. The yield functions of the detected anions exhibit signatures of dissociative electron attachment (DEA), initiated by shape (<3 eV) and core-excited (>5 eV) resonances. These parent anion precursor states decompose into a variety of negatively charged fragments with one of the most dominant channels being dehydrogenation, i.e., abstraction of the H radical with the excess electron remaining on the cysteine-like moiety. While this reaction is operative only within the low energy range, S- and SH- appear from both resonance regions. From energy considerations it is shown that formation of S- below 3 eV must be associated with a conversion of cysteine into alanine. The present results demonstrate the capacity of electrons at subexcitation energies to effectively change the nature of amino acids.