Photoion mass spectrometry of five amino acids in the 6-22 eV photon energy range

A photoionization mass spectrometry study in the 6-22 eV photon energy region of five amino acids, glycine-h5 and its -d5 isotopologue, α-alanine, β-alanine, α-aminoisobutyric acid and α-valine, revealed VUV-induced degradation pathways of these important biological molecules. The fragmentation patterns, ionization energies and ion appearance energies are reported, many for the first time, and are compared with results of electron impact and other studies. Assignment of ion peaks and determination of fragment ion formation channels were assisted by mass spectral data on deuterated isotopologues of the three proteinaceous amino acids studied. Thermochemical data, coupled with the observed ion appearance energies, was also useful in clarifying dissociative photoionization pathways. Ion pair formation appears to occur in certain low energy dissociation processes. Isomeric interconversion between α-alanine and β-alanine cations does not occur up to 20 eV excitation energy. Some astrophysical implications concerning the prospects for amino acid observation and survival in the interstellar medium and in meteorites are briefly discussed