Investigation of the Gas-Phase Amino Acid Alanine by Synchrotron Radiation Photoelectron Spectroscopy

Valence and C1s core level photoelectron spectra of gaseous alanine have been recorded with synchrotron radiation. Using ab initio Green's Function calculations of the vertical outer valence ionization energies and CMS-Xα calculations of the orbital ionization cross-sections, it is possible to account well for the features of both the new h ν = 92 eV valence photoelectron spectrum and also its differences with an earlier h ν = 21.2 eV spectrum. Good agreement may be achieved by considering just the contribution of a single molecular conformation. This agrees with previous experimental findings, but conflicts with calculations which suggest that a range of molecular conformations would coexist in an equilibrium sample. A study of the valence photoelectron spectrum of the amino acid threonine complements that of alanine, but unlike the latter is limited by the effects of thermal decomposition of the sample. The C1s core level spectrum of alanine is reported and its peaks are assigned to ionization of the three C atoms in the molecule. A fourth minor peak that is observed is tentatively assigned to a peptide C ONH 2 linkage which may be formed between alanine monomers.