The photoelectron spectra of amino-acids : A survey

Photoelectron spectra, excited by He I radiation, of the following amino acids were measured:- alanine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, histidine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophane, tyrosine, 4-hydroxyproline, β-alanine, γ-aminobutyric acid and δ-aminovaleric acid. The spectra showed a number of bands of low ionization energy (<13 eV) and these have been assigned to photoionizations from the NH₂ and COOH groups. For the α-amino acids with complex side-chains extra bands arising from easily-ionizable electrons in aromatic rings or atomic lonepairs, such as sulphur or oxygen atoms, were also identified. Perturbation arguments together with the spectra of closely related molecules have been used to confirm the assignments. During measurement, decomposition occurred for aspartic acid, asparagine and histidine, while glutamic acid and glutamine cyclized.