Investigation of the amino acids glycine, proline, and methionine by photoemission spectroscopy.

The valence and core level photoelectron spectra of glycine, proline, and methionine in the gas phase have been investigated by VUV and soft X-ray radiation. The outer valence band photoemission spectra are similar to previously reported He I spectra, although relative peak intensities are different due to the different photon energy. We extended the spectral range to include the inner valence region. The carbon, nitrogen, and oxygen 1s as well as the sulfur 2p core level spectra of these amino acids have been measured and the states identified. Valence band spectra of proline have been recorded as a function of temperature, and they provide information about the relative populations of the lowest energy conformers.