An X-ray absorption study of glycine, methionine and proline

The gas phase photoabsorption spectra of glycine, methionine and proline have been measured at the carbon, nitrogen and oxygen K edges, and spectra have been calculated at all three edges for glycine. The agreement between theory and experiment is excellent. The glycine spectra at all edges are similar to published electron energy loss spectra [M.L. Gordon, G. Cooper, C. Morin, T. Araki, C.C. Turci, K. Kaznatcheev, A.P. Hitchcock, J. Phys. Chem. A 107 (2003) 6144], but show more structure due to the higher resolution. The carbon K edge spectra of methionine and proline are assigned by comparison with glycine and with published calculations [K. Kaznacheyev, A. Osanna, C. Jacobsen, O. Plashkevych, O. Vahtras, H. Ågren, V. Carravetta, A.P. Hitchcock, J. Phys. Chem. A 106 (2002) 3153]. The nitrogen K edge spectra of glycine and methionine are similar, which facilitates the assignments. The spectrum of proline is significantly different, but similar to recently published data. The difference occurs because proline is a secondary amine, while glycine and methionine are primary amines. The oxygen K edge spectra of all three compounds are similar to one another, and resemble the spectrum of formic acid, the simplest carboxylic acid.