Journal of Electron Spectroscopy and Related Phenomena 155 (2007) 47–53 An X-ray absorption study of glycine, methionine and proline O. Plekan a,1 , V. Feyer b , R. Richter a , M. Coreno b,d , M. de Simone c,d , K.C. Prince a,c,∗ , V. Carravetta e a Sincrotrone Trieste, in Area Science Park, I-34012 Basovizza, Trieste, Italy b CNR-IMIP, Montelibretti, Rome I-00016, Italy c CNR Laboratorio Nazionale TASC-INFM, I-34012 Basovizza, Trieste, Italy d INSTM, Trieste, Italy e Institute of Chemical Physical Processes-CNR, Via Moruzzi 1, 56124 Pisa, Italy Available online 19 November 2006 Abstract 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. ˚ Agren, 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. © 2006 Elsevier B.V. All rights reserved. Keywords: X-ray absorption; Glycine; Proline; Methionine 1. Introduction The electronic structure of organic molecules of biological significance is a subject of increasing interest, partly because methods of theoretical calculation are reaching ever higher lev- els of sophistication and precision, and can predict the structure and function of complex molecules. Any calculation must be tested against experimental data to determine its reliability, and molecules in the gas phase represent a very suitable benchmark as they are not influenced by solid state effects. However, even for smaller bio molecules like amino acids, their low vapour pres- sure may make it difficult to achieve sufficient sample densities to permit some spectroscopic measurements without thermally decomposing the compound. For this reason there is a dearth of information about the electronic structure of amino acids in the gas phase. Glycine and some related compounds have been examined by electron energy loss spectroscopy (EELS) ∗ Corresponding author. Tel.: +39 0403758584; fax: +390403758565. E-mail address: Prince@elettra.trieste.it (K.C. Prince). 1 Permanent address: Institute of Electron Physics, 88017 Uzhgorod, Ukraine. [1] and the carbon, nitrogen and oxygen K edge loss spectra reported. These are equivalent to the NEXAFS (near edge X- ray absorption fine structure) spectra, for the conditions under which they were taken. The NEXAFS spectra of glycine and proline in solution have also been measured as a function of pH [2,3], and compared with calculated spectra of gas phase glycine. At low and moderate pH, the spectra resemble those of solid glycine, where the compound is known to be zwitterionic. In basic solution, the spectra resemble the calculated gas phase spectra. The gas phase spectra of proline at the N and O edges have been reported recently by Marinho et al. [4]. In addition the solid state carbon NEXAFS spectra of all natural amino acids have been measured by Kaznacheyev et al. [5] and compared with theory. The calculations were carried out for the zwitteri- onic forms of the amino acids, and so the nitrogen and oxygen K edge spectra are not comparable with gas phase data, but these calculations should give rather accurate predictions of the spectra of the carbon skeleton of amino acids. The solid state NEXAFS spectra of glycine, methionine and proline at the C, N and O edges have been reported by Zubavichus et al. [6,7]. 0368-2048/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2006.11.004