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Journal of Chemical Physics / Volume 131 / Issue 3 / ARTICLES / Biological Molecules, Biopolymers, and Biological Systems

J. Chem. Phys. 131, 035103 (2009); doi:10.1063/1.3168393 (11 pages)

Electronic structure of aromatic amino acids studied by soft x-ray spectroscopy

Wenhua Zhang1,2, Vincenzo Carravetta2, Oksana Plekan3, Vitaliy Feyer3, Robert Richter3, Marcello Coreno4, and Kevin C. Prince3,5

1Department of Theoretical Chemistry, School of Biotechnology, Royal Institute of Technology, S-10691 Stockholm, Sweden and Hefei National Laboratory for Physical Science at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China
2Institute of Chemical Physical Processes, CNR, via Moruzzi 1, 56124 Pisa, Italy
3Sincrotrone Trieste, Area Science Park, I-34012 Basovizza, Trieste, Italy
4CNR-IMIP, Montelibretti, Rome I-00016 Italy
5Laboratorio Nazionale TASC, CNR-INFM, 34012 Trieste, Italy

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(Received 27 March 2009; accepted 12 June 2009; published online 17 July 2009)

The electronic structure of phenylalanine, tyrosine, tryptophan, and 3-methylindole in the gas phase was investigated by x-ray photoemission spectroscopy (XPS) and near edge x-ray absorption fine structure (NEXAFS) spectroscopy at the C, N, and O K-edges. The XPS spectra have been calculated for the four principal conformers of each amino acid, and the spectra weighted by the Boltzmann population ratios calculated from published free energies. Instead of the single peaks expected from the stoichiometry of the compounds, the N 1s core level spectra of phenylalanine and tryptophan show features indicating that more than one conformer is present. The calculations reproduce the experimental features. The C and O 1s spectra do not show evident effects due to conformational isomerism. The calculations predict that such effects are small for carbon, and for oxygen it appears that only broadening occurs. The carbon K-edge NEXAFS spectra of these aromatic amino acids are similar to the published data of the corresponding molecules in the solid state, but show more structure due to the higher resolution in the present study. The N K-edge spectra of tryptophan and 3-methylindole differ from phenylalanine and tyrosine, as the first two both contain a nitrogen atom located in a pyrrole ring. The nitrogen K-edge NEXAFS spectra of aromatic amino acids do not show any measurable effects due to conformational isomerism, in contrast to the photoemission results. Calculations support this result and show that variations of the vertical excitation energies of different conformers are small, and cannot be resolved in the present experiment. The O NEXAFS spectra of these three aromatic compounds are very similar to other, simpler amino acids, which have been studied previously.

© 2009 American Institute of Physics

Article Outline

  1. INTRODUCTION
  2. EXPERIMENTAL AND THEORETICAL METHODS
  3. CORE LEVEL PHOTOEMISSION: THEORETICAL COMPUTATION AND EXPERIMENTAL RESULTS
  4. NEXAFS SPECTRA
    1. Carbon K -edge NEXAFS spectra
    2. Nitrogen K -edge NEXAFS spectra
    3. Oxygen K -edge NEXAFS spectra
  5. CONCLUSIONS

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KEYWORDS and PACS

Keywords

core levels, electronic structure, EXAFS, isomerism, molecular biophysics, proteins, stoichiometry, XANES, X-ray photoelectron spectra

PACS

  • 87.15.Pc

    Electronic and electrical properties

  • 87.15.M-

    Spectra of biomolecules

ARTICLE DATA

Permalink
http://link.aip.org/link/doi/10.1063/1.3168393

PUBLICATION DATA

ISSN:

0021-9606 (print)  
1089-7690 (online)

Publisher:

$abstract.society.value is a Member of CrossRef American Institute of Physics

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