Core level study of alanine and threonine

Area of Science:

• Physical Chemistry
• Spectroscopy
• Computational Chemistry

Background:

• Amino acids alanine and threonine exist in various spatial arrangements called conformers.
• Understanding how molecular structure influences spectroscopic signals is crucial for chemical analysis.
• Previous studies have explored the electronic structure of simpler amino acids like glycine.

Purpose of the Study:

• To investigate the influence of conformers on core-level X-ray photoemission spectra (XPS) and near-edge X-ray absorption fine structure (NEXAFS) of alanine and threonine.
• To compare experimental spectroscopic data with theoretical calculations for accurate interpretation.
• To elucidate the differences in conformer sensitivity between photoemission and photoabsorption techniques.

Main Methods:

• Gas-phase XPS and NEXAFS measurements were performed at the carbon, nitrogen, and oxygen K edges.
• Theoretical calculations employing approximations were used to model various conformers of alanine and threonine.
• Results were validated by comparing calculations for proline with higher-level theoretical data.

Main Results:

• Anomalous broadening in nitrogen 1s photoemission peaks was observed and attributed to conformer populations.
• Carbon K-edge NEXAFS spectra showed similarities to glycine, with assigned resonances.
• Oxygen K-edge spectra displayed characteristic peaks assigned to oxo and hydroxyl groups, with additional features for threonine.

Conclusions:

• Conformer effects are clearly observable in photoemission spectra but less distinct in NEXAFS spectra.
• The differing sensitivity is explained by energetic shifts of core-hole states and unoccupied orbitals, causing partial cancellation in NEXAFS.
• This study highlights the complementary nature of XPS and NEXAFS in probing molecular structure and conformer populations.