Conformers of gaseous proline

Area of Science:

• Computational Chemistry
• Molecular Spectroscopy
• Biochemistry

Background:

• L-proline is a unique amino acid with a cyclic pyrrolidine ring, influencing its structural and chemical properties.
• Understanding the conformational landscape of L-proline is essential for its biological function and for interpreting experimental data.

Purpose of the Study:

• To accurately determine the geometries, energies, and spectroscopic properties of L-proline conformers.
• To identify the most populated conformers of L-proline under various conditions.
• To investigate the role of intramolecular hydrogen bonds in stabilizing L-proline conformers.

Main Methods:

• Ab initio electronic structure calculations were performed to model L-proline.
• Calculations included corrections for electron correlation, core correlation, and relativistic effects.
• The atoms-in-molecules (AIM) technique was used to analyze bonding and hydrogen bonds.

Main Results:

• Eighteen conformers of neutral L-proline were studied, with four identified as significantly populated at low to moderate temperatures.
• The relative energies and structural differences, including ring puckering and hydrogen bonding (N.HO or NH.O), were precisely calculated.
• Accurate rotational constants, centrifugal distortion constants, vibrational frequencies, and infrared intensities were obtained for spectral characterization.

Conclusions:

• The study provides a comprehensive theoretical characterization of L-proline conformers, aiding experimental identification.
• Intramolecular hydrogen bonds play a key role in the stability of the low-energy conformers.
• The calculated spectroscopic parameters are valuable for experimental validation and understanding L-proline's behavior.