Ab initio and DFT conformational study of proline dipeptide

• Published in: Journal of molecular structure. Theochem Vol. 675; no. 1; pp. 37 - 45
• Main Author: Kang, Young Kee
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.04.2004
• Subjects: Ab initio and B3LYP calculations; Ac-Pro-NHMe; Cis–trans isomerization; Solvation; Structure; Ac-Pro-NHMe; Ab initio and B3LYP calculations; Cis–trans isomerization; Solvation; Structure
• ISSN: 0166-1280; 1872-7999
• DOI: 10.1016/j.theochem.2003.12.031

We report here the results on conformational free energies of Ac-Pro-NHMe calculated using the ab initio and density functional methods with the self-consistent reaction field (SCRF) theory at HF and B3LYP levels with 6-31G(d), 6-31G(d,p), and 6-31+G(d) basis sets to investigate the conformational preference of proline depending on the cis/ trans peptide bonds and down/up puckerings along the backbone torsion angle ψ in the gas phase, chloroform, and water. There are no significant differences in the potential energy surfaces and in the optimized structures for local minima and transition states at the HF level with the three basis sets. However, the differences in atomic charges with the three basis sets appear to be responsible for altering relative energies of local minima and transition states as well as the rotational barriers of the cis–trans isomerization for the X-Pro bond. The populations of the intramolecularly hydrogen-bonded conformation C decreased and the populations of the polyproline-like conformation F and the α-helical conformation A increased as the solvent polarity increased, which is in good agreement with CD and NMR experiments. The comparison of the populations of backbone and cis conformations and the rotational barriers for the cis–trans isomerization computed at the three levels and those of experiments in chloroform and water indicates that the HF/6-31+G(d) level with the conductor-like PCM SCRF method appears to be the appropriate method in describing the cis–trans isomerization of the X-Pro peptide bond in solution.