Ab initio conformational analysis of alanine

by Cao, M.; Newton, Susan Quinlyn; Pranata, Julianto; Schäfer, Lothar

The ab initio geometries of 13 conformations of alanine were optimized without any constraints at the HF/6-31G** HF/6-311G**, MP2/6-31G**, and MP2/6-311G** levels of theory. By comparing structures devoid of electron correlation (i.e. HF-optimized) with the corresponding structures in which correlation was switched on (i.e. MP2-optimized), the structural effects of electron correlation can be detected. It is found that the magnitude of the effects (up to 0.03 Aring, bond distances and about 3deg in angles) follows the sequence C-O>C-N>C-C, and H-O-C>H-N-C>H-C-C. For the assignment of the spectroscopic data in a previous microwave study of alanine, it was an important finding that, at the HF-computational level, only the 6-31G** geometry of conformation 1 agreed with one of two experimental sets of rotational constants and dipole moment components. In contrast, we find that, at the MP2 computational level, a second less stable conformer, form 5, is also in good agreement with the same set of rotational constants and dipole components. Thus, observation of conformer 1 rather than form 5 can be established only by reference to additional evidence; e.g. the calculated energies, the gas electron diffraction data of alanine, and the most stable form found for glycine. Finally, in a comparison of alanine with alanine dipeptide, the N-C-C=O torsional angles of the characteristic low energy regions of peptides, C7eq, C5C7ax, alphaR, and alphaL, are found close to the minimum energy regions of the free acid studied here.

Journal of Molecular Structure-Theochem
Start Page
2210-2728; 0166-1280