AB-INITIO CONFORMATIONAL-ANALYSIS OF ALANINE

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 Angstrom in bond distances and about 3-degrees 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, C5, C7ax, alpha(R), and alpha(L), are found close to the minimum energy regions of the free acid studied here.