Ab initio conformational study of N-acetyl-L-proline-N',N'-dimethylamide: a model for polyproline.

We report here the results on N-acetyl-l-proline-N',N'-dimethylamide (Ac-Pro-NMe2) as a model for polyproline at the HF/6-31+G(d) level with the conductor-like polarizable continuum model of self-consistent reaction field methods to figure out the conformational preference and cis-trans isomerization of polyproline in the gas phase, chloroform, methanol, and water. The second methyl substitution at the carboxyl amide end results in different backbone structures and their populations from those of N-acetyl-L-proline-N-methylamide (Ac-Pro-NHMe). In particular, all conformations with the C7 hydrogen bond between acetyl and amide ends, which is the most probable conformations of Ac-Pro-NHMe in the gas phase and in nonpolar solvents, disappeared for Ac-Pro-NMe2 even in the gas phase due to the lack of amide hydrogen. The dominant conformation for Ac-Pro-NMe2 is the polyproline II structure with the trans prolyl peptide bond in the gas phase and in solutions. In methanol, the population of the polyproline I structure with the cis prolyl peptide bond is calculated to be larger than that in water, which is consistent with experiments. It should be noted that Ac-Pro-NMe2 has higher rotational barriers for the cis-trans isomerization of the Ac-Pro peptide bond than Ac-Pro-NHMe in the gas phase and in solutions, which could be due to the lack of the intramolecular hydrogen bond between prolyl nitrogen and carboxyl N-H group for the transition state of Ac-Pro-NMe2. The rotational barriers for Ac-Pro-NMe2 are increased with the increase of solvent polarity, as seen for Ac-Pro-NHMe.