Imide cis-trans isomerization of N-acetyl-N′-methylprolineamide and solvent effects

We report here the results on conformational free energies of N-acetyl-N'-methylprolineamide (Ac-Pro-NHMe) at the HF, MP2, and B3LYP levels of theory with the 6-31+G(d) basis set to see how well the nb initio and density functional computations with the self-consistent reaction field (SCRF) method would reproduce the observed solvent effects on the cis-trans isomerization of imide bond. The augmentation of the prolyl amide cis-isomer population with increased solvent polarity was reproduced by the MP2 or B3LYP free energies using the SCRF solvation free energies. Furthermore, the energy barriers for amide isomerization that were calculated for Ac-Pro-NHMe in different solvents reflected the experimentally observed trend for dimethylacetamide in which the energy for rotation augmented with increased solvent polarity. However, it should be noted that neither the isodensity polarizable continuum model (IPCM) nor the self-consistent isodensity polarizable continuum model (SCI-PCM) predicted properly both solvent effects. Our results indicate the need for improved solvation theory.