Effects of the phi[NHCO] retromodification on dehydroalanine dipeptide.

The potential energy surface of the phi [NHCO] delta Ala dipeptide has been computed using ab initio quantum mechanical calculations at different theoretical levels. Three degenerate minima were found and characterized. The most favoured conformation is stabilized by an intramolecular hydrogen bonding interaction. The other two minima correspond to helical and malonamide-like conformations, being 4.6 kcal/mol and 6.9 kcal/mol less stable than the lowest energy conformation, respectively. Influence of solvent effects on the relative stabilities of the different conformations have been accounted using SCRF calculations. Two implicit solvent models have been considered: water and CCl4. The results indicate that in the both cases the conformation with an intramolecular hydrogen bonding interaction retains its lowest energy.