Conformational preferences of the asparagine residue. Gas-phase, aqueous solution, and chloroform solution calculations on the model dipeptide

The conformational preferences of the asparagine dipeptide have been determined in the gas phase using ab initio calculations at the HF/6-31G(d) and MP2/6-31G(d)levels. Geometry optimizations lead to 17 different minima, the lowest energy structure being characterized by a C-7,C-eq backbone orientation and a gauche conformation for the side chain. Self-consistent reaction-field (SCRF) calculations using a polarized continuum method have been performed in order to evaluate the effects of the solvent on the conformational preferences of the compound under study. Water and chloroform were the solvents chosen for this purpose. Results show that water solvent exerts a larger influence on the conformational properties of the asparagine dipeptide than chloroform solvent. However, in both solvents the lowest energy structure is the same as that in the gas phase. Finally, the side chain conformations of the characterized minima have been analyzed and compared with experimental data. The gauche was the most populated conformer in the three investigated environments. This feature is in excellent agreement with the experimental data reported for peptides and proteins.