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 d etermined in the gas phase using ab initio calculations at the HF/6-31 G(d) and MP2/6-31G(d)levels. Geometry optimizations lead to 17 differe nt 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 c ontinuum method have been performed in order to evaluate the effects o f 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 conf ormational properties of the asparagine dipeptide than chloroform solv ent. However, in both solvents the lowest energy structure is the same as that in the gas phase. Finally, the side chain conformations of th e characterized minima have been analyzed and compared with experiment al data. The gauche was the most populated conformer in the three inve stigated environments. This feature is in excellent agreement with the experimental data reported for peptides and proteins.