A conformational study of the alpha-L-aspartate-containing dipeptide

The conformational preferences of the alpha-L-aspartate-containing dipeptid e were investigated by ab initio calculations. The structures of the minima were generated by full geometry optimization at the HF/6-31G(d) and HF/6-3 1+G(d) levels of 27 starting geometries, resulting from the systematic comb ination of the three minima associated with the flexible dihedral angles ph i, psi, and chi(1) The energies of the resulting minima were computed at th e MP2/6-31+G(d) level. Selected minima were used as starting points for geo metry optimization at the MP2/6-31+G(d) level. The conformational behavior of this compound was markedly different from that of the model dipeptides c omposed of common alpha-amino acids. Thus, the charged side chain produces substantial changes in the potential energy hypersurface with respect to th ose observed in other compounds with neutral polar side chains, such as the L-asparagine-containing dipeptide.