Ab initio calculations on blocked Pro-Ala dipeptides

The effect of the chirality of the amino acid at position i + 2 on a beta-turn was investigated by a grid scan ab initio calculation on the Ac-L-Pro-L-Ala-NH2 and Ac-L-Pro-D-Ala-NH2 blocked dipeptides. Th6-31G basis set was used to estimate the effect of the alanyl side chain on the conformation of the peptide backbone in a blocked dipeptide as a simple, but complete model for a reverse turn. This study provides a quantum mechanical evaluation of the ability of the NH at the i + 3 residue to form the I-I-bond that closes the 10 membered ring which stabilizes the turn, The lowest energy of all 64 probed conformations of the D-Ala containing peptide corresponded to a good type II beta-turn with a hydrogen bond distance between the acetyl oxygen and the amide terminal hydrogen of 2.21 Angstrom. A comparison with the nonblocked dipeptide ab initio study indicates that the presence of the end blocks enhances the propensity of the D-Ala-containing dipeptide for a type II beta-turn, but does not seem to enhance the propensity of the L-Ala-containing dipeptide for a type I beta-turn. The energies and geometric parameters for the lowest four optimized conformations identified by the grid scan search for each molecule have been calculated. (C) 1998 Elsevier Science B.V.