Conformation of N-acetyl- l-pro- d-ala- N-methyl tripeptide empirical, semi-empirical MO and AB initio MO calculations

Abstract

Conformational energies of the title peptide calculated by the empirical program packages GROMOS, CHARMM and DISCOVER (the latter two also with the AMBER force field), semiempirical MO scheme AMI, and the ab initio STO-3G basis set are compared. The lowest energies were obtained for either of the C7C7 (axial and equatorial) conformations except by CHARMM with incorporated AMBER force field, by which the βII conformation is preferred. The relative energy differences of these three conformations are within 3–5 kcal mol −1. Energy minimizations of the peptide surrounded by 121 SPC type water molecules give the lowest energy of hydration for the βII bend. The calculated energies are compared with the results of experimental conformational analysis.