AB-INITIO CALCULATIONS ON PRO-ALA AND PRO-GLY DIPEPTIDES

The geometries of the dipeptides L-Pro-L-Ala, L-Pro-D-Ala and L-Pro-Gl y were investigated by a grid scan ab initio calculation. The 6-31G ba sis set was used to estimate the effect of the alanyl side-chain on th e conformation of the peptide backbone and to provide a computational basis for the interpretation of known physical-chemical properties of larger peptides that contain these dipeptides. These calculations furn ish a direct quantum mechanical assessment of the energetic consequenc es of a methyl side-chain in the if i + 2 position of a turn. The resu lts of the calculation support the current view that the presence of a D-Ala residue in the i + 2 position favors a type II beta-turn over a type I beta-turn conformation, while L-Ala has the opposite effect. T otal and relative energies for all the optimized conformations identif ied by the grid search are given and geometric parameters (bond length s, bond angles and dihedral angles) and net atomic charges have been c alculated.