Calculation of the Phi-Psi maps for alanyl and glycyl dipeptides with different additive and non-additive molecular mechanical models

Conformational energies are calculated for the glycyl and alanyl dipeptides using four different molecular mechanical models: (1) the Cornell et al. additive force field with phi and psi dihedral parameters V-n/2 set to 0; (2) the standard Cornell et al. additive force field; (3) an alternative version of (2) with phi and psi parameters optimized to fit high level ab initio energies calculated for all-alpha(r) and all-beta alanine tetrapeptides; and 4) a non-additive variation of (1) which includes atom-centered charge polarization. Molecular mechanical energies are calculated with each model for low energy conformations of glycyl and alanyl dipeptides and compared with high level ab initio calculations. In addition, phi-psi maps are calculated and plotted for all four models and compared.