PEPTIDE MODELS .1. TOPOLOGY OF SELECTED PEPTIDE CONFORMATIONAL POTENTIAL-ENERGY SURFACES (GLYCINE AND ALANINE DERIVATIVES)

N-Formylglycinamide (For-Gly-NH2), N-formyl-L-alaninamide (For-L-Ala-NH2), and N-formyl-D-alaninamide (For-D-Ala-NH2) were subjected to conformational studies by molecular mechanics (MM) and ab initio SCF methods. Both methods predicted the corresponding-gamma (usually labeled as C-7eq) conformations to be the global minimum on the Ramachandran map, E(phi,psi). The number of minima and their approximate location obtained by MM corresponded to those that one might have expected on the basis of multidimensional conformation analysis. However, only the ab initio SCF study was capable of properly describing the effects associated with excessive repulsive or attractive interactions that lead to the annihilation and creation of certain critical points. Consequently, the topology of the MM and SCF conformational potential energy surfaces, E(phi,psi), were found to be remarkably different for the three diamide systems investigated in this work.