AB-INITIO SCF AND FORCE-FIELD CALCULATIONS ON LOW-ENERGY CONFORMERS OF 2-ACETYLAMINO-2,N-DIMETHYLPROPANAMIDE

The understanding of the conformational properties of alpha,alpha-disubstituted amino acids is related to the design of structural mimics for proteins. More specifically, 2-amino-2-methylpropanoic acid has been widely used in constructing helical peptide modules. In this paper, we report the results of ab initio SCF calculations with 3-21G, 6-31G* and 6-31G** basis sets on four low-energy conformations of 2-acetylamino-2,N-dimethylpropanamide. The results are used to assess the reliability of empirical potentials used in force-field methodologies. For this purpose, averaged multiple conformations' atomic point charges were determined with the 6-31G* basis set. The results point to some important deficiencies in current force-fields. Thus, the relative energies of dipeptide conformations with excessive repulsive or excessive attractive interactions are not well described by some empirical potentials.