Exploration of the full conformational space of N-acetyl-L-glutamine-N-methylamide. An ab initio and density functional theory study

Abstract

The full conformational space of N-acetyl-L-glutamine-N-methylamide was explored by ab initio (RHF/3-21G and RHF/6-31G(d)) and DFT (B3LYP/6-31G(d)) computations. On the Ramachandran hypersurface of four independent variables, E = E(φ,ψ,χ1,χ2), 59 conformers were located instead of the expected 34 = 81 stable structures. The relative stabilities of the various conformers were analyzed in terms of side chain/backbone interactions covering different hydrogen bonding types by using the theory of atoms in molecules (AIM) and molecular electrostatic potentials (MEPs). The theoretical results were compared with some experimental data (NMR and X-ray).