Exploration of the Four-Dimensional-Conformational Potential Energy Hypersurface of N-Acetyl-l-aspartic Acid N‘-Methylamide with Its Internally Hydrogen Bonded Side-Chain Orientation

Side-chain conformational potential energy hypersurfaces have been generated and analyzed for each of the nine possible backbone conformers of N-acetyl-l-aspartic acid-N‘ methylamide. A total of 37 out of the 81 possible conformers were found and optimized at the B3LYP/6-31G(d) level of theory. The relative energies as well as the stabilization exerted by the side-chain on the backbone have been calculated, at this level of theory, for the 37 optimized conformers. Various backbone-backbone (N−H···OC) and backbone-side-chain (N−H···OC; N−H···OH) hydrogen bonds were analyzed. The appearance of the notoriously absent αL backbone conformer was attributed to such a backbone-side-chain (BB−SC) hydrogen bonds as well as a very unusual backbone-backbone (BB−BB) hydrogen bond.