Fig. 2. Ramachandran-type plots of minimal model structures of oligo-N-methylalanine. (a) Ramachandran-type plot of acetyl-N-methyl-l-alanine dimethylamide (a minimal model of l-NMA oligomers). (b) The DFT-optimized structure of l-NMA. The lowest-energy conformer in the Ramachandran-type plot in Fig. 2a was geometry optimized. Hydrogen pairs on non-neighboring carbons showing medium to strong NOE signals are connected using red arrows. The hydrogens with NOE signals are shown in yellow. (c) The crystal structure of acetyl-N-methyl-l-alanine with a C-terminal Boc-piperazine. Hydrogen atoms are omitted for clarity. For (b and c), atoms and bonds other than the l-NMA residue are shown in gray. (d) Ramachandran-type plot of acetyl-N-methyl-d-alanine dimethylamide (a minimal model of d-NMA oligomers).
