Comprehensive Analysis of Energy Minima of the 20 Natural Amino Acids

Energy minima of the 20 natural amino acids (capped by a peptide bond at both the N and the C termini, CH3-C(=O)-N(H)-(H)C-alpha(R)-C(=O)-N(H)-CH3), were obtained by ab initio geometry optimization. Starting with a large number of minima, quickly generated by MarvinView, geometry optimization at the HF/6-31G(d,p) level of theory reduced the number of minima, followed by further optimization at the B3LYP/apc-1 and MP2/cc-pVDZ levels, which caused some minima to disappear and some stable minima to migrate on the Ramachandran map. There is a relation between the number of minima and the size and the flexibility of the side chain. The energy minima of the 20 amino acids are mainly located in the regions of beta(L), gamma(L), delta(L), and ?L of the Ramachandran map. Multipole moments of atoms occurring in the fragment [-NH-C-alpha-C(=O)-] common to all 20 amino acids were calculated at the three levels of theory mentioned above. The near parallelism in behavior of these moments between levels of theory is beneficial toward estimating moments with the more expensive B3LYP and MP2 methods from data calculated with the cheaper HF method. Finally, we explored the transferability of properties between different amino acids: the bond length and angles of the common fragment [-NH-C-alpha(H alpha C beta)-C'(=O)-] in all amino acids except Gly and Pro. All bond lengths are highly transferable between different amino acids, and the standard deviations are small.