Aspartate: An interesting model for analyzing dipole-ion and ion pair interactions through its oppositely charged amine and acid groups.

Abstract

Anionic species of aspartic acid, Asp- , having a zwitterionic backbone and a deprotonated side chain, appears to be a good example for analyzing dipole-ion and ion pair interactions. Density functional theory calculations were herein performed to investigate the low energy conformers of Asp- embedded in a dielectric continuum modeling an aqueous environment, through a scan of the potential energy as a function of the side chain (χ1 , χ2 ) torsion angles. The most energetically favorable conformers having g+ g- and g- g+ side chain orientations are found to be stabilized by charge-enhanced intramolecular H-bonding involving the positively charged ( ) and the two negatively charged (COO- ) groups. These conformers were further used to analyze Asp- + nW clusters (W: water, n = 1 or 3), and Asp- /Asp- pair formation. COO- groups were found to be the most attractive sites for hosting a water molecule (binding energy: -6.0 ± 1.5 kcal/mol), compared to groups (binding energy: -4.7 ± 1.1 kcal/mol). Energy separation between g+ g- and g- g+ conformers increases upon explicit hydration. Asp- /Asp- ion pairs, stabilized by the interaction between the group of a partner and the COO- group of the other, shows a quite constant binding energy (-8.1 ± 0.2 kcal/mol), whatever the pair type, and the relative orientation of the two interacting partners. This study suggests a first step to achieve a more realistic image of intermolecular interactions in aqueous environment, especially upon increasing concentration. It can also be considered as a preliminary attempt to assess the interactions of the Lys+ …Asp- /Glu- ion pairs stabilizing intra- and interchain interactions in proteins.