Electrostatically defying cation-cation clusters: can likes attract in a low-polarity environment?

Like-charge ion pairing is commonly observed in protein structures and plays a significant role in biochemical processes. Density functional calculations combined with the conductor-like polarizable continuum model were employed to study the formation possibilities of doubly charged noncovalently linked complexes of a series of model compounds and amino acids in the gas phase and in solution. Hydrogen bond interactions were found to offset the Coulombic repulsion such that cation-cation clusters are minima on the potential energy surfaces and neither counterions nor solvent molecules are needed to hold them together. In the gas phase the dissociation energies are exothermic, and the separation barriers span from 1.7 to 15.6 kcal mol(-1). Liquid-phase computations indicate that the separation enthalpies of the cation-cation complexes become endothermic in water and nonpolar solvents with dielectric constants of ≥7 (i.e., the value for THF). These results reveal that electrostatically defying noncovalent complexes of like-charged ions can overcome their Coulombic repulsion even in low-polarity environments.