Like-Charge Attraction of Molecular Cations in Water:
Subtle Balance between Interionic Interactions and Ionic Solvation
Effect
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Abstract
Despite strong electrostatic repulsion,
like-charged ions in aqueous
solution can effectively attract each other via ion–water interactions.
In this paper we investigate such an effective interaction of like-charged
ions in water by using the 3D-RISM-SCF method (i.e., electronic structure
theory combined with three-dimensional integral equation theory for
molecular solvents). Free energy profiles are calculated at the CCSD(T)
level for a series of molecular ions including guanidinium (Gdm<sup>+</sup>), alkyl-substituted ammonium, and aromatic amine cations.
Polarizable continuum model (PCM) and mean-field QM/MM free energy
calculations are also performed for comparison. The results show that
the stability of like-charged ion pairs in aqueous solution is determined
by a very subtle balance between interionic interactions (including
dispersion and π-stacking interactions) and ionic solvation/hydrophobic
effects and that the Gdm<sup>+</sup> ion has a rather favorable character
for like-charge association among all the cations studied. Furthermore,
we investigate the like-charge pairing in Arg-Ala-Arg and Lys-Ala-Lys
tripeptides in water and show that the Arg-Arg pair has a contact
free-energy minimum of about −6 kcal/mol. This result indicates
that arginine pairing observed on protein surfaces and interfaces
is stabilized considerably by solvation effects