Like-charge attraction of molecular cations in water: subtle balance between interionic interactions and ionic solvation effect.
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(+)), 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(+) 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.
Department of Chemistry, Graduate School of Science, Kyoto University , Kyoto 606-8502, Japan.
Taichi
T
Inagaki
Shinji
S
Aono
Hiroshi
H
Nakano
Takeshi
T
Yamamoto
D000588:Amines / Q000737:chemistry; D000595:Amino Acid Sequence; D064751:Ammonium Compounds / Q000737:chemistry; D002412:Cations / Q000737:chemistry; D004151:Dipeptides / Q000737:chemistry / Q000378:metabolism; D019791:Guanidine / Q000737:chemistry; D057927:Hydrophobic and Hydrophilic Interactions; D008958:Models, Molecular; D009842:Oligopeptides / Q000737:chemistry / Q000378:metabolism; D011789:Quantum Theory; D012997:Solvents / Q000737:chemistry*; D013816:Thermodynamics; D014867:Water / Q000737:chemistry*
D000588:Amines; D064751:Ammonium Compounds; D002412:Cations; D004151:Dipeptides; D009842:Oligopeptides; D012997:Solvents; D014867:Water; C076685:arginylarginine; D019791:Guanidine