The molecular origin of like-charge arginine-arginine pairing in water.

Molecular dynamics simulations show significant like-charge pairing of guanidinium side chains in aqueous poly-arginine, while this effect is absent in aqueous poly-lysine containing ammonium-terminated side chains. This behavior of the guanidinium group is revealed also by protein database searches, having important biochemical implications. Combination of molecular dynamics simulations with explicit solvent and ab initio calculations employing a polarizable continuum model of water allows one to rationalize the formation of contact ion pairs between guanidinium cations in terms of individual interactions at the molecular level.