Like-Charge Ion Pairing in Water: An Ab Initio Molecular Dynamics Study of Aqueous Guanidinium Cations

The existence of like-charge guanidinium-guanidinium contact ion pairs in water is established by ab initio molecular dynamics simulations. Despite direct electrostatic repulsion, a contact ion pair is observed between two guanidinium cations, stabilized primarily by their amphiphilic behavior and van der Waals interactions. In a control simulation performed for two aqueous ammonium cations, no such contact ion pair is formed. This is the strongest computational evidence so far for the existence of specific contact ion pairing between guanidinium cations in water, with important implications for biological processes involving arginine-rich proteins.