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.