Guanidinium cations pair with positively charged arginine side chains in water

Using capillary electrophoresis and molecular dynamics simulations, we directly demonstrate that guanidinium cations exhibit an affinity for the positively charged side chains of arginine-containing peptides in aqueous solutions, while no such effect is observed for sodium cations and/or lysine. Such a counterintuitive pairing between these two positively charged moieties has implications for destabilization and aggregation of arginine-rich proteins in guanidinium salt solutions.