Using capillary electrophoresis and molecular dynamics simulations we directly demonstrate 
that guanidini-um 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 counter-intuitive pairing between these two positively 
charged moieties has implications for destabiliza-tion and aggregation of arginine-rich 
proteins in guanidinium salt solutions.