First publication in PNAS!
Very thorough joint experimental (NMR, SAXS) & theoretical (umbrella-sampling, MD, mean field modeling) work of a group around Mikael Lund and his collaborators provided strong support on unusual self-association of highly charged poly-arginine chains. Note that like-charged species normally repel each other. This behavior is unique for arginine, and diminishes upon mutation to lysine (another positively charged amino acid). On the other end, purely poly-lysine chains behave as standard polyelectrolytes.
Long poly-arginine segment are found in many cell penetrating peptides, also known as arginine-rich peptides (RRPs), which can spontaneously traverse biological membranes in so called direct mode. Therefore this unusual binding motif may have a potential biological role.
Note that the rare like-charge attraction has its origin already on the level of guanidinium cations. link1, link2, research
The origin of this story goes back to 2012, to Pavel Jungwirth‘s group in Prague, when Mario Vazdar did first MD simulations of many deca-arginine and deca-lysine chains, and we have quantified the observed like-charged association. The motivation was the obsess by guanidinium and arginine systems, and a talk of Robin Curtis (University of Manchester) on his DLS experiments from around 2010.