We knew that guanidinium cation is an unique species, which desire our attention. Its ability to form cation-cation pairs is manifested in aqueous solutions of guanidinium salts as well as between arginine sidechains in proteins. 
Such arginine magic was pointed out to lead to an unexpected attraction between polyarginine chains with consequences to the action of arginine-rich cell penetrating peptides.
For more, check our recent review in Accounts of Chemical Research.
doi: 10.1021/acs.accounts.8b00098
Computer modeling of complex systems 