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Hi Jan, please note, Arg-Arg proximity is not always repulsive: guanidinium groups can associate bridged by H-bonds and interactions with water molecules or neighboring amino acids. There are many examples of these unusual Arg formations, see for reference: Neves, Yeager and Abagyan (2012) "Unusual Arginine Formations in Protein Function and Assembly: Rings, Strings and Stacks", J. Phys. Chem. B 116, 7006−7013 Hope this is helpful, Andrey