Molecular dynamics simulations of histidine-based dipeptides in water show that a protonated
histidine side chain group has a propensity for forming like-charged contact pairs with another
protonated histidine or with arginine. This effect is of similar strenght to that in observed
previously for arginine-arginine pairing. Even stronger contact pairs are formed in singly
protonated or deprotonated dihistidine, where stacking of aromatic rings is not weakened by
Coulomb repulsion between the side chains. Qualitatively the same pairing behavior is also
observed in a mixed solution of imidazole and imidazolium chloride.