Do Cation-π Interactions Exist in Bacteriorhodopsin?

Metal ions are essential to the structure and physiological functions of bacteriorhodopsin.Experimental evidence suggests the existence of specific cation binding to the negatively charged groups of Asp85 and Asp212 via an electrostatic interaction.However,only using electrostatic force is not enough to explain the role of the metal cations because the carboxylate of Asp85 is well known to be protonated in the M intermediate.Considering the presence of some aromatic amino acid residues in the vicinity of the retinal pocket,the existence of cation-πinteractions between the metal cation and aromatic amino acid residues is suggested.Obviously,introduction of this kind of interaction is conducive to understanding the effects of the metal cations and aromatic amino acid residues inside the protein on the structural stability and proton pumping of bacteriorhodopsin.