Do cation-pi interactions exist in bacteriorhodopsin?

Metal ions are essential to the structure and physiological functions of ba cteriorhodopsin. Experimental evidence suggests the existence of specific c ation binding to the negatively charged groups of Asp85 and Asp212 via an e lectrostatic interaction. However, only using electrostatic force is not en ough to explain the role of the metal cations because the carboxylate of As p85 is well known to be protonated in the M intermediate. Considering the p resence of some aromatic amino acid residues in the vicinity of the retinal pocket, the existence of cation-pi interactions between the metal cation a nd aromatic amino acid residues is suggested. Obviously, introduction of th is kind of interaction is conducive to understanding the effects of the met al cations and aromatic amino acid residues inside the protein on the struc tural stability and proton pumping of bacteriorhodopsin.