Effects of pH and acetylation on Hg2+-induced purple to blue transition in bacteriorhodopsin

Effects of Hg2+ ions on the absorption spectrum of bacteriorhodopsin have been measured at different pH values and after acetylation. UV-difference spectroscopy and CD spectra show that Hg2+-induced color change is essentially similar to that caused by removal of cations or acidification. The ability of Hg2+-induced purple-to-blue transition is pH-dependent and exhibits a maximum at pH 5.5. Acetylation influences the absorption in the same way as Hg2+ ions and accelerates Hg2+-induced purple to blue transition. All these results strongly suggest that the Hg2+ effect is not a specific binding but just a replacement of intrinsic cations on the membrane surface, where they form a double layer. The composition of the double layer determines the surface pH on the membrane, which affects the color of the bacteriorhodopsin.