Effects of pH and acetylation on Hg(2+)-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.