PH-DEPENDENCE OF THE DARK EQUILIBRIUM BETWEEN 13-CIS-BACTERIORHODOPSIN AND TRANS-BACTERIORHODOPSIN OF PURPLE MEMBRANES

The relative content of 13-cis- and trans-bacteriorhodopsins* in purple membranes adapted to darkness, determined according to the amount of the intermediate M412 formed under illumination, varies depending on the pH. An increase in the proportion of trans-bacteriorhodopsin with decreasing pH occurs simultaneously with the disappearance of its ability for transmembrane proton transport. The increase in the proportion of 13-cis-bacteriorhodopsin at high pH values is correlated with the appearance of its ability for transmembrane transport of protons. It is concluded that the dark changes in the isomeric composition of retinal residues in the purple membranes are due to a change in the degree of protonation of the groups serving as proton acceptors in the photosynthetic cycle of 13-cis- and trans-bacteriorhodopsin. It is also suggested that the rate of the dark trans half-arrow-right-over-half-arrow-left 13-cis-isomerization of the chromophore depends on the degree of protonation of all the groups.