Electrokinetic charge of the anesthetic-induced bR₄₈₀ and bR₃₈₀ spectral forms of bacteriorhodopsin

The translational and rotational electrokinetics of the anesthetic-induced spectral transitions bR₅₆₈ → bR₄₈₀ → bR₃₈₀ of bacteriorhodopsin have been investigated. Formation of the bR₄₈₀ form is associated with an increase of the purple membrane negative electrokinetic charge, while the transformation of bR₄₈₀ into bR₃₈₀ is accompanied by a decrease of the membrane negative charge as compared to that of the 480 nm-absorbing form. Removal of anesthetics leads to the back transitions bR₄₈₀ → bR₅₆₈ and (in part) bR₃₈₀ → bR₅₆₈; however, the electrokinetic charge of the native membranes is not restored. A strong decrease in the electric y polarizability and the appearance of a slow polarizability component are also observed in anesthetic-treated membranes. Comparison with the electrokinetic behaviour of partially delipidated membranes and with that of liposomes composed of purple membrane total lipids suggests that: (i) anesthetic molecules partition mainly at the protein/lipid interface inducing irreversible rearrangement of the boundary lipid layer, and (ii) different mode(s) or site(s) of interaction are responsible for the spectral and surface charge effects. The data are compatible with the hypothesis of anesthetics acting through partial dehydration of the membrane surface.