Equilibria between multiple spectral forms of bacteriorhodopsin effect of delipidation, anesthetics and solvents on their pH dependence

Abstract

In bacteriorhodopsin, there exist acid-base equilibria between four major spectral forms with λ max at 570, 605, 460 and 380 nm, respectively. The apparent p K a of these spectral titration in purple membranes are compared with those observed for delipidated bacteriorhodopsin and anesthetic-treated purple membranes. The general behavior is that in the absence of appropriate solvation (or protection) by lipids, the pH range within which the spectral transitions occur is greatly reduced. In addition, halogenated general anesthetics together with few previously reported reagents are far from being the only types of lipid-soluble molecule able to produce such an effect. It can be mimicked with a series of commonly used organic solvents including acetone, alcohols and alcanes. We thus propose that they act following the same ‘hole punching’ mechanism which enable bulk water ionic solutes to come close enough to the chromophore to switch on or off the dipoles that are responsible for switching from one color to another.