LIGHT-INDUCED PROTON DISSOCIATION AND ASSOCIATION IN BACTERIORHODOPSIN

Abstract— Light-induced proton release and uptake by acetylated and unmodified bacteriorhodopsin were measured. Bacteriorhodopsin, when illuminated, shows a net proton release at neutral and alkaline pH's, but in acidic pH, it shows an uptake of protons. In the presence of high concentrations of guanidine hydrochloride, light caused only proton release even in acidic pH and the maximum extent of the release was one proton per bacteriorhodopsin molecule around pH 8.
Acetylation of bacteriorhodopsin caused no alteration in the absorption spectrum of purple complex (bR₅₇₀) and M₄₁₂-intermediate, but decreased the decay rate of the M₄₁₂-intermediate. Light-induced release of protons was not observed even in neutral pH values, and only the proton uptake was noticed by acetylated purple membrane fragments. In high concentrations of guanidine hydrochloride, no proton uptake or release by illumination was observed. Vesicles were reconstituted from acetylated purple membrane. These vesicles had almost no ability for light-induced proton transport. The role of amino group(s) in light-induced proton release and transport through the purple membrane is discussed.