Effect of Local Conditions on Heterogeneous Reactions in the Bacteriorhodopsin Membrane: An Electrochemical View

Abstract

Bacteriorhodopsin, the light‐harvesting protein in the purple membrane of Halobacterium halobium, has been extensively studied in the past decade as a model for light energy transduction. Flash photolysis studies have established several transient reaction intermediates. A number of laboratories have attempted to correlate pulsed light‐induced electrical relaxation in the bacteriorhodopsin membrane with the corresponding spectral relaxation. Although the electrical data were shown to roughly correlate with the spectral data as long as both were measured in the same laboratory, data from different laboratories rarely agree. This paper analyzes the problem from an electrochemical point of view: the proton translocation across the membrane is treated as two coupled interfacial proton transfer reactions in a three‐phase system (one membrane phase and two aqueous phases). In view of the existence of two exposed surfaces of bacteriorhodopsin, the concept of “local reaction conditions” is proposed to emphasize the unique situation of the two coupled heterogeneous reactions that are subject to different reaction conditions at the two membrane surfaces. Variations of these local conditions may cause only localized conformational changes rather than global changes involving the entire pigment molecule. The present analysis cautions against the indiscriminate use of spectral data alone as a means of identifying photointermediates and emphasizes that meaningful comparisons of electrical data with spectral data can only be done when both are measured under identical local conditions.