Electric‐Field Effects in 13‐Demethyl‐11,14‐Epoxyretinal‐Bacteriorhodopsin Films

Abstract— We report the results of experiments on the application of electric fields across thin, dry Alms of a bacteriorho‐dopsin (BR) analog pigment in which the retinal chro‐mophore has been replaced with 13‐demethyl‐11,14‐epoxyretinal. As previously observed in other BR variants with low Schiff‐base pK values, this pigment exhibits protonation and deprotonation of the Schiff base under an applied electric field, depending on the initial Schiff‐base protonation state or effective pH. At low effective pH, a fast (<200 μs) deprotonation reaction dominates. At high pH, an apparently different mechanism leads to Schiff‐base protonation on the time scale of seconds. Comparison of our results with a simple model suggests that the external field causes a shift in the pK of the Schiff base by1–2 pH units.