Electric-field effects in 13-demethyl-11,14-epoxyretinal-bacteriorhodopsinfilms

We report the results of experiments on the application of electric fields across thin, dry films of a bacteriorhodopsin (BR) analog pigment in which the retinal chromophore has been replaced with 13-demethyl-11,14-epoxyretin al. As previously observed in other BR variants with low Schiff-base pK val ues, this pigment exhibits protonation and deprotonation of the Schiff base under an applied electric field, depending on the initial Schiff-base prot onation state or effective pH. At low effective pH, a fast (<200 mu s) depr otonation 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 by 1-2 pH units.