Controlling the pKa of the bacteriorhodopsin Schiff base by use of artificial retinal analogues

Artificial bacteriorhodopsin pigments based on synthetic retinal analogues carrying an electron-withdrawing CF3 substituent group were prepared. The effects of CF3 on the spectra, photocycles, and Schiff base pKa values of the pigments were analyzed. A reduction of 5 units in the pKa of the Schiff base is observed when the CF3 substituent is located at the C-13 polyene position, in the vicinity of the protonated Schiff base nitrogen. The results lead to the unambiguous characterization of the (direct) titration of the Schiff base in bacteriorhodopsin and to the conclusion that the deprotonation rate of the Schiff base during the photocycle (i.e., the generation of the M412 intermediate) is determined by a structural change in the protein.