ELECTRIC-FIELD-INDUCED SCHIFF-BASE DEPROTONATION IN D85N MUTANT BACTERIORHODOPSIN

The application of an external electric field to dry films of Asp-85 - -> Asn mutant bacteriorhodopsin causes deprotonation of the Schiff bas e, resulting in a shift of the optical absorption maximum from 600 nm to 400 nm. This is in marked contrast to the case of wild-type bacteri orhodopsin films, in which electric fields produce a red-shifted produ ct whose optical properties are similar to those of the acid-blue form of the protein. This difference is due to the much weaker binding of the Schiff-base proton in the mutant protein, as indicated by its low pK of approximate to 9, as compared with the value pK approximate to 1 3 in the wild type. Other bacteriorhodopsins with lowered Schiff-base pK values should also exhibit a field-induced shift in the protonation equilibrium of the Schiff base. We propose mechanisms to account for these observations.