The application of an external electric field to dry films of Asp-85 → Asn mutant bacteriorhodopsin causes deprotonation of the Schiff base, 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 bacteriorhodopsin films, in which electric fields produce a red-shifted product 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 ≃9, as compared with the value pK ≃ 13 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.
|Original language||English (US)|
|Number of pages||4|
|Journal||Proceedings of the National Academy of Sciences of the United States of America|
|Publication status||Published - Oct 15 1996|
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