Resonance Raman spectra of the acidified and deionized forms of bacteriorhodopsin

The 568-nm absorption band of light-adapted bacteriorhodopsin (BR) shifts to 605 nm at pH 2, forming BR605A, and it shifts back to 565 nm at pH 0, forming BR565A. We have obtained resonance Raman spectra of BR605A and BR565A using purple membrane samples that have been suspended in a rotating Raman cell with a polyacrylamide gel. Raman spectra were also obtained of purple membrane in deionized solutions (BR605D). The spectra of BR605A and BR605D are very similar, and they correspond closely with the Raman spectrum of dark-adapted BR, which contains an approximately equal mixture of 13-cis and all-trans retinal protonated Schiff-base chromophores. This shows that BR605A and BR605D are not homogeneous molecular species but contain a mixture of pigment molecules with both 13-cis and all-trans retinal isomers. The Raman spectrum of BR565A is nearly identical to that of light-adapted BR, demonstrating that BR565A contains an all-trans protonated Schiff-base chromophore. These data provide constraints on the possible structural changes that can be invoked to explain the spectral shifts induced in the acid and deionized species.