Bacteriorhodopsins with chromophores modified at the β‐ionone site

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Abstract
The binding to bacterioopsin [from Halobacterium holobium] of the all-trans isomers of retinal analogs lacking the 6-membered ring and differing in length of the conjugated chain, and the light-driven action of the proton pump of the resulting bacteriorhodopsin analogs was studied. The opsin shifts in these modified bacteriorhodopsins are all around 2700 cm-1 and do not depend on the number of double bonds in the chromophore. These experimental results suggest that the 4800 cm-1 opsin shift in unmodified bacteriorhodopsin consists of a contribution of .apprx. 2700 cm-1 due to the interaction of the protonated Schiff-base with the counterion. The extra 2100 cm-1 shift in bacteriorhodopsin is due to the specific interaction of the cyclohexene ring and the protein. Only the bacteriorhodopsin analog with the same number of conjugated double bonds in the chromophore as bacteriorhodopsin itself shows light-driven proton pump action.