Bacteriorhodopsins with chromophores modified at the β-ionone site: formation and light-driven action of the proton pump

The binding to bacterioopsin of the all-trans isomers of retinal analogues lacking the six-membered ring and differing in length of the conjugated chain, as well as the light-driven action of the proton pump of the resulting bacteriorhodopsin analogues, were 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 about 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 analogue with the same number of conjugated double bonds in the chromophore as bacteriorhodopsin itself shows light-driven proton pump action.