Bacteriorhodopsin. The influence of the cyclohexene-ring methyls

Article abstract of DOI:10.1002/recl.19871060402

Four ring-demethylated retinals, viz. 1,1',5-tridemethylretinal, 1,1'-didemethylretinal, 1,5-didemethylretinal and 1-demethylretinal, have been synthesized via new and simple schemes.The properties of these modified retinals, their protonated Schiff bases and the corresponding bacteriorhodopsins have been studied and compared with the native system.These bacteriorhodopsin analogues have also been tested for their proton-pump efficiencies.A large decrease in proton-pump activity was found for the analogues lacking the 5-methyl group.On the whole, the opsin shifts of the modified bacteriorhodopsins were much lower than those of the native system.UV-Vis and 1H NMR data support a planar 6-s-trans conformation for the demethylated retinals in solution rather than a twisted 6-s-cis conformation (torsion angle 40-60 deg) as found in retinal.This explains the lower opsin shift and the better fit of these demethylated retinals in bacteriorhodopsin's binding site, which, in its native form, contains a 6-s-trans chromophore.