Acetylenic retinals form functional bacteriorhodopsins but do not form bovine rhodopsins

Article abstract of DOI:10.1021/ja00331a040

Three acetylenic retinals bearing a triple bond between the carbon atoms 7 and 8, and carrying no, two, or three methyl groups on the cyclohexene ring, were synthesized and various isomers obtained by illumination and subsequent chromatographic separation. The introduction of the acetylenic bond leads to a different geometrical arrangement of the cyclohexene ring with respect to the polyene side chain. Attempts to reconstitute rhodopsins and isorhodopsins with any of these compounds failed. Bacteriorhodopsins formed with the 13-cis and the all-trans isomers of all three acetylenic retinals showing absorption maxima between 490 and 530 nm. The increasing number of methyl groups on the ring correlated with an increasing red shift of the absorption maximum of the chromoproteins. This same correlation was found for the proton-pumping activity of the acetylenic bacteriorhodopsins. The results are in agreement with the fact that in rhodopsin a covalent binding at the carbonyl group (Schiff's base formation) and also a fixation at the cyclohexene ring of retinal occurs. The changed geometry of the acetylenic retinals, therefore, prevents binding. In bacteriorhodopsin, which lacks a specific cyclohexene ring binding site, the acetylenic retinals form active molecules. However, the contribution of the ring and its substituents to the function of bacteriorhodopsin is significant.