Photochemical and functional properties of bacteriorhodopsins formed from 5,6-dihydro- and 5,6-dihydrodesmethylretinals

Abstract

5,6-Dihydroretinal and 5,6-dihydro-1,1,5,9,13-desmethylretinal are synthesized, and their all-trans isomers are shown to form pigment analogues (lambda max at 475 and 460 nm, respectively) of bacteriorhodopsin (purple membrane protein). The shift of the absorption maximum od the pigment from that of the protonated Schiff base of the chromophore for 5,6-dihydrobacteriorhodopsin is small compared to that of the native pigment, suggesting that negative charges similar to those controlling the lambda max of visual pigment rhodopsin exist near the cyclohexyl ring. Both pigment analogues undergo reversible light-induced spectral shifts reflecting cyclic photoreactions of the pigments. These results indicate that the absence of the C-5--C-6 double bond and of the five methyl groups of retinal does not abolish the photochemistry of these pigment analogues and strongly suggest that these structural features are not directly required for the photoreactions of native bacteriorhodopsin. The apparent rates of the photochemical transformations of these artificial pigments are quite different from those of bacteriorhodopsin. A working hypothesis is proposed for the photocycle of the pigment analogues, which includes a slower light-induced cycling rate (for the light-adapted pigments) than that of native bacteriorhodopsin and an increased rate of dark adaptation. When incorporated into egg lecithin vesicles both pigment analogues show proton pumping ability, again indicating that the missing double bond and the methyl groups are not structurally required for the function of the pigments.