Properties of an Analogue Pigment of Bacteriorhodopsin Synthesized with Naphthylretinal

The artificial pigment of bacteriorhodopsin was synthesized from aii-fra/u-naphthylretinal [(all-E)-3-methyl-7-(2-naphthyl)-2,4,6-octatrienal] and bacterioopsin in membranes. Two kinds of pigments were formed with all-fra/iy-naphthylretinal; one has its absorption maximum at 503 nm (Np-bR₅₀₃) and another, at 442 nm (Np-bR.₄₄₂). When the retinal/protein ratio on mixing is smaller, the larger amount of Np-bR₅₀₃ is formed. The equimolar mixture produces Np-bR₄₄₂. Successive addition of small amounts of naphthylretinal yields almost 100% of Np-bR₅₀₃. The all-trans isomer of naphthylretinal was extracted from both pigments. After reduction of the pigments with sodium borohydride, addition of all-trans-retinal did not produce bacteriorhodopsin. This indicates that the binding site of naphthylretinal in both pigments is the same as that of trans-bR. But the CD spectra of both pigments were different from each other; that of Np-bR₅₀₃ represents characteristic exciton CD bands, while that of Np-bR₄₄₂ shows only a positive one. When incorporated into lipid vesicles, both pigments showed a light-dependent proton pumping activity.