Role of Retinal Isomerizations and Rotations in the Photocycle of Bacteriorhodopsin

Artificial bacteriorhodopsin (bR) pigments based on synthetic retinal analogues with selectively blocked single and double bonds were prepared. It was shown that rotations around single bonds C₁₂-C₁₃ and C₁₀-C₁₁ and isomerizations of C₁₁=C₁₂ and C₉=C₁₀ are not required either for initiating the photocycle of all-trans-bR or for forming its M₄₁₂ intermediate. The results are discussed in light of mechanisms for the primary event (based on the C₁₃=C₁₄ isomerization) involving a concerted double-bond and single-bond rotation around adjacent C,C bonds. Similarly, the photoreaction of the 13-cis isomer of bacteriorhodopsin does not require isomerization about the C₁₁=C₁₂ double bond or rotation around C₁₂-C₁₃. It is also shown that 13-cis ⇔ all-trans (light-dark adaptation) reaction of bacteriorhodopsin does not involve additional rotations or isomerizations involving the C₉-C₁₃ section of the molecule.