Preparation and photolysis of deuterium‐labelled rhodopsin analogues

The compounds 18‐ and 19‐trideutero‐11‐cis‐retinal, > 99% pure, with over 98% and 99% trideuterium incorporation were synthesized together with the corresponding all‐trans derivatives. Rhodopsins were prepared from bovine opsin and 18‐ and 19‐trideutero‐11‐cis‐retinal. 18‐ and 19‐trideutero‐all‐trans‐retinal were isolated as oximes after photolysis of the corresponding rhodopsins. The oximes showed the same deuterium incorporation as the starting 18‐ and 19‐trideutero‐11‐cis‐retinals and as those oximes obtained by denaturation of 18‐ and 19‐trideuterorhodopsin in the dark. These results demonstrate that during rhodopsin photolysis no detectable exchange of D or H occurs at carbon atoms 18 and 19. These facts, together with Resonance Raman data of 18‐trideutero‐bathorhodopsin and 5‐demethyl‐bathorhodopsin, establish that the primary step in rhodopsin photochemistry is not a phototautomerization involving the 5‐CH3 group. With respect to the possibility that the primary step is a phototautomerization involving the 9‐CH3 group, the situation is less clear. At this time tile experimental evidence of the various research disciplines is insufficiently conclusive to establish unambiguously which of the two possibilities, cis‐trans photoisomerization or phototautomerization involving the 9‐CH3 group, is the primary event in visual photochemistry.