Theory of rhodopsin activation: Probable charge redistribution of excited state chromophore

The unicellular eukaryote Chlamydomonas reinhardtii is a phototactic alga which swims toward or away from light by using rhodopsin as its photoreceptor. The blind mutant FN68 lacks retinal and therefore does not undergo phototaxis, but this property is readily restored upon incubation of the microorganism in media containing retinal analogs. Incorporation of chromophores having fixed double bond geometry indicates that specific isomerizations of the 7-ene, 9-ene, 11-ene, 13-ene and 15-ene (C=N⁺H) are unnecessary. Phototaxis is also restored by numerous retinal analogs including short acyclic aldehydes. Phototaxis is even restored by hexenal and hexanal. An attractive hypothesis is that charge redistribution in the excited state of the chromophore triggers the activation of rhodopsin.