Probing for the threshold energy for visual transduction: Red-shifted visual pigment analogs from 3-methoxy-3-dehydroretinal and related compounds

While azulenic retinal analogs failed to yield a red-shifted visual pigment analog, the 9-cis isomers of the push-pull polyenals 3-methoxy-3-dehydrore tinal and 14F-3-methoxy-3-dehydroretinal yielded iodopsin pigment analogs w ith absorption maxima at, respectively, 663 and 720 nm. The former gave a r elatively stable bathe product (700 nm) and was able to activate transducin , A lower activity was observed for the latter. One possible explanation fo r the combined results is that the excitation energies of these red-shifted pigments are approaching the threshold energy for visual transduction (alt hough at this time we cannot rigorously exclude a role of the added F-atom in reducing the transducin activity).