11-cis retinal

From Ganfyd

A key compound for vision, related to vitamin A, it is activated on the absorption of a proton in the visual cycle forming the all-trans retinal and so triggering the phototransduction cascade. More accurately it is actually bound to an opsin such as rhodopsin as a Schiff base as in the diagram on the left. The colour of the visual pigments is related to the delocalisation of the π electrons along the double bonds. When visible light hits the 11-cis retinal chromophore, a p electron is promoted to a higher-energy orbital, allowing free rotation about the bond between carbon atom 11 and carbon atom 12 of the retinal molecule. About half the time, this rotation leads to the isomerization of retinal when the p electron returns to the lower-energy orbital, generating all-trans retinal. It is said that a single photon will generate a nerve impulse from a rod cell so presumably in the micro-environment of a rhodopsin molecule this isomerization is encouraged more than in vitroNeeds citation .