11-cfs-retinal

Because all these cone cells use 11-cfs-retinal as a substrate trigger, it has long been suspected that there must be three different opsin proteins. Recent work has begun to establish how the opsins vary the spectral sensitivity of the cone cells, even though all of them have the same kind of light-absorbing chromophore. 

The all-trans forms of the vitamins A predominate but 11-cfs-retinal is the light-absorbing chromophore of the visual pigments... 

All-frans-Vitamin A is then isomerized to its 11-ds-Vitamin A isomer. After the isomerization, the 11-cfs-Vitamin A is oxidized back to 11-ds-retinal, which forthwith recombines with the opsin protein to form rhodopsin. The regenerated rhodopsin is then ready to begin the cycle anew, as illustrated in the figure. 

In the dark retinal in rhodopsin is in the 11-cfs-configuration. Irradiation of rhodopsin leads to a series of conformational changes which can be noted by the disappearance and appearance of various intermediates of different colours. 

Theoretically, 16 stereoisomers of vitamin A (and of the retinal) can be considered since the side chain bears 4 double bonds. According to Pauling,the possibility of forming ds double bonds at C-7 or C-11 has to be discarded on account of the steric hindrance existing in these forms which would no longer allow the resonance phenomena. However, two hindered vitamins A (and two retinals) having 11-cis configurations could be synthetized (the cfs-11 and m-as-ll,13). Construction of the models shows that distortion of the side chain may partly reduce the possibility of resonance for these compounds. However, their existence is undeniable. 

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