= 11-cis-retinal
\b[11-cis-retinal] is a geometrical form of <retinal> with a cis configuration at the carbon-11/carbon-12 double bond. In vertebrate rod <rhodopsin>, it acts as the bound <chromophore> that absorbs light. <Photoisomerization> toward <all-trans-retinal> triggers a change in the receptor's state, linking molecular geometry to <photoreceptor> signaling.<Crystal Structure of Rhodopsin: A G Protein-Coupled Receptor by Krzysztof Palczewski and others>[Page 739: bound 11-cis chromophore, photoisomerization and receptor activation.]{ref}
The name identifies a configuration, not a different number of atoms. <11-cis-retinal> and <all-trans-retinal> have the same molecular composition but differ in the arrangement around a double bond. Replacing a cis arrangement by trans is therefore an example of <cis-trans isomerism>, rather than cleavage of the retinal molecule into smaller fragments.
The $2.8$ angstrom <rhodopsin> crystal structure showed the retinal-binding pocket in the dark-state receptor. Bound <11-cis-retinal> helps maintain its inactive arrangement; its behavior depends on the surrounding <protein>, so free retinal and retinal inside a receptor need not have identical optical properties.<Crystal Structure of Rhodopsin: A G Protein-Coupled Receptor by Krzysztof Palczewski and others>[Abstract, retinal pocket and chromophore–protein interactions.]{ref}
The <visual cycle> regenerates the usable 11-cis supply. In <RPE65>-deficient mice, 11-cis retinoids and normal rod pigment were absent despite retained rod <opsin>. This distinguishes making the receptor chain from supplying the correctly configured light-sensitive component.<Rpe65 is necessary for production of 11-cis-vitamin A in the retinal visual cycle by T. M. Redmond and others>[Abstract: retinoid composition, rod opsin, rhodopsin and functional phenotype.]{ref}
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