 11-cis-retinal

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.^[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.^[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.^[ref]