Chromophore

The part of a molecule responsible for absorption in a specified spectral region. Its electronic excited states and local environment influence the optical response.

A light-absorbing aldehyde related to vitamin A, used as a chromophore in rhodopsin-family proteins. Its local packing and bonding connect light absorption to structural changes.

All-trans-retinal is the geometrical form of retinal whose relevant chain double bonds have trans configurations. In rod vision it is formed from the bound 11-cis-retinal chromophore after light absorption by rhodopsin. The change in geometry helps initiate receptor activation, while later processing of retinal participates in the visual cycle.^[ref]

“All-trans” describes cis-trans isomerism. It does not mean that the entire molecule is a perfectly straight, rigid line, or that the double bonds have become single bonds. The carbon skeleton and elemental composition are preserved during the idealized isomerization; arrangement changes.

The visual system must subsequently restore a pigment-compatible retinal supply. After bleaching, the retinal-derived material undergoes biochemical processing involving the photoreceptor and retinal pigment epithelium; rebuilding rod rhodopsin requires 11-cis-retinal, not merely any molecule labeled retinal. The RPE65 knockout illustrates the dependence on chemical form: all-trans retinyl esters accumulated while 11-cis retinoids were absent and normal rod responses were lost.^[ref]

Retinyl esters are distinct chemical species from all-trans-retinal. The experiment demonstrates a disrupted conversion pathway; it should not be paraphrased as though the accumulated ester and the newly photoisomerized aldehyde were the same compound.

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]