Frans-Retinal complex

The trans-retinal complex with opsin (called metarho-dopsin-ll) is less stable than the c/s-retinal complex, and it dissociates into opsin and frans-retinal. This change in geometry triggers a response in the rod nerve cells, which is transmitted to the brain and perceived as vision. 

The two molecules have different shapes because of their different structures. The 11-ds-retinal has a fairly curved shape, and the parts of the molecule on either side of the cis double bond tend to lie in different planes. Because proteins have complex and specific three-dimensional shapes (tertiary structures), 11-ds-retinal associates with the protein opsin in a particular manner. All-frans-retinal has an elongated shape, and the entire molecule tends to lie in a single plane. This different shape for the molecule, compared with that for the 11-ds isomer, means that all-frans-retinal will have a different association with the protein opsin. 

At some time after the 11-ds-retinal-opsin complex receives a photon, a message is received by the brain. It was originally thought that either the isomerization of 11-czs-retinal to all-frans-retinal or the conformational change of the opsin protein was an event that generated the electrical message sent to the brain. Current research. 

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