Isomerization of Retinal:
Photochemical events in vision involve the protein opsin
and the cis/trans isomers of retinal. The cis-retinal fits into
a receptor site of opsin.
Upon absorption of a photon of light in the visible range,
cis-retinal can isomerize to all-trans-retinal. In the cis-retinal,
the hydrogens (light gray in the molecular model on the left)
are on the same side of the double bond (yellow in the molecular
In the trans-retinal, the hydrogens are on opposite sides
of the double bond. In fact, all of the double bonds are in the
trans-configuration in this isomer: the hydrogens, or hydrogen
and -CH3, are always on opposite sides of the double bonds (hence,
the name "all-trans-retinal").
Note how the shape of the molecule changes as a result of
this isomerization. The molecule changes from an overall bent
structure to one that is more or less linear. All of this is
the result of trigonal planar bonding (120 o bond
angles) about the double bonds.
Compare Cis and Trans-Retinal- Chime
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