Isomerization in the Vision Process

The conversion of a cis isomer to the trms isomer (or vice versa) is a process called cis-trans isomerization. Such conversions are crucial in the vision process. The molecules in the retina that respond to light are rhodopsin, which has two components called 11-cw-retinal and opsin. Retinal is the light-sensitive component, and opsin is a protein molecule. Upon receiving a photon in the visible region, 11-cw-retinal isomer-izes to all-trans retinal by breaking a carbon-carbon tt bond. With the TT bond broken, the atoms connected by the carbon- 

The primary event in the vision process is the conversion of 11-cw-retinal to the aU-trans isomer of rhodopsin. The double bond at which the isomerization occurs is between carbon number 11 and carbon number 12. For simplicity, most of the H atoms are omitted. In the absence of light, this transformation takes place about once in a 1000 years  

These two structures could only be interconverted through rotation of one of the —CHCl gronps by 180° abont the C=C bond axis. This wonld necessitate the breaking of the TT bond between the two carbon atoms. The properties of the two molecules are quite different. As discnssed in Section 4.2, the cis isomer has a relatively large dipole moment (1.89 D), whereas the more symmetric trans isomer has a dipole moment of zero. This difference is reflected in the boihng points, which are 333.5 K and 320.7 K for the cis and trans isomers, respectively. 

Another class of stereoisomers are optical isomers, or enantiomers, which are non-superimposable mirror images. ( Snperimposable means that if one structure is laid over the other, the positions of aU the atoms will match.) Like geometric isomers, optical isomers come in pairs. However, the optical isomers of a compound have identical physical and chemical properties, such as melting point, boihng point, dipole moment, and chanical reactivity toward molecules that are not enantiomers themselves. Enantiomers differ from each other in their interactions with plane-polarized hght, as we will see. 

The most common occurrences of optical isomerism occur in molecules that contain carbon atoms bonded to four different elements or molecular fragments. In the compound CHFClBr, for example, the geometry about the central carbon atom is tetrahedral (Ns = 4, no lone pairs). Two enantiomers are possible  

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