Double bonds in retinal

The so-called torsion model by Kakitani and Kakitani [203] was developed to explain the high quantum yield of photoisomerization in rhodopsins as compared to isomerization yields observed in model protonated Schiff bases. The main feature of this model is the twist around double bonds in retinal when it is bound to opsin, which would make this substrate strained to be near the transition state for isomerization (Fig. 17). The most important twist as estimated by this model would be around the 11,12 double bond, although other double bonds would simultaneously be twisted to some extent ([204] and refs, therein). The protein was considered to be responsible for inducing the selective twist. This model predicted a... 

On the basis of this simple model we would like to suggest as a possibility that through the interaction of retinal with opsin a torsion is induced about a double bond in retinal, resulting in an increase in the isomerization quantum yield. 

In lipid metabolism, ds-trans isomerism is particularly important. For example, double bonds in natural fatty acids (see p.48) usually have a as configuration. By contrast, unsaturated intermediates of p oxidation have a trans configuration. This makes the breakdown of unsaturated fatty acids more complicated (see p. 166). Light-induced cis-trans isomerization of retinal is of central importance in the visual cycle (see p.358). 

A number of geometric isomers of retinol exist because of the possible cis-trans configurations around the double bonds in the side chain. Fish liver oils contain mixtures of the stereoisomers synthetic retinol is the all-trans isomer. Interconversion between isomers readily takes place in the body. In the visual cycle, the reaction between retinal (vitamin A aldehyde) and opsin to form rhodopsin only occurs with the 11 -cis isomer. 

The primary chemical reaction in vision, the one responsible for generating an impulse in the optic nerve, involves ds-trans isomerization around one of the double bonds in the retinal portion of rhodopsin. When rhodopsin is active (that is, when it can respond to visible light), the double bond between carbon atoms 11 and 12 of the retinal (ll- >retinal) has the cis orientation. Under the influence of light, an isomerization reaction occurs at this double bond, producing M-trans-Tetinal. Because the all-traw5 form of retinal cannot bind to opsin, zA-lrans-retinal and free opsin are released. As a result of this reaction, an electrical impulse is generated in the optic nerve and transmitted... 

Cis-trans isomerism is important in several biological processes, one of which is vision. When light strikes the retina, a cis double bond in the compound retinal (structurally related to vitamin A) is converted to a trans double bond. The conversion triggers a chain of events that finally results in our being able to see. 

The four frans double bonds in the side chain of retinal are labeled a-d. Double bond c (between carbons 11... 

Vitamin A is an example of a biologically important compound for which a number of cis—trans isomers are possible. There are four carbon-carbon double bonds in the chain of carbon atoms bonded to the substituted cyclohexene ring, and each has the potential for cis-trans isomerism. Thus, 2 = 16 cis—trans isomers are possible for this structural formula. Vitamin A is the all trans isomer. The enzyme-catalyzed oxidation of vitamin A converts the primary hydroxyl group to a carbonyl group of an aldehyde to give retinal, the biologically active form of the vitamin ... 

In polyenes, single bonds go to double bonds and vice versa at excitation. This means that a di-trans-isomerization takes place around double bonds (that is, double bonds in the ground state). A polyene called retinal is involved in the vision... 

Vision begins when light rays (photons) are focused by the eye s lens on to the retina, the layer of cells lining the inside of the eye ball. The retina contains large numbers of photoreceptor cells known as rods and cones. The ends of the rods and cones contain a molecule called rhodopsin which consists of a protein (opsin) covalently bonded to a purple pigment molecule called retinal. Structural changes which occur around a carbon-carbon double bond in the retinal component of the rhodopsin trigger a series of chemical reactions that eventually result in vision. 

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