ISOMERISM AND THE VISUAL PIGMENTS 

 DIFFERENCES BETWEEN CIS AND TRANS ISOMERS 



The differences in properties between cis and trans isomers are best 

 illustrated with reference to the carotenoids. These substances, con- 

 taining 40 carbon atoms per molecule can be considered to be built 

 up from eight isoprene (CHg^CH— C(CH3)=CH2) units. The 

 main structural characteristic of the carotenoids is a reversal of 

 direction of the C5 (isoprene) units at the centre of the molecule — *as 

 if four C5 units had combined head to tail to form a C20 unit, two of 

 which then combined tail to tail to give a C40 carotenoid' (good win, 

 1953). 



The skeletal basis of the carotenoid (C40) molecule in terms of 

 'isoprene (Cs) units' 



The following remarks, though applying specifically to the caro- 

 tenoids are also applicable to the closely related 'half carotenoids,' 

 i.e. C20 units, such as the retinenes and the vitamins A and also to 

 polyenes in general. 



The dl\-trans configuration for a carotenoid molecule is one which 

 involves least strain. Therefore, it is not surprising that, with a few 

 exceptions, the natural carotenoids possess entirely trans configura- 

 tions. By suitable procedures (described in the next section) the 

 naturally occurring ^\\-trans carotenoids can be converted into a 

 mixture of isomers and these can be separated by chromatographic 

 analysis. In all cases it is found that the mixtures obtained by iso- 

 merization exhibit certain common features with respect to the 

 original 2i\\-trans forms : 



1. They are more soluble. 



2. Their melting points are lower. 



3. They absorb at shorter wavelengths (3-4 m/^) in the visible 

 region than the parent 2i\\-trans isomer. 



4. Their extinction coefficients (intensities of absorption) are 

 lower. 



5. They are often characterized by the development of a new 

 absorption band in the near ultraviolet. These new bands are 

 called *c/5 peaks.' 



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