CHAPTER V 

 Cis-trans isomerism of carotenoids 



Cis-trans isomerism of the type exhibited by simple ethylenic compounds 

 is also observed in the carotenoid series. Since a carotenoid contains several 

 carbon-carbon double bonds, a considerable number of geometrically isomeric 

 forms are possible. Thus a polyene of the formula R(CH==CH)9R' containing 9 

 double bonds can theoretically exist in 512 different cis-trans isomeric forms. 



The fact that bixin occurs in two isomeric forms was discovered by 

 Herzig and Faltis^ in 1923. It was proved by Karrer and co-workers^ in 1929 

 that these two forms were cis-trans isomers. Bixin is the more labile form. It is 

 easily converted into /so-bixin which in view of its greater stability is regarded 

 as the trans-iorm. Later, Kuhn and Winterstein^ found that crocetin, the 

 chief pigment of saffron, is accompanied by a small quantity of a geometrical 

 isomer, /50-crocetin. /so-crocetin is a labile compound which can readily be 

 converted into the more stable crocetin by means of iodine or other catalytic 

 agents. 



In 1935, GiLLAM and El Ridi'* found that on repeated chromatographic 

 adsorption of homogeneous ^-carotene, two zones are obtained. The upper 

 zone consists of j3-carotene while the lower zone contains a new pigment, 

 pseudo-a-carotene. It is possible that this isomer of j3-carotene is not formed 

 during adsorption, as assumed by Gillam and El Ridi, but spontaneously in 

 the solution of the pigment^. 



In recent years, the reversible interconversion of carotenoids believed to 

 be due to cis-trans isomerism has been investigated in detail, particularly by 

 Zechmeister and his collaborators. Although a considerable body of experi- 

 mental material is already available, especially with regard to the spectro- 

 scopic and chromatographic properties of the different isomers, this field of 

 work awaits further development, since the majority of interconversion products 

 have not yet been isolated in crystalline form. Only a brief summary of these 

 investigations will be given here. The properties of the different isomers are 

 described in later chapters dealing with the parent carotenoids. 



On the basis of recent studies employing x-ray analysis^, spectral analysis 

 and chromatographic analysis, it may be assumed that with a very few ex- 

 References p. 4.2. 



