CIS-TRANS ISOMERISM 39 



ceptions (e.g. bixin, pro-y-carotene, pro-lycopene) the natural carotenoids all 

 possess entirely /^-ans-configurations. This is not surprising since the trans- 

 configurations are those with the smallest energy content and the greatest 

 stability. Thus natural j5-carotene is beheved to have the following structure. 



CH3 CH3 H H 



C 2C 3C 



/\ y\y^ 



CH, C— C C 



I l|| ' ' 



CH, C H CH3 



CH2 CH3 



On the basis of theoretical considerations, Zechmeister, Pauling and 

 collaborators^ concluded that not all ethylenic groups of a carotenoid molecule 

 are capable of taking part in cis-trans isomerisation, but only those of the type 

 -C(CH3) = CH- and the double bond in the centre of the molecule. In the 

 case of ^-carotene only the 3-, 5-, 6-, 7-, and 9-double bonds could thus be 

 involved in cis-trans isomerism. The remaining ethylenic groups are beheved 

 always to assume a ^ra«s-configuration owing to steric hindrance. 



Isomerisation can be brought about in the following ways.- a) refluxing of a 

 solution of the carotenoid in an organic solvent, b) melting of the crystals, 

 c) treatment with iodine^, d) treatment with acids, and e) illumination. The 

 separation of the isomerisation products is carried out by means of chromato- 

 graphic analysis. 



All the transformation products of natural /raws-carotenoids so far obtained 

 exhibit the following common features^": 



1. The colour intensity of the pigment solution decreases as a result of 

 isomerisation. 



2. The isomerisation products are more soluble than the starting materials. 



3. The melting points of cis-isomers are lower than those of the pigments 

 with complete /r^ws-configurations. 



4. The isomerisation products often revert to the parent pigment with 

 complete /raws-configuration on crystallisation. Others crystallise inhomo- 

 geneously as shown by the fact that fresh solutions of the crystals give rise to 

 several zones on chromatography. This is the case for instance, with pseudo-a- 

 carotene, neo-carotene and neo-a-carotene^i. 



5. If the molecule contains one or more asymmetric carbon atoms, isomeri- 

 sation is often accompanied by large changes in optical rotation. 



6. The strength of adsorption of the isomerised carotenoids on the chro- 

 matogram column differs considerably from that of the /raws-pigments. 



7. The isomerisation products always absorb at shorter wavelengths in the 

 References p. /).2. 



