4° 



CIS-TRANS ISOMERISM 



V 



visible region of the spectrum than the parent carotenoids with complete 

 trans-conhguvsition. If the labile isomerisation products are treated with iodine, 

 the absorption maxima are again displaced to longer wavelengths, but not as 

 far as the location of the absorption maxima of the original /mws-carotenoids. 

 This is explained by Zechmeister and his coworkers as due to the fact that 

 in these isomerisations an equilibrium is always established so that the reverse 

 change to the original trans-pigment is never complete. 



8. The extinction coefficients of the isomerisation products are lower than 

 those of the parent carotenoids with complete ^raws-configurations. 



9. The isomerisation products are often characterised by the appearance of 

 a new maximum in the ultra-violet spectrum. Following Zechmeister and 

 PoLGAR these new maxima are termed "cis-peaks". This phenomenon is 

 illustrated in the schematic figure below : 



ca. 3-^mu. 



Fig. 2. 



Carotenoid with complete ^raws-configuration 

 Isomerisation product 



With all the isomerisation products so far examined, the centre of the cis- 

 peak lies at a distance of 142 (±2) m// from the long-wave maximum (in 

 hexane solution). 



For a theoretical interpretation of the "cis-peak", compare the review by 

 Zechmeisteri*^. 



A more detailed discussion of this subject is beyond the scope of this mono- 

 graph. The properties of the different isomers are briefly described in later 

 sections dealing with the corresponding parent carotenoids. As a guide to the 

 relevant literature all the carotenoids so far examined for czs-^yaws isomerisation 

 are summarised in the following table. 

 References p. 42. 



