PROPERTIES OF TPIE CAROTENOIDS 



651 



have been marshalled to assist in the interpretation of the structure of 

 the stereoisomers. The absorption data on shifts of the absorption maxi- 

 mum in the visual area have been of use in establishing the number of cis 

 linkages. When the maximum has been shifted about 5 m/x ( ± 1 m^i) , it is 

 reasonably certain that the compound is a mono-m derivative. Similarly, 

 in the case of the di-m compounds, the new absorption maximum is about 

 10 m/x lower than that for the aW-trans form. In the case of pro-7-caro- 

 tene with 5 cis linkages, the shift of the maximum in the visual area is 31 ; 

 for prolycopene with 6 cis bonds it is 34, and in all-as-lycopene having 7 

 cis linkages, the change in location of the absorption maximum is 38 mn. 



The information gained from the cis-peak change aids in establishing not 

 only the number but also the location of the cis bonds. The effect of a 

 given cis linkage varies with its position in the molecule. As noted earlier, 

 the greatest effects are upon a centrally located mono-a's derivative. 



Chiefly on the basis of data on absorption curves, Zechmeister'*^^ has sug- 

 gested the structure of a number of isomers. The probable structures of a 

 number of the well-known isomers are summarized in Table 34. 



Table 34 

 Probable Structural Relationships of Some Stereoisomers of a Number of 



Carotenoid Sets" 



" Adapted from L. Zechmeister, Chem. Revs., 34, 267-34-4 (1946). 



