12 CHEMICAL CONSTITUTION AND BIOLOGICAL PROPERTIES 49 



the structure of the carotenoid. One advantage of the method of thermal 

 degradation, however, is that the products formed give some indication of the 

 relative position of the side-chain methyl groups. Thus Van Hasselt^^ obtained 

 w-xylene from bixin. According to Kuhn and Winterstein^*, the xylene is 

 derived from a part of the aliphatic chain. 



•••=CH-C=CHCH=CH-C=--- CH=C-CH3 



II / \ 



CH, CH, > CH3— C CH 



CH-CH 



A similar observation was made by Zechmeister and von Cholnoky^" 

 with capsanthin. Later, Kuhn and Winterstein^^ re-investigated thermal 

 decompositions and isolated 2 : 6-dimethylnaphthalene from different caro- 

 tenoids. This must be derived from the central part of the polyene chain. 



•••CH CH CH CH 



y yv y\/v 



CH CH C-CHj CH C C-CHj 



HoC-C CH CH HoC^C C CH 



CH CH--^ CH CH 



II. DETERMINATION OF OPTICAL ROTATION 



Optical activity can be used to decide the question as to whether the 

 carotenoid molecule has a symmetrical or unsymmetrical structure. The 

 C-line of mercury (656.3 mfi) is often employed as light source as proposed by 

 Zechmeister and Tuzson^^^ Kuhn, Winterstein and Lederer, on the other 

 hand, recommend a quartz cadmium lamp as a more powerful Hght source^^. 



12. relationships between chemical constitution and 

 biological properties 



As was explained on p. 13, the relationship between the vitamin A potency 

 of a carotenoid and its structure appears to be governed by the principle that 

 high vitamin A activity depends on the presence of an unsubstituted jS-ionone 

 ring. In this way, conclusions can be drawn from the physiological activity of 

 a polyene pigment regarding its content of jS-ionone rings. It must be re- 

 membered, however, that some carotenoids (e.g. ^-carotene di-epoxide), which 

 do not contain an unsubstituted /3-ionone ring, nevertheless possess vitamin 

 A-activity because they undergo certain transformations in the animal organ- 

 ism (cf . p. 148) 34 and that some growth-promoting properties are also exhibited by 

 partly demethylated and acetylenic analogues of vitamin A (cf. p. 14, footnote). 

 References p. 51-52. 

 Carotenoids 4 



