CAROTENOIDS 



(ii) oL-carotene. Although a-carotene is often associated with 

 fi-carotene in green leaves it is quite frequently absent, for neither 

 spinach, nettle^*, artichoke, nor barley ^^ leaves contain a-carotene. 

 68 per cent, of the species examined by MacKinney^ contained a- 

 carotene in their leaves in amounts varying from " a trace " to 35 per 

 cent, of the total carotene fraction. Kuhn and Lederer's^* earlier 

 value for the a-carotene content of horse-chestnut leaves falls within 

 this range ; similar values are reported for the leaves of Japanese 

 plants. ® 



a-carotene 



(iii) y-carotene. There is no convincing evidence that y -carotene 

 occurs to any appreciable extent in green leaves except in those of 

 the Californian marsh dodder {Cuscuta salina)^^, in which it is the 

 major carotenoid component ; 20-25 mg. of crystalline y-carotene 

 was isolated from 1 kg. of these leaves compared with only 12*5 mg. 

 of p-carotene. Kuhn and Brockmann^^ elucidated the structure of 

 y-carotene : 



(iv) cis'isomers. It is obvious that molecules with structures such as 

 the carotenoids possess, have many spatial possibilities. Since the other 

 carotenes reported to occur in leaves are stereoisomers of the parent 

 ^-carotene, it will be convenient at this point to discuss briefly the 

 stereochemistry of the carotenoids. 



Gillam and El Ridi^^ in 1935-6 were the first to draw attention to 

 the lability of carotenoids in solution, and since then Zechmeister ^ • and 

 his co-workers have rehivestigated the problem at great length and in 

 1944 Zechmeister produced an authoritative review on the subject. 

 The diW-trans form of a carotenoid, the most stable form because of its 



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