CAROTENOIDS 



Neo-^-carotene A 

 theo-^' carotene B 



Neo-fi- carotene U 

 Neo-fi- carotene T 



^ >4//-trans-/3-corote/7e 



Fig. 2. — Illustrating the naming of stereoisomers of carotenoids according to the 

 positions they occupy on an adsorption column relative to the parent 

 all-trons compound. 



Not all the double bonds in a carotenoid molecule are capable of 

 trans -^ cis rotation because of steric hindrance ; only 5 double 

 bonds in p-carotene can rotate thus : 



Me 

 Me Me Me Me 



AII-tran$-p-carotene (rotation can only take place around 

 double bonds 3, 5, 6, 7, 9). 



As a result of this restriction, the term ** all-m " applied to caro- 

 tenoids is not the antithesis of " 2\\-trans'' In an " aW-trans " molecule 

 every double bond does have a /ra/w-configuration, v^^hereas in an ** ali- 

 os " molecule all possible trans -^ cis rotations have occurred, but 

 there are always present a number of double bonds M^ith the *' trans- '* 

 configurations. 



Occasionally the most abundant naturally occurring carotenoid has 

 a m-configuration. Zechmeister terms this a " pro-carotenoid," e.g., 

 pro-y-carotene : the naturally occurring a\\-trans forms are given no 

 special designation. 



The demonstration of the ease with which carotenoids undergo 

 stereoisomerization makes it very difficult now to assess some of the 

 earlier work on the carotenoids present in plant tissues ; extractions 

 were often carried out with some vigour using hot solvents and with no 

 precautions to exclude bright sunlight. Thus a number of claims to 

 have isolated a series of new and unidentified carotenoids must be 

 treated with caution until re-investigations under modern conditions 

 rule out cis -^ trans isomerization. 



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