II. CHEMISTRY AND INDUSTRIAL PREPARATION 



15 



given carotenoid Imiits its number of cis-trans isomers. These restrictions 

 limit the total possible isomers for j8-carotene (sym.) to 20, for a-carotene 

 (unsym.) to 32, for 7-carotene (unsym.) to 64, and for cryptoxanthin 

 (misym.) to 32. 



It has now been well established'^ that most of the naturally occurring 

 provitamin A carotenoids exist in an sdl-lrans configuration. The all-^rans 

 configuration for /3-carotene is shown in structure I in which double bonds 



\/ H H H H H CH, H CH, H CH, 



,C^ I I I I I I I I I l/CH, 



H,C^ /C I I I i I I i I I II" I 



C I H CH3 H CHj H H H H H C^^^CH, 

 HaCH, I C" 



HaC H, 



I All-ira?is-/3-carotene 



3, 5, 6, 7, and 9 are capable of existing in both cis and trans isomers, and 

 with the exception of double bonds 1 and 11, which are fixed, 2, 4, 8, and 

 10 are sterically hindered and exist only in trans form. Table V gives a set 

 of twenty possible isomers of jS-carotene. 



TABLE V 

 Possible cis-trans Isomers of /3-Carotene 



The aW-trans configuration of provitamin A carotenoids is preserved only 

 when they are freshly extracted from natural sources or kept in the crys- 

 talline form. In solution some of the sterically unhindered double bonds 

 slowly change configuration, and a mixture of several cis-trans isomers is 

 produced. This change can be brought about more rapidly when the solu- 

 tions are refluxed in the dark or exposed to light in the presence of catalytic 

 amounts of iodine. The change can be easily detected spectroscopically, 

 since there is a decrease in color intensity and the absorption maxima 

 shift from the visible toward the ultraviolet region of the spectrum. In 



'' L. Zechmeister, Vitamins and Hormones 7, 57 (1949). 



