PROPERTIES OF THE CAROTENOIDS 621 



(4) Stereochemical Behavior of the Carotenotds 



a. Introduction. The phenomenon of stereochemical or geometrical 

 isomerism has been recognized for a number of years as a property of 

 organic compounds possessing a double bond. Thus, the interconversion 

 of oleic and elaidic acids has long been explained as a trans-cis rearrange- 

 ment. However, the application of this type of isomerism to the carote- 

 noids has been recognized only recently. The first interpretation of 

 such a relationship among the carotenoids was furnished by Karrer and his 

 co-workers^*^ in 1929. They suggested that the second fonii of bixin, dis- 

 covered by Herzig and Faltis**^ six years earlier, and the original bixin 

 had a cis-trans relationship. 



In 1935, Gillam and El Ridi**^ were the first workers to point out the 

 stereochemical isomerism in the C40 carotenoids. When homogeneous /3- 

 carotene was washed from an alumina column with petroleum ether and 

 then readsorbed on such a column, it was found to separate into two bands 

 which showed the characteristics of /S- and of ct-carotene (pseudo-a-caro- 

 tene), respectively. On subsequent elution of either band followed by 

 rechromatography, two similar zones w^ere again formed. Such isomeriza- 

 tion was therefore shown to be reversible and never complete. These 

 workers named the new isomer "pseudo-a-carotene" ; they recognized it as 

 being distinct from a-carotene. On similar treatment, a-carotene exhibited 

 a like phenomenon, but the new pigment possessed maxima at shorter 

 instead of at longer wave lengths. In further studies on the cis-trans 

 rearrangements of 18-carotene, Gillam and co-workers ^^^-^^^ proved the 

 reversibility of the change, and discussed the possibility of geometrical 

 isomerism. 



Although the British workers attributed this isomerism to the action of 

 the adsorbent in the chromatographic column, subsequent work of Zech- 

 meister and his collaborators has demonstrated that the changes are 

 spontaneous, and are independent of the process of adsorption. Thus, it 

 was shown that capsanthin solutions undergo a spontaneous and reversible 

 change.*" Furthermore, it was almost simultaneously demonstrated that 

 similar changes could be produced, in /3-carotene, lycopene, and crypto- 

 xanthin solutions, which were not related to the adsorption process. ^^^-^^^ 

 Finally, where such a cis-trans rearrangement was accompanied by a 

 simultaneous alteration in optical rotation, Zechmeister and Tuzson*^" 

 were able to demonstrate the change without the use of the Tswett column 



^8* A. E. Gillam and M. S. El Ridi, Nature, 136, 914-915 (1935). 



^86 A. E. Gillam and M. S. El Ridi, Biochem. J., 30, 1735-1742 (1936). 



«7 A. E. Gillam, M. S. El Ridi, and S. K. Kon, Biochem. J., 31, 1605-1610 (1937). 



^88 L. Zechmeister and P. Tuzson, Nature, I4I, 249-250 (1938). 



^89 L. Zechmeister and P. Tuzson, Biochem. J., 32, 1305-1311 (1938). 



«« L. Zechmeister and P. Tuzson, Ber., 72, 1340-1346 (1939). 



