534 VI. CAROTENOIDS AND RELATED COMPOUNDS 



H3C CH3 H3C CHj 



\ / \ / 



C Q C 



C40H56 ' H?C C— OH + HeC CH? 



I I I 



H^C C = 0=C CHj 



\ / \ / / 



C CH3 H3C c = o 



H? \ 



OH 



o-Carotene Geronie Isogeronic 



aeid acid 



hydrogenation, 11 molecules of hydrogen are absorbed, as in the case of 

 /S-carotene. "* Furthermore, the empirical formula is the same as for ^- 

 carotene, i.e., C40H56. The structure proposed by Karrer et al.^''^ has been 

 repeatedly confirmed and is now considered to be firmly established. It is 

 frequently referred to as j8, a '-carotene. 



H3C CHj H,C CHj 



p CH3 Cn3 CH3 Cn3 p 



/\ hh|hhhIhhhhIhhhIhhh/\ 

 HjC c-c:c-c:C'C:C'C:C'C:c-C;C-C:C-C:C-C:C-C* CHj 



I II II 



H?C C C CHz 



\ / \ / \ / 



C CH3 H3C c 



H2 Hj 



a-Carotene 



One marked difference between a- and j8-carotene is the fact that the 

 former compound is optically active, while the latter pigment has no such 

 property. The optical isomerism of a-carotene is readily explainable by 

 the presence of asymmetry on carbon 6' (indicated by star). However, in 

 spite of the close relationship in structure of a- and jS-carotenes, Hunter and 

 Scott "^ were unable to bring about a transformation of the a- to the jS- 

 ionone ring with sodium isopropoxide. a-Carotene has a strong provita- 

 min A activity. The best evidence indicates that it is about 50% as 

 potent as /3-carotene.^''^~^*^ 



b. Occurrence. As indicated in the discussion of /3-carotene, a-carotene 

 is wideh^ distributed in nature, although it comprises a much smaller propor- 

 tion of the total carotenoid than is the case with /3-carotene. The highest 



"' R. Kuhn and E. F. Moller, Z. angew. Chem., 47, 145-149 (1934). 



"5 R. F. Hunter and A. D. Scott, Biochem. J., 38, 211-213 (1944). 



i^« H. V. Euler, P. Karrer, H. Hellstrom, and M. Rydbom, Heh. Chim. Acta, I4, 839- 

 842 (1931). 



1'^ R. Kulin and H. Brockmann, Ber., 64, 1859-1864 (1931). 



'■* R. Kuhn, H. Brockmann, A. Scheunert, and M. Schieblich, Z. physiol. Chem., 221, 

 129-136 (1933). 



1" H. Brockmann, Z. angew. Chem., 47, 523-529 (1934). 



ISO H. Brockmann and M. L. Tecklenburg, Z. physiol. Chem., 221, 117-128 (1933). 



*** H. J. Deuel, Jr., E. Sumner, C. Johnston, A. Polgdr, and L. Zechmeister, Arch. 

 Biochem., 6, 157-161 (1945). 



