514 VI. CAROTENOIDS AND RELATED COMPOUNDS 



of these, which they called cc-carotene, was strongly dextro-rotatory ( [af^ = 

 34.0° in benzene), ^^ while the second preparation, which they designated as 

 /3-carotene, was found to be optically inactive. Similar results were ob- 

 tained simultaneously and independently by Karrer et a/.^^-^^ and by Rosen- 

 heim and Starling,^- who reported that crystalline carotene prepared from 

 carrots contained an optically active, as well as an optically inactive, com- 

 ponent. Further confirmation of the existence of a- and /3-isomers of caro- 

 tene was presented by van Stolk, Guilbert, and Penau.^^ 



The presence of a third isomer, 7-carotene, was discovered by Winter- 

 stein and Ehrenberg,^^ and was confirmed somewhat later by Kuhn and 

 Brockmann.^^'^^ Although this pigment was not described as early as the 

 a- or /3-isomers, inasmuch as it normally occurs to the extent of only 0.1% 

 in carotene preparations from most sources, it may account for as much as 

 50% of the total carotenoids in some instances; this is the case with the 

 pigments in the fruit peel of the Moluccan plant, Gonocaryum pyriforme.^'^ 



(1) ^-Carotene 



a. Structure. Because of the fact that /3-carotene is the carotenoid 

 most widely distributed in nature and also the one most readily prepared 

 in pure form, it was the first member of this group for which the structiu'al 

 relationships were understood. The structure was largely established 

 through the pioneer work of Karrer and associates, ^'^-'^^ Kuhn and von 

 Euler, as well as of Zechmeister and co-workers.^^ Furthermore, it is the 

 carotenoid which possesses the greatest provitamin A activity. 



The empirical formula of the several carotenes has been shown repeatedly 

 to agree with the relationship C40H66 as first postulated by Willstatter and 

 Mieg.*" Kuhn and Winterstein^^ and also Karrer and Salomon^^ estab- 

 lished the fact that the carotenoids possess a series of conjugated double 

 bonds. The total number of double bonds in the /3-carotene molecule was 



*° P. Karrer, A. Helfenstein, H. Wehrli, B. Pieper, and R. Morf, Helv. Chitn. Acta, 

 14, 6U-632 (1931). 



^' P. Karrer, H. v. Euler, and H. Hellstrom, Arkiv Ketn. Mineral. Geol., BIO, No. 15, 

 1-6 (1931). 



62 O. Rosenheim and W. W. Starling, Chemistry & Industry, 50, 443 (1931). 



6' D. van Stolk, J. Guilbert, and H. Penau, Chemie et industrie, Special No., 550-553S 

 (March, 1932); Chem. Abst., 26, 3826 (1932); Compt. rend., 193, 209-210 (1931). D. van 

 Stolk, J. Guilbert, H. Penau, and H. Simonnot, Bidl. soc. chim. biol., 13, 616-635 (1931). 



" A. Winterstein and U. Ehrenberg, Z. physiol. Chem., 207, 25-34 (1932). 



66 R. Kuhn and H. Brockmann, Natiirwissenschaften, 21, 44 (1933). 



66 R. Kuhn and H. Brockmann, Ber., 66, 407-410 (1933). 



6^ A. Winterstein, Z. physiol. Chem., 21.9, 249-252 (1933). 



68 P. Karrer and R. Morf, Helv. Chim. Acta, 16, 625-641 (1933). 



69 L. Zechmeister, L. v. Cholnoky, and V. Vrabely, Ber., 61, 566-568 (1928). 

 «o R. Willstatter and W. Mieg, Ann., 355, 1-28 (1907). 



61 R. Kuhn and A. Winterstein, Helv. Chim. Acta, 11, 427-431 (1928). 

 " P. Karrer and H. Salomon, Helv. Chim. Acta, 11, 513-525 (1928). 



