CHEMICAL STRUCTURE OF THE CAROTENOIDS 471 



noids not containing oxygen (hydrocarbons, C40H56) be called carotenes; 

 all carotenoids containing oxygen (in a carbonyl or hydroxyl group) be 

 called xanthophylls; and the most abundant compound of this class in 

 leaves, C4oH64(OH)2, be called luteol. Bogert (1938), on the other hand, 

 proposed that the term carotenol be used as a generic name for all 

 hydroxyl derivatives of carotenes, while the term xanthophyll should 

 either be eliminated altogether or reduced to its original meaning 

 ("leaf-xanthopyll"). This suggestion has been followed in the present 

 discussion. Furthermore, we are using the ending -ol (rather than in) 

 also in the designation of individual carotenols (e. g., luteol and not 

 lutein; fucoxanthol and not fucoxanthin). 



Carotene, C40H56, occurs in several isomeric forms. Only two have 

 been definitely identified in green leaves, according to Mackinney's 

 (1935) analysis of 59 species. Both contain 11 double bonds and two 

 ionone rings. In the most abundant isomer, jS-carotene, all double bonds 

 are conjugated (formula 17.1). The a-carotene, with a double bond in 

 one ionone ring displaced into the 5-6 position, and thus not conjugated 

 with the rest, has been identified by Mackinney (1935) in 40 species, in 

 concentrations up to 35% of the total carotene. Other carotenes, found 

 for example in flowers, and perhaps present in traces in leaves as well 

 (7-carotene, l3^copene) have one or both ionone rings open, with one or 

 two additional double bonds accounting for the isomerism with /3-carotene. 



H3C CH3 H3C CH3 



C HH HHH HHHH HHH HH C 



/2'\ II III I I I I 111 II /2\ 



H,C3' I'C C=C— C=C— C=C— C=C— C=C— C=C— C=C— C=C— C=C Ci 3CH2* 



I II I I t I I II I 



H2C4' 6'C CH3 CH3 CH3 CH3 C6 4CH2 



C CH3 H3C CH2 



Ho 



Formula 17.1. /3-Carotene. 



The asterisks indicate the positions of OH groups in luteol. 



The splitting by hydrolysis of the middle double bond in /3-carotene 

 (indicated by arrow) produces vitamin A. This vitamin may be present 

 as such in green leaves, as suggested, for instance, by Joyet-Lavergne 

 (1937). The alternation of single and double bonds and the arrangement 

 of methyl side chains in carotene suggest a relationship with isoprene, 

 CH2=C(CH3) — CH=CH2, the parent substance of rubber, terpenes, 

 and probably also of phytol (page 439). Phytol has the same carbon 

 skeleton (C20) as has vitamin A. A genetic relation between phytol and 

 the carotenoids was deemed possible by Willstatter and Mieg (1907) and 



