TERPENOIDS AND STEROIDS 



149 



It will be noted that the molecule is symmetrical on each side of the dotted line and may 

 be viewed as formed by joining two diterpene radicals of the phytyl type. Variations are 

 introduced by double bonds and functional groups such as hydroxyl and carboxyl. As 

 double bonds are added, opportunites for cis-trans isomerism are introduced, and many 

 of the problems in carotenoid chemistry arise from difficulty in distinguishing and sepa- 

 rating geometrical isomers of this type. Native carotenoids are believed to be all trans, 

 but isomerization may occur on isolation. 



Cyclization at one or both ends of the carbon chain gives rise to the other two funda- 

 mental types of carotenoids - i. e. : 



If only one /3-ionone type ring is present, it is written to the left. Some other com- 

 pounds (e. g. bixin, crocetin) are known which have fewer than 40 carbon atoms but are 

 classed with the carotenoids because peculiarities in their structure suggest that they have 

 been derived from degradation of carotenoids rather than built up from smaller units. 



No general function can be assigned to the carotenoids. There is some indication 

 that they function as light receptors for phototropism. As flower pigments they may play 

 a role in attracting insects, but most attention has been given to their possible function as 

 leaf chloroplast pigments. To some extent light absorbed by carotenoids can be transferred 

 to chlorophyll and used in photosynthesis. There is also good evidence that the carotenoids 

 protect chlorophyll against photodestruction by short wavelength light — i. e. at a wavelength 

 of about 400 mp. where both carotenoids and chlorophylls have strong absorption maxima. 

 Albino plants lack both carotenoids and chlorophyll under normal conditions of growth but 

 in dim light they are able to accumulate chlorophyll. In normal light the chlorophyll is 

 rapidly destroyed since carotenoids are not present to protect it (26). 



The most widespread carotenoid is /3-carotene, which may make up as much as 

 0. 1% of dried green leaves. Lutein is the most important leaf xanthophyll and may occur 

 in green leaves at a greater concentration than /3-carotene. Most carotenoids have the 

 same central carbon chain as /3-carotene and differ only in the portions corresponding to 

 the two rings. Therefore in the formulas of Table 12, the complete structure is given 

 only for /3-carotene and the straight chain represented by a dotted line unless it differs 

 from that of /3-carotene. 



