Chapter 17 

 THE ACCESSORY PIGMENTS 



A. The Carotenoids* 



The occurrence and concentration of carotenoids in leaves and algae 

 was described in chapter 15. The present chapter deals with their 

 chemical properties. For general information and bibliography, refer- 

 ence should be made to the monographs of Palmer (1922), Zechmeister 

 (1934), Lederer (1934), and Bogert (1938); and for the xanthophylls to 

 Strain (1938). 



1. Chemical Structure 



Carotenoids are pigments — usually yellow, red or orange — whose 

 chemical structure is characterized by a long, straight, unsaturated 

 carbon chain, often terminated at one or both ends by an ionone ring 

 (hexamethylene ring with one double bond). The carbon chain has an 

 affinity for similar chains in fats and lipides and accounts for the lipophilic 

 properties of the carotenoids. Some carotenoids contain no polar groups 

 at all; others possess one or several hydroxyls or carbonyls in the terminal 

 groups, which increase their solubility in alcohols, but are not enough to 

 bring about solubility in water. 



Most carotenoids contain 40 carbon atoms, and 11 or more double 

 bonds. Most or all of the double bonds are conjugated. The long 

 chain of conjugated double bonds (so-called "polyene chain") is a 

 chromophore and is responsible for the color of the carotenoids. 



The nomenclature of the carotenoids is somewhat confused. This 

 applies particularly to the term "xanthophyll." This name, meaning 

 "leaf -yellow," was introduced by Berzelius (1837) as a counterpart to 

 chlorophyll, the "leaf -green." Later, one of the two yellow pigments of 

 the leaves was found to be identical with carotene from carrot roots, and 

 the use of the name xanthophyll was restricted to the other one. Still 

 later, pigments similar to leaf xanthophyll were found in algae and in 

 many animal tissues, and also were called xanthophylls. On the other 

 hand, the "leaf xanthophyll" itself proved to be a mixture of several 

 pigments, the most common of them being identical with luteol, the 

 coloring matter of egg yolk. Strain (1938) suggested that all carote- 



* Bibliography, page 480. 



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