CHAPTER I 





DEFINITIONS AND NOMENCLATURE Wv ^ ^ y 

 INTRODUCTION ^X^ ^^ 



The yellow alcohol-soluble pigments of autumn leaves were called 

 xanthophylls by Berzelius^ in 1837. Fremy^ and Stokes^ were the 

 first to show that these pigments also occurred in green leaves, a possi- 

 bility that had been first envisaged as early as 1827. * Later, following 

 the classical work of Tswett^ on the chromatographic separation of 

 leaf xanthophylls which showed them to be a complex mixture of "poli- 

 chromes ", these pigments were divided into two classes : one very 

 soluble in hydrocarbon solvents was called '* carotenes " and the other, 

 much less soluble in these solvents but very soluble in ethanol was 

 called " xanthophylls " ; the two classes were grouped together 

 under the general term " carotenoids ". The developments in 

 recent years have made desirable the extension and standardization of 

 these terms. Most of the ambiguities have recently been resolved in 

 statements from the " Union Internationale de Chimie " " and the 

 Committee on Biological Nomenclature of The National Research 

 Council of America. ' 



DEFINITION OF CAROTENOIDS 



Karrer's definition of carotenoids, accepted by the " Union inter- 

 national de Chimie," states that " Carotenoids are yellow to red 

 pigments of aliphatic or alicyclic structure, composed of isoprene 

 units (usually eight) linked so that the two methyl groups nearest the 

 centre of the molecule are in positions 1 : 6 whilst all other lateral 

 methyl groups are in positions 1:5; the series of conjugated double 

 bonds constitutes the chromophoric system of the carotenoids." 



- C=-CH- CH — CH- C =- CH-CH =i= CH - CH — C- CH --= CH — CH ^^ C — 



I I : I I 



9^3 CH, cent^.or CH, CH3 



V'"' "Ad). molecule '■ (jy, 



e> ^ ^ 



<& 



Arrangement of methyl groups around centre of a carotenoid molecule 



1 



