Introduction 



The term carotenoids refers to a group of pigments, yellow to red in colour, 

 which are widely distributed in the vegetable and animal kingdoms, and are 

 distinguished by the following features : they are generally composed of isoprene 

 residues, usually eight, arranged in such a way that in the middle of the molecule 

 two methyl groups are present in i : 6 positions, while all other side-chain methyl 

 groups occupy i : 5 positions. The general structure of the carotenoids is of the 

 aliphatic or aliphatic-alicyclic type and their chromophoric systems contain 

 numerous conjugated carbon-carbon double bonds. 



All carotenoids are soluble in fats and lipoids; the term lipochronies is 

 derived from this property. The only water-soluble carotenoids are those which, 

 owing to the presence of acidic groups (e.g. carboxyl or enol groups), are able 

 to form water-soluble alkali salts, or have acquired lyophilic properties by 

 esterification with sugar residues (e.g. in crocin). 



In view of their general chemical structure, carotenoids may be regarded 

 as a sub-group of the polyene pigments. However, the latter also include 

 pigments not composed of isoprene residues, but containing an unbranched 

 aliphatic chain of conjugated double bonds (e.g. the diphenylpolyenes). 



A revised nomenclature for carotenoids has recently been proposed*, but in 

 the present monograph the individual pigments are mostly referred to by the 

 names given to them by their discoverers. 



The great interest which the carotenoids have aroused during the last 

 twenty years is conditioned not only by their interesting chemical structure 

 but also by their biological and physiological importance. Several of these 

 pigments are pro-vitamins of vitamin A and thus play an essential part in the 

 animal and human organism. Their significance in the vegetable kingdom has 

 so far been less thoroughly investigated but there can be little doubt that here 

 also they fulfil important functions. 



The Nomenclature of the Carotenoid Pigments (Report of the Committee on Bio- 

 chemical Nomenclature of the National Research Council, accepted by the Nomenclature, 

 Spelling and Pronunciation Committee of the American Chemical Society — Chem. Eng. 

 News 24 (1946) 1235. — Report of the 'Commissions de Reforme de la Nomenclature de 

 Chimie organique at de Chimie biologique'. London, July 1947). 



