CHAPTER IV 

 The chemical constitution of the carotenoids 



Approximately 80 natural carotenoids are at present known. The constitu- 

 tions of about 35 have been completely or largely elucidated and they are all 

 closely related chemically. They all belong to the class of polyenes, their most 

 characteristic structural feature being the large number of conjugated double 

 bonds. Another characteristic feature is that of the 50 carotenoids the empirical 

 formulae of which are known, 45 contain 40 carbon atoms, and only 5 have a 

 different number of carbon atoms in the molecule. 



The fact that there is some connection between the carotenoids and isoprene 

 was first recognised by Willstatter and Mieg^. To-day we know that isoprene 

 is a constituent unit of the carotenoid pigments, which may be regarded as 

 consisting of 8 isoprene molecules. It is characteristic of all carotenoids that 

 the arrangement of the isoprene residues becomes reversed in the centre of the 

 carotenoid molecule so that the central methyl groups occupy i : 6 instead of 

 1:5 positions^. The formula of lycopene is an example of this structural 

 principle : 



CHa CHq CHi CHo 



C CH3 CHj CH3 CH3 C 



/ ■ \ I I I \ 



CH CH-CH4:CH-C=CHCH:^iCH-C=CHCHiiCHCH = C-CHit:CHCH=C-CH:|:CH-CH CH 



CHg C*CH3 . H3C*C CH2 



X. / Lycopene \ y, 



CH2 CHj 



The important principle of the "reversal" of the central isoprene units has 

 given rise to the hypothesis that a carotenoid molecule may be formeid in the 

 plant by the combination of two identical residues, e.g. two partially de- 

 hydrogenated phytyl groups, by a linking of the two terminal carbon atoms. 



The following table, which contains the structural formulae of all naturally 

 occurring carotenoids of known structure, shows the close structural relationship 

 between these compounds. Formally, they can all be related to lycopene. 

 y-Carotene, /3-carotene and a-carotene can be formed from lycopene by ring 

 closure at one or both ends of the molecule, while bixin and crocetin can be 

 References p. jy. 



