CHAPTER VI 

 Methods of elucidating the constitution of carotenoids 



The elucidation of the constitution of the carotenoids is no easy task, and 

 although carotene and some of its congeners have been known for a very long 

 time, it is only within the last 20 years that it has been possible to obtain some 

 insight into the chemical structure of these compounds. The following sections 

 are meant to provide a short summary of the principal methods which have 

 been used for this purpose. For detailed descriptions of the experimental 

 methods the original literature should be consulted. 



I. DETERMINATION OF THE NUMBER OF DOUBLE BONDS 



The most characteristic structural feature of the polyene pigments is the 

 large number of double bonds in the molecule. In determining the constitution 

 of a carotenoid it is important to be able to establish the number of carbon- 

 carbon double bonds with small amounts of material (about 5 mg). Useful 

 information can be obtained by first determining the absorption spectrum of 

 the carotenoid. The relationships between the number of double bonds and the 

 absorption spectrum have been fully investigated and a knowledge of one of 

 these properties enables one to make predictions about the other. More exact 

 information is provided by quantitative measurements of the addition of 

 hydrogen, halogen, or iodine chloride, or of oxygen. The most accurate values 

 are obtained from catalytic hydrogenation, but the other methods have some- 

 times been used to confirm the results obtained. 



Catalytic hydrogenation can be carried out on the macro- or microscale. In 

 either case, all the double bonds in the molecule react including the carbonyl 

 group. Epoxide groups are also reduced during catalytic hydrogenation with 

 the formation of hydroxyl groups^. 



Colloidal platinum^, platinum oxide^, palladium oxide^, or platinum ad- 

 sorbed on Kieselguhr* can be used as catalysts. 



The following are suitable solvents: acetic acid (free from higher homo- 

 logues), ethyl acetate, acetic acid-ethanol mixtures, cvc/ohexane, hexane, 

 decalin, etc. Carotenoids are often so sparingly soluble that they have to be 

 References p. 51-52. 



