50 METHODS OF ELUCIDATING CONSTITUTION VI 



13. RELATIONSHIPS BETWEEN CHEMICAL CONSTITUTION 



AND COLOUR 



It has been mentioned before that the colour of a carotenoid is one of its 

 most important characteristics, and that it is often possible to make deductions 

 regarding the structure of a polyene pigment from its absorption spectrum. 

 A great deal of material is available in this field and many useful relation- 

 ships have been established^^. 



The shortest wavelength selective absorption in the visible region so far 

 observed with a natural carotenoid is shown by auroxanthin (maxima at 454 

 and 423 m^ in carbon disulphide solution, cf. p. 196). The longest wavelength 

 selective absorption in the visible region is shown by torularhodin (maxima at 

 582, 541, and 502 m// in carbon disulphide solution*, cf. p. 330). 



The light absorption properties are determined by the following factors: 



a) Number and type of double bonds, 



b) Number and type of carbonyl groups, 



c) Number of epoxide groups, 



d) Number and position of carboxyl groups, 



e) Number of hydroxyl groups, 



f) Steric configuration of the carotenoid. 



In recent years the detailed relationships between the constitution and colour 

 of the carotenoids have been investigated particularly by Kuhn and co- 

 workers^^, by Hausser and Smakula^^ and by Karrer and co-workers^. 

 These investigations are discussed in more detail in the following chapter. 



14. COMPARISON WITH PARTIALLY SYNTHETIC POLYENE 



PIGMENTS 



Recently, comparison with partially synthetic carotenoids has often been 

 used successfully for elucidating the structure of carotenoids of unknown 

 constitution. An example is provided by the comparison of j3-citraurin (p. 219) 

 with ;S-apo-2-carotenal (p. 144) which, according to Karrer and Solmssen^* 

 differ only by the presence of an extra hydroxyl group in |8-citraurin. This 

 comparison lead to the complete elucidation of the constitution of ^-citraurin. 



Karrer and Jucker have recently succeeded in elucidating the structure 

 of numerous carotenoids by comparison with partially synthetic pigments. 

 Thus the following pairs of natural and partially synthetic pigments were 

 proved to be identical : flavoxanthin and xanthophyll epoxide, antheraxanthin 

 and zeaxanthin mono-epoxide, violaxanthin and zeaxanthin di-epoxide, citro- 

 xanthin and mutatochrome, etc. 



* In this connection tlie partially synthetic dehydrolycopene is of interest. It exhibits 

 a long wave absorption maximum in CSj at 601 m/z (p. 122). 



References p. 51-52. 



