602 VI. CAROTENOIDS AND RELATED COMPOUNDS 



(6) Azafrinone. This compound is the diketone of azafrin and has a 

 formula of C27H36O4. It forms orange-red platelets from a concentrated 

 solution, but separates from more dilute solutions in star-shaped clusters of 

 needle-like crystals. It melts at 191°C. and is optically inactive. The 



CH3 CHj CH; 



HjC CH3 



\ X 



/ \^H hIhhhIhhhhIhhii 

 HjC C • C : c • C : C • c : c • C : C • C : C • c : C • C : c • c-OH 

 I I 



HzC C = 



\ / \ 

 C CH3 



H2 



Azafrinone 



absorption spectrum corresponds to that of its methyl ester (C28H38O4). It 

 reacts with only one hydroxylamine molecule, forming a monoxime which 

 crystallizes in platelets and melts at 194°C. Azafrinone aldehyde origi- 

 nates on chromic acid degradation of /3-oxy carotene.^" 



6. Properties of the Carotenoids 



(!) Melting Points and Optical Rotation 



The carotenoids represent a group of substances which are solids at 

 ordinary temperatures, having characteristic melting points which are 

 usually between 150° and 200°C. However, in certain cases values as low 

 as 80°C. and as high as 300°C. have been recorded. Data on the melting 

 points, specific rotation, vitamin A activity, and double bonds for some of 

 the more common carotenoids are summarized in Tables 12 and 13. 



(2) Spectral Absorption 



One of the most important physical properties of the carotenoids which 

 helps in their identification is their spectral absorption. The intensity 

 with which light of various wave lengths is absorbed is characteristic of 

 each derivative and, in fact, even varies with the different stereochemical 

 isomers. With the recent development of simple instruments for the 

 quantitative measurement of the absorption spectra (such as the Beckman 

 spectrophotometer and the Coleman photoelectric spectrophotometer), 

 wide application of the extinction values at wave lengths of maximum 

 absorption have been made for the quantitative determination of the 

 carotenoids. Such procedures are exceedingly accurate and relatively 

 simple. 



The extinction coefficient (or simply the extinction value) is usually 



"^ R. Kuhn and H. Brockmann, Unpublished work, cited by L. Zechmeister, Die 

 Carotinoide, Springer, Berlin, 1934, pp. 144, 271. 



