174 CAROTENOIDS CONTAINING HYDROXYL GROUPS XI 



chromophoric systems. Zerewitinoff determinations show that the oxygen 

 atom is present as a hydroxyl group. Since rubixanthin exhibits no vitamin A 

 activity, the hydroxyl group must be substituted in the /?-ionone ring, since 

 compounds containing an unsubstituted ^-ionone ring generally possess growth 

 promoting properties. For reasons of analogy, Kuhn and Grundmann assume 

 that the hydroxyl group is present in position 3. Although the formula of rubi- 

 xanthin has thus not been finally proved, it appears the most probable according 

 to present knowledge, and is in complete agreement with all the properties of 

 the pigment. 



Properties 



Crystalline form: Rubixanthin crystallises from a mixture of benzene and 

 petroleum ether in orange-red needles, and from a mixture of benzene and 

 methanol in dark red needles with a copper-like lustre. 



Melting point: 160°. 



Solubility: The pigment is easily soluble in benzene and chloroform, but, 

 only sparingly soluble in alcohols and petroleum ether. 



Spectral properties: 



Solvent: Absorption maxima: 



Carbon disulphide 533 494 461 rafi 



Chloroform 509 474 439 m/x 



Ethanol 496 463 433 m^ 



Petroleum ether 495.5 463 432 m/i 



Hexane 494 462 432 m[i 



Optical activity: Rubixanthin is optically inactive. 



Partition test: On partition between petroleum ether and 90 % methanol, 

 the pigment is found in the upper layer. If 95 % methanol is used, however, 

 the pigment is hypophasic. 



Chromatographic behaviour: Owing to the presence of one hydroxyl group, 

 rubixanthin is very much more strongly adsorbed on calcium hydroxide than 

 carotenoid hydrocarbons. On the other hand, rubixanthin is more weakly 

 adsorbed on zinc carbonate or alumina than phytoxanthins containing 2 

 hydroxyl groups (zeaxanthin, xanthophyll, etc.). The separation of rubixanthin 

 from cryptoxanthin is very tedious as these two compounds show almost 

 identical adsorption properties. 

 References p. 214-21'j. 



