192 CAROTENOIDS CONTAINING HYDROXYL GROUPS XI 



Antheraxanthin crystallises from methanol or a mixture of benzene and 

 methanol in needles or thin plates, m.p. 205°. By the action of acidic chloro- 

 form, mutatoxanthin and a little zeaxanthin are formed from antheraxanthin. 

 On shaking an ethereal solution of the pigment with concentrated aqueous 

 hydrochloric acid, a blue colouration is produced after a period of time. 



Solvent: Absorption maxima: 



Carbon disulphide 510 478 mpL 



Chloroform 490.5 460.5 m/x 



Recently, Tappi and Karrer'^^ have isolated a cis isomer of anthera- 

 xanthin from Lilium candidum. The formulation of the pigment is based on the 

 following facts. Cis antheraxanthin has a lower melting point than trans 

 antheraxanthin and its absorption maxima are displaced towards shorter 

 wavelengths by 4-6 m/i. Spectral measurements show that on irradiation with 

 ultraviolet light and on treatment with iodine the new isomer is partly 

 isomerised to trans antheraxanthin, though the amount of available material 

 was insufhcient to isolate the latter in a cystalline state. On treatment with 

 chloroform containing traces of hydrochloric acid, cis antheraxanthin is con- 

 verted into mutatoxanthin which is also obtained from trans antheraxanthin 

 under these conditions. 



Cis antheraxanthin is the first natural carotenoid epoxide with a partial 

 cis configuration. The small difference in the location of the absorption maxima 

 of cis and trans antheraxanthin suggests that only one double bond in the 

 former has a czs-configuration. The large difference in melting points, on the 

 other hand, suggests that there is a considerable difference in molecular shape 

 and that the double bond involved in the geometrical isomerism may perhaps 

 be situated in the centre of the polyene chain. 



Cis antheraxanthin crystallises from methanol in long, yellow-red needles, 

 m.p. 110° (uncorr., evacuated capillary). It is easily soluble in benzene, carbon 

 disulphide and ether, fairly soluble in warm methanol and very sparingly 

 soluble in petroleum ether. 



Solvent: Absorption maxima: 



Carbon disulphide 506 476 m/^ 



Benzene 487 457 m/z 



Ethanol 472 445 m^ 



On shaking an ethereal solution with concentrated hydrochloric acid, the 

 latter assumes a fairly stable blue colouration. 



References p. 214— 2ij. 



