226 CAROTENOI DS CONTAINING CARBONYL GROUPS XII 



concentrated under reduced pressure in nitrogen, and the pigments are saponified 

 with aqueous potassium hydroxide and then separated into epiphasic and hypo- 

 phasic fractions. Myxoxanthin is obtained from the epiphase and myxoxanthophyll 

 can be extracted from the hypophase (p. 228). 



The epiphasic petroleum ether extract is washed free from alkaU, dried, and 

 allowed to stand for a short time during which a considerable quantity of j3-caro- 

 tene separates. The mother liquors are evaporated to dryness in vacuum, the residue 

 is chromatographed on alumina, and the pigment of the central zone of the chro- 

 matogram is chromatographed again from alumina. The myxoxanthin is then 

 dissolved in a mixture of ether and methanol, the solution is concentrated, and 

 colourless impurities are separated out by cooling. On further concentration and 

 cooling, the mother liquors yield myxoxanthin. For further purification, the 

 pigment is repeatedly crystallised from a mixture of pyridine and methanol. 



Chemical Constitution 

 CH, CH, CH3 CH3 



C CH3 CH3 CH3 CH3 CH 



/\ II I I \ 



CH, C-CH=CH-C=CHCH=CH-C=CHCH=CHCH=C-CH=CHCH=C-CH=CH-CH CH 



r II i 11 



CH, C-CH3 H3C-C CH 



\ / M3'xoxanthin( ?) \y' 



CHs, CO 



The constitution of myxoxanthin has not yet been fully elucidated. 

 However, the investigations of Heilbron and Lythgoe^^ and Karrer and 

 Rutschmann23 make the above formula very probable. 



Elementary analysis gave the empirical formula C40H54O. On microhydro- 

 genation, myxoxanthin absorbed 12 mols of hydrogen, while myxoxanthin 

 oxime requires 13 mols of hydrogen for saturation. This shows that the free 

 pigment contains 12 double bonds and one carbonyl group. The latter can be 

 reduced by means of aluminium isopropoxide and isopropylalcohol to give a 

 secondary alcohol, myxoxanthol. This agrees in its spectral properties with 

 y-carotene and rubixanthin and must therefore contain the same, or a very 

 similar, chromophoric system. Since myxoxanthin exhibits vitamin A activity, 

 it must further contain an unsubstituted j8-ionone ring. For reasons of analogy 

 (e.g. astacene), it is assumed that the carbonyl group is cross-conjugated with 

 the system of conjugated double bonds. (Carotenoid ketones which contain a 

 ketone group attached terminally to the unsaturated system generally exhibit 

 an absorption spectrum with 3 bands, whereas astacene and myxoxanthin 

 exhibit only one band) . 



Properties 



Myxoxanthin crystallises from a mixture of pyridine and methanol in deep 

 violet prisms, m.p. 168-169°. The pigment is easily soluble in a mixture of 

 chloroform and ether, but only sparingly soluble in chloroform alone. On 



References p. 253—255* 



