8 XANTHOPHYLL 201 



potassium hydroxide and washed free from alkali. The solvent is removed by- 

 distillation in a stream of carbon dioxide until the volume is reduced to 300 ml; 

 on cooling the solution, part of the xanthophyll crystallises out. Further quantities 

 of pigment can be obtained from the mother liquors after further concentration 

 and addition of petroleum ether. The remaining mother liquors are finally taken up 

 in methanol and diluted with water under petroleum ether, when the remainder 

 of the pigment is precipitated. The total yield amounts to up to 6 g. The preparations 

 obtained in this way differ widely with regard to rotation and m.p. The pigment 

 can be purified bv repeated crystallisation from methanol or chromatography on 

 zinc carbonate from benzene solution. For other methods of preparation cf. Kuhn, 

 WiNTERSTEiN and LedererI"!, Miller^°2 and Zechmeister and Tuzson^"^. 



Chemical Constitution 



CHo CHo CHj Cxlo 



\V \V 



C CHo CHo CHo CHo C 



/\ I I I r /\ 



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



HOCH C-CHa H,C-C 3'CHOH 



\4/ Xanthophyll \a'/ 



CHj CH 



The constitution of xanthophyll was elucidated mainly by Karrer and co- 

 workers^"*. The empirical formula of xanthophyll was determined by Will- 

 STATTER and MiEG^"^. Zechmeister and Tuzson^"^ established the presence 

 of II double bonds by catalytic hydrogenation. This was confirmed by Pumme- 

 rer and Rebmann^"', who showed that the pigment absorbs 11 mols of iodine 

 chloride. The spectral properties of xanthophyll are similar to those of a-caro- 

 tene, indicating the presence of similar chromophoric systems. According to 

 Karrer, Helfenstein and Wehrli^*'^, the two oxygen atoms are present as 

 hydroxyl groups, which can be quantitatively determined by the method of 

 Zere'witinoff. These findings were confirmed by the preparation of a number 

 of esters and of a monomethyl ether of xanthophyll (Karrer and co- 

 workers^^^'ii"). Furthermore, Karrer, Zubrys and Morf^ were able to 

 oxidise perhydroxanthophyll to a diketone, thus proving that the two hydroxyl 

 groups are secondary and do not below to enol groupings. The number of side- 

 chain methyl groups was determined by means of chromic acid and perman- 

 ganate oxidations^^^. Karrer and co-workers^^^ obtained further evidence 

 for the structure of xanthophyll by the potassium permanganate oxidation of 

 the pigment, which yielded a:a-dimethylsuccinic acid and dimethylmalonic 

 acid. No geronic acid or a:a-dimethylglutaric acid were formed and it was 

 concluded from the result that xanthophyll differs from carotene in the struc- 

 ture of the two carbon rings. The two hydroxyl groups are contained in the 

 rings and the two most likely positions are the carbon atoms 3 or 4, and 3' or 

 References p. 214-217. 



