CHEMICAL RELATIONS BETWEEN CAROTINOIDS 175 



place in a high vacuum. No similar explanation is offered for the 

 divergence of the molecular weight and elementary composition deter- 

 minations from the theoretical values. It is not apparent from the 

 description given that any less care was taken in purifying the crys- 

 tals for these analyses than was taken by Willstiitter and Micg in 

 preparing the plant xanthophyll for a like purpose. 



When these results are viewed from a strictly chemical standpoint 

 it is difficult to escape the conclusion that the purest preparations 

 were not free from admixture with a substance of high molecular 

 weight which is lower in carbon and higher in hydrogen than the 

 xanthophyll. This conclusion is supported by Willstiitter and 

 Escher's own statement that the 4 grams of crude pigment contained 

 a large proportion of wax-like material which had solubility prop- 

 erties similar to the pigment. On the other hand, it is hardly pos- 

 sible that the high melting point of the egg yolk pigment in compari- 

 son with the plant xanthophyll, which led Willstiitter and Escher to 

 call the pigment xanthophyll "b," was due to the impossibility of 

 removing the unknown impurity. The presence of a wax of high 

 molecular weight would undoubtedly alter the melting point of the 

 pure pigment but would lower, rather than raise it. There is cer- 

 tainly no evidence that the preparations used for the melting point 

 determinations were of any higher purity than those used for the 

 elementary analyses. 



The other chemical properties of the egg yolk xanthophyll leave no 

 doubt as to its identity in these respects with the plant xanthophyll. 

 The actual solubility of the two pigments in various solvents is iden- 

 tical. The crystalline form from various solvents agrees perfectly. 

 Both pigments form a violet colored crystalline iodide (showing that 

 Kiihne's failure to obtain the blue iodine lipochrome reaction was not 

 due to any peculiar characteristic of the pigment). The phase test 

 applied to the purified pigment shows complete correspondence with 

 the xanthophyll group of carotinoids, the pigment being practically 

 quantitatively hypophasic between petroleum ether and 80-90 per 

 cent ethyl or methyl alcohol. Finally, the spectroscopic absorption 

 bands of the two pigments are identical in every respect when meas- 

 ured under the same conditions. 



As pointed out in Chapter IV there is also a biological basis for 

 rejecting Willstiitter and Escher's conclusion that the egg yolk xan- 

 thophyll is an isomer of plant xanthophyll, unless it be assumed that 

 the plant xanthophyll from which the egg yolk pigment is derived is 



