i84 CAROTENOIDS CONTAINING HYDROXYL GROUPS XI 



present indicate that zeaxanthin is an isomer of xanthophyll. The nature of 

 the isomeric relationship was estabHshed by the investigations of Karrer and 

 co-workers^^'^^ who showed that xanthophyll is derived from a-carotene, whereas 

 zeaxanthin is the corresponding derivative of /5-carotene. By the dry distillation 

 of the pigment, Kuhn and Winterstein^" obtained toluene, w-xylene and 

 2 :6-dimethylnaphthalene. By ozonisation and potassium permanganate oxida- 

 tion of zeaxanthin, Karrer and co-workers^^ obtained a : a-dimethylsuccinic 

 acid. The arguments applied in the case of xanthophyll regarding the position 

 of the hydroxyl groups are therefore also valid for zeaxanthin (cf. p. 201). The 

 number of side-chain methyl groups and double bonds was determined by Kuhn 

 and co-worker s^2. 



In 1938, Karrer and co-workers provided further confirmation for the 

 constitution of zeaxanthin by preparing /9-citraurin from zeaxanthin by partial 

 oxidative degradation with potassium permanganate^^. 

 CHo CHo 



C CH3 CH3 CH3 CH3 



/\ I I II 



CH„ C-CH=CH-C=CHCH=CH-C=CHCH=CHCH=C-CH=CHCH=C-CHO 



I II 



HOCH C-CHg 



\ / ^-Citraurin 



CHa 



Properties and Physical Constants 



Crystalline form: Zeaxanthin crystallises from methanol in long, yellow 

 plates and clusters which, in contrast to xanthophyll, do not occlude solvent. 

 The pigment crystalHses especially well from ethanol in short thick rombic 

 prisms. From a mixture of carbon disulphide, petroleum ether and ether, zea- 

 xanthin separates in clustered needles. 



Melting point: 205° (uncorr.)^*, 215.5° (corr.)^^. 



Solubility: 1 g zeaxanthin dissolves in about 1.5 1 of boiling methanol. The 



pigment is almost insoluble in petroleum ether and hexane. Its solubility in ether, 



chloroform, carbon disulphide and pyridine is somewhat greater. A suspension 



of the pigment in glacial acetic acid becomes homogeneous on addition of hexane. 



Spectral properties (cf. Fig. 8, p. 350) : 



Solvent: Absorption maxima: 



Carbon disulphide 517 482 450 m// 



Chloroform 495 462 429 m/i 



Ethanol 483 451 423.5 m/z 



Petrol 483.5 451.5 423 mn 



Methanol 480.5 449.5 421.5 m/< 



Quantitative extinction measurements : In carbon disulphide, Kuhn and 

 Smakula^^; in ethanol and carbon disulphide, Hausser and Smakula^' and 

 Hausser38. 



References p. 214-217. 



