6 VIOLAXANTHIN 195 



glutaric acid. These results established the relationship of violaxanthin to the 

 xanthophyll and zeaxanthin series (of. p. 201). Karrer and co-workers^^ showed 

 that violaxanthin absorbs 10 mols of hydrogen on catalytic hydrogenation. 

 The position of the absorption maxima indicated that all 10 double bonds 

 must be conjugated*. 



According to Karrer and Rutschmann^^ only two of the four oxygen 

 atoms in the violaxanthin molecule are present as hydroxyl groups; the nature 

 of the other two oxygen atoms could not be established**. Eventually Karrer 

 and JucKER were able to identify violaxanthin with their synthetic zeaxanthin 

 di-epoxide. The constitution of violaxanthin was thus proved (cf. p. 190). 



Properties 



Violaxanthin crystallises from methanol in yellow-orange prisms, or from 

 carbon disulphide in reddish-brown spears, m.p. 200°. It is easily soluble in 

 ethanol, methanol, carbon disulphide and ether, bu t almost in soluble in 

 petroleum ether. 



Solvent: Absorption maxima: 



Carbon disulphide 501 470 440 m^ 



Chloroform 482 451.5 424 m^ 



Petrol 472 443 417.5 m/i 



Ethanol 471.5 442.5 417.5 ran 



Methanol 469 440 415 m/< 



(cf. Fig. 9, p. 351) 



For quantitative extinction measurements, cf. Hausser and Smakula^^. 

 Violaxanthin gives a very characteristic, stable deep-blue colouration on 

 shaking an ethereal solution with 20% hydrochloric acid***. The pigment is 

 separated from other phytoxanthins by chromatographic adsorption from 

 benzene solution on zinc carbonate or calcium carbonate. The specific rotation 

 in chloroform is [aj^d = +35°- 



Violaxanthin is converted into mutatoxanthin, auroxanthin and zeaxanthin 

 under the influence of dilute acids®^. These reactions are discussed on p. 62. 



Derivatives 



Perhydroviolaxanthin: This compound is obtained on catalytic reduction of 

 violaxanthin in ethanol. It is a colourless, weakly leavo-rotatory oil^^. In view 



With regard to apparent contradictions concerning the number of double bonds, 

 cf. Helv. chim. Acta 28 (1945) 300. 



With regard to the contradictory data concerning the number of hydroxyl groups, 

 cf. Helv. chim. Acta 28 (1945) 300. 

 *** For further colour reactions cf. R. Kuhn and A. Winterstein, Ber. 64 (1931) 326. 



References p. 214— 21';. 



