2 RHODOXANTHIN 223 



Zerewitinoff determinations indicated the presence of one hydroxyl group, 

 but this result must be due to partial enolisation of the ketone, since no acetyl 

 derivative could be obtained by the action of acetic anhydride in pyridine. 

 (This latter result could be due to the presence of a tertiary hydroxyl group, 

 but this would not be in accord with the general properties of the pigment) . 



The constitution of rhodoxanthin has been confirmed by investigations of 

 Karrer and Solmssen^^ in the course of which the pigment was converted into 

 zeaxanthin via its dihydro-derivative, and the relationship between the two 

 pigments was thus established. These investigations have already been described 

 on p. 182. 



Properties 



Crystalline form: Rhodoxanthin crystallises from a mixture of benzene and 

 methanol (1:4) in dark violet needles, combined in rosettes. From aqueous 

 p^Tidine or ethanol it is obtained in thin, finely branched rods. If an ethanolic 

 solution of rhodoxanthin is allowed to evaporate slowly, the pigment crystallises 

 in well-formed leaflets. 



Melting point: 219° (corr., evacuated capillary). 



Solubility: The pigment is very easily soluble in pyridine, easily soluble in 

 benzene and chloroform, very sparingly soluble in ethanol and methanol, and 

 insoluble in petrol, hexane, and petroleum ether. 



spectral properties: 



Solvent Absorption maxima 



Carbon disulphide 564 525 491 m/i 



Chloroform 546 510 482 m/i 



Benzene 542 503,5 474 m/< 



Ethanol 538 496 (very difiuse) 



Petrol 524 489 458 m/i 



Petroleum ether 521 487 456 m^ 



Hexane 524 489 458 m/* 



(cf. Fig. 13 and 15, p. 353 and 354) 



Solutions of the pigment in petrol are yellow-red, solutions in methanol 

 wine-red. The same difference was observed in the case of capsanthin. Zech- 

 MEiSTER and PoLGAR^^ ascribe this phenomenon to the polar nature of the 

 alcohol. 



Colour reactions: Rhodoxanthin dissolves in concentrated sulphuric acid 

 with a deep blue colour. On treating a solution of the pigment in chloroform 

 with antimony trichloride, an intense blue-violet colouration is observed. On 

 shaking an ethereal solution of rhodoxanthin with 25% hydrochloric acid, 



References p. 253-255. 



