252 CAROTENOIDS CONTAINING CARBONYL GROUPS XII 



The formula for capsorubin proposed by Zechmeister and von Chol- 

 NOKY^''^' ^"^ has not been completely established, but is in complete accord 

 with all the properties of the pigment. The molecular formula of capsorubin is 

 Qo-^6o04- It contains 9 conjugated double bonds and- 2 carbonyl groups. 

 Acetylation shows the presence of two hydroxyl groups which, for reasons of 

 analogy, are assigned to the same positions as in xanthophyll (p. 201) zea- 

 xanthin (p. 183) and capsanthin (p. 242). The products of oxidation with 

 chromic acid indicate the presence of 4 side-chain methyl groups. 



Properties^^^' io» 



Capsorubin crystallises from a mixture of benzene and petrol in violet- 

 red needles. From carbon disulphide the pigment is obtained in rhombic plates. 

 It is readily soluble in alcohol and acetone, more sparingly soluble in ether, 

 benzene and carbon disulphide, and almost insoluble in petroleum ether. The 

 chromatographic behaviour of capsorubin is similar to that of capsanthin. It is 

 well adsorbed on calcium carbonate from carbon disulphide and is found above 

 capsanthin on the column. Melting point 201° (corr.). 



On treating an ethereal solution of capsorubin with concentrated hydro- 

 chloric acid, the latter immediately assumes a violet colouration, which later 

 turns deep blue. With 25 % hydrochloric acid, no colour change is observed. 

 The pigment is entirely hypophasic in the partition test. 



Solvent Absorption maxima 



Carbon disulphide 541.5 503 468 m^ 



Benzene 520 486 455 m/z 



Petrol 506 474 444 m/i 



Capsorubin diacetate C44Hg408: 



This compound is obtained by the acetylation of capsorubin in pyridine 

 with acetyl chloride. The ester crystallises from methanol in square leaflets, 

 m.p. 179° (corr.). It is readily soluble in petrol, benzene and methanol. 



Cis-Trans Isomers 



Zechmeister and von Cholnoky^^" examined the behaviour of capsorubin 

 on treatment with iodine, on heating or on standing over long periods. They 

 found that the pigment undergoes similar transformations as capsanthin (cf. p. 

 248). They were not able, however, to isolate any of the transformation 

 products, which they regard as cis-trans isomers, in the crystalline state. The 

 only data available are the absorption spectra and optical rotations. 

 References p. 253-255. 



