298 CAROTENOIDS OF UNCERTAIN STRUCTURE XIV 



The solvent is removed by distillation and the residue is repeatedly adsorbed on 

 calcium hydroxide. Only in this way is it possible to separate rhodopin from rhodo- 

 violascin and rhodovibrin. Finally the pigment is re-crystallised from a mixture of 

 carbon disulphide and petroleum ether. 



The structure of rhodopin is not yet definitely known. It has the molecular 

 formula C^oHggO or C4QH56O. Zerewitinoff determinations showed the presence 

 of one active hydrogen atom, but no acetyl derivative of rhodopin could be 

 prepared. Microhydrogenation indicated the presence of 12 double bonds, 

 which are probably all conjugated. No carbonyl group was detected and on 

 reduction of the pigment with zinc dust and acetic acid no change in the spec- 

 trum was observed. Methoxyl determinations gave a negative result. Karrer 

 and KoENiG^^ attempted to obtain some information regarding the structure 

 of rhodopin by oxidative degradation with permanganate. The amount of 

 material available was, however, too small for the identification of the degrada- 

 ation products. 



Rhodopin crystallises from a mixture of carbon disulphide and petroleum 

 ether in deep red crystals which appear as small clusters of needles and prisms 

 under the microscope. Melting point: 171° (after previous sintering). Rhodopin 

 exhibits epiphasic behaviour on partition between petroleum ether and 90 % 

 methanol. 



Solvent Absorption maxima 



Carbon disulphide 547 508 478 m/f 



Chloroform 521 486 453 mfi 



Petroleum ether 501 470 440 m// 



Ethanol (absolute) 505 474 (445)m/i 



(cf. Fig. 26, p. 358) 



3. RHODOVIBRIN 



During the chromatographic purification of rhodopin on calcium hydroxide, 

 Karrer and Solmssen* obtained another hitherto unknown carotenoid for 

 which they proposed the name rhodovibrin. Rhodovibrin is adsorbed somewhat 

 more strongly than rhodopin on the chromatographic column and can thus be 

 separated from the latter. Repeated chromatographic adsorption is necessary, 

 however, as the two pigments are only separated with difficulty. Rhodovibrin 

 crystallises form a mixture of carbon disulphide and petroleum ether in small 

 deep-red crystal clusters which are almost indistinguishable in appearance from 

 those of rhodopin and melt at 168°. The absorption maxima are located at 556 

 and 517 m// in carbon disulphide solution. The pigment could not be obtained 

 in a completely pure state as it is only present in very small quantities in 



The literature is summarised under rhodopin, p. 297; cf. H. Koenig, Dissertation, 

 Zurich, 1940, p. 29. 



References p. 341-343. 



