3 p-CAROTENE 149 



be concluded that they are partly deoxygenated to /?-caroteneormutatochrome* 

 in the animal organism, since according to all previous experience, vitamin A 

 activity requires the presence of an unsubstituted /5-ionone ring in the caro- 

 tenoid**. 



Cis-trans-Isomers of ^-Carotene^^^' ^^' 



In 1935, GiLLAM and El Ridi^^^ observed that several zones are developed 

 during the chromatographic adsorption of pure /S-carotene. They ascribed 

 this phenomenon to a change of the pigment during adsorption, and were 

 able to isolate a transformation product which they termed pseudo-a-carotene. 

 This compound exhibited the characteristic properties of a carotenoid, melted 

 at 166°, and showed epiphasic behaviour, no optical activity and vitamin A 



potency. 



Solvent: Absorption maxima: 



Carbon disulphide 507 477 m// 



Chloroform 486 456 m/t 



Petroleum ether 477 446 m/t 



Ethanol 478 447 m/^ 



More recently, Zechmeister and co-workers^^'' have investigated these 

 changes and have shown that they are not produced by adsorption, but that 

 polyene pigments generally can be isomerised by dissolution, melting of the 

 crystals, or certain other operations (cf . p. 39) . From a number of considerations, 

 including the fact that these transformations are reversible, Zechmeister 

 concluded that m-^rans-isomerism is involved. About 10 zones are observed 

 in the transformation chromatogram and the individual pigments are regarded 

 as stereoisomers of /5-carotene. Only one of these pigments has so far been 

 obtained in the crystalline state, and has been termed neo-/5-carotene U. It is 

 rather more strongly adsorbed chromatographically than /3-carotene, is 

 somewhat more soluble in organic solvents, and exhibits epiphasic behaviour 

 on partition between petroleum ether and methanol. Thus 250 mg of /9-carotene 

 yielded 41 mg of neo-/5-carotene U, m.p. 122-123° (corr., block). According to 

 Zechmeister and collaborators^^, neo-^-carotene U contains one cis-double 

 bond whereas the pseudo-a-carotene obtained by Gillam and El Ridi^^^ may 

 contain two czs-double bonds. 



Solvent: Absorption maxima of neo-^-carotene U: 



Carbon disulphide 512.5 478.5 m/^ 



Benzene 494 461 m^ 



Petroleum ether 481 450 m/i 



Chloroform 493.5 461 m/j, 



Ethanol 482 450.5 m/t 



* Early investigations showed that mutatochrome exhibits vitamin A activity. 

 See, however, footnote p. 14. 



References p. 16^-ijo. 



