y-CAROTENE 163 



Chemical Constitution 

 CHo CH, CH3 CH, 



C CH3 CH3 CH3 CH3 c^ 



/\ II I I \ 



CH, C-CH=CH-C=CHCH=CH-C=CHCH=CHCH=C-CH=CHCH=C-CH=CH-CH CH 



I II II I 



CHj C-CHg ^ HaC-C CH, 



\ / y-Carotene \ y^ 



CH, CH, 



The elucidation of the constitution of this pigment was especially difficult 

 in view of the small amount of material available. After establishing the carbon 

 and hydrogen content of y-carotene, Kuhn and Brockmann^^^ proved the 

 presence of 12 double bonds by means of catalytic hydrogenation. y-Carotene 

 therefore contains one isocyclic ring. The absorption spectrum indicates that 

 only II of the 12 double bonds are conjugated. By ozonisation of the pigment 

 Kuhn and Brockmann obtained 0.85 mol of acetone, and concluded that one 

 end of the molecule must have an open-chain structure. No geronic acid could 

 be isolated, so that the presence of a /5-ionone ring has not been finally proved. 

 However, the vitamin A activity (cf. p. 15) of y-carotene supports the proposed 

 formula since it is known that only compounds containing an unsubstituted 

 j5-ionone ring show growth-promoting properties. 



Properties 



Crystalline form: y-Carotene crystallises from a mixture of benzene and 

 methanol in microscopic dark red prisms with a blue lustre. On rapid crystal- 

 lisation, the pigment is obtained in more lightly coloured needles. 



Melting point^^^: 178° (corr., in vacuum)!^^; 176.5° (corr.)^^*. 



Solubility: y-Carotene is less soluble in the usual solvents than the ^S-isomer. 



Spectral properties: 



Solvent: Absorption maxima: 



Carbon disulphide 533.5 496 463 m// 



Chloroform 508.5 475 446 mfi 



Benzene 510 477 447 m// 



Petrol . .' 495 462 431 m^ 



Hexane 494 462 431 ran 



(cf. Fig. 5, p. 349) 



Quantitative extinction measurements: Kuhn and Brockmanni*^, 



A. WiNTERSTEiN and U. Ehrenberg, {Z. physiol. Chem. 20J (1932) 25) ascribed the 

 above formula to a pigment which they had isolated from Convallaria majaiis. Subsequent 

 investigations showed, however, that this pigment was a mixture and not identical with 

 y-carotene. 



References p. 165-170. 



