I50 CAROTENOID HYDROCARBONS OF KNOWN CONSTITUTION X 



Neo-^-carotene U shows vitamin A activity (cf. p. 15), but of a lower order 

 than that of natural /S-carotene. 



The other isomers could not be prepared in the crystalline state. They were 

 characterised by their absorption maxima in petroleum ether solution, and by 

 their positions in the chromatogram. In the following table, the compounds are 

 enumerated in the sequence in which they occur in the chromatogram. 



These are followed by a few other transformation products to which no 

 names have been assigned. Natural |S-carotene with complete ^rans-configuration 

 occurs between neo-/9-carotene V and neo-/S-carotene A in the chromatogram. 

 According to Polgar and Zechmeister^**', neo-y5-carotene B is identical with 

 pseudo-a-carotene. 



, 4. a-CAROTENE C4oHg6 



History 



1931 a-Carotene is discovered simultaneously by Kuhn and Lederer^*^ and 

 Karrer and co-workers^^^. The new pigment is an isomer of /5-carotene 

 and generally accompanies the latter in vegetable and animal materials. 



1933 Karrer and Walker^^^ introduce calcium hydroxide and calcium oxide 

 as new adsorbents for the chromatographic separation of epiphasic 

 polyene pigments. Pure a-carotene is thus prepared for the first time. 



1933 Karrer, More and Walker^** elucidate the constitution of a-carotene. 



Occurrence 



a-Carotene is almost as widely distributed in the vegetable kingdom as the 

 /5-isomer. It is present in varying amounts in most carotene preparations and 

 is concentrated in the mother liquors during recrystallisation. 



Mackinney^*^ examined the presence of a-carotene in green parts of plants, 

 especially leaves, and found that the following plants contain a-carotene: 

 References p. 165-1^0. * 



