CAROTENOIDS IN LAND PLANTS 



low energy content, can undergo trans -> cis isomerization in 

 solution, yielding a complex equilibrium mixture of isomers. These 

 changes are accelerated by light, heat, and by the addition of iodine 

 or acid. Any m-isomer similarly treated will produce the same 

 equilibrium mixture as the dX\-trans isomer. These isomers can be 

 separated chromatographically and are named according to the posi- 

 tions they occupy relative to the parent dXX-tram substance on an 

 adsorption column. Any m-isomer is named by prefixing " neo- " to 



15-0 



380 400 420 440 460 460 500 520 

 Wavelength fm/i) 



Fig. I. — The absorption spectrum in hexane of a-c^rotene, p-carotene and 

 X-carotene. {From Zechmeister, L (1944) Chem. Rev., 34, 267.) 



the name of the parent compound and then adding the letters " T, u, 

 V," etc., or " A, B, c," etc., according to its relative position on the 

 column, T, u, v, if adsorbed above, and a, b, c, if adsorbed below the 

 all-trans form. A glance at the diagram (Fig. 2) will make this clear. 

 It is interesting to note that Gillam and El Ridi's isomer of (3-carotene, 

 pseudo-a-carotene, is, according to Zechmeister's notation, neo-p- 

 carotene B. 



9 



