The isomerization of carotenes

EARLIER experiments on the chromatographic adsorption of fl-carotene on Tswett columns of alumina or calcium hydroxide have shown that this pigment readily undergoes change into another carotenoid having a distinctly different absorption spectrum [Gillam & El Ridi, 1935]. Later work resulted in the isolation of a new crystalline carotenoid exhibiting an absorption spectrum different from that of fl-carotene but identical with that of a-carotene. Further investigation and analysis showed this to be a carotene isomeride differing from ac-carotene in certain properties and it was therefore named pseudo-a-carotene (+oc-carotene) [Gillam & El Ridi, 1936]. A similar study of the adsorption of oc-carotene indicated that this pigment could also be changed into yet another new entity (differentiated by means of its absorption spectrum) which was provisionally called neocarotene. On crystallization it was found that the absorption spectrum had reverted to that ofthe parent ac-carotene. The properties of the crystals were however quite different from those of either ac-carotene or c-carotene, being in fact those of another new carotenoid which has been named neo-oc-carotene, although the absorption spectra of all three are identical [Gillam et al. 1937]. At the time that these experiments were carried out it was concluded that the adsorption process itself was responsible for the isomeriza- tion of both the ac and the ,8-carotenes, although Zechmeister & Cholnoky [1937],