The isomerization of carotenes by chromatographic adsorption

Abstract

Previous work on the carotene of butter showed that adsorption on alumina or Ca(OH)2 separates the pigment into 2 parts which simulate [alpha]- and [beta]-carotene respectively. It has now been found that what was apparently [alpha]-carotene is largely made up of a new pigment spectroscopically identical with it. The new pigment[long dash]pseudo-[alpha]-carotene[long dash]has been prepared by repeated chromatographic adsorption of [beta]-carotene on alumina. It has m.p. 166[degree], is isomeric with [beta]-carotene, and on readsorption is reconverted largely into this compound. Although spectroscopically identical with [alpha]-carotene it differs in having no rotation, and in adsorption properties. Hence it is probably not a racemic form of [alpha]-carotene. It is suggested that the mechanism of formation is a displacement of a terminal double bond out of the conjugated system by the adsorption process. Preliminary exps. with [alpha]-carotene indicate that this also can be changed by adsorption into another carotenoid ("neo-carotene") the absorption maxima of which are displaced even further towards the violet than those of [alpha]-carotene.[long dash]In using the chromatographic method for the separation of carotenoids care must be taken to differentiate between genuine separations and transformations brought about by the process itself.