Studies of γ-Carotene and Related Carotenoids: I. γ-Carotene. II. Pro-γ-Carotene. III. Gazaniaxanthin. IV. Micro-Determinations of the Isopropylidene Group in Carotenoids

A series of investigations have been carried out with γ-carotene and related carotenoids. In “monkey flowers” (Mimulus longiflorus) a carotenoid occurs which apparently differs only in melting point from γ -carotene described "by Kuhn and Brockmann. The available evidence indicates that this compound is indeed γ-carotene but the cause of the melting point difference remains anomalous. A new carotenoid, the second of its type, possessing four to five cis double bonds in the molecule has been isolated from the fruit of Butia capitata and Pyracantha angustifolia and its properties described. The compound, a hydrocarbon, has been termed pro- γ -carotene because of its relation to γ -carotene. Its most remarkable property is the almost instantaneous change in the spectrum upon treatment with iodine. The change is caused by the formation of a mixture of stereoisomors of the carotenoid, in which the all trans form predominates but in which the pro-carotenoid is absent. The stereoisomerization of pro-γ-carotene under the influence of several catalysts and experimental conditions has been studied chromatographically. Experiments with gazaniaxanthin, discovered by Schor in the flowers of Gazania rigens, have indicated that this carotenoid is dihydro-rubixanthin. The stereoisomerization of the compound has been studied but the chromatographic separation of the stereoisomers is extremely difficult. The spectral curve of the substance exhibits the “cis peak" phenomenon on isomerization. Isopropylidene determinations by ozonolysis have been carried out with a number of carotenoids. Gazaniaxanthin, which probably possesses an isopropyl group, behaves as if about 0.9 “isopropylidene” group were present while β-carotene seemingly possesses 0.3 “isopropylidene.” These results have been correlated with the fact that the method is known to indicate the presence of some “isopropylidene” when an isopropyl group is present.