Carotenes, alicyclic

Fig. 26.9. Summary of biogenesis or alicyclic carotenes (Britton, 1993 modified and used with permission of the copyright owner. Chapman Hall, London).

Determination of the adsorption characteristics of a carotenoid is useful in its identification. This can be accomplished by tic or by column or paper chromatography. The adsorption characteristics of a carotenoid are influenced by a number of structural features. For acyclic carotenes, adsorption affinity increases with an increasing number of double bonds. In contrast, the presence of mono- or dlcyclic end groups lessens the adsorptive affinity of the compound compared to that of the corresponding acyclic carotene. The structural differences between alicyclic and aromatic rings also result in a difference in adsorptive properties of these compounds. The adsorp-... 

Xanthophylls are the main carotenoids of plants. They primarily arise as products of biochemical oxidation (hydroxylation and epoxidation) of carotenes. Xanthophylls derived from acyclic carotenes occur in foods in small quantities. For example, tomatoes contain as minor pigments 1,2-epoxylycopene, 5,6-epoxylycopene, 1,2-epoxyphytoene and some other compounds. Much more common are monohydroxysubstituted alicyclic derivatives of carotenes called cryptoxanthins. Most plant materials contain small amounts of a-cryptoxanthin also called zeinoxanthin, derived from a-carotene (9-182) and P-cryptoxanthin, derived from... 

Many carotenoids contain alicyclic rings, and in higher plants two types are found, the p and the e, as typified by jS-carotene and a-carotene, respectively (see Figure 4.4). These rings are formed by separate pathways, probably by proton loss from alternative positions in the same carbonium ion precursor, which has been created by proton attack at C-2 of the acyclic precursor. These reactions have been explored in a series of elegant experiments with D2O, which have also elucidated the stereochemistry of attack at C-2 and the behavior of the methyl substituents at C-1 (see Ref. 43). The pathways from acyclic to cyclic carotenes in plants are shown in Figure 4.4. Some of these conversions have been demonstrated with cell extracts of higher plant tissues, especially the conversion of lycopene to /3-carotene. ... 

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