Carotenoid structure

WOODALL A A, BRITTON G and JACKSON M J (1997) Carotenoids and protection of phospholipids in solution or in liposomes against oxidation by peroxyl radicals relationship between carotenoid structure and protective ability , Biochim Biophys Acta, 1336, 575-86. 

It has been established that carotenoid structure has a great influence in its antioxidant activity for example, canthaxanthin and astaxanthin show better antioxidant activities than 3-carotene or zeaxanthin. 3- 3 3-Carotene also showed prooxidant activity in oil-in-water emulsions evaluated by the formation of lipid hydroperoxides, hexanal, or 2-heptenal the activity was reverted with a- and y-tocopherol. Carotenoid antioxidant activity against radicals has been established. In order of decreasing activity, the results are lycopene > 3-cryptoxanthin > lutein = zeaxanthin > a-carotene > echineone > canthaxanthin = astaxanthin. ... 

In dark conditions, the spontaneous isomerization of carotenoids occurs in solution the rate is dependent on temperature, solvent, and carotenoid structure. In the case of P-carotene, 13-di-P-carotene was formed approximately three times faster than the 9-cis- isomer at room temperature and at 150°C. ... 

The determination of the absolute configuration of a carotenoid is only possible by circular dichroism (CD) measurement. The spectrum interpretation can only be done by comparison with reference or model compounds with known chiralities. The sample requirement is as low as 5 to 50 pg, but CD facilities are not so commonly available. Buchecker and Noack reported experimental aspects and discussion of the relationships of carotenoid structures and CD spectra. 

Furthermore, the mechanism shown in Figure 12.1 considers only the all-tnmv-carotcnoid form as the initial compound however, although the all-tran.v-isomer predominates, d.v-isomcrs are also commonly found in model solutions and even more frequently in food systems, since these isomers are in equilibrium in the solution. Therefore, the initial carotenoid system often contains a mixture of isomers, whose composition changes according to the carotenoid structure, solvent, and heat treatment. For example, the isomerization rate of P-carotene is higher in nonpolar solvents, e.g., petroleum ether and toluene, than in polar solvents (Zechmeister 1944). 

Thus, the carotenoid acts as a catalyst deactivating 02. Many different carotenoids have been studied to investigate the influence of different carotenoid structural characteristics on the ability to quench ()2. Much of this work has been carried out in organic solvents with some typical results, taken from Conn et al. (1991), Rodgers and Bates (1980), and Edge et al. (1997) as shown in Table 14.1. 

The rate of reaction was found to be virtually independent of the carotenoid structure, which is in contrast to electron transfer reactions (see Section 14.3.2). 

Mortensen, A. and Skibsted, L.H. 1997a. Importance of carotenoid structure in radical scavenging reactions. J. Agric. Food. Chem. 45 2970-2977. 

Food Colorants/Carotenoids Structure and Solubility. Food, Nutrition, and Health 410, University of British Columbia, http //www.agsci.ubc.ca/courses/fnh/410/colour/ 3 30.htm... 

A few examples to render tetrapyrrolic compounds less phototoxic can be found in the hterature. In one approach, carotenoid structures were employed for the synthesis of some carotenoporphyrin derivatives [92-94]. Figure 8 shows two stuctures by way of example. Due to similar photophysical properties of the two structural components, the excited triplet state of the porphyrin is quenched by the carotenoid moiety, thus inhibiting the formation of singlet oxygen, while its fluorescence capabilities are still preserved. Biodistribution studies revealed enhanced uptake into tumour tissue [39,93,95]. However, microscopy studies have shown that such compounds are associated with connective tissues in the tumors rather than with cancerous cells indicating low specificities for mahgnant transformation [96]. 

Figure 9.2. The inherent metabolic flexibility of the isoprenoid pathway leading to the synthesis of some carotenoid pigments. Genes coding for two enzymes capable of acting on carotenoid structures were introduced into Escherichia coli which had already been transformed to give it the capacity to make p,p-carotene. Both of the two introduced new enzymes (one shown with red arrows and the other with blue arrows) acted on multiple substrates because of their lack of specificity. The resulting matrix of transformations means that nine different products can be made by just two tailoring enzymes. (Adapted from Umeno et al. ° who used data from Misawa et al. °)...

New Structures and Stereochemistry.—New Carotenoid Structures. A mutant strain of Rhizobium lupini contains a new nor-carotenoid, 2, 3 -trans-dihydroxy-2-nor-/3,/3-carotene-3,4-dione (1)." The wild-type R. lupini, when cultured in the presence of the cyclization inhibitors nicotine or CPTA, produced three new monocyclic carotenoids, 2,3-h-ans-dihydroxy-/3,-caroten-4-one (2), 3-hydroxy-/3,(/ -caroten-4-one (3), and p,tf/-carotene-2,3-trans-dio (4)," which were characterized by m.s. and n.m.r. The light absorption and mass spectra of a carotenoid from Rhodopseudomonas capsulata allowed its identification" as demethylspheroidenone [l-hydroxy-3,4-didehydro-l,2,7, 8 -tetrahydro- /f,(/f-caroten-2-one (5)]. 

Physical Methods.—Electronic absorption, mass, n.m.r., and increasingly c.d. spectra are used routinely in the elucidation of new carotenoid structures and the characterization of synthetic products. Spectroscopic data for individual carotenoids may be found in many of the papers already cited. The papers quoted in this section are those which are concerned largely or entirely with one or more of the physical methods used for the separation, assay, and spectroscopic analysis of carotenoids and related compounds. 

Figure 3 Selected carotenoid structures from bacteria, algae, plants, and animals, and of precursors and metabolic products with biologic function. The lUPAC numbering is given for lycopene (top right).

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