Isomeric /3-carotene

Pesek, C.A., Warthesen, J.J., and Taoukis, PS., A kinetic model for equilibration of isomeric 3-carotenes, J. Agric. Food Chem, 38, 41, 1990. 

CA Pesek, JJ Warthesen, PS Taoukis. A kinetic model for equilibrium of isomeric /3-carotenes. J Agric Food Chem 38 41-45,1990. 

Figure F2.4.1 Liquid chromatography/mass spectrometry (LC/MS) analysis of isomeric carotenes in a hexane extract from 0.5 ml human serum. Positive ion electrospray ionization MS was used on a quadrupole mass spectrometer with selected ion monitoring to record the molecular ions of lycopene, p-carotene, and a-carotene at m/z (mass-to-charge ratio) 536. A C30 HPLC column was used for separation with a gradient from methanol to methyl-ferf-butyl ether. The a -trans isomer of lycopene was detected at a retention time of 38.1 min and various c/ s isomers of lycopene eluted between 27 and 39 min. The all-frans isomers of a-carotene and P-carotene were detected at 17.3 and 19.3 min, respectively.

Hoffmaim-La Roche has produced -carotene since the 1950s and has rehed on core knowledge of vitamin A chemistry for the synthesis of this target. In this approach, a five-carbon homologation of vitamin A aldehyde (19) is accompHshed by successive acetalizations and enol ether condensations to prepare the aldehyde (46). Metal acetyUde coupling with two molecules of aldehyde (46) completes constmction of the C q carbon framework. Selective reduction of the internal triple bond of (47) is followed by dehydration and thermal isomerization to yield -carotene (21) (Fig. 10). 

In the BASF synthesis, a Wittig reaction between two moles of phosphonium salt (vitamin A intermediate (24)) and C q dialdehyde (48) is the important synthetic step (9,28,29). Thermal isomerization affords all /ra/ j -P-carotene (Fig. 11). In an alternative preparation by Roche, vitamin A process streams can be used and in this scheme, retinol is carefully oxidized to retinal, and a second portion is converted to the C2Q phosphonium salt (49). These two halves are united using standard Wittig chemistry (8) (Fig. 12). 

PApo-8 -carotenal. The specifications of this colorant. (38) were discussed earlier. P-Apo-8 -carotenal has provitamin activity with 1 g of the colorant equal to 1,200,000 lU of vitamin A. Like all crystalline carotenoids, it slowly decomposes ia air through oxidatioa of its coajugated double boads and thus must be stored ia sealed coataiaers uader an atmosphere of iaert gas, preferably under refrigeration. Also like other carotenoids P-apo-8 -carotenal readily isomerizes to a mixture of its cis and trans stereoisomers when its solutions are heated to about 60°C or exposed to ultraviolet... 

Conjugation is crucial not only for the colors we see in organic molecules but also for the light-sensitive molecules on which our visual system is based. The key substance for vision is dietary /3-carotene, which is converted to vitamin A by enzymes in the liver, oxidized to an aldehyde called 11-frans-retinal, and then isomerized by a change in geometry of the C11-C12 double bond to produce 11-cis-retinal. 

Although a -trans- and 9-cis-RA are only minor metabolites of ROL and (3-carotene, they display 100-1000-fold higher biological activity. Whereas all-trans-RA binds only to RARs, 9-cis-RA binds both RARs and RXRs. The stereoisomer of all-irarcs-RA, 13-cis-RA, exhibits a much lower affinity for RARs and RXRs and exerts its molecular effects mostly through its isomerization into all-irarcs-RA. 

Four cis isomers of P-carotene (13,15-di-di-, 15-cis-, l3-cis-, and 9-cis-) and three of a-carotene (15-di-, 13-di-, and 9-cis-) were formed during heating of their respective dll-trans carotene crystals at 50,100, and 150°C. Isomerization catalyzed by heat was considered as a reversible first-order degradation reaction — a trans-to-cis conversion two- to three-fold slower than the backward (cis-to-trans) reaction (Table 4.2.6). The 9-cis- and 13-di- were the major P-carotene isomers formed and the 13 -cis- formed at a two- to three-fold faster rate than O-cw-P-carotene. In this system, a-carotene showed lower stability than P-carotene (Table 4.2.6). The activation energy (EJ was not reported since practically no degradation was observed... 

On the other hand, isomerization of sil-trans P-carotene was found to be comparatively faster in a model containing methyl fatty acid and chlorophyll heated at 60°C (Table 4.2.6), resulting in 13-cw-P-carotene as the predominant isomer. The first-order degradation rate of P-carotene significantly decreased with the increased number of double bonds in the methyl fatty acid, probably due to competition for molecular oxygen between P-carotene and the fatty acid. Since the systems were maintained in the dark, although in the presence of air, the addition of chlorophyll should not catalyze the isomerization reaction. 

Among thermal processes, canning caused the largest trans-to-cis isomerization of provitamin A carotenoids, increasing the total cis isomers by 39% for sweet potatoes, 33% for carrots, 19% for collards, 18% for tomatoes, and 10% for peaches 13-di-P-carotene was the isomer formed in highest amonnts. ... 

Losses of 45 to 48% in the P-carotene contents and formation of cis isomers were also verified by pasteurization of carrot juice at 110 and 120°C for 30 sec. No significant effects on trans-to-cis isomerization of a- and P-carotene isomers were observed after acidification and heating of carrot juice at 105°C for 25 sec. In addition, an increase of only 3% in the cis isomers of provitamin A carotenoids was observed after orange juice pasteurization. " ... 

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. ... 

Chen, B.H., Chen, T.M., and Chien, J.T., Kinetic model for studying the isomerization of a- and (3-carotene during heating and illumination, J. Agric. Food Chem., 42, 2391, 1994. 

Aman, R., Schieber, A., and Carle, R., Effects of heating and illumination on trans-cis isomerization and degradation of (3-carotene and lutein in isolated spinach chlo-roplasts, J. Agric. Food Chem., 53, 9512, 2005. 

Chen, B.H. and Huang, J.H., Degradation and isomerization of chlorophyll a and (3-carotene as affected by various heating and illumination treatments, Food Chem., 62, 299, 1998. 

Beyer, R, Mayer, M., and Kleinig, K., Molecular oxygen and the state of geometric isomerism of intermediates are essential in the carotene desaturation and cyclization reactions in daffodil chromoplasts, Eur. J. Biochem. 184, 141, 1989. 

Masamoto, K. et al.. Identification of a gene required for cis-to-trans carotene isomerization in carotenogenesis of the cyanobacterium Synechocystis sp. PCC 6803, Plant Cell Physiol. 42, 1398, 2001. 

Hu, Y., Heshimoto, H., Moie, G., Hengartner, U., and Koyama, Y. 1997. Unique properties of the ll-cis and 11,11 -di-civ isomers of P-carotene as revealed by electronic absorption, resonance Raman and 1H and 13C NMR spectroscopy and by HPLC analysis of their thermal isomerization. J. Chem. Soc. Perkin Trans. 2 2699-2710. 

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