Carotene, apo

As described in the preceding paragraphs, oxidation products of carotenoids can be formed in vitro as a result of their antioxidant or prooxidant actions or after their autoxidation by molecular oxygen. They can also be found in nature, possibly as metabolites of carotenoids. Frequently encountered products are the monoepoxide in 5,6- or 5, 6 -positions and the diepoxide in 5,6 5, 6 positions or rearrangement products creating furanoid cycles in the 5,8 or 5, 8 positions and 5,8 5, 8 positions, respectively. Products like apo-carotenals and apo-carotenones issued from oxidative cleavages are also common oxidation products of carotenoids also found in nature. When the fission occurs on a cyclic bond, the C-40 carbon skeleton is retained and the products are called seco-carotenoids. 

A similar system, but with a more hindered porphyrin (tetramesitylporphyrin = tetraphenylporphyrin bearing three methyl substituents in ortho and para positions on each phenyl group), was tested for P-carotene oxidation by molecular oxygen. This system was chosen to slow the oxidation process and thus make it possible to identify possible intermediates by HPLC-DAD-MS analysis. The system yielded the same product families as with lycopene, i.e., (Z)-isomers, epoxides, and P-apo-carotenals, together with new products tentatively attributed to diapocarotene-dials and 5,6- and/or 5,8-epoxides of P-apo-carotenals. The oxidation mechanism appeared more complex in this set-up. 

Analogously prepared are the / -D-glucosyl ester of 8 -apo-/ -carotene-8 -oic acid (as imidazolide and triazolide, obtained in 81 and 66% yield, respectively) and vitamin A acid (as triazolide, obtained in 87% yield) tl95]... 

Piekara-Sady, L., A. S. Jeevarajan et al. (1995). ENDOR study of the (7,7 -dicyano)- and (7 -phenyl)-7 -apo-carotene radical cations formed by UV photolysis of carotenoids adsorbed on silica gel. J. Chem. Soc. Faraday Trans. 91 2881-2884. 

Wang, X. D. et al. (1991). Enzymatic conversion of beta-carotene into beta-apo-carotenals and retinoids by human, monkey, ferret and rat tissue. Arch. Biochem. Biophys. 285(1) 8-16. 

In addition to oxidation of retinol, retinoic acid may be formed by the /3-oxidation of apo-carotenals arising from the asymmetric cleavage of /3-carotene (Section 2.2.2.1). 

The Reaction Specificity of Carotene Dioxygenase Whereasthe principal site of carotene dioxygenase attack is the 15,15 -central bond of p-carotene, there is evidence that asymmetric cleavage also occurs, leading to formation of 8 -, 10 -, and 12 -apo-carotenals, as shown in Figure 2.4. These apo-carotenals are metabolized by oxidation to apo-carotenoic acids, which are substrates for /3-oxidation to retinoic acid and a number of other metabolites. 

Colors Several different colors used in dressings include (3-carotene, apo-carotenal, FD C colors, tumeric, and titanium dioxide. They are added to make pourable dressings eye appealing and to augment naturally occurring colors. 

TABLE 22. H NMR chemical shift differences (ppm) of olefinic protons of (aU- )-7 -aiyl-7 -apo- -carotens and yS./J-carotene ... 

Some of the differences between the electronic spectroscopy of simple polyenes and that of carotenoids are illustrated in Fig. 4 which compares the low-temperature absorption and fluorescence spectra of three heptaenes hexadecaheptaene (7a), a synthetic C30 spheroidene (7b), and an apo-carotene (7c). Spectra of hexadecaheptaene are well resolved with well-defined electronic origins, allowing the... 

The major differences between the optical spectroscopy of model polyenes and short apo-carotenes and the longer, more conjugated carotenoids employed in photobiology are illustrated in Fig. 5. Most striking is the aoss-over from the S,(2 Ag) So(l A ) emission observed for the apo-heptaene (7c) to die dominant, S,( 1 B ) - So( 1 Ag) fluorescence of the analogous apo-nonaene (9). Similar changes... 

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