Carotenoid compounds structures

Some carotenoids have structures containing fewer than 40 carbon atoms and derived formally by loss of part of the C40 skeleton. These compounds are referred to as apocarotenoids when carbon atoms have been lost from the ends of the molecule or as norcarotenoids when carbon atoms have been lost formally from within the chain. These modifications are caused by oxidative degradation at the level of the terminal rings either by nonspecific mechanisms (lipoxygenase, photo-oxidation) or by... 

Degraded Carotenoids. Several compounds have been described and characterized which are related structurally to carotenoids and may be biodegradation products of carotenoids. The structure of the vitamin A dimer kitol has been determined as... 

Figure 22-5 Structures and partial biosynthetic pathways for a few of the more than 600 known carotenoid compounds. The origin of some hydrogen atoms from mevalonate is shown, using the numbering for mevalonate. The numbering system for C40 carotenoids is also indicated.

Leuenberger, H. G., Boguth, W., Widmer, E., and Zell, R. 1976. Synthesis of optically active natural carotenoids and structurally related compounds. I. Synthesis of chiral key compound (4A 6A)-4-hydioxy-2,2,6-trimethylcyclohexanone. Helvetica Chimica... 

Degraded Carotenoids. Syntheses have been reported for several compounds structurally related to carotenoids. Many of the procedures used may be relevant to the construction of carotenoid end-groups. 

Mayer, H., and Rlittimann, A.. Synthesis of optically active, natural carotenoids and structurally related compounds. Part 4. Synthesis of (3E,37f,6 /f)-lutein, Helv. Chim. Acta, 63, 1451, 1980. 

Carotenoids represent one of the broadest groups of natural antioxidants (over 600 characterized structurally) with significant biological effects and numerous industrial applications. Lycopene is a typical acyclic carotene that serves as a starting metabolite for formation of carotenoid derivatives via specific routes (p-carotene, torulene, etc.). Xanthophylls include hydroxy-, methoxy- oxo-, epoxy-, carboxy-, and aldehydic groups (torularhodin, zeaxanthin, astaxanthin, etc.), which results in a broad structural variety of carotenoid compounds. 

Figure 2A. SFC-FID chromatograms of carotenoids separated on DB-17 stationary phase. See Figure 1 for key to compound structures.

Many compounds with fewer than 40 carbon atoms, but with carotenoid-like structures, are found in nature. Synthesis of these compounds, sometimes called apocarotenoids, appears to occur mainly by catabolism of carotenoids (Parry and Horgan, 1991). Apocarotenoids in the range of C9-C13 are found in plant essential oils, but others, ranging up to C30 compounds, are essentially nonvolatile. However, knowledge of the biochemistry of carotenoid metabolism is limited (Parry and Horgan, 1991). In vitro, photooxidation of carot-... 

Most carotenoid compounds are tetraterpenoids formally containing eight isoprene units. They owe their colour to the chain of conjugated double bonds, which occurs in several basic structures and their combinations. 

The absorption and transport processes of many of the phytochemicals present in food are complex and not fully understood, and prediction of their bioavailability is problematic. This is particularly true of the lipid-soluble phytochemicals. In this chapter the measurement of carotenoid bioavailability will be discussed. The carotenoids serve as an excellent example of where too little understanding of food structure, the complexity of their behaviour in foods and human tissues, and the nature and cause of widely different individual response to similar intakes, can lead to misinterpretation of study results and confusion in our understanding of the relevance of these (and other) compounds to human health. 

It will be appreciated that the delivery of nutrients from foods is attenuated by the structure of the food and the way in which it is digested. Thus, delivery from the food structure occurs over the same timescale as gastric emptying. Carotenoids, and other compounds, isolated from the food structure are generally emptied from the stomach and absorbed more rapidly. These different rates of delivery may have profound effects on subsequent metabolism. 

Subsequent cyclizations, dehydrogenations, oxidations, etc., lead to the individual naturally occurring carotenoids, but little is known about the biochemistry of the many interesting final structural modifications that give rise to the hundreds of diverse natural carotenoids. The carotenoids are isoprenoid compounds and are biosynthesised by a branch of the great isoprenoid pathway from the basic C5-terpenoid precursor, isopentenyl diphosphate (IPP). The entire biosynthesis takes place in the chloroplasts (in green tissues) or chromoplasts (in yellow to red tissues). 

FIGURE 3.3.1 Chemical structures of carotenoid oxidation products occurring in nature. The compound numbers correspond to those cited in Britton, G. et al., Carotenoids Handbook ... 

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. 

Bioactive compounds, such as carotenoids have strong antioxidative properties and are used as efficient radical scavengers. In some natural sources several carotenoid isomers can be found, which differ in their biochemical activities such as bioavailability or antioxidation potency. Knowing the structure and concentration of each stereoisomer is crucial for an understanding of the effectiveness... 

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