Carotenoid study

Carotenoid oxidation products, as carotenoids, may exert protective or detrimental effects on human health. Efforts must be made to try to identify them in vivo where they may appear in lower quantities than carotenoids. Studies of abiotic systems can provide great support for their identification and the comprehension of their stability and reactivity. 

Since it is easier to control and change the conditions of carotenoid studies carried out in model systems, information on degradation kinetics (reaction order model, degradation rate, and activation energy) and products formed are often derived from such studies. 

A large set of results obtained in recent years for various carotenoids (see, e.g., Simonyi et al. (2003) for review) suggests that planarity of the carotenoid molecule is crucial for aggregation. This hypothesis is supported by the observation that zeaxanthin and astaxanthin, both fairly planar molecules, form aggregates more readily than other carotenoids. Moreover, zeaxanthin and astaxanthin are the only two carotenoids studied so far that can, depending on preparation conditions, form exclusively either H- or J-aggregates (Billsten et al. 2005, Kopsel et al. 2005, Avital... 

From Table 14.6 it can be seen that, with the exception of astaxanthin (ASTA), the rate constants for the electron transfer reactions decrease for each carotenoid in the order 9-phenanthryl peroxyl > 1-naphthyl peroxyl > 2-naphthyl peroxyl. This order of reactivity should be related to the reduction potentials of the radicals, with 9-phenanthryl peroxyl having the highest reduction potential. The same order of reactivity for these three arylperoxyl radicals reacting with Trolox was shown by Neta and coworkers (Alfassi et al. 1995). The reactivities of all the carotenoids studied are similar... 

The work by Hill et al. also noted differences for ASTA compared with the other carotenoids studied. Its radical cation was not formed initially from CC1302 but was formed solely through the proposed addition radical. Unfortunately, LYC could not be studied due to its insolubility in TX 100 micelles. However, since LYC appears, from its quenching of 02 and its protection against N02 to be the most efficient natural carotenoid antioxidant, we repeated this work using 4% TX 405 TX 100 (4 1) mixed micelles for both 0-CAR and LYC (unpublished) and have observed LYC behaving in a different manner to the other carotenoids as there appears to be no conversion of the adduct to the radical cation. 

Fig. 1. All-trans-carotenoids studied in this contribution. They are labelled M9, Mil, M13 and M15 according the number of conjugated double bonds.

The interaction of carotenoids and carotenoid radicals with other anti-oxidants is of importance with respect to anti-oxidative and possibly pro-oxidative reactions of carotenoids. All the radical cations of the carotenoids studied reacted with vitamin C so as to repair the carotenoid (e.g. in methanol, CAR t AscH CAR + AscH -I- H ). hi polar environments the vitamin E radical cation is deprotonated (TOH —> TO -i- H ) and TO does not react with carotenoids, whereas in non-polar environments, TOH is converted into TOH by hydrocarbon carotenoids, hi aU solvents studied, singlet oxygen is efficiently quenched by carotenoids that have appropriate low-lying triplet energy levels O -i- CAR -> Oj -i- CAR". However, such reactions are stiU to be observed in vivo. 

Mortensen et al. (1997) and Everett et al. (1996) used pulse radiolysis to generate NO and showed that, for the five carotenoids studied ( 8-carotene, astaxanthin, lutein, zeaxanthin and lycopene), only electron transfer could be observed ... 

This is consistent with the data ofHill et al. (1995) who have shown that the of/S-car " in TX 100 is blue-shifted by 104 nm to 936 nm compared with 1040 nm in hexane (Dawe and Land, 1975). Such differences were found for all the carotenoids studied by Hill etal. (1995). 

Proposed structures and exhaustive references to some 560 naturally occurring carotenoids studied until 1986, including trivial and semisystematic IUPAC names [2] have been compiled [7], as well as more recent additions [12]. A critical, selective treatment with key references will soon appear [13]. Extensive books on isolation and analysis including worked examples [1], applied spectroscopy [14], total synthesis [15] and biosynthesis and metabolism [16] have been published. 

Capsanthin (16) and capsorubin (16a), the colourants in paprika oleoresin, although not produced by commercial synthesis have been prepared in the course of carotenoid studies (ref. 58). Capsanthin has been synthesised from p-citraurin ( 3-hydroxy-p-apo-8 -carotenal ) which is available from zeaxanthin (3R, 3R )-p-carotene-3,3 -diol), by oxidation with potassium permanganate (ref. 59). 

Carotenoid pigments also present other spectroscopic properties such as fluorescence and absorption of energy in the infrared (IR) region. Fluorescence is a property rarely present in the carotenoids, and in fact, only a few carotenoids fluoresce when they are excited at appropriated wavelengths (e.g., phytofluene). Therefore, fluorescence spectroscopy is not frequently used in carotenoid studies. Similarly, the use of... 

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