Carotenoid radical reactions

The fundamental chemistry of carotenoid radicals and the reactions with oxidizing agents, peroxy radicals, etc., is important for evaluating the proposed actions of... 

Carotenoid radicals — Many of the important oxidations are free-radical reactions, so a consideration of the generation and properties of carotenoid radicals and of carbon-centered radicals derived from carotenoids by addition of other species is relevant. The carotenoid radicals are very short-lived species. Some information has been obtained about them by the application of radiation techniques, particularly pulse radiolysis. Carotenoid radicals can be generated in different ways. "... 

In the carotenoid radicals, the unpaired electron is highly delocalized over the conjugated polyene chromophore. This has a stabilizing effect and also allows subsequent reactions. The cation and anion radicals can be detected by their characteristic spectral properties, with intense absorption in the near-infrared region. 

The dioxygen molecule exists in two forms a triplet or ground state in which it is a stable biradical and a singlet or excited state in which it is not a radical. Reactions of carotenoids with singlet oxygen have already been presented in this chapter and we now focus on the reactions of carotenoids and oxygen in the ground or triplet state. 

Studies on carotenoid autoxidation have been performed with metals. Gao and Kispert proposed a mechanism by which P-carotene is transformed into 5,8-per-oxide-P Carotene, identified by LC-MS and H NMR, when it is in presence of ferric iron (0.2 eq) and air in methylene chloride. The P-carotene disappeared after 10 min of reaction and the mechanism implies oxidation of the carotenoid with ferric iron to produce the carotenoid radical cation and ferrous iron followed by the reaction of molecular oxygen on the carotenoid radical cation. Radical-initiated autoxidations of carotenoids have also been studied using either radical generators like or NBS.35... 

Martin, H.D. et al.. Chemistry of carotenoid oxidation and free radical reactions. Pure Appl. Chem., 71, 2253, 1999. 

In the context of diagenesis in recent anoxic sediments, reduced carotenoids, steroids, and hopanoids have been identified, and it has been suggested that reduction by sulhde, produced for example, by the reduction of sulfate could play an important part (Hebting et al. 2006). The partial reduction of carotenoids by sulfide has been observed as a result of the addition of sulfide to selected allylic double bonds, followed by reductive desulfurization. This is supported by the finding that the thiol in allylic thiols could be reductively removed by sulhde to produce unsaturated products from free-radical reactions (Hebting et al. 2003). 

The reaction of CARs with free radicals is much more complex and depends mostly on the nature of the free radical [RO ] rather than on the CAR. Certainly, at least four processes have been reported. Of course, in all four processes, the unpaired electron of the free radical is transferred to the CAR so that a new, carotenoid radical (or CAR adduct radical) is produced. ... 

The rate constants for the reactions of the arylperoxyl radicals with carotenoids were determined from the first-order kinetics of the formation of the carotenoid radicals produced (using a range of carotenoid concentrations). The three arylperoxyl radicals were all observed to react with carotenoids to yield the carotenoid radical cations via electron transfer. 

El-Agamey, A. and McGarvey, D.J. 2003. Evidence for a lack of reactivity of carotenoid radicals towards oxygen A laser flash photolysis study of the reactions of carotenoids with acylperoxyl radicals in polar and non-polar solvents. J. Am. Chem. Soc. 125 3330-3340. 

El-Agamey, A, Cantrell, A, Land, EJ, McGarvey, DJ, and Truscott, TG, 2004a. Are dietary carotenoids beneficial Reactions of carotenoids with oxy-radicals and singlet oxygen. Photochem Photobiol Sci 3, 802-811. 

Although LOX activity is important to the plant s defense against pathogens, there are negative aspects of the enzyme in foods. LOX activity and the resulting fatty acid hydroperoxide products initiate free radical chains that modify proteins (particularly residues of Trp, His, Cys, Tyr, Met, and Lys) as well as vitamins or their precursors (e.g., carotene and tocopherol). Evidence of such free radical reactions is often visibly observed as loss of carotenoid/chlorophyll pigments in improperly blanched frozen foods. Another consequence of these free radical reactions is the development of potent off-flavors, many of which originate from decomposition of the fatty acid hydroperoxide products. 

As mentioned above, the natural photosynthetic reaction center uses chlorophyll derivatives rather than porphyrins in the initial electron transfer events. Synthetic triads have also been prepared from chlorophylls [62]. For example, triad 11 features both a naphthoquinone-type acceptor and a carotenoid donor linked to a pyropheophorbide (Phe) which was prepared from chlorophyll-a. The fluorescence of the pyropheophorbide moiety was strongly quenched in dichloromethane, and this suggested rapid electron transfer to the attached quinone to yield C-Phe+-Q r. Transient absorption studies at 207 K detected the carotenoid radical cation (kmax = 990 nm) and thus confirmed formation of a C+-Phe-QT charge separated state analogous to those formed in the porphyrin-based triads. This state had a lifetime of 120 ns, and was formed with a quantum yield of about 0.04. The lifetime was 50 ns at ambient temperatures, and this precluded accurate determination of the quantum yield at this temperature with the apparatus employed. 

Black, H.S. and Lambert, C.S., Radical reactions of carotenoids and potential influence on UV carcinogenesis, in Oxidants and Antioxidants in Cutaneous Biology. Current Problems in Dermatology, vol. 29, Thiele, J. and Eisner, P., Eds., Karger, Basel, 2001, p. 157. 

The C +-P-Q state in triads such as 35 eventually recombines to the groimd state, unless it is harvested by subsequent reactions. The simplest possible recombination pathway involves electron transfer from the quinone radical anion directly to the carotenoid radical cation. However, this pathway can be very slow, even if thermodynamics are very favorable, because of the weak electronic coupling between the radical ions. In some cases, charge recombination has been found to fol-... 

As indicated in Figure I, wild-type bacterial reaction centers also contain a carotenoid polyene. This polyene is not involved as a donor or acceptor in the normal electron transfer sequence, although carotenoid radical cations have been observed spectroscopically in photosynthetic preparations under certain conditions [18,19]. In many of the artificial photosynthetic systems which will be discussed below, the carotenoid is used as a convenient secondary electron donor. Carotenoids do perform two important functions in photosynthesis. They provide photoprotection from singlet oxygen damage, and act as light-gathering antennas for the special pair (see Sections III and IV). 

Miscellaneous Physical Chemistry. Various aspects of the physical chemistry of /3-carotene and related carotenoids have been reported, including several theoretical calculations related to spectroscopic properties,investigations of carotenoid triplet states and triplet energies,studies of carotenoid radical ions, and examination of electron-transfer reactions between carotenoids and chlorophyll Two reviews offer brief surveys of the year s literature on the photochemistry of... 

Martin, H-D., Ruck, C., Schmidt, M., Sell, S., Beutner, S., Mayer, B., and Walsh, R. 1999. Chemistry of carotenoid oxidation and free radical reactions. Pure Appl. Chem. 71, 2253-2262. 

It is well established that unsatnrated fatty acids undergo oxidation, via a radical reaction mechanism. Carotenoids undergo similar reactions and indeed do this so readily they can act as antioxidants in food materials. This antioxidant ability of carotenoids derives from their ability to form a resonance stabilised free radical. In certain controlled conditions chemical oxidation of carotenoids can give rise to epoxide formation and isomerisation of this to a furanoxide (Wong,... 

Wu Y, Piekara-Sady L and Kispert LD(1991) Photochemically generated carotenoid radicals on Nafion film and silica gel An EPR and ENDOR study. Chem Phys Lett 180 573-577 Yeates TO, Komiya H, Chirino A, Rees DC, Allen JP and Feher G (1988) Structure of the reaction center from Rhodobacter sphaeroides R-26 and 2.4.1 Protein-cofactor (bacterio-chlorophyll, bacteriopheophytin, and carotenoid) interactions. Proc Natl Acad Sci USA 85 7993-7997 Young AJ (1991) The photoprotective role of carotenoids in higher plants. Physiol Plant 83 702-708 Young AJ and Frank HA (1996) Energy transfer reactions involving carotenoids Quenchingofchlorophyll fluorescence. J Photochem Photobiol B Biol 36 3-15... 

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. 

---