Carotenoid radicals generation

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. "... 

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... 

Carotenoid radical intermediates generated electrochemically, chemically, and photochemically in solutions, on oxide surfaces, and in mesoporous materials have been studied by a variety of advanced EPR techniques such as pulsed EPR, ESEEM, ENDOR, HYSCORE, and a multifrequency high-held EPR combined with EPR spin trapping and DFT calculations. EPR spectroscopy is a powerful tool to characterize carotenoid radicals to resolve -anisotropy (HF-EPR), anisotropic coupling constants due to a-protons (CW, pulsed ENDOR, HYSCORE), to determine distances between carotenoid radical and electron acceptor site (ESEEM, relaxation enhancement). 

Wu, Y., L. Piekara-Sady et al. (1991). Photochemically generated carotenoid radicals on Nafion film and silica gel An EPR and ENDOR study. Chem. Phys. Lett. 180 573-577. 

These assays measure the level of protection provided to the naturally occurring carotenoid derivative crocin from bleaching by the radical generator AAPH. The assay was originally suggested by Bors and others (1984) and modified by Tubaro and others (1998), who used it to show that plasma antioxidant capacity is deeply influenced by the consumption of wine. The addition of a sample containing chain-breaking antioxidants results in the decrease in the rate of crocin decay. The sample is monitored for 10 min at 443 nm. 

Finally, radical cations can be generated in solution by different types of pulse radiolysis225. Like PET, this is inherently a method for transient spectroscopic observations, but it has proved to be invaluable in investigations of dimer cations, e.g of polyenes, which form spontaneously upon diffusion of radical cations in the presence of an excess of the neutral parent compound, but a discussion of the electronic structure of such species is beyond the scope of this review. Pulse radiolysis is of interest in the present context because it allows the observation of large carotenoid radical cations which are difficult to create in solid-state or gas-phase experiments... 

The nature of the medium, hydrophilic or lipophilic, in which the antioxidant is to be effective is a further important question, which has been tackled with respect to human plasma by Yeum et al.465 They used ABAP as a hydrophilic radical generator and 2,2 -azobis(4-methoxy-2,4-dimethylvaleronitrile) as a lipophilic radical generator. In the former case, the rates of consumption of antioxidants decreased in the order, ascorbic acid > a-tocopherol > uric acid > lycopene > lutein > cryptoxan-thin > /1-carotene, whereas, in the latter case, a-tocopherol and carotenoids were depleted at similar rates, ahead of ascorbic and uric acid. The behaviour of melanoidins of different types under such conditions would be of interest. 

The decay of the carotenoid radical cation absorption of C +-P-C6o occurs on the micro second time scale in the frozen glass. It is accompanied by the rise of C-P-Ceo generated by charge recombination of the C -P-Ceo biradical, which is formed with a quantum yield of 0.07. The major component of the decay of the - C-P-Ceo transient has a time constant of 10 ps, which is a typical lifetime for a carotenoid triplet state. The absorption of C -P-Ceo " at 77 K does not decay exponentially, but an average decay rate of 7.5 x 10 s may be calculated from the data [155]. Time-resolved experiments have allowed detection of the EPR resonances of the C +-P-C6o biradical and C-P-Ceo- The spin-polarization of the carotenoid triplet spectrum verifies formation of this state by the radical pair... 

Our results support the hypothesis that IO2 may trigger astaxanthin biosynthesis, and that carotenoids serve an antioxidant role in the yeast by reacting with peroxyl radicals. Generation of IO2 may explain the increase in carotenoid formation mediated by O2 and blue light in P. rhodozyma (44,45). Short light pulses of 15 minutes resulted in transient increases in astaxanthin levels in P. rhodozyma, but the highest carotenoid yields were obtained by continuous illumination. 

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... 

While the generation of carotenoid radical cations was originally achieved by radiolytic processes they have now been prepared photochemically (Tinkler et al., 1996), chemically (Ding et al, 1988), and via electrochemical methods (Grant et al., 1988). In the electrochemical study both one-electron and two-electron oxidation was observed with the two-electron oxidation subsequently producing the radical cation via reactions of the type ... 

The sulfonyl radical CHjSOO also reacts with carotenoids to generate both the cation radical and some pre-cation radical intermediate species (Everett et al, 1996 Mortensen et al., 1997). In the recent study the loss of the ground state absorption led to complex kinetics and these results seem best interpreted by considering that an intermediate (possibly an ion pair) leads to the carotene radical and the adduct [CAR—RSOj] , which has absorption overlapping that of CAR, then undergoes a bimolecular process to give some unidentified product(s). 

A comprehensive resonance Raman study of carotenoid cation radicals generated from canthaxanthin (16) and various apocarotenoids is referred to [136]. The resonance Raman spectra of the carotenoid cation radicals were in general similar to the resonance Raman spectra of the excited triplet state of the carotenoid. Upon formation of the cation radical of canthaxanthin (16) and the apocarotenoids investigated, the C=C stretching vibrations were decreased by 30-40 cm 1 whereas the C-C stretching vibrations were increased by 15-30 cm 1 relative to the parent... 

Recent work in the area has concentrated on the reactions of carotenoids with peroxyl radicals, generated mainly by the thermal decomposition of azo-initiators that lead to a variety of products. " Most of these products seem to be apocarotenals or apocarotenons of various chain lengths produced by cleavage of a double bond in the polyene chain, such as P-apo-12 -carotenal, P-apo-14 -carotenal, P-apo-lO-carotenal, and P-apo-13-carolenone. Kennedy and Liebler " reported that 5,6-epoxy-p,p-carotene and 15,15 -epoxy-P,P-carotene and several unidentified polar products were formed by the peroxyl radical oxidation of P-carotene by the peroxyl radicals. 

The quenching reactions often generate another radical species, usually an antioxidant radical or radical ion, (though addition radicals are also possible) and these can then go on to react with other biomolecules or radicals. For example, carotenoid radical cations have been shown to oxidise the amino acids tyrosine and cysteine, so have pro-oxidant ability [141]. 

Because the carotenoids favour hydrophobic domains they are generally localised in the membranes and lipoproteins of animal cells. In this location they can influence the oxidation of membrane lipids and prevent the passage of free radicals from one cellular compartment to another. Thus, DNA in the nucleus is protected from intracellularly generated ROS by (at least) the nuclear membrane and from extracellular ROS by a number of membranes. Should ROS reach the nucleus, base oxidation can occur. The base most susceptible to oxidation is guanine, although all other bases can also be affected. The cell has the ability to detect damaged bases, excise them. 

Addition — The addition of a radical species such as a peroxy radical ROO or the hydroxyl radical HO" to the polyene chain could generate a carotenoid-adduct radical CAR + ROO —> CAR - OOR. 

Convincing evidence indicates that ROS generated both endogenously and also in response to diet and lifestyle factors may play a significant role in the etiology of atherosclerosis and CHD. Indeed, free radicals are responsible for LDL oxidation, which is involved in the initiation and promotion of atherosclerosis. Thus, protection from LDL oxidation by antioxidants such as carotenoids may lead to protection against human CHD. 

A second highly soluble diphosphate derivate, 3.17, was also produced (solubility 29mg/mL) its efficacy in an in vitro cancer agent was screened, and it proved to be the most active carotenoid ever tested in this system (Hix et al. 2005), and more potent than Cardax, 3.19 (Hix et al. 2004). Overall, the second-generation compounds showed increased promise over the prototypes in certain contexts, particularly those in which immediate radical scavenging by highly potent and soluble compounds are required. 

Spin trapping EPR technique and UV-Vis spectroscopy have been used (Polyakov et al. 2001b) to determine the relative rates of reaction of carotenoids with OOH radicals formed by the Fenton reaction in organic solvents. The Fe3+ species generated via the Fenton reaction... 

Carotenoid neutral radicals are also formed under irradiation of carotenoids inside molecular sieves. Davies and Mims ENDOR spectra of lutein (Lut) radicals in Cu-MCM-41 were recorded and then compared with the simulated spectra using the isotropic and anisotropic hfcs predicted by DFT. The simulation of lutein radical cation, Lut +, generated the Mims ENDOR spectrum in Figure 9.7a. Its features at B through E could not account for the experimental spectrum by themselves, so contribution from different neutral radicals whose features coincided with those of the experimental... 

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