Carotenoid radical cations

Ding, R., J. L. Grant et al. (1988). Carotenoid cation radicals produced by the interaction of carotenoids with iodine. J. Phys. Chem. 92 4600-4606. 

Jeevarajan, A. S., L. D. Kispert et al. (1993b). An ENDOR study of carotenoid cation radicals on silica-alumina solid support. Chem. Phys. Lett. 209 269-274. 

Khaled, M., A. Hadjipetrou et al. (1990). Electrochemical and electron paramagnetic resonance studies of carotenoid cation radicals and dications Effect of deuteration. J. Phys. Chem. 94 5164—5169. 

Piekara-Sady, L., M. M. Khaled et al. (1991). Comparison of the INDO to the RHF-INDO/SP derived EPR proton hyperfine couplings for the carotenoid cation radical Experimental evidence. Chem. Phys. Lett. 186 143-148. 

Moreover, carotenoid cation radicals can be formed as a result of oxidation of carotenoids by iron ions, Fe(III) (Equation 15.9) (Polyakov et al., 2001) ... 

One possible mechanism responsible for cooperative action of antioxidants is reduction of a semi-oxidized carotenoid by another antioxidant. Carotenoid cation radicals can be reduced, and therefore recycled to the parent molecule, by a-tocopherol, ascorbate, and melanins (Edge et al., 2000b El-Agamey et al., 2004b) (Figure 15.5). Interestingly, lycopene can reduce radical cations of other carotenoids, such as astaxanthin, (3-carotene, lutein, and zeaxanthin (Edge et al., 1998). 

Jeevarajan AS, Kispert LD, Chumanov G, Zhou C and Cotton TM (1996c) Resonance Raman study of carotenoid cation radicals. Chem Phys Lett 259 515-522 Jeevarajan AS, Kispert LD, Avdievich N1 and Forbes MDE (1996d) Role of excited singlet state in the photooxidation of carotenoids a time-resolved Q-band EPR study. J Phys Chem 100 669-671... 

It was noted above that laser flash photolysis of carotene in chloroform as solvent, led to carotenoid cation radical production and a corresponding transient absorption in the infrared. However, Mortensen and Skibsted (1996b) have also shown that, in carbon tetrachloride as solvent, whilst the parent carotenoid was bleached, no infrared absorbing species arose. Possibly the neutral carotene radical (CAR ) was produced via hydrogen atom transfer ... 

Overall it is interesting to compare the different series of peroxyl radicals discussed above. The alkylperoxyl radicals react only very slowly to yield adducts and/or neutral carotene radicals by hydrogen atom transfer, the arylperoxyl radicals and NOj give carotenoid radical cations only and CCljO and CHjSOO give carotenoid cation radicals and, for most carotenoids, an intermediate which decays to... 

Grant JL, Kramer VJ, Ding R and Kispert LD (1988) Carotenoid cation radicals Electrochemical, optical and EPR study. J AmerChem Soc 110 2151-2157 Gust D, Moore TA, Moore AL, Jori G and Reddi E (1993) The photochemistry of Carotenoids Some photosynthetic and photomedical aspects. Ann New York Acad Sci 691 32-47 Hill TJ, Land EJ, McGarvey DJ, Schalch W, Tinkler JH and Tmscott TG (1995) Interactions between carotenoids and the CCI3O2 radical. J Am Chem Soc 117 8322-8326 Jeevarajan AS, Kispert LD, Chumanov G, Zhou C and Gotten TM (1996) Resonance Raman study of carotenoid radical cations. Chem Phys Lett 259 515-522 Lafferty J, Roach AC, Sinclair RS, Tmscott TG and Land EJ... 

Carotenoid cation radicals, which have lost one 71-electron from the polyene chain, have been demonstrated in PS2 [36-39], Moreover, the presence of carotenoid dications (lacking two 71-electrons from the polyene chain) has been suggested in photosynthetic reaction centres [40]. 

A carotenoid cation radical is in principle formed when one 71-electron from the polyene chain is removed. A priori delocalisation of the unpaired electron would be predicted. 

The electrochemical properties of carotenoids has been reviewed [102,103]. Electrochemical data for several naturally occurring carotenoids including oxidation potentials, reaction rate constants and kinetic equilibrium constants were discussed. Conventional electrochemical techniques such as cyclic voltammetry (CV) were treated. The oxidation potential of neutral carotenoids, corresponding to the formation of cation radicals were 0.50 - 0.72 V versus SCE, and that of carotenoid cation radicals, corresponding to the formation of dications were 0.52 - 0.95 V versus SCE. Further details are available [104,105],... 

For the characterisation of carotenoid cation radicals, there are four different spectroscopic techniques that have been most widely used ... 

The application of these techniques for studying carotenoid cation radicals are treated below. 

A characteristic feature of carotenoid cation radicals is their electronic absorption in the NIR region, e.g. [36,40,55,103,116,117], in contrast to... 

ENDOR is a spectroscopic method used in studies of free radicals [122], and is used in modem studies on carotenoid cation radicals. 

The orientation of a carotenoid cation radical on solid support can be studied by ENDOR [123]. Pulsed ENDOR has been used for studies on carotenoid cation radicals in PS2 [38],... 

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... 

Time-resolved resonance Raman was used in an investigation of carotenoid cation radicals and triplet states [137]. 

Spectroscopic and electrochemical properties of carotenoid cation radicals are already referred to. 

The effects of the length of the polyene chain and acceptor substituents on the stability of carotenoid cation radicals have been studied [117]. For esters of conjugated carotenoid carboxylic acids, the stability of then-cation radicals decreased with increasing length of the polyene chain. For conjugated aldehydes or nitriles there was no profound effect. 

Reactions of carotenoid cation radicals were treated under antioxidants. 

Neutral carotenoid Cation radical Dication Car Car+ Car2+... 

Whereas free radicals like carotenoid cation radicals are not amenable to NMR analysis, was the first complete NMR analysis of p-carotene dication (11) recently accomplished by our group [11]. 

The equilibrium between neutral carotenoid, cation radical and dication was already discussed [142,143], More recently the effect of electrolytes and temperature on carotenoid dications were studied [40]. The stability of the P-carotene (1) dication at -25°C in CHCI3 was remarkable, showing a decrease of less than 20% during 2h, as based on NIR spectra [11]. 

Carotenoid cation radicals are treated above. Considered here are spinless cations. 

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