P-Carotene radical cation

Also employing the hybrid density functional theory method (B3LYP) on the P-carotene radical cation, more refined results were obtained when taking rotation of the C6-C7-single bond into consideration [95], Fig. 21. 

The current picture of the p-carotene radical cation is thus delocalisation of the unpaired spin delocalised over the entire conjugated... 

Carotenoids, in particular P-carotene, are well-known free-radical quenchers [78]. It has been shown that peroxy radicals can oxidise P-carotene to the corresponding radical cations [78]. Absorption spectra demonstrate the appearance of broad bands in the range 600-1000 nm with = 910 nm (e = 9.4 x lOVM/cm) during pulse radiolysis of solutions of P-carotene in tert-butyl alcohol - water mixtures containing nitrate [79]. These bands were attributed to P-carotene radical cations generated by electron abstraction from the substrate by NO radicals ... 

Mortensen and Skibsted (1996b) have shown that laser flash photolysis of carotenoids in chloroform leads to immediate bleaching of the carotene absorption and the concomitant formation of two near infrared absorbing species (A = 920 ran and 1000 nm for )3-Ctirotene). The species absorbing at about 1000 nm is the carotene radical cation ()3-car ) and, as with the carotene/CCljOj system noted above, the p-car is formed from the other species. The nature of the other species is not defined although an adduct/ion pair or a neutral carotene radical is proposed. 

Although carotene radical cations are too short-lived to determine their reversible reduction potentials directly, the (relative ease( of electron transfer for seven carotenoids has been determined by pulse radiolysis. In the series astaxanthin > 3-apo-8-8 -carotenal > canthaxantin > lutein > zeaxanthin > 3-carotene > lycopene, lycopene is the strongest reductant. It can, for example, reduce lutein cation radicals to lutein, whereas P-carotene cannot (Edge et al., 1998). 

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... 

Comproportionation equilibrium constants for Equation 9.3 between dications and neutral molecules of carotenoids were determined from the SEEPR measurements. It was confirmed that the oxidation of the carotenoids produced n-radical cations (Equations 9.1 and 9.3), dications (Equation 9.2), cations (Equation 9.4), and neutral ir-radicals (Equations 9.5 and 9.6) upon reduction of the cations. It was found that carotenoids with strong electron acceptor substituents like canthaxanthin exhibit large values of Kcom, on the order of 103, while carotenoids with electron donor substituents like (J-carotene exhibit Kcom, on the order of 1. Thus, upon oxidation 96% radical cations are formed for canthaxanthin, while 99.7% dications are formed for P-carotene. This is the reason that strong EPR signals in solution are observed during the electrochemical oxidation of canthaxanthin. 

Gao et al. (2006) considered the data on an electron double resonance spectra of the cation-radical in conjunction with the results of calculation within the DFT. The authors established that the methyl group at the double bond of the cyclohexene ring is responsible for deprotonation of the P-carotene cation-radical. This route of proton elimination produces the most stable radical leaving the Jt-conjugation system to be intact. Deprotonation at the polyene methyl groups would... 

Fig. (21). Potential energy curve for rotation around the C6-C7 and C6 -C7 bonds (A), and the effect of this rotation on selected bond distances (B) of p-carotene cation radical. Reprinted with permission from [95], Copyright 2001 American Chemical Society.

Under particular conditions, P-carotene (1) serves as an alternative electron donor in the PS2 reaction centre, coping with highly oxidised intermediates formed during water oxidation. This redox role of P-carotene (1) in PS2 is unique amongst photosynthetic reaction centres. An overview on the electron transfer processes in PS2, emphasising those involving carotenoids, has recently been presented [97]. Methods employed for studies of the P-carotene cation radical (12) in PS2 include Resonance Raman [36,98,99], FT-IR [100], ENDOR [38] and EPR [37] spectroscopy, see under Methods below. 

The properties recorded for P-carotene cation radical in PS2 compare well with those of P-carotene cation radical prepared by other methods. 

Cation radical formation of P-carotene (1) and other carotenoids were formed by photoexcitation at 308 nm in CC14 solution [106]. Cation radical fonnation of p-carotene (1) via the triplet state in chloroform... 

A hypsochromic shift with increasing solvent polarity is observed. Absorption curves for the cation radical and dication of [3-carotene (1) are reported [109], see Fig. 23. The molar extinction coefficients for neutral p-carotene (1) and the charged species 11 and 12 are of comparable magnitude, Table 1, Fig. 23. INDO/S calculations for these spectra were performed and discussed. 

The EPR spectrum of the P-carotene cation radical (12) imbedded in a mesoporous silica matrix was recently reported [123],... 

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]. 

It is interesting to note that the dication structure 11 elucidated by NMR incorporates elements predicted by theoretical calculations including a pair of charged solitons (47, 48) and reversal of double and single bonds in the central part of the molecule (47, 48). Rotated C-6,7 and C-6 ,7 single bonds as calculated for the P-carotene cation radical (12), Fig. 21, was established for the dication 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]. 

It was observed that electrochemical oxidation of all-trans P-carotene (1) and canthaxanthin (16) in CH2CI2 leads to significant trans-cis isomerisation [105]. It was suggested that the isomerisation mechanism involved cation radicals and/or dications which could easily undergo geometrical isomerisation. This proposal was supported by AMI molecular orbital calculations, which showed that the energy barrier from trans to cis is much lower in the cation radical and dication species than in the neutral carotenoid [105]. 

Already in 1975 were anion radicals shown to exhibit strong NIR absorption at ca. 150 nm shorter wavelengths than cation radicals [118]. The radical anions of p-carotene (1) and lycopene (24) exhibited w,x in hexane solution at 880 nm and at 950 nm respectively. Subsequent studies resulted in similar conclusions. The wavelength maxima of the absorption bands were linearly dependent on the number of conjugated double bonds [151]. Hypsochromic shifts were observed in methanol relative to hexane solutions [118,151],... 

Figure 5.2.8 Near-infrared spectrum of the P-carotene cation radical as obtained by pulse radiolysis in N O-saturated micelles (Hill, 1997).

NO is known to be a moderately strong one-electron oxidant with the redox potential ° (NO /NO ") = + 1.04 V). The absolute rate constant [NO + P-carotene] is (1.1 0.1) X 10 M/s. The antioxidant properties of P-carotene not only reflect rates of free-radical scavenging, but also the reactivity of the resultant carotenoid radicals. Radical cations formed are highly resonance-stabilised and therefore relatively unreactive. 

The alumina surface is an extremely versatile and widely used support for studies in many areas of chemistry. To complete the review of the literature in the past two years, Lefondeur et al used EPR to study the paramagnetic properties of nickel nanoparticles deposited on alumina, while Konovalova et al used ID and 2D ESEEM and pulsed ENDOR to study the nature of the adsorbed canthaxanthin and 8 -apo-P-caroten-8 -al radical cations on an activated silica-alumina surface. Both of these excellent and thorough papers describe in detail the interpretations of the EMR data in relation to the role of the surface. 

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