Canthaxanthin cation radical

Piekara-Sady, L., S. A. Jeevarajan et al. (1993). An ENDOR study of canthaxanthin cation radical in solution. Chem. Phys. Lett. 207 173-177. 

EPR data for the canthaxanthin cation radical (46), more recently generated in organic matrices, has been published [123-125]. A symmetrical unresolved EPR line at 9 GHz was assigned to a 7i-radical cation with electron density throughout the polyene chain [124],... 

The ENDOR spectrum of the canthaxanthin cation radical (46) generated electrochemically in CH2CI2 was first reported in 1993 [129], The isotropic hyperfine coupling constants were in reasonable agreement with calculated values. 

The stoichiometry corresponds to a previous report for crystals of the complex from a benzene solution [115]. Cation radicals of (3-carotene (1) and canthaxanthin (16) were again reported to be selectively formed in solutions by using iodine [109],... 

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... 

An intermediate role of cation radicals in cis-trans isomerisation of carotenoids has been considered [138]. AMI molecular orbital calculations show that the energy barrier of cis-trans isomerisation are much lower in cation radicals ( 20 kcal/mol) and dications ( 0 kcal/mol) than in neutral carotenoids ( 55 kcal/mol). HPLC analyses of the product mixture after bulk electrolysis of [3-carotene (1), canthaxanthin (16) and apocarotenoids showed the presence of 5-cis, 13-cis, 9-cis, 9,13-dicz s and all-trans isomers [138]. 

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]. 

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. 

The Davies pulsed ENDOR spectrum of canthaxanthin oxidized on silica-alumina measured in the temperature range of 3.3-80K showed no lineshape changes, which is in agreement with previous 330 GHz EPR studies of canthaxanthin radical cations (Konovalova et al. 1999). This implies very rapid rotation of the methyl groups down to 3.3 K. 

FIGURE 9.8 HF-EPR spectra of canthaxanthin radical cation adsorbed on silica-alumina (solid line)—experimental spectrarecorded at5 K (dotted line)—simu lated spectra using g-tensor values g = 2.0032 and g =gyy = 2.0023 and linewidth of 13.6G. (From Konovalova, T.A., J. Phys. Chem. B, 103, 5782, 1999. With permission.)... 

The 327-670 GHz EPR spectra of canthaxanthin radical cation were resolved into two principal components of the g-tensor (Konovalova et al. 1999). Spectral simulations indicated this to be the result of g-anisotropy where gn=2.0032 and gi=2.0023. This type of g-tensor is consistent with the theory for polyacene rc-radical cations (Stone 1964), which states that the difference gxx gyy decreases with increasing chain length. When gxx-gyy approaches zero, the g-tensor becomes cylindrically symmetrical with gxx=gyy=g and gzz=gn. The cylindrical symmetry for the all-trans carotenoids is not surprising because these molecules are long straight chain polyenes. This also demonstrates that the symmetrical unresolved EPR line at 9 GHz is due to a carotenoid Jt-radical cation with electron density distributed throughout the whole chain of double bonds as predicted by RHF-INDO/SP molecular orbital calculations. The lack of temperature... 

Comparison of g-Values for Various Radical Cations with Those Observed for Canthaxanthin Radical Cation... 

Konovalova, T. A., J. Krzystek et al. (1999). 95-670GHz EPR studies of canthaxanthin radical cation stabilized on a silica-alumina surface. J. Phys. Chem. B 103 5782-5786. 

Electronic Absorption Spectroscopy. Absorption spectra have been obtained for radical cations and anions generated from a number of carotenoids [phytoene (7,8,ll,12,7, 8, ll, 12 -octahydro-i//,i/ -carotene) (135) and canthaxanthin ()3,/3-carotene-4,4 -dione) (130)] and related polyenes [7,7 -dihydro- -carotene (131),... 

Miscellaneous Physical Chemistry. A kinetic study has been made of the electrochemical reduction of /8-carotene. The photoelectron quantum yield spectrum and photoelectron microscopy of /3-carotene have been described. Second-order rate constants for electron-transfer reactions of radical cations and anions of six carotenoids have been determined. Electronic energy transfer from O2 to carotenoids, e.g. canthaxanthin [/8,/3-carotene-4,4 -dione (192)], has been demonstrated. Several aspects of the physical chemistry of retinal and related compounds have been reported, including studies of electrochemical reduction, the properties of symmetric and asymmetric retinal bilayers, retinal as a source of 02, and the fluorescence lifetimes of retinal. Calculations have been made of photoisomerization quantum yields for 11-cis-retinal and analogues and of the conversion of even-7r-orbital into odd-TT-orbital systems related to retinylidene Schiff bases. ... 

ENDOR and NMR studies in conjunction with theoretical AMI and/or INDO studies (in particular RHF-INDO/SP) have contributed greatly to the understanding of the carotenoid radical cation and the description of the charge delocalization along the polyene chain (Piekara-Sady et al., 1991 Hand et al., 1993 Piekara-Sady et al., 1993,1995). An improved crystal structure of j3-carotene reported by Senge et al. (1992) has been used and provided the basis for the success of some of the theoretical descriptions. ENDOR studies have also been successfully performed on /3-carotene and canthaxanthin radicals produced photochemically on Nafion films and silica gel (Piekara-Sady et al., 1991 Wu et al., 1991), and... 

Experimental optical spectra in the NMR region are reported for protonated adducts of (3-carotene (1), canthaxanthin (16) and an apocarotenal measured in benzene, CH2CI2 and acetonitrile solutions at room temperature [116]. Extinction coefficients were estimated. For 13-carotene (1) in the presence of 0.29 vol% trifluoroacetic acid at A,max 1022 nm in CH2CI2, s = 1.86 105 was determined. The position of A ax for protonated 1 was in the same region as that of radical cations and was only slightly dependent on the solvent. 

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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