Carotenoids radicals

MORTENSEN A, SKIBSTED L H (1997) Relative stability of carotenoid radical cations and homologue tocopheroxyl radicals. A real time kinetic study of antioxidant hierarchy. FFBS Letters, 417, 261-6. 

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. 

El-Agamey, A., Carotenoid radical chemistry and antioxidant/pro-oxidant properties. Arch. Biochem. Biophys., 430, 37, 2004. 

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... 

Burke M, Edge R, Land EJ, McGarvey DJ, and Truscott TG. 2001. One-electron reduction potentials of dietary carotenoid radical cations in aqueous micellar environments. FEBS Letters 500(3) 132-136. Bystritskaya EV and Karpukhin ON. 1975. Effect of the aggregate state of a medium on the quenching of singlet oxygen. Doklady Akademii Nauk SSSR 221 1100-1103. 

In this chapter, various EPR techniques that have been used to study the carotenoid radical cations and neutral radicals will be described. These methods with references are given in Table 9.1. 

Photoinduced electron transfer in frozen solutions Carotenoid radical cations formed by chemical oxidation with I2... 

EPR studies of host-guest complexes of carotenoids Measuring distances between carotenoid radicals and distant metals in matrices by using ESEEM methods and pulsed EPR relaxation techniques EPR studies of radical cations on activated alumina and silica-alumina... 

The ferric ion is often used to form the carotenoid radical cation. However, care must be taken to control the concentration of the ferric ion relative to that of the carotenoid. Several existing equilibria have been studied by EPR, as well as NMR, LC-MS, and optical techniques. These studies have shown the following equilibria (Scheme 9.2) depending on the concentrations of Fe3+, Fe2+, and Ck relative to that of the neutral carotenoid and its radical cation and dication. 

Adsorption of carotenoids on activated silica-alumina results in their chemical oxidation and carotenoid radical formation. Tumbling of carotenoid molecules adsorbed on solid support is restricted, but the methyl groups can rotate. This rotation is the only type of dynamic processes which is evident in the CW ENDOR spectrum. 

Carotenoid radical formation and stabilization on silica-alumina occurs as a result of the electron transfer between carotenoid molecule and the Al3+ electron acceptor site. Both the three-pulse ESEEM spectrum (Figure 9.3a) and the HYSCORE spectrum (Figure 9.3b) of the canthaxanthin/ A1C13 sample contain a peak at the 27A1 Larmor frequency (3.75 MHz). The existence of electron transfer interactions between Al3+ ions and carotenoids in A1C13 solution can serve as a good model for similar interactions between adsorbed carotenoids and Al3+ Lewis acid sites on silica-alumina. 

Using different DFT functionals and basis sets (Focsan et al. 2008, Lawrence et al. 2008) it was confirmed that the isotropic ()-methyl proton hyperfine couplings do not exceed 9MHz for the carotenoid radical cation, Car-. DFT calculations of neutral carotenoid radicals, Car formed by proton loss (indicated by ) from the radical cation, predicted isotropic P-methyl proton couplings up to 16 MHz, a fact that explained the large isotropic couplings observed by ENDOR measurements for methyl protons in UV irradiated carotenoids supported on silica gel, Nafion films, silica-alumina matrices, or incorporated in molecular sieves (Piekara-Sady et al. 1991, 1995, Wu et al. 

CW ENDOR spectrum measurements carried out at 120 K (the optimum temperature for measuring resolved CW ENDOR powder spectra of carotenoid radicals) shows resolved lines from the P-methyl hfc (Piekara-Sady et al. 1991,1995, Wu et al. 1991, Jeevarajan et al. 1993b) (see Figure 9.5). The lines above 19 MHz are due to neutral radicals according to DFT calculations (Gao et al. 2006). 

Determination of g-tensor components from resolved 327-670 GHz EPR spectra allows differentiation between carotenoid radical cations and other C-H jt-radicals which possess different symmetry. The principal components of the g-tensor for Car"1 differ from those of other photosynthetic RC primary donor radical cations, which are practically identical within experimental error (Table 9.2) (Robinson et al. 1985, Kispert et al. 1987, Burghaus et al. 1991, Klette et al. 1993, Bratt et al. 1997) and exhibit large differences between gxx and gyy values. 

Photo-oxidation of carotenoids in Ni-MCM-41 produces an intense EPR signal (Figure 9.11) with -value 2.0027 due to the carotenoid radical another, less intense EPR signal, with =2.09 is attributed to an isolated Ni(I) species produced as a result of electron transfer from the carotenoid molecule to Ni(II). It has been reported that Ni(I) ions prepared upon reduction of Ni(II)-MCM-41 by heating in a vacuum or in dry hydrogen exhibits an EPR spectrum with , =2.09 and N=2.5... 

Figure 9.13 compares X-band EPR spectra of Fe-MCM-41 before (a) and after (b) and (c) carotenoid adsorption. The sample with incorporated Car exhibits a signal with g=2.0028 + 0.0002, characteristic of carotenoid radical cation prior to irradiation (Figure 9.13b). Irradiation of the samples at 365 nm (77 K) increases the Car 1 signal intensity (Figure 9.13c). The X-band experiments (Figure...

Carotenoids incorporated in metal-substituted MCM-41 represent systems that contain a rapidly relaxing metal ion and a slowly relaxing organic radical. For distance determination, the effect of a rapidly relaxing framework Ti3+ ion on spin-lattice relaxation time,and phase memory time, Tu, of a slowly relaxing carotenoid radical was measured as a function of temperature in both siliceous and Ti-substituted MCM-41. It was found that the TM and 7) are shorter for carotenoids embedded in Ti-MCM-41 than those in siliceous MCM-41. 

D is the dipole-dipole interaction between the slow relaxing carotenoid radical and the fast relaxing Ti3+ ion r is the interspin distance... 

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). 

Deng, Y. (1999). Carotenoid radical cations and dications studied by electrochemical, optical, and flow injection analysis Lifetime, extended chain conjugation, and isomerization properties. Ph.D dissertation, The University of Alabama, Tuscaloosa, AL. 

Deng, Y., G. Gao et al. (2000). Effects of polyene chain length and acceptor substituents on the stability of carotenoid radical cations. J. Phys. Chem. B 104 5651-5656. 

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