Carotenoid cation radical ENDOR

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. 

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],... 

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... 

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. 

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. 

Konovalova, T. A., S. A. Dikanov et al. (2001a). Detection of anisotropic hyperfine components of chemically prepared carotenoid radical cations ID and 2D ESEEM and pulsed ENDOR study.. /. Phys. Chem. B 105 8361-8368. 

Piekara-Sady, L., A. S. Jeevarajan et al. (1995). ENDOR study of the (7,7 -dicyano)- and (7 -phenyl)-7 -apo-carotene radical cations formed by UV photolysis of carotenoids adsorbed on silica gel. J. Chem. Soc. Faraday Trans. 91 2881-2884. 

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... 

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