chapter  9
30 Pages

9Chapter Applications of EPR Spectroscopy to Understanding Carotenoid Radicals

Resonance (SEEPR) Techniques .......................................................................................... 161 9.3 Time-Resolved EPR (TREPR) ............................................................................................. 162 9.4 Photoinduced Electron Transfer in Frozen Solutions ........................................................... 163 9.5 Chemically Formed Carotenoid Radical Cations ................................................................. 164 9.6 Spin Trapping EPR Method .................................................................................................. 165 9.7 Supramolecular Complex Formation .................................................................................... 167 9.8 Carotenoid Interaction with Surroundings: ESEEM Method............................................... 168

9.8.1 Bonding of b-Carotene to Cu2+ in Cu-MCM-41 ...................................................... 168 9.9 EPR on Activated Silica-Alumina ........................................................................................ 169 9.10 DFT Calculations to Interpret EPR Spectra ......................................................................... 169 9.11 b-Methyl Protons from CW ENDOR: Advantage of Pulsed Davies

and Mims ENDOR ............................................................................................................... 172 9.12 a-Protons from HYSCORE Analysis ................................................................................... 174 9.13 g-Anisotropy: High-Field g-Tensor Resolution ..................................................................... 175 9.14 High-Field EPR Measurements of Metal Centers ................................................................ 176

9.14.1 Carotenoids in Ni-MCM-41 ...................................................................................... 176 9.14.2 Carotenoids in Fe-MCM-41...................................................................................... 178

9.15 Relaxation by Metals: Distance Measurements .................................................................... 181 9.16 Effect of Distant Metals on g-Tensor .................................................................................... 184 9.17 Dimers Detected by g-Tensor Anisotropy Variation ............................................................ 184 9.18 Conclusions ........................................................................................................................... 185 Acknowledgments .......................................................................................................................... 185 References ...................................................................................................................................... 185

Carotenoids (Car) are known antioxidants. Extensive electrochemical studies in solution have established the low oxidation potentials and demonstrated the formation in various media of carotenoid

radical cations (Car•+), dications (Car2+), and the loss of H+ to form the carotenoid neutral radical (#Car•) (Gao et al. 1996, Jeevarajan et al. 1996a) according to the following equations:

•Car Car e E

(9.1)

• 2Car Car e E

(9.2)

2Car Car 2Car K

(9.3)

dp 2Car Car H

#Car Car H K

(9.5)

# #Car e Car E

(9.6)

It has been demonstrated (Mairanovski et al. 1975, Park 1978, Grant et al. 1988, Chen 1991, Khaled 1992, Jeevarajan et al. 1994a-c, Jeevarajan 1995, Jeevarajan and Kispert 1996, Jeevarajan et al. 1996a, Gao et al. 1997, Deng 1999, Liu and Kispert 1999, Hapiot et al. 2001, Konovalov et al. 2002) that great care must be taken to eliminate any traces of water or oxygen during the electrochemical studies, in order to obtain reproducible results. Accurate oxidation potentials could be deduced (Hapiot et al. 2001) only if ts were made to cyclovoltammograms (CV) recorded over six orders of magnitude of sweep times. The more traditional way of recording CV (Mairanovskii et al. 1975, Park 1978, Grant et al. 1988, Chen 1991, Khaled 1992, Jeevarajan et al. 1994a-c, Jeevarajan 1995, Jeevarajan and Kispert 1996, Jeevarajan et al. 1996a, Gao et al. 1997, Deng 1999, Liu and Kispert 1999, Hapiot et al. 2001, Konovalov et al. 2002) gave oxidation potentials some 50-100 mV lower.