One-electron transfer product of quinone addition to carotenoids EPR and optical absorption studies

It was shown by EPR and W-VIS studies that one-electron transfer reactions between carotenoids (beta -carotene, 8 ' -apo-beta -caroten-8 ' -al, cantha xanthin) and quinones [2,3-dichloro-5,6-dicyano- 1,4-benzoquinone (DDQ), te trachlorobenzoquinone (CA)] include the formation of a charge-transfer comp lex (CTC), which exists in equilibrium with an ion-radical pair (Car(.)+ Q( .-)). UV spectra display a new absorption band in the near m region (at 103 0 nm) for the beta -carotene-DDQ mixture which is assigned to a CTC. The ab sorption maximum of this band gradually shifts from 1030 nm to shorter wave lengths and finally corresponds to that of the beta -carotene radical catio n (1000 nm). A new absorption band at 340 nm, detected for all systems unde r study, is attributed to the product of quinone addition to carotenoid. Th e EPR spectra of the Car-DDQ mixture measured at 77 K when [Car] less than or equal to [DDQ] exhibit a structureless singlet line with g = 2.0066 +/- 0.0002 which is attributed to a CTC. Increasing the temperature gives rise to a new five-line signal with g = 2.0052 +/- 0.0002 and hyperfine coupling constant of 0.6 G due to the DDQ radical anion. At room temperature stable radicals (more than 24 h) with g = 2.0049 +/- 0.0002 and DeltaH(p-p) = 4.3 C were detected. Electron nuclear double resonance (ENDOR) results indicat e that these species contain both nitrogen and chlorine atoms. We attribute these signals to a carotenoid-quinone radical adduct. For chloranil parama gnetic species are generated only after irradiation. Both the carotenoid ra dical cation and the quinone radical anion were observed in this case. Anal ysis of the isolated reaction product made using the Beilstein test, H-1-NM R and IR spectroscopies suggests that the major product is a carotenoid-hyd roquinone ether. A mechanism of the Car-Q adduct formation is proposed. (C) 2001 Elsevier Science B.V. All rights reserved.