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

Abstract

It was shown by EPR and UV–VIS studies that one-electron transfer reactions between carotenoids (β-carotene, 8′-apo-β-caroten-8′-al, canthaxanthin) and quinones [2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), tetrachlorobenzoquinone (CA)] include the formation of a charge-transfer complex (CTC), which exists in equilibrium with an ion–radical pair (Car +⋯Q −). UV spectra display a new absorption band in the near IR region (at 1030 nm) for the β-carotene–DDQ mixture which is assigned to a CTC. The absorption maximum of this band gradually shifts from 1030 nm to shorter wavelengths and finally corresponds to that of the β-carotene radical cation (1000 nm). A new absorption band at 340 nm, detected for all systems under study, is attributed to the product of quinone addition to carotenoid. The EPR spectra of the Car–DDQ mixture measured at 77 K when [Car]≤[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 Δ H p–p=4.3 G were detected. Electron nuclear double resonance (ENDOR) results indicate that these species contain both nitrogen and chlorine atoms. We attribute these signals to a carotenoid–quinone radical adduct. For chloranil paramagnetic species are generated only after irradiation. Both the carotenoid radical cation and the quinone radical anion were observed in this case. Analysis of the isolated reaction product made using the Beilstein test, 1 H -NMR and IR spectroscopies suggests that the major product is a carotenoid–hydroquinone ether. A mechanism of the Car–Q adduct formation is proposed.