EPR and ENDOR studies of carotenoid–solid Lewis acid interactions

Abstract

EPR and ENDOR results provide strong support for the formation of carotenoid radical cations on activated alumina and silicaÈalumina as a consequence of electron transfer from the adsorbed carotenoid molecules to the Lewis acid sites on these surfaces. This assignment is conÐrmed by the correlation between the AlIII Lewis acid site content on the activated surfaces and the maximum concentration of the generated carotenoid radical cations. The results are compared with those obtained for a solution of carotenoid and which serves as a model system. The ENDOR line at ca. 6.5 MHz obtained for the carotenoids AlCl 3 , adsorbed on solid supports and solution, is attributed to the high-frequency feature of a hyperÐne doublet, carotenoidÈAlCl 3 centred about the 27Al Larmor frequency. ENDOR detection of the hyperÐne doublet, instead of the single line at the Larmor frequency, indicates the formation of strong complexes between carotenoid molecules and Lewis acid sites on the surface.

Chemistry of carotenoid neutral radicals

“…Due to the fact that β-methyl groups rapidly rotate relative to the microfrequency even at 5 K and 670 GHz, 61 resolved ENDOR spectra of carotenoid radicals have been observed in frozen solutions and in powders for carotenoids adsorbed on Nafion film, 55,58 silica gel and on silica-alumina. 57 Further study 62 has shown that the electron is transferred from the carotenoid molecules adsorbed on the activated alumina or silica-alumina to the surface Al 3+ Lewis acid site, since 27 Al couplings were detected. Adsorption of carotenoids in activated silicaalumina 63,64 results in the formation of radical cations by electron transfer between carotenoid molecules and the Al 3+ electron acceptor.…”

“…66 But each methyl groups rotates, and this rotation is the only type of dynamic processes which is evident in the CW ENDOR spectra. 62,65 Davies pulsed ENDOR, 330 GHz EPR 61 , ESEEM and HYSCORE measurements 65 have been used in combination to examine the structures and features of these adsorbed radicals. Davies pulsed ENDOR and 330 GHz EPR 61 measurements from 3.3 to 80 K showed no lineshape changes, implying very rapid rotation of the methyl groups down to 3.3 K.…”

“…24,63,86 This explained the large isotropic couplings observed in ENDOR measurements of irradiated carotenoids supported on silica gel, Nafion films, silica alumina matrices or incorporated in molecular sieves. 55,56,58,62,79,81 The most energetically favorable proton loss from the radical cations occurs from methylene or methyl groups usually situated on the terminal rings, extending the conjugated system. As a consequence of deprotonation, the unpaired electron spin distribution changes so that larger methyl proton hyperfine constants occur for the neutral radicals (13-16 MHz) than for the radical cation (7-10 MHz).…”

Electrochemical and EQCM investigation of a selenium derivatized carotenoid in the self-assembled state at a gold electrode

“…Thus, EPR and ENDOR studies evidenced the occurrence of electron transfer from adsorbed carotenoids to the Lewis acid sites of various solid supports [14]. Photochemical generation of the cation radicals with carotenoids adsorbed on silica gel was also scrutinized [15].…”

Electron paramagnetic resonance analyses of biotransformation reactions with cytochrome P-450 immobilized on mesoporous molecular sieves

“…This material can be modified with different metals such as Cu II , Ti IV , Al III , etc., which give to it additional properties during the process of electron transfer (Jeevarajan et al 1993;Konovalova and Kispert 1998).…”