| Abstract
| - Pulsed electron nuclear double resonance (ENDOR) and two-dimensional (2D)-hyperfine sublevel correlationspectroscopy (HYSCORE) studies in combination with density functional theory (DFT) calculations revealedthat photo-oxidation of natural zeaxanthin (ex Lycium halimifolium) and violaxanthin (ex Viola tricolor) onsilica−alumina produces the carotenoid radical cations (Car•+) and also the neutral carotenoid radicals (#Car•)as a result of proton loss (indicated by #) from the C4(4‘) methylene position or one of the methyl groups atposition C5(5‘), C9(9‘), or C13(13‘), except for violaxanthin where the epoxide at positions C5(5‘)−C6(6‘)raises the energy barrier for proton loss, and the neutral radicals #Car•(4) and #Car•(5) are not observed. DFTcalculations predict the largest isotropic β-methyl proton hyperfine couplings to be 8 to 10 MHz for Car•+,in agreement with previously reported hyperfine couplings for carotenoid π-conjugated radicals with unpairedspin density delocalized over the whole molecule. Anisotropic α-proton hyperfine coupling tensors determinedfrom the HYSCORE analysis were assigned on the basis of DFT calculations with the B3LYP exchange-correlation functional and found to arise not only from the carotenoid radical cation but also from carotenoidneutral radicals, in agreement with the analysis of the pulsed ENDOR data. The formation of the neutralradical of zeaxanthin should provide another effective nonphotochemical quencher of the excited state ofchlorophyll for photoprotection in the presence of excess light.
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