Pulsed EPR and DFT Characterization of Radicals Produced by Photo-Oxidation of Zeaxanthin and Violaxanthin on Silica−Alumina

Abstract

Pulsed electron nuclear double resonance (ENDOR) and two-dimensional (2D)-hyperfine sublevel correlation spectroscopy (HYSCORE) studies in combination with density functional theory (DFT) calculations revealed that photo-oxidation of natural zeaxanthin (ex Lycium halimifolium) and violaxanthin (ex Viola tricolor) on silica-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 at position 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. DFT calculations predict the largest isotropic beta-methyl proton hyperfine couplings to be 8 to 10 MHz for Car*+, in agreement with previously reported hyperfine couplings for carotenoid pi-conjugated radicals with unpaired spin density delocalized over the whole molecule. Anisotropic alpha-proton hyperfine coupling tensors determined from 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 carotenoid neutral radicals, in agreement with the analysis of the pulsed ENDOR data. The formation of the neutral radical of zeaxanthin should provide another effective nonphotochemical quencher of the excited state of chlorophyll for photoprotection in the presence of excess light.

Influence of pH on the decay of β-carotene radical cation in aqueous Triton X-100: A laser flash photolysis study

“…(4)) [24,46]. Similar results were reported for proton loss from the zeaxanthin radical cation (Chart S1) [47].…”

“…both pathways will form carotenoid neutral radicals (b-CAR Å and HO-b-CAR-H Å ) (Scheme 3). Recently, Kispert et al [24,[46][47][48][49][50] have extensively investigated the stabilities of different carotenoid neutral radicals formed via proton loss from various positions. For example; based on theoretical calculations and experimental results, the most energetically favorable positions for proton loss from b-CAR-H Å+ , to form b-CAR Å , are 4, 4 0 and methyl groups at 5 and 5 0 positions ( Fig.…”

Water soluble biocompatible vesicles based on polysaccharides and oligosaccharides inclusion complexes for carotenoid delivery

“…These processes have been previously studied in detail in homogeneous solutions (Polyakov et al, 2001b;. Carotenoid radicals, that are intermediate products in these reactions, attract significant attention since their properties are extremely important for understanding the molecular mechanisms of carotenoid activity Konovalova, Dikanov, Bowman, & Kispert, 2001;Focsan et al, 2008). Another fundamental property of the carotenoids is their ability to form aggregates in the presence of water.…”

EPR Study of the Astaxanthin n-Octanoic Acid Monoester and Diester Radicals on Silica–Alumina

“…6,7 Carotenoid (1.7 mg of astaxanthin, 2.3 mg of n-octanoic acid monoester, or 2.6 mg of n-octanoic acid diester of astaxanthin) solutions in 0.5 mL of CH 2 Cl 2 (6 × 10 −3 M) were degassed in 4 mm quartz EPR tubes by three freeze−pump−thaw cycles. Fresh activated silica−alumina was then added to the carotenoid solution in the EPR tube until the carotenoid solution became clear.…”

“…This charge-transfer quenching mechanism has also been observed in light harvesting minor complexes CP24, CP26, and CP29. 3−5 Recently, an additional step has been proposed: 6 proton loss from Zea •+ to form the #Zea • neutral radical (proton loss indicated by #) which could act as a radical trap and be a very efficient quencher of chlorophyll excited states. Lutein radical cation could give a similar quenching reaction because proton loss can readily occur from its cyclohexene ends, but the terminal rings of 9′-cis neoxanthin or violaxanthin radical cations cannot readily lose a proton.…”