Carotenoid Radical Research Articles

Relative stability of carotenoid radical cations and homologue tocopheroxyl radicals. A real time kinetic study of antioxidant hierarchy

Real time detection following laser flash photolysis of transient carotenoid radical cations and tocopheroxyl radicals formed in chloroform and bleaching of the carotenoids has allowed interaction between carotenoids and tocopherols to be studied. It is found that α-, β-, and γ-tocopherol reduce all the carotenoid radical cations investigated whereas the δ-tocopheroxyl radical can be reduced by lycopene and β-carotene. Astaxanthin, canthaxanthin, and β-apo-8′-carotenal radical cations are scavenged rapidly by all four tocopherol homologues whereas the other carotenoid radical cations react much more slowly with the tocopherols. The results allow the antioxidant hierarchy to be established: α-tocopherol>lycopene∼β-tocopherol∼γ-tocopherol>β-carotene>zeaxanthin∼δ-tocopherol>lutein>echinenone≫canthaxanthin∼β-apo-8′-carotenal>astaxanthin.

Importance of Carotenoid Structure in Radical-Scavenging Reactions

The rate of (parallel) reaction between carotenoids and phenoxyl radical to yield (i) carotenoid/phenoxyl adducts and (ii) carotenoid radical cations (in effect regenerating the phenol by reduction) has been found to increase for an increasing number of (coplanar) conjugated double bonds in the carotenoid and to decrease in the presence of hydroxy and especially keto groups. The reactions to yield the carotenoid radical have half-lives of tenths of milliseconds, and when studied by transient visible and near-infrared absorption spectroscopy using nanosecond laser flash photolysis for radical generation, the relative rates in di-tert-butyl peroxide/benzene (7/3, v/v) at 20 °C were lycopene (1.66), β-carotene (1), zeaxanthin (0.79), lutein (0.70), and echinenone (0.68), while canthaxanthin and β-apo-8‘-carotenal hardly reacted and astaxanthin not at all. When formed, the carotenoid radicals are rather stable (second-order decay on a millisecond time scale). The lower the energy of the near-infrared transiti...

Free Radical Transients in Photobleaching of Xanthophylls and Carotenes

Carotenoicls in chloroform and carbon tetrachloriclc photobleach upon nanosecond laser flash photolysis in two steps: instantaneously and in a second-order reaction. The rate constant for second-order reaction (first-order in a solvent derived radical and first-order in (excess) ccirotenoid) is largest for carotenes (9.8·108 M-1 s-1 for β-carotene), intermediate for hydroxylated carotenoids, and smallest for carbonyl containing carotenoids (1.0·108 M-1 s-1 for astaxanthin) in chloroform at 20°C. Near infrared, ibsorbing transients are formed concomitant with pliotohleaching in chloroform (not detected in cxbon tetrachloride). A species formed instantaneously is tentatively identified as either a carotenoid/solvent adduct or an ion-pair. A second species is formed by decay of the instantaneously formed species and is identified as the carotenoid radical cation. This species is formed in a first-order reaction with a rate constant of approx. 5·104 s-1 and absorbing at longer wavelength than the precursor. The lifetime (second-order decay) of the interniediates appears to be longest for the carotenoids with the longest conjugated system. The results indicate that carotenes are better antioxidants than xantliophylls as the carotenes, at least in the present lipophilic solvents, react faster with free radicals.

Carotenoid Radical Cations, Dications and Radical Trications.

Carotenoid Radical Cations, Dications and Radical Trications.

Electrochemical, EPR and AM1 studies of acetylenic and ethylenic carotenoids

It has been shown, by cyclic voltammetry (CV) and simultaneous electrochemical and electron paramagnetic resonance (SEEPR) measurements of β-carotene (I) and 15,15′-didehydro-β-carotene (II) in dichloromethane solution, that the central triple bond increases the oxidation potentials by 237 ± 5 mV and decreases the solution EPR peak-to-peak linewidth (ΔHpp) by 1.7 G. Two synthetic carotenoids, 7,7′-diphenyl-7,7′-diapocarotene (III) and 7,7′-diphenyl-15,15′-didehydro-7,7′-diapocarotene (IV), close analogues of I and II, were also studied by CV and SEEPR methods. In contrast to compound I, two separate oxidation waves are observed for III, a feature previously found only for carotenoids containing electron-withdrawing substituents. The increase in oxidation potential of IV compared to III was similar to that of IIvs.I, but the decrease in ΔHpp(2.7 G) was considerably larger. Because the radical cation of III is adsorbed strongly on the platinum electrode in dichloromethane, complete analysis using the DigiSim® program was not possible; however, the oxidation potentials could be estimated. Structural parameters of neutral I–IV and their dications were calculated using the AM1 method, and the bond lengths and bond angles of I and II are in good agreement with crystallographic data. The magnitude of the EPR parameters, ΔHpp and the g factors, for the carotenoid radical cations of I–IV indicate a polyene π-radical cation structure with little unpaired electron density occurring near the terminal groups. The radical cation and dication deprotonate by loss of a proton from the chain methyl groups, with the former forming a polyene π-radical.

Investigation of Carotenoid Radical Cations and Triplet States by Laser Flash Photolysis and Time-Resolved Resonance Raman Spectroscopy: Observation of Competitive Energy and Electron Transfer

The first nanosecond time-resolved resonance Raman study of carotenoid radical cations is reported for the polyenes septapreno-β-carotene and 7,7‘-dihydro-β-carotene. In addition, previously unreported resonance Raman spectra of the ground and triplet states of these molecules are reported. The radical cations were generated following electron transfer quenching of triplet 1-nitronaphthalene in methanol and Triton X-100 micelles. The quenching of triplet 1-nitronaphthalene by these carotenoids involves solvent-dependent competition between energy transfer and electron transfer, and for both carotenoids, estimates are given for the efficiencies of these two processes in methanol and hexane. The resonance Raman spectra of septapreno-β-carotene ground and triplet states are consistent with spectra reported previously for other carotenoids. However, the resonance Raman spectrum of the triplet state of 7,7‘-dihydro-β-carotene displays an intensity profile not found in the triplet spectra of other carotenoids. In addition, the resonance Raman spectrum of the radical cation of 7,7‘-dihydro-β-carotene is quite distinct from that of septapreno-β-carotene. These observations are attributed to differences in electronic structure arising from septapreno-β-carotene having an odd number of conjugated double bonds while 7,7‘-dihydro-β-carotene, unusually, has an even number.

Coordinated Photoinduced Electron and Proton Transfer in a Molecular Triad

Excitation of carotenoid-porphyrin-quinone (C-P-Q) triads yields the porphyrin first excited singlet state, which decays by electron transfer to give a C-P[sup [center dot]+]-Q[sup [center dot]-] charge-separated state. Competing with rapid charge recombination is electron transfer from the carotenoid to produce a long-lived C[sup [center dot]+]-P-Q[sup [center dot]-] species. High quantum yields of the final state require tuning of electronic and thermodynamic factors to favor forward electron transfer over charge recombination. Triad 1 illustrates a new strategy for slowing charge recombination based on coupling photoinduced electron transfer to a change in proton chemical potential. The quantum yields and lifetimes of the final charge-separated states in the triads were assessed by monitoring the transient carotenoid radical cation absorptions. The results demonstrate that the yield of charge separation in multicomponent molecular photovoltaics can be increased by a coordinated electron and proton transfer process. It is also interesting that in 1 a substantial fraction of the intramolecular redox potential produced by photoinduced electron transfer is transformed into proton chemical potential. Elaboration of this concept could lead to photoinduced generation of proton motive force in a heterogeneous system. 24 refs., 3 figs.

ENDOR study of the (7′,7′-dicyano)- and (7′-phenyl)-7′-apo-β-carotene radical cations formed by UV photolysis of carotenoids adsorbed on silica gel

The radical cations of two synthetic carotenoids, 7′-7′-dicyano-7′-apo-β-carotene (I) and 7′-phenyl-7′-apo-β-carotene (II), have been prepared by UV irradiation of samples adsorbed on silica gel. The methyl proton hyperfine coupling constants were deduced from ENDOR powder spectra and assigned by RHF-INDO/SP calculations. The powder EPR spectra of the carotenoid radical cations formed photolytically by electron transfer are unresolved, with a peak-to-peak linewidth of 15 ± 1 G, irrespective of the donor or acceptor character of the terminal substituents.

Photochemically generated carotenoid radicals on Nafion film and silica gel: an EPR and ENDOR study

Carotenoid cation radicals have been observed at 120 K, by EPR and proton ENDOR measurements, to be formed upon 77 K photolysis of thin films of Nafion or silica gel coated with the carotenoids, β-carotene and canthaxanthin. The powder ENDOR spectra consist of resolvable lines due to couplings larger than 2.8 MHz but smaller than 17 MHz assigned to the methyl protons of the carotenoid cation radical and to an α-proton of the planar polyene chain.

Singlet photochemistry in model photosynthesis: identification of charge separated intermediates by Fourier transform and CW-EPR spectroscopies

Intramolecular electron transfer from the photoexcited state of a carotenoid-porphyrin-diquinone tetrad was studied by selective laser excitation with both Fourier transform and CW EPR spectroscopies. It is shown that the electron transfer occurs from the singlet state of the porphyrin constituent to produce the terminal benzoquinone radical anion and the carotenoid radical cation. This tetrad molecule can maintain the charge-separated state for a substantial period of time ({approximately} 1 {mu}s), allowing the characterization of the short-lived radicals. The derivative-like spectrum of the quinone radical anion and its dependence on the turning angle of the microwave pulse indicates that electron transfer proceeds via the singlet state.

Oxidation of Quercetin by Carotenoid Radical Generated in Illuminated Spinach Chloroplasts: The Effect of Ascorbate on Quercetin Oxidation

Oxidation of Quercetin by Carotenoid Radical Generated in Illuminated Spinach Chloroplasts: The Effect of Ascorbate on Quercetin Oxidation

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Dye-sensitized photo-oxidations of liposomes are inhibited by the presence of carotenoids in the membrane. Some carotenoid bleaching occurs during the reaction. It was shown that carotenoids are capable of inhibiting free-radical-induced lipid peroxidation in liposomes as well as singlet oxygen-induced lipid peroxidation. There is little direct effect of lipid peroxides on carotenoid bleaching during the photo-oxidative reactions. The carotenoid bleaching observed during the dye-photosensitized oxidations of the liposomes appears to be due to interactions between the carotenoid pigments and either radical intermediates from type I reactions or direct chemical reaction with singlet oxygen in a type II reaction. It was concluded that carotenoid pigments function as protective agents by quenching triplet sensitizers, singlet oxygen, and radical intermediates.

Flash-induced carotenoid radical cation formation in Photosystem II

Light excitation of chloroplasts at low temperature produces absorption changes ( ΔA) with a large positive peak at 990 nm and a bleaching around 480 nm. ΔA at 990 nm rises with t 1 2 = 0.6 ms at 20–77 K and remains largely stable. This signal is not observed when Photosystem II (PS II) photochemistry is blocked by reduction of the primary plastoquinone. It is observed also in purified PS II particles, in which case it could be shown that during a sequence of short flashes, the absorption at 990 nm rises in parallel with plastoquinone reduction measured at 320 nm. In chloroplasts the light-induced 990-nm ΔA at 77 K is increased under oxidizing conditions (addition of ferricyanide) and upon addition of 2-(3-chloro-4-trifluoromethyl)anilino-3,5-dinitrothiophene (ANT2p). At 21°C, flash excitation of chloroplasts or of PS II particles induces only a very small ΔA at 990 nm, even when this is measured with a 100-ns time resolution or when the material is preilluminated. In both materials, however, a large flash-induced ΔA takes place when various lipophilic anions are added. After a flash the signal rises in less than 100 μs and its decay varies with experimental conditions; the decay is strongly accelerated by benzidine. The difference spectrum measured in PS II particles includes a broad peak around 990 nm and a bleaching around 490 nm. These absorption changes are attributed to a carotenoid radical cation formed at the PS II reaction center. It is estimated that in the presence of lipophilic anions at room temperature, one cation can be formed by a single flash in 80% of the reaction centers. At cryogenic temperature approx. 8% of the PS II reaction centers can oxidize a carotenoid after a single flash.

Rapid fluorescence changes observed in chloroplasts: Their relationship to the O 2 evolving system

The time course of fluorescence yield was measured either during or after brief flashes given in a sequence to dark-adapted algae or isolated chloroplasts. 1. (A) The fluorescence kinetics during short (2–12μs) flashes indicated two light-driven reactions of similar quantum yield, one increasing the fluorescence yield, the other quenching it. The characteristics of the quencher were consistent with those of a carotenoid radical or triplet. 2. (B) The oscillations of fluorescence yield attained in each flash in a sequence given to dark-adapted chloroplasts were eliminated by 100 mM methylamine, which does not alter O 2 production during the flashes. This indicates that the fluorescence yield during a flash, although influenced by the S states, is not directly controlled by them. 3. (C) Measurements of the fluorescence yield from 80 to 1000 μs after individual saturating flashes in a sequence indicated the following: 1. 1. A previously unseen 35-μs rise occurs after the flash. The rise correlates with a lag in System II recovery. It is not eliminated by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), is accentuated by NH 2OH and is only a little dependent upon the S states. 2. 2. For the first two flashes in a sequence the kinetics of fluorescence recovery correspond to the kinetics of relaxation of the O 2 system. The recovery after the first flash was faster than after the second. The recoveries were not changed by the addition of 10 mM NH 2OH. 3. 3. The recovery kinetics are complex (at least biphasic). Under all conditions studied, including a varying degree of reduction of the acceptor Q, there appeared to be a significant, 200–250-μs component. 4. (D) The results are interpreted in terms of a single type of System II trap. The oscillations of the fluorescence yield and the 35-μs rise are governed by movement of charge on the donor side of System II. Fluorescence behavior at longer times is governed primarily by the state of the acceptor Q.