Carotenoid Radical Cations Research Articles

Photooxidation of Carotenoids in Mesoporous MCM-41, Ni-MCM-41 and Al-MCM-41 Molecular Sieves

Photooxidation of β-carotene and canthaxanthin in mesoporous MCM-41, Ni-MCM-41, and Al-MCM-41 molecular sieves was studied by 9−220 GHz electron paramagnetic resonance (EPR) and 9 GHz electron nuclear double resonance (ENDOR). X-ray powder diffraction (XRD) measurements established that the MCM-41 pore size (33 A) was large enough to accommodate carotenoids. Mesoporous MCM-41 molecular sieves are found to be promising hosts for long-lived photoinduced charge-separation between carotenoid radical cations (Car•+) and the MCM-41 framework. Incorporating metal ions into siliceous MCM-41 enhances efficiency of carotenoid oxidation. The photoyield and stability of generated carotenoid radical cations increased in the order MCM < Ni-MCM < Al-MCM. Formation of carotenoid radical cations within the Me-MCM-41 is due to electron transfer between incorporated carotenoid molecules and metal ions, which act as electron acceptor sites. Detected EPR signals of Ni(I) species provide direct evidence for the reduction of Ni...

One-electron reduction potentials of dietary carotenoid radical cations in aqueous micellar environments

The one-electron reduction potentials of the radical cations of five dietary carotenoids (β-carotene, canthaxanthin, zeaxanthin, astaxanthin and lycopene) in aqueous micellar environments have been obtained from a pulse radiolysis study of electron transfer between the carotenoids and tryptophan radical cations as a function of pH, and lie in the range of 980–1060 mV. These values are consistent with our observation that the carotenoid radical cations oxidise tyrosine and cysteine. The decays of the carotenoid radical cations in the absence of added reactants suggest a distribution of exponential lifetimes. The radicals persist for up to about 1 s, depending on the medium.

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

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.

Characterisation of carotenoid radical cations in liposomal environments: interaction with vitamin C

Pulse radiolysis was used to generate the radical cations of β-carotene and two xanthophylls, zeaxanthin and lutein, in unilamellar vesicles of dipalmitoylphosphatidyl choline. The rate constants for the reaction (repair) of these carotenoid radical cations with the water-soluble vitamin C were found to be similar (∼1×10 7 M −1 s −1) for β-carotene and zeaxanthin and somewhat lower (∼0.5×10 7 M −1 s −1) for lutein. The results are discussed in terms of the microenvironment of the carotenoids and suggest that for β-carotene, a hydrocarbon carotenoid, the radical cation is able to interact with a water-soluble species even though the parent hydrocarbon carotenoid is probably entirely in the non-polar region of the liposome.

Chlorophyll and carotenoid radicals in photosystem II studied by pulsed ENDOR.

The stable carotenoid cation radical (Car(*+)) and chlorophyll cation radical (Chl(Z)(*+)) in photosystem II (PS II) have been studied by pulsed electron nuclear double resonance (ENDOR) spectroscopy. The spectra were essentially the same for oxygen-evolving PS II and Mn-depleted PS II. The radicals were generated by illumination given at low temperatures, and the ENDOR spectra were attributed to Car(*)(+) and Chl(Z)(*+) on the basis of their characteristic behavior with temperature as demonstrated earlier [Hanley et al. (1999) Biochemistry 38, 8189-8195]: i.e., (a) the Car(*)(+) alone was generated by illumination at < or =20 K, while Chl(Z)(*+) alone was generated at 200 K, and (b) warming of the sample containing the Car(*+) to 200 K resulted in the loss of the signal attributable to Car(*+) and its replacement by a spectrum attributable to the Chl(Z)(*+). A map of the hyperfine structure of Car(*+) in PS II and in organic solvent was obtained. The largest observed hyperfine splitting for Car(*+) in either environment was in the order of 8-9 MHz. Thus, the spin density on the cation is proposed to be delocalized over the carotenoid molecule. The pulsed ENDOR spectrum of Chl(Z)(*)(+) was compared to that obtained from a Chl a cation in frozen organic solvent. The hyperfine coupling constants attributed to the beta-protons at position 17 and 18 are well resolved from Chl(Z)(*+) in PS II (10. 8 and 14.9 MHz) but not in Chl a(*+) in organic solvent (12.5 MHz). This suggests a more defined conformation of ring IV with respect to the rest of the tetrapyrrole ring plane of Chl(Z)(*+) than Chl a(*+) probably induced by the protein matrix.

Diet Potentiates the UV-Carcinogenic Response to β-Carotene

The role of β-carotene as an anticancer agent has been questioned as a result of clinical trials in which the incidence of nonmelanoma skin cancer was unchanged in patients receiving a β-carotene supplement and in β-carotene-supplemented smokers who suffered a significant increase in lung cancer occurrence. In laboratory studies, β-carotene-supplemented semidefined diets, in contrast to earlier studies employing commercial closed-formula diets, not only failed to provide a protective effect to ultraviolet (UV) carcinogenesis but resulted in significant exacerbation. A rationale for this distinct carcinogenic response to β-carotene rests with the stability of the carotenoid radical cation, believed to be dependent on the presence of other antioxidants for rapid repair, and suggests that response to β-carotene depends on the presence and interaction with other dietary factors. Here, we report that diet potentiates β-carotene-mediated exacerbation of UV carcinogenesis. Although the dietary factor(s) responsible for this effect is unidentified, these studies underscore the potential risk of β-carotene supplementation in free-living populations where dietary status is widely varied.

Electrochemical properties of natural carotenoids

Abstract Carotenoids are widely distributed among plants, certain bacteria and animals, serving for light harvesting and photoprotection in photosynthesis and as antioxidants. Electrochemical data (such as oxidation potentials, reaction rate constants, kinetic equilibrium constants) for 12 naturally occurring carotenoids are summarized in this paper. Electrochemical parameters were estimated by simulating experimental cyclic voltammograms (CVs). Conventional electrochemical techniques (cyclic voltammetry and Osteryoung square-wave voltammetry) were used in these studies. CVs of some natural carotenoids are presented and discussed. The dependence of redox potentials and other kinetic parameters on the structures of carotenoids is summarized. The oxidation potential of neutral carotenoid ( E ° 1 ), which corresponds to the formation of cation radicals, varies from 0.50 to 0.72 V versus SCE. The oxidation potential of carotenoid cation radical ( E ° 2 ), which corresponds to the formation of a dication, usually lies in the region of 0.52–0.95 V versus SCE. The electrochemical data presented in this article should be helpful in the study of reaction processes in model systems or actual photosynthesis reaction centers.

Effects of Polyene Chain Length and Acceptor Substituents on the Stability of Carotenoid Radical Cations

The stability of radical cations of three series of carotenoids substituted with terminal ester, aldehyde, and cyano groups and with different numbers of backbone double bonds was studied by electrochemical and optical methods. The ethyl esters are 8‘-apo-β-caroten-8‘-oate (I), 6‘-apo-β-caroten-6‘-oate (II), and 4‘-apo-β-caroten-4‘-oate (III); the aldehydes are 8‘-apo-β-caroten-8‘-al (IV), 6‘-apo-β-caroten-6‘-al (V), and 4‘-apo-β-caroten-4‘-al (VI); and the cyano compounds are 8‘-apo-β-caroten-8‘-nitrile (VII), 6‘-apo-β-caroten-6‘-nitrile (VIII), and 4‘-apo-β-caroten-4‘-nitrile (IX). Cyclic voltammetry (CV) and Osteryoung square wave voltammetry (OSWV) results indicate that the stability of carotenoid radical cations depends on the number of conjugated chain double bonds. For the esters, the longer the olefin chain, the more unstable the radical cations. In contrast, for the aldehydes and the nitriles, the stability of the radical cations is similar or varies slightly with backbone chain length. The half-...

The reduction potential of the β-carotene [rad]+ /β-carotene couple in an aqueous micro-heterogeneous environment

There is a resurgence of interest in the role of electron transfer reactions involving β-carotene in photosynthesis. There is also current debate on the health benefits of dietary carotenoids and the possible deleterious effects on certain sub-populations such as smokers. The impact of dietary carotenoids on health may well be also related to radical reactions. A key parameter in biological systems is therefore the one-electron reduction potential of the carotenoid radical cation, now reported for the first time in a model biological aqueous environment. The value obtained is 1.06±0.01 V and is sufficiently high to oxidise cell membrane proteins, but is low enough to repair P 680 + in the photosynthetic reaction centre.

EPR spin trapping detection of carbon-centered carotenoid and β-ionone radicals

Free radical intermediates were detected by the electron paramagnetic resonance spin trapping technique upon protonation/deprotonation reactions of carotenoid and β-ionone radical ions. The hyperfine coupling constants of their spin adducts obtained by spectral simulation indicate that carbon-centered radicals were trapped. The formation of these species was shown to be a result of chemical oxidation of neutral compounds by Fe 3+ or I 2 followed by deprotonation of the corresponding radical cations or addition of nucleophilic agents to them. Bulk electrolysis reduction of β-ionone and carotenoids also leads to the formation of free radicals via protonation of the radical anions. Two different spin adducts were detected in the reaction of carotenoid polyenes with piperidine in the presence of 2-methyl-2-nitroso-propane (MNP). One is attributable to piperidine radicals (C 5H 10N ) trapped by MNP and the other was identified as trapped neutral carotenoid (β-ionone) radical produced via protonation of the radical anion. Formation of these radical anions was confirmed by ultraviolet-visible spectroscopy. It was found that the ability of carotenoid radical anions/cations to produce neutral radicals via protonation/deprotonation is more pronounced for unsymmetrical carotenoids with terminal electron-withdrawing groups. This effect was confirmed by the radical cation deprotonation energy (H D) estimated by semiempirical calculations. The results indicate that the ability of carotenoid radical cations to deprotonate decreases in the sequence: β-ionone > unsymmetrical carotenoids > symmetrical carotenoids. The minimum H D values were obtained for proton abstraction from the C(4) atom and the C(5)-methyl group of the cyclohexene ring. It was assumed that deprotonation reaction occurs preferentially at these positions.

Kinetics and mechanism of the primary steps of degradation of carotenoids by acid in homogeneous solution.

The kinetics of reaction between trifluoroacetic acid as an acid of medium strength and the carotenoids beta-carotene, zeaxanthin, canthaxanthin, and astaxanthin has been examined in detail including the effects of dioxygen, acid concentration, and carotenoid structure. Reaction between acid and carotenoid leads to species absorbing in the red and near-infrared (NIR) spectral regions, intermediates that subsequently disappear. ESR experiments clearly show that these species are not carotenoid radicals, although their NIR absorption is similar to the absorption of carotenoid radical cations. Under most reaction conditions, the disappearance of carotenoids follows pseudo-zero-order kinetics, whereas the reaction order is >1 with respect to acid, and the long-lived (hours) intermediates are suggested to be mono- (700 nm) and diprotonated carotenoid ( approximately 950 nm). Acid induces cis/trans-isomerization via the protonated intermediates, which also decay to nonradical species with shorter conjugated systems-most probably carotenoid esters. Slow protonization of the methine carbon is the primary step in the degradation, but dioxygen increases the rate as a result of formation of a charge-transfer complex with the carotenoids as indicated by a red-shift of the NIR absorption bands. Carotenoids with carbonyl groups (astaxanthin and canthaxanthin) have slower rates of degradation than beta-carotene and zeaxanthin, indicating preferential nondegradative protonation of the carbonyl groups.

Effect of Electrolytes and Temperature on Dications and Radical Cations of Carotenoids: Electrochemical, Optical Absorption, and High-Performance Liquid Chromatography Studies

The effect of supporting electrolytes and temperature on the behavior of dications and radical cations of carotenoids is studied. Cyclic voltammograms (CVs) of canthaxanthin (I) at 23 and −25 °C show that Car•+ of I has similar stability during the time of the CV scan, when using tetrabutylammonium perchlorate (TBAPC), tetrabutylammonium tetrafluoroborate (TBATFB), or tetrabutylammonium hexafluorophosphate (TBAHFP) as supporting electrolyte. However, the stability of Car2+ decreases when using TBAPC or TBATFB; β-carotene (II) shows similar behavior. The CV of I at −25 °C shows a strong cathodic wave (wave 6) near −0.15 V (vs Ag) with an intensity about half that of the neutral oxidation wave when TBAPC or TBATFB is the supporting electrolyte. When TBAHFP is used, wave 6 (ca. −0.05 V vs Ag) is ca. 8 times weaker than when TBAPC or TBATFB is used. This wave results from the reduction of a species that may be a decay product of Car2+ of I. Our results show that these electrolytes commonly used in electrochem...

Low-Temperature Optical and Resonance Raman Spectra of a Carotenoid Cation Radical in Photosystem II

Low-temperature (77 K) illumination of manganese-depleted Synechocystis PCC 6803 photosystem II core complexes caused the reversible photooxidation of a carotenoid, forming a carotenoid cation radical with an absorbance maximum at 984 nm. Resonance FT-Raman spectra obtained with 1064 nm excitation gave a spectrum characteristic of carotenoid cation radicals in solution. This is the first example of a resonance Raman spectrum of a carotenoid cation radical in a protein. The carotenoid photooxidation requires prior chemical oxidation of cytochrome b559 which implicates the carotenoid in the secondary electron-transfer pathway of photosystem II that may play a role in photoprotection. The possible nature of the pathway and the structure of the carotenoid cation radical are discussed.

Carotenoid oxidation in photosystem II.

The oxidation of carotenoid upon illumination at low temperature has been studied in Mn-depleted photosystem II (PSII) using EPR and electronic absorption spectroscopy. Illumination of PSII at 20 K results in carotenoid cation radical (Car+*) formation in essentially all of the centers. When a sample which was preilluminated at 20 K was warmed in darkness to 120 K, Car+* was replaced by a chlorophyll cation radical. This suggests that carotenoid functions as an electron carrier between P680, the photooxidizable chlorophyll in PSII, and ChlZ, the monomeric chlorophyll which acts as a secondary electron donor under some conditions. By correlating with the absorption spectra at different temperatures, specific EPR signals from Car+* and ChlZ+* are distinguished in terms of their g-values and widths. When cytochrome b559 (Cyt b559) is prereduced, illumination at 20 K results in the oxidation of Cyt b559 without the prior formation of a stable Car+*. Although these results can be reconciled with a linear pathway, they are more straightforwardly explained in terms of a branched electron-transfer pathway, where Car is a direct electron donor to P680(+), while Cyt b559 and ChlZ are both capable of donating electrons to Car+*, and where the ChlZ donates electrons when Cyt b559 is oxidized prior to illumination. These results have significant repercussions on the current thinking concerning the protective role of the Cyt b559/ChlZ electron-transfer pathways and on structural models of PSII.

Surface Modification of TiO2 Nanoparticles with Carotenoids. EPR Study

EPR measurements demonstrate efficient charge separation on carotenoid-modified titanium dioxide nanoparticles (7 nm). Strong complexation of carotenoids containing terminal carboxy groups (−CO2H) with the TiO2 surface leads to electron transfer from the adsorbed carotenoid molecule to the surface trapping site. For these systems, EPR signals of the carotenoid radical cations Car•+ and the electrons trapped on the TiO2 are observed before irradiation (77 K). Their UV−visible spectra show an absorption band with a maximum near 650 nm that is characteristic of the trapped electrons. Surface modification of the TiO2 by other carotenoids results in the formation of a complex with an optical absorption band near 545 nm. These systems form charge-separated pairs [Car•+···TiO2(e-tr)surf. TiO2(e-tr)latt.] only upon 365−600 nm illumination at 77 K. Complexation of the TiO2 colloids with carotenoids enhances spatial charge separation, shifts the absorption threshold into the visible region, and thus greatly improves the reducing ability of the semiconductor. Photoreduction of acceptor molecules such as 2,5-dichloro-1,4-benzoquinone, nitrobenzene, and oxygen is demonstrated.

Photovoltaic response of carotenoid-sensitized electrode in aqueous solution: ITO coated with a mixture of TiO2 nanoparticles, carotenoid, and polyvinylcarbazole

Photovoltaic responses were observed upon irradiating (>420 nm) an ITO electrode spin-coated with a mixture of TiO2 nanoparticles, polyvinylcarbazole, and carotenoid (canthaxanthin or β-carotene, Car) and immersed in an aqueous KCl solution also containing hydroquinone (QH2). The anodic photocurrent involves electron transfer from the excited state of the carotenoid (Car*) to the TiO2 conduction band; the resulting carotenoid radical cation (Car˙+) is reduced by QH2. The photocurrent was nearly constant during one hour of continuous irradiation, which indicates that these carotenoids are stable at the electrode/electrolyte interface, presumably because of rapid regeneration of Car from Car˙+ during the electron transfer cycle. In contrast, when the aqueous electrolyte solution was saturated with O2, a cathodic photocurrent was observed; this reversal of polarity is attributed to electron transfer from Car* to O2, and subsequent reduction of the resulting Car˙+ by electrons from the TiO2 conduction band.

Absorption and electroabsorption spectra of carotenoid cation radical and dication

Radical cations and dications of two carotenoids astaxanthin and canthaxanthin were prepared by oxidation with FeCl 3 in fluorinated alcohols at room temperature. Absorption and electroabsorption (Stark effect) spectra were recorded for astaxanthin cations in mixed frozen matrices at temperatures about 160 K. The D 0→D 2 transition in cation radical is at 835 nm. The electroabsorption spectrum for the D 0→D 2 transition exhibits a negative change of molecular polarizability, Δ α=−1.2·10 −38 C·m 2/V (−105 A 3), which seems to originate from the change in bond order alternation in the ground state rather than from the electric field-induced interaction of D 1 and D 2 excited states. Absorption spectrum of astaxanthin dication is located at 715–717 nm, between those of D 0→D 2 in cation radical and S 0→S 2 in neutral carotenoid. Its shape reflects a short vibronic progression and strong inhomogeneous broadening. The polarizability change on electronic excitation, Δ α=2.89·10 −38 C·m 2/V (260 A 3), is five times smaller than in neutral astaxanthin. This value reflects the larger energetic distance from the lowest excited state to the higher excited states than in the neutral molecule.

Relative One-Electron Reduction Potentials of Carotenoid Radical Cations and the Interactions of Carotenoids with the Vitamin E Radical Cation

Pulse radiolysis studies have been used to determine the electron-transfer rate constants between various pairs of carotenoids, one of which is present as the radical cation. These dietary carotenoids include those of importance to vision, namely zeaxanthin and lutein. These results have suggested the order of relative ease of electron transfer between six carotenoids. Additional experiments, involving electron transfer between astaxanthin (ASTA), β-apo-8‘-carotenal (APO), and vitamin E (TOH), lead to the following order in terms of relative ease of electron transfer for the seven carotenoid radical cations studied: astaxanthin > β-apo-8‘-carotenal > canthaxanthin > lutein > zeaxanthin > β-carotene > lycopene, such that lycopene is the strongest reducing agent (the most easily oxidized) and astaxanthin is the weakest, and the radical cations of the visual carotenoids, lutein (LUT) and zeaxanthin (ZEA), are reduced by lycopene (LYC) but not by β-carotene (β-CAR). Work on 7,7‘-dihydro-β-carotene (77DH) and...

EPR and ENDOR studies of carotenoid–solid Lewis acid interactions

EPR and ENDOR results provide strong support for the formation of carotenoid radical cations on activated alumina and silica–alumina as a consequence of electron transfer from the adsorbed carotenoid molecules to the Lewis acid sites on these surfaces. This assignment is confirmed 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 AlCl3, which serves as a model system. The ENDOR line at ca. 6.5 MHz obtained for the carotenoids adsorbed on solid supports and carotenoid–AlCl3 solution, is attributed to the high-frequency feature of a hyperfine doublet, centred about the 27Al Larmor frequency. ENDOR detection of the hyperfine 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.

Comparative mechanisms and rates of free radical scavenging by carotenoid antioxidants

The comparative mechanisms and relative rates of nitrogen dioxide (NO 2 ⋅), thiyl (RS ⋅) and sulphonyl (RSO 2 ⋅) radical scavenging by the carotenoid antioxidants lycopene, lutein, zeaxanthin, astaxanthin and canthaxanthin have been determined by pulse radiolysis. All the carotenoids under study react with the NO 2 ⋅ radical via electron transfer to generate the carotenoid radical cation (Car ⋅+). In marked contrast the glutathione and 2-mercaptoethanol thiyl radicals react via a radical addition process to generate carotenoid-thiyl radical adducts [RS-Car] ⋅. The RSO 2 ⋅ radical undergoes both radical addition, [RSO 2-Car] ⋅ and electron abstraction, Car ⋅+. Both carotenoid adduct radicals and radical cations decay bimolecularly. Absolute rate constants for radical scavenging were in the order of ∼10 7–10 9 M −1 s −1 and follow the sequence HO(CH 2) 2S ⋅>RSO 2 ⋅>GS ⋅>NO 2 ⋅. Although there were some discernible trends in carotenoid reactivity for individual radicals, rate constants varied by no greater than a factor of 2.5. The mechanism and rate of scavenging is strongly dependent on the nature of the oxidising radical species but much less dependent on the carotenoid structure.