Β-carotene Radical Cation Research Articles

Investigations of β-carotene radical cation formation in infrared matrix-assisted laser desorption electrospray ionization (IR-MALDESI).

Radical cationization of endogenous hydrocarbons in cherry tomatoes was previously reported using infrared matrix-assisted laser desorption electrospray ionization (IR-MALDESI), a mass spectrometry imaging technique that operates at ambient conditions and requires no sample derivatization. Due to the surprising nature of this odd-electron ionization, subsequent experiments were performed on β-carotene to determine the amount of radical cationization across different sampling conditions. β-Carotene was analyzed across a variety of sample states using IR-MALDESI followed by Orbitrap mass spectrometric analysis: first, as a standard in ethanol in a well plate; second, as particulates on printer paper; and third, as particulates covered by an ice matrix. These techniques were also performed with a β-carotene standard either in solution with a reducing agent (ascorbic acid) or with ascorbic acid in the electrospray solution. Tandem mass spectrometry confirmed the presence of the radical cation of β-carotene by comparing fragments against NIST and METLIN databases. It was always analyzed as a radical cation when sampled from solution, where ascorbic acid increased radical cation abundance when in solution with β-carotene. Mixed-mode ionization between radical cationization and proton adduction was observed from dried particulates using IR-MALDESI. There are several potential mechanisms for β-carotene radical cationization prior to IR-MALDESI analysis, with multiphoton ionization, thermal degradation, and/or reaction with oxygen appearing to be the most logical explanations. Furthermore, although not the primary cause, changing certain aspects of sample conditions can result in significant mixed-mode ionization with competing protonation.

Kaempferol Binding to Zinc(II), Efficient Radical Scavenging through Increased Phenol Acidity.

Zinc(II) enhances radical scavenging of the flavonoid kaempferol (Kaem) most significantly for the 1:1 Zn(II)-Kaem complex in equilibrium with the 1:2 Zn(II)-Kaem complex both with high affinity at 3-hydroxyl and 4-carboxyl coordination. In methanol/chloroform (7/3, v/v), 1:1 Zn(II)-Kaem complex reduces β-carotene radical cation, β-Car•+, with a second-order rate constant, 1.88 × 108 L·mol-1·s-1, while both Kaem and 1:2 Zn(II)-Kaem complex are nonreactive, as determined by laser flash photolysis. In ethanol, 1:1 Zn(II)-Kaem complex reduces the 2,2-diphenyl-1-picrylhydrazyl radical, DPPH•, with a second-order rate constant, 2.48 × 104 L·mol-1·s-1, 16 times and 2 times as efficient as Kaem and 1:2 Zn(II)-Kaem complex, respectively, as determined by stopped-flow spectroscopy. Density functional theory calculation results indicate significantly increased acidity of Kaem as ligand in 1:1 Zn(II)-Kaem complex other than in 1:2 Zn(II)-Kaem complex. Kaem in 1:1 Zn(II)-Kaem complex loses two protons (one from 3-hydroxyl and one from phenolic hydroxyl) forming 1:1 Zn(II)-(Kaem-2H) during binding with Zn(II), while Kaem in 1:2 Zn(II)-Kaem complex loses one proton in each ligand forming Zn(II)-(Kaem-H)2, as confirmed by UV-vis absorption spectroscopy. Zn(II)-(Kaem-2H) is a far stronger reductant than Kaem and Zn(II)-(Kaem-H)2 as determined by cyclic voltammetry. Significant rate increases for the 1:1 complex in both β-Car•+ scavenging by electron transfer and DPPH• scavenging by hydrogen atom transfer were ascribed to decreases of ionization potential and of bond dissociation energy of 4'-OH for deprotonated Zn(II)-(Kaem-2H), respectively. Increased phenol acidity of plant polyphenols by 1:1 coordination with Zn(II) may explain the unique function of Zn(II) as a biological antioxidant and may help to design nontoxic metal-based drugs derived from natural bioactive molecules.

Genistein Binding to Copper(II)-Solvent Dependence and Effects on Radical Scavenging.

Genistein, but not daidzein, binds to copper(II) with a 1:2 stoichiometry in ethanol and with a 1:1 stoichiometry in methanol, indicating chelation by the 5-phenol and the 4-keto group of the isoflavonoid as demonstrated by the Jobs method and UV-visible absorption spectroscopy. In ethanol, the stability constants had the value 1.12 × 1011 L2∙mol−2 for the 1:2 complex and in methanol 6.0 × 105 L∙mol−1 for the 1:1 complex at 25 °C. Binding was not detected in water, as confirmed by an upper limit for the 1:1 stability constant of K = 5 mol−1 L as calculated from the difference in solvation free energy of copper(II) between methanol and the more polar water. Solvent molecules compete with genistein as demonstrated in methanol where binding stoichiometry changes from 1:2 to 1:1 compared to ethanol and methanol/chloroform (7/3, v/v). Genistein binding to copper(II) increases the scavenging rate of the stable, neutral 2,2-diphenyl-1-picrylhydrazyl radical by more than a factor of four, while only small effects were seen for the short-lived but more oxidizing β-carotene radical cation using laser flash photolysis. The increased efficiency of coordinated genistein is concluded to depend on kinetic rather than on thermodynamic factors, as confirmed by the small change in reduction potential of −0.016 V detected by cyclic voltammetry upon binding of genistein to copper(II) in methanol/chloroform solutions.

Regeneration of β-Carotene from Radical Cation by Eugenol, Isoeugenol, and Clove Oil in the Marcus Theory Inverted Region for Electron Transfer.

The rate of regeneration of β-carotene by eugenol from the β-carotene radical cation, an initial bleaching product of β-carotene, was found by laser flash photolysis and transient absorption spectroscopy to be close to the diffusion limit in chloroform/methanol (9:1, v/v), with a second-order rate constant (k2) of 4.3 × 109 L mol-1 s-1 at 23 °C. Isoeugenol, more reducing with a standard reduction potential of 100 mV lower than eugenol, was slower, with k2 = 7.2 × 108 L mol-1 s-1. Regeneration of β-carotene following photobleaching was found 50% more efficient by eugenol, indicating that, for the more reducing isoeugenol, the driving force exceeds the reorganization energy for electron transfer significantly in the Marcus theory inverted region. For eugenol/isoeugenol mixtures and clove oil, kinetic control by the faster eugenol determines the regeneration, with a thermodynamic backup of reduction equivalent through eugenol regeneration by the more reducing isoeugenol for the mixture. Clove oil, accordingly, is a potential protector of provitamin A for use in red palm oils.

Radicals formed from proton loss of carotenoid radical cations: A special form of carotenoid neutral radical occurring in photoprotection

In an organized assembly in Arabidopsis thaliana plant, proton loss from the radical cation of zeaxanthin (Zea+) was found to occur under intense illumination, a possible component in photoprotection. A stable neutral radical is formed because of the favorable proton loss at C4(4′) position(s) of the terminal ends of Zea+ that extends the unpaired spin density distribution (notation Zea(4) or Zea(4′) by symmetry). Proton loss from the radical cation of β-carotene (β-car+) to available proton acceptors was also detected in a PSII sample upon irradiation. A controversial optical absorption peak at 750nm predicted by DFT was attributed to β-carotene neutral radical formed by proton loss at C4(4′) position(s) situated on the terminal end(s) (notation β-car(4) or β-car(4′) by symmetry), also detected in solid copper-containing MCM-41 molecular sieves (Cu-MCM-41) and by hydrogen atom transfer from β-carotene to hydroxyl radical. Unlike the PSII organized assembly where proton loss occurs from the terminal ends of the radical cation, in the Cu-MCM-41 unorganized assembly proton loss occurs from all positions including the methyl groups on the polyene chain forming neutral radicals with different EPR couplings, different unpaired spin density distribution and different optical absorption peaks. We emphasize here the properties of these neutral radicals for various carotenoids formed under high-light conditions and in different media (solution, solid siliceous materials, and photosynthetic samples).

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

The identification of the spectral information of carotenoid neutral radicals is essential for studying their reactivities towards O2 and thereby evaluating their role in the antioxidant–prooxidant properties of the corresponding carotenoid. Recently, it was reported that β-carotene neutral radical (β-CAR) has an absorption maximum at 750nm. This contradicts the results of many reports that show carotenoid neutral radicals (CAR) absorb in the same or near to the spectral region as their parent carotenoids. In this manuscript, the influence of pH on the decay of β-carotene radical cation (β-CAR-H+), generated in an aqueous solution of 2% Triton X-100 (TX-100), was investigated, employing laser flash photolysis (LFP) coupled with kinetic absorption spectroscopy, to identify the absorption bands of the β-carotene neutral radicals. By increasing the pH value of the solution, the decay of β-CAR-H+ is enhanced and this enhancement is not associated with the formation of any positive absorption bands over the range 550–900nm. By comparing these results with the literature, it can be concluded that β-carotene neutral radicals most probably absorb within the same spectral range as that of β-carotene. The reaction pathways of the reaction of β-CAR-H+ with −OH have been discussed.

Efficient scavenging of β-carotene radical cations by antiinflammatory salicylates

The radical cation generated during photobleaching of β-carotene is scavenged efficiently by the anion of methyl salicylate from wintergreen oil in a second-order reaction approaching the diffusion limit with k2 = 3.2 × 10(9) L mol(-1) s(-1) in 9 : 1 v/v chloroform-methanol at 23 °C, less efficiently by the anion of salicylic acid with 2.2 × 10(8) L mol(-1) s(-1), but still of possible importance for light-exposed tissue. Surprisingly, acetylsalicylate, the aspirin anion, reacts with an intermediate rate in a reaction assigned to the anion of the mixed acetic-salicylic acid anhydride formed through base induced rearrangements. The relative scavenging rate of the β-carotene radical cation by the three salicylates is supported by DFT-calculations.

β-Carotene Radical Cation Addition to Green Tea Polyphenols. Mechanism of Antioxidant Antagonism in Peroxidizing Liposomes

Green tea polyphenols, (-)-epicatechin (EC), (-)-epigallocatechin (EGC), (-)-epicatechin gallate (ECG), and (-)-epigallocatechin gallate (EGCG), all showed antioxidative effect in liposomes for lipid oxidation initiated in the lipid phase (antioxidant efficiency EC > EGCG > ECG > EGC) or in the aqueous phase (EC ≫ EGC > EGCG > ECG) as monitored by the formation of conjugated dienes. For initiation in the lipid phase, β-carotene, itself active as an antioxidant, showed antagonism with the polyphenols (EC > ECG > EGCG > EGC). The Trolox equivalent antioxidant capacity (TEAC EGC > EGCG > ECG > EC) correlates with the lowest phenol O-H bond dissociation enthalpy (BDE) as calculated by density functional theory (DFT). Surface-enhanced Raman spectroscopy (SERS) was used to assess the reducing power of the phenolic hydroxyls in corroboration with DFT calculations. For homogeneous (1:9 v/v methanol/chloroform) solution, the β-carotene radical cation reacted readily with each of the polyphenol monoanions (but not with the neutral polyphenols) with a rate approaching the diffusion limit for EC as studied by laser flash photolysis at 25 °C monitoring the radical cation at 950 nm. The rate constant did not correlate with polyphenol HOMO/LUMO energy gap (DFT calculations), and β-carotene was not regenerated by an electron transfer reaction (monitored at 500 nm). It is suggested that the β-carotene radical cation is rather reacting with the tea polyphenols through addition, as further evidenced by steady-state absorption spectroscopy and liquid chromatography-mass spectroscopy (LC-MS), in effect preventing regeneration of β-carotene as an active lipid phase antioxidant and leading to the observed antagonism.

Phenol Acidity and Ease of Oxidation in Isoflavonoid/β-Carotene Antioxidant Synergism

Regeneration of β-carotene from the β-carotene radical cation by the 4'-propylpuerarin anion (second-order rate constant=1.5×10(9) L mol(-1) s(-1) in methanol/chloroform=1:9 (v/v) solution at 25 °C as determined by laser flash photolysis) was found to be marginally slower than regeneration by the 7-propylpuerarin anion (2.3×10(9) L mol(-1) s(-1)), in agreement with the 7-propylpuerarin anion being more reducing (E'=0.56 V vs NHE) than the 4'-propylpuerarin anion (E'=1.01 V vs NHE). The potentials were calculated from E°=1.12 and 1.44 V (vs NHE) as determined by cyclic voltametry in aqueous solution and pKa=9.51 and 7.23 obtained previously for 7-propylpuerarin and 4'-propylpuerarin, respectively. The less reducing but more acidic 4'-propylpuerarin showed less antioxidant activity in liposome of pH 7.4, but more significant antioxidant synergism with β-carotene than the more reducing but less acidic 7-propylpuerarin for oxidation initiated in the liposome lipid phase. Electrostatic effects are concluded to be important in the regeneration of β-carotene from the radical cation in the water/lipid interface because approximately 50% of 4'-propylpuerarin is present as the anion, whereas only 0.5% of 7-propylpuerarin is present as the anion. In contrast, penetration of the undissociated phenolic group into the lipid phase, more significant for 7-propylpuerarin than for 4'-propylpuerarin according to the calculated water/lipid partition coefficients, becomes important for the chain-breaking action in lipid oxidation of the puerarin derivatives as models for (iso)flavonoids and their glycosides.

Direct Observation of the β-Carotene Reaction with Hydroxyl Radical

Hydroxyl radical reacts readily with β-carotene following submicrosecond laser photolysis of N-hydroxypyridine-2(1H)-thione (N-HPT) as a "photo-Fenton" reagent generating hydroxyl and thiyl radicals in acetonitrile:tetrahydrofuran (4:1, v/v) solution. On the basis of spectral evidence, and supported by kinetic considerations and thermodynamic calculations, a short-lived transient species, detected by time-resolved absorption spectroscopy with an absorption maximum at ∼750 nm and a lifetime of ∼150 ns at 25 °C under anaerobic conditions, is suggested to be the long-sought neutral β-carotene radical formed by hydrogen-atom abstraction. The transient spectrum is different from the spectra of the β-carotene radical cation (∼1000 nm absorption maximum with a millisecond lifetime), the β-carotene radical adducts (∼520 nm, several microsecond lifetime), the β-carotene radical cation ion pair (∼750 nm, several hundred microsecond lifetime), and the β-carotene radical anion (∼880 nm, a few tens of microsecond lifetime). In parallel, β-carotene reacts with the thiyl radical to yield a sulfur radical adduct with absorption maximum at ∼520 nm with a lifetime of 3.0 μs. For astaxanthin and canthaxanthin, the reaction with the thiyl radical dominates and the neutral radical is hardly formed in agreement with the less reducing properties of these 4,4'-diketo carotenoids without the reactive 4,4'-hydrogens.

Thermodynamic versus Kinetic Control of Antioxidant Synergism between β-Carotene and (Iso)flavonoids and Their Glycosides in Liposomes

Antioxidant synergism (or antagonism) between plant (iso)flavonoids (daidzein, baicalein, and quercetin) or their glycosides (puerarin, baicalin, and rutin) and β-carotene in phosphatidylcholine liposomes (pH 7.4) with oxidation initiated thermally by the lipophilic free radical initiator 2,2'-azobis(2,4-dimethylvaleronitrile) (AMVN) and followed by the formation of conjugated dienes did not depend simply on the bond dissociation enthalpy (BDE) of the phenol O-H bond or the HOMO/LUMO energy gap based on density functional theory (DFT) calculations. Rate of regeneration of β-carotene from the β-carotene radical cation as the one-electron oxidation product of the lipid phase antioxidant by the monoanion form of the (iso)flavonoids in homogeneous (1:9 v/v methanol/chloroform) solution, as studied by laser flash photolysis and occurring on a microsecond time scale with biphasic kinetics, was in better agreement with the observed nonadditive antioxidative effects. However, correcting the observed (pseudo)-first-order rate constant for β-carotene regeneration for water/lipid distribution of the (iso)flavonoids provided an almost correct ordering of the (iso)flavonoids, according to the nonadditive effects with β-carotene on lipid oxidation.

Formation of Carotenoid Neutral Radicals in Photosystem II

beta-Carotene radicals produced in the hexagonal pores of the molecular sieve Cu(II)-MCM-41 were studied by ENDOR and visible/near-IR spectroscopies. ENDOR studies showed that neutral radicals of beta-carotene were produced in humid air under ambient fluorescent light. The maximum absorption wavelengths of the neutral radicals were measured and were additionally predicted by using time-dependent density functional theory (TD-DFT) calculations. An absorption peak at 750 nm, assigned to the neutral radical with a proton loss from the 4(4') position of the beta-carotene radical cation in Cu(II)-MCM-41, was also observed in photosystem II (PS II) samples using near-IR spectroscopy after illumination at 20 K. This peak was previously unassigned in PS II samples. The intensity of the absorption peak at 750 nm relative to the absorption of chlorophyll radical cations and beta-carotene radical cations increased with increasing pH of the PS II sample, providing further evidence that the absorption peak is due to the deprotonation of the beta-carotene radical cation. Based on a consideration of possible proton acceptors that are adjacent to beta-carotene molecules in photosystem II, as modeled in the X-ray crystal structure of Guskov et al. Nat. Struct. Mol. Biol. 2009, 16, 334-342, an electron-transfer pathway from a beta-carotene molecule with an adjacent proton acceptor to P680*+ is proposed.

Detection of the D0→D1 transition of β-carotene radical cation photoinduced in photosystem II

The D0→D1 absorption band of a β-carotene radical cation in the near-infrared (NIR) region was detected for the first time in photosystem II (PSII). PSII-enriched membranes and isolated reaction center (RC) complexes (D1/D2/Cytb559) from spinach were illuminated at 80 and 150 K, respectively, in the presence of electron acceptors. In both preparations, UV-Vis-NIR difference spectra upon illumination exhibited a medium-intensity band at ∼1450 nm along with a strong band at ∼990 nm. The latter band has been assigned to the D0→D2 transition of the radical cation of the β-carotene in the secondary electron transfer pathway in PSII. These NIR bands disappeared at 210 K in the PSII membranes, and diminished their intensities in the RC complexes partially depleted of carotenoid. The absence or diminish of the β-carotene cation with little change in the formation of chlorophyll cations under these conditions were also confirmed by detecting light-induced FTIR difference spectra in the mid-IR region. From these results, it was concluded that the NIR band observed at ∼1450 nm arose from the D0→D1 transition of the β-carotene radical cation. It was shown that the observed band in the RC complexes was a mixture of the band of one β-carotene cation (Car507+) at 1464 nm and that of the other cation (Car489+) at a wavelength shorter than 1434 nm, indicating that the D0→D1 transition is sensitive to the protein environment. It is proposed that the position and the relative intensity of the D0→D1 band together with the well-known D0→D2 band can be useful monitors to investigate the properties of the radical cation and the molecular interaction of β-carotene in the PSII proteins.

Effect of polar solvents on β-carotene radical precursor

β-Carotene forms radicals in chloroform upon photo-excitation (i) in the femtosecond time-scale by direct electron ejection into chloroform and (ii) in the microsecond time-scale by secondary reactions with chloroform radicals formed in the faster reactions. The precursor for β-carotene radical cation decays in a second-order reaction in the mixed solvents, with a rate decreasing for increasing dielectric constant of cosolvent (acetic acid < ethanol < acetonitrile∼methanol). The precursor is assigned as an ion pair from which the β-carotene radical cation is formed in neat chloroform, but in more polar solvents it reacts at least partly through disproportionation in a bimolecular reaction promoted by the presence of ions. The stabilization of the radical precursor by increased solvent polarity, allowing for deactivation of the precursor by an alternative reaction channel, is discussed in relation to the balance of pro- and antioxidative properties of β-carotene at lipid/water interfaces.

Density Functional Theory Study of the β-Carotene Radical Cation and Deprotonated Radicals

The beta-carotene radical cation and deprotonated neutral radicals were studied at the density functional theory (DFT) level using different density functionals and basis sets: B3LYP/3-21G, SVWN5/6-31G*, BPW91/DGDZVP2, and B3LYP/6-31G**. The geometries, total energies, spin distributions, and isotropic and anisotropic hyperfine coupling constants of these species were calculated. Deprotonation of the methyl group at the double bond of the cyclohexene ring of the carotenoid radical cation at 5 or 5' produces the most stable neutral radical because of retention of the pi-conjugated system while less stable deprotonation at 9 or 9' and 13 or 13' of the chain methyl groups causes significant distortion of the conjugation. The predicted methyl hyperfine coupling constants of 13-16 MHz of the neutral radicals are in good agreement with the previous electron nuclear double resonance (ENDOR) spectrum of photolyzed beta-carotene on a solid support. DFT calculations on the beta-carotene radical cation in a polar water environment showed that the polar environment does not cause significant changes in the proton hyperfine constants from those in the isolated gas-phase molecule. DFT calculated methyl proton hyperfine coupling constants of less than 7.2 MHz are in agreement with those reported for the radical cation in photosystem II (PS II) and those found in the absence of UV light for the radical cation on a silica alumina matrix.

Construction and Characterization of Genetically Modified Synechocystis sp. PCC 6803 Photosystem II Core Complexes Containing Carotenoids with Shorter π-Conjugation than β-Carotene

Beta-carotene has been identified as an intermediate in a secondary electron transfer pathway that oxidizes Chl(Z) and cytochrome b(559) in Photosystem II (PS II) when normal tyrosine oxidation is blocked. To test the redox function of carotenoids in this pathway, we replaced the zeta-carotene desaturase gene (zds) or both the zds and phytoene desaturase (pds) genes of Synechocystis sp. PCC 6803 with the phytoene desaturase gene (crtI) of Rhodobacter capsulatus, producing carotenoids with shorter conjugated pi-electron systems and higher reduction potentials than beta-carotene. The PS II core complexes of both mutant strains contain approximately the same number of chlorophylls and carotenoids as the wild type but have replaced beta-carotene (11 double bonds), with neurosporene (9 conjugated double bonds) and beta-zeacarotene (9 conjugated double bonds and 1 beta-ionylidene ring). The presence of the ring appears necessary for PS II assembly. Visible and near-infrared spectroscopy were used to examine the light-induced formation of chlorophyll and carotenoid radical cations in the mutant PS II core complexes at temperatures from 20 to 160 K. At 20 K, a carotenoid cation radical is formed having an absorption maximum at 898 nm, an 85 nm blue shift relative to the beta-carotene radical cation peak in the WT, and consistent with the formation of the cation radical of a carotenoid with 9 conjugated double bonds. The ratio of Chl(+)/Car(+) is higher in the mutant core complexes, consistent with the higher reduction potential for Car(+). As the temperature increases, other carotenoids become accessible to oxidation by P(680)(+).

Modulation of Dietary Vitamins E and C Fails to Ameliorate β-Carotene Exacerbation of UV Carcinogenesis in Mice

β-Carotene is a strong singlet oxygen quencher and, under most conditions, exhibits strong antioxidant properties. Based on these properties, and a number of epidemiological studies, it was suggested that an above average intake of the carotenoid might reduce cancer risks. Earlier studies had found that β-carotene, when added to commercial closed-formula rodent diets, provided significant photoprotection to ultraviolet light (UV) carcinogenesis. However, clinical trials found that β-carotene supplementation evoked no change in incidence of nonmelanoma skin cancer and that smokers suffered a significant increase in lung cancer incidence. Further, recent studies, employing β-carotene-supplemented semidefined diets, not only failed to find a photoprotective effect, but significant exacerbation of UV carcinogenic expression resulted. Based on the relative electron transfer rate constants for interactions between β-carotene, a-tocopherol (vitamin E), and vitamin C, a mechanism was proposed for the repair of β-carotene radical cation, a strongly oxidizing radical resulting from β-carotene interactions with many oxidizing species. It was theorized that vitamin C repaired the carotenoid radical cation. As mice have no nutritional requirement for vitamin C and smokers are known to exhibit low levels of the vitamin, it was suggested that differences in the relative levels of vitamin C in closed-formula rations (no vitamin C) in which photoprotection occurred, and semidefined diets (containing vitamin C) in which exacerbation resulted, might account for the differences in response. Hairless mice were fed β-carotene-supplemented semidefined diets containing varying levels of vitamins E and C (either increasing their concentrations or reducing them to reflect levels found in closed-formula rations) and subjected to a UV carcinogenesis protocol. Increasing levels of vitamins E and C did not ameliorate β-carotene exacerbation of UV carcinogenesis. Nor did elimination of vitamin C from the diet. Reduced levels of dietary vitamin E augmented β-carotene exacerbation of UV carcinogenic expression, suggesting vitamin E and β-carotene interaction. It is concluded that the photoprotective effect of β-carotene reported earlier by others, or the noninjurious effect of β-carotene found in our studies with closed-formula rations, might depend on interaction with other dietary factors that are either absent, or present in ineffectual concentrations, in the semidefined diet in which exacerbation of UV carcinogenesis occurs. Those factors could be other carotenoids, their isomers, or some yet unidentified phytochemical(s).

Photoionization of β-carotene via electron transfer from excited states to chlorinated hydrocarbon solvents. A picosecond transient absorption study

Time-resolved photoinduced (λexc 266, 355 and 532 nm) formation of the radical cation of β-carotene in the chlorinated solvents n-butylchloride, dichloromethane, 1,2-dichloroethane, chloroform and carbon tetrachloride, was studied with 5–10 ps time resolution. The radical cation is formed via electron transfer from the electronically excited solute to the solvent and is characterized by an absorption band with maximum at 900–950 nm. The process is efficient (ϕ > 0.12) and fast (t < 10 ps), which excludes involvement of the triplet state. It is suggested that the electron transfer proceeds from either the vibrationally excited singlet 2A−g (l 2) or from the “forbidden” 1B−u state. The formation times of the carotene radical cation in the chlorinated hydrocarbon solvents are on the order of their diffusion times.

Scavenging of acetylperoxyl radicals and quenching of triplet diacetyl by β-carotene: mechanisms and kinetics

β-Carotene scavenges triplet diacetyl generated by laser flash photolysis with a second-order rate constant of 9.1±0.9×10 9 M −1 s −1 in deaerated benzene at 20°C. In the presence of oxygen diacetyl dissociates to generate acetylperoxyl radicals. It is demonstrated that diacetyl does not dissociate to any appreciable extent in the absence of oxygen. The acetylperoxyl radical is scavenged by β-carotene with second-order rate constant 9.2±0.6×10 8 M −1 s −1 in aerated benzene at 20°C to give an adduct between the acetylperoxyl radical and β-carotene, whereas no evidence of oxidation of β-carotene by the strongly oxidizing acetylperoxyl radical to give the β-carotene radical cation is found. This adduct decays with first-order rate constant 1.35±0.16×10 3 s −1 to give (presumably) a β-carotene epoxide and the acetyloxyl radical.

Density Functional Theory Study of the β-Carotene Radical Cation

The hybrid density functional theory method B3LYP is employed to study the β-carotene radical cation. The radical is characterized by means of its geometry, spin distribution, and isotropic and anisotropic hyperfine coupling constants. It is shown that the spin is delocalized over the whole π-conjugated system, including the double bonds of the headgroups. This delocalization results in methyl hyperfine coupling constants lower than 9 MHz, in excellent agreement with recent experimental couplings of the carotene radical in Photosystem II and in vitro, but in conflict with previous theoretical calculations. It is also demonstrated that rotation of the headgroups can affect the properties of the radical, in particular the spin delocalization to the ring.