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

Gardenia yellow pigment: Extraction methods, biological activities, current trends, and future prospects

Food coloring plays a vital role in influencing consumers' food choices, imparting vibrant and appealing colors to various food and beverage products. Synthetic food colorants have been the most commonly used coloring agents in the food industry. However, concerns about potential health issues related to synthetic colorants, coupled with increasing consumer demands for food safety and health, have led food manufacturers to explore natural alternatives. Natural pigments not only offer a wide range of colors to food products but also exhibit beneficial bioactive properties. Gardenia yellow pigment is a water-soluble natural pigment with various biological activities, widely present in gardenia fruits. Therefore, this paper aims to delve into Gardenia Yellow Pigment, highlighting its significance as a food colorant. Firstly, a thorough understanding and exploration of various methods for obtaining gardenia yellow pigment. Subsequently, the potential functionality of gardenia yellow pigment was elaborated, especially its excellent antioxidant and neuroprotective properties. Finally, the widespread application trend of gardenia yellow pigment in the food industry was explored, as well as the challenges faced by the future development of gardenia yellow pigment in the field of food and health. Some feasible solutions were proposed, providing valuable references and insights for researchers, food industry professionals, and policy makers.

Conformation-dependent antioxidant properties of β-carotene

The antioxidant capacity of β-carotene has been studied in terms of H-atom abstraction reactions using quantum chemical methods. These oxidation reactions are studied for the all-trans as well as 15,15'-cis isomers (15Z) of β-carotene, as the latter is only ∼10 kJ mol^-1 less stable than the all-trans isomer in the gas phase and about 9 kJ mol^-1 less stable in aqueous solution. Hydrogen abstraction from the rotamers obtained through C-C single and double bond rotations has been shown to play an important role in determining the antioxidant capacity of β-carotene. Hydrogen abstraction from the C4 and C5-CH₃ positions of the β-ionone rings and the C7 and C9 positions along the polyene chain of β-carotene by the hydroxyl radical have been studied. In the all-trans form the most favorable H-atom abstraction reaction occurs at the C4 position of the terminal regions of the polyene π-system of β-carotene, closely followed by hydrogen abstraction from the C5 methyl position. The H-atom abstraction reactions are more exothermic in water than in the gas phase due to solvation energies for the water product.

Study of the properties of carotenoids and key carotenoid biosynthesis genes from Deinococcus xibeiensis R13

To investigate the properties of carotenoids from the extremophile Deinococcus xibeiensis R13, the factors affecting the stability of carotenoids extracted from D. xibeiensis R13, including temperature, illumination, pH, redox chemicals, metal ions, and food additives, were investigated. The results showed that low temperature, neutral pH, reducing agents, Mn²⁺ , and food additives (xylose and glucose) can effectively improve the stability of Deinococcus carotenoids. The carotenoids of D. xibeiensis R13 exhibited strong antioxidant activity, with the scavenging rate of hydroxyl radicals reaching 71.64%, which was higher than the scavenging efficiency for 1,1-diphenyl-2-picrylhydrazyl free radicals and 2,2'-azino-bis (3-ethyl-benzothiazoline-6-sulfonic acid) free radicals (44.55 and 27.65%, respectively). In addition, the total antioxidant capacity reached 0.60 U/ml, which was 2.61-fold that of carotenoids from the model strain Deinococcus radiodurans R1. Finally, we predicted the gene clusters encoding carotenoid biosynthesis pathways in the genome of R13 and identified putative homologous genes. The key enzyme genes (crtE, crtB, crtI, crtLm, cruF, crtD, and crtO) in carotenoid synthesis of D. xibeiensis R13 were cloned to construct the multigene coexpression plasmids pET-EBI and pRSF-LmFDO. The carotenoid biosynthesis pathway was heterologously introduced into engineered Escherichia coli EBILmFDO, which exhibited a higher yield (7.14 mg/L) than the original strain. These analysis results can help us to better understand the metabolic synthesis of carotenoids in extremophiles.

Carotenoids: Importance in Daily Life-Insight Gained from EPR and ENDOR

Carotenoids are indispensable molecules for life. They are present everywhere in plants, algae, bacteria whom they protect against free radicals and oxidative stress. Through the consumption of fruits and vegetables and some carotenoid-containing fish, they are introduced into the human body and, similarly, protect it. There are numerous health benefits associated with the consumption of carotenoids. Carotenoids are antioxidants but at the same time they are prone to oxidation themselves. Electron loss from the carotenoid forms a radical cation. Furthermore, proton loss from a radical cation forms a neutral radical. In this mini-review, we discuss carotenoid radicals studied in our groups by various physicochemical methods, namely the radical cations formed by electron transfer and neutral radicals formed by proton loss from the radical cations. They contain many similar hyperfine couplings due to interactions between the electron spin and numerous protons in the carotenoid. Different EPR and ENDOR methods in combination with DFT calculations have been used to distinguish the two independent carotenoid radical species. DFT predicted larger coupling constants for the neutral radical compared to the radical cation. Previously, INDO calculations miss assigned the large couplings to the radical cation. EPR and ENDOR have aided in elucidating the physisorb, electron and proton transfer processes that occur when carotenoids are adsorbed on solid artificial matrices, and predicted similar reactions in aqueous solution or in plants. After years of study of the physicochemical properties of carotenoid radicals, the different published results start to merge together for a better understanding of carotenoid radical species and their implication in biological systems. Up until 2008, the radical chemistry in artificial systems was elucidated but the correlation between quenching ability and neutral radical formation was an inspiration to look for these radical species in vivo. In addition, the EPR spin-trapping technique has been applied to study inclusion complexes of carotenoids with different delivery systems.

Power Law Distribution Concerning Absolute Free Energies of Linear Sulfur Chains, Polythiazyls, Polyisoprenes, Linear trans-Polyenes, and Polyynes

After quantum calculations of absolute free energies of polyatomic sulfur derivatives or hydrocarbon oligomer compounds, we have studied the variations of G _n as a function of the number of repeating units n (sulfur atoms or hydrocarbon units, n = 2 to about 60), and observed that these values can be correlated by (a) a nearly perfect linear relationship, G _n = An + B (A and B, constants), with a high accuracy which enables an extrapolation for higher values of n, and (b) a power law:, δ G = G _n/ n - G_{( n-1)} /( n - 1) = C/ n ^d ( C and d, constants). From comparison of variations of the C-C bond lengths, we show that the conjugation of trans-polyenes (up to C₆₀H₆₂) is more important than for polyynes (up to C₆₀H₂).

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 750 nm. 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 550-900 nm. 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.

EPR study of the astaxanthin n-octanoic acid monoester and diester radicals on silica-alumina

The radical intermediates of the n-octanoic monoester and n-octanoic diester of astaxanthin were detected by pulsed EPR measurements carried out on the UV-produced radicals on silica-alumina artificial matrix and characterized by density functional theory (DFT) calculations. Previous Mims ENDOR for astaxanthin detected the radical cation and neutral radicals formed by proton loss from the C3 (or C3') position and from the methyl groups. Deprotonation of the astaxanthin neutral radical formed at the C3 (or C3') position resulted in a radical anion. DFT calculations for astaxanthin showed that the lowest energy neutral radical forms by proton loss at the C3 (or C3') position of the terminal ring followed by proton loss at the methyl groups of the polyene chain. Contrary to astaxanthin where proton loss can occur at either end of the symmetrical radical, for the diester of astaxanthin, this loss is prevented at the cyclohexene ends and is favored for its methyl groups. The monoester of astaxanthin, however, allows formation of the neutral radical at C3' and prevents its formation at the opposite end where the ester group is attached. At the terminal ring without the ester group attached, migration of proton from hydroxyl group to carbonyl group facilitates resonance stabilization, similarly to already published results for astaxanthin. However, cw EPR shows no evidence of a monoester radical anion formed. This study suggests the different radicals of astaxanthin and its esters that would form in a preferred environment, either hydrophobic or hydrophilic, depending on their structure.

The metal cation chelating capacity of astaxanthin. Does this have any influence on antiradical activity?

In this Density Functional Theory study, it became apparent that astaxanthin (ASTA) may form metal ion complexes with metal cations such as Ca⁺², Cu⁺², Pb⁺², Zn⁺², Cd⁺² and Hg⁺². The presence of metal cations induces changes in the maximum absorption bands which are red shifted in all cases. Therefore, in the case of compounds where metal ions are interacting with ASTA, they are redder in color. Moreover, the antiradical capacity of some ASTA-metal cationic complexes was studied by assessing their vertical ionization energy and vertical electron affinity, reaching the conclusion that metal complexes are slightly better electron donors and better electron acceptors than ASTA.

Carotenoid radical formation: dependence on conjugation length

The relative energy of carotenoid neutral radicals formed by proton loss from the radical cations of linear carotenoids has been examined as a function of conjugation length from n = 15 to 9. For a maximum conjugation length of n = 15 (bisdehydrolycopene, a symmetrical compound), proton loss can occur from any of the 10 methyl groups, with proton loss from the methyl group at position C1 or C1' being the most favorable. In contrast, the most energetically favorable proton loss from the radical cations of lycopene, neurosporene, spheroidene, spheroidenone, spirilloxanthin, and anhydrorhodovibrin occurs from methylene groups that extend from the conjugated system. For example, decreasing the conjugation length to n = 11 (lycopene) by saturation of the double bonds C3-C4 and at C3'-C4' of bisdehydrolycopene favors proton loss at C4 or C4' methylene groups. Saturation at C7'-C8' in the case of neurosporene, spheroidene, and spheroidenone (n = 9, 10, 11) favors the formation of a neutral radical at the C8' methylene group. Saturation of C1-C2 by addition of a methoxy group to a bisdehydrolycopene-like structure with conjugation of n = 12 or 13 (anhydrorhodovibrin, spirilloxanthin) favors proton loss at the C2 methylene group. As a consequence of deprotonation of the radical cation, the unpaired electron spin distribution changes so that larger β-methyl proton couplings occur for the neutral radicals (13-16 MHz) than for the radical cation (7-10 MHz), providing a means to identify possible carotenoid radicals in biological systems by Mims ENDOR.

Free radical formation in novel carotenoid metal ion complexes of astaxanthin

The carotenoid astaxanthin forms novel metal ion complexes with Ca(2+), Zn(2+), and Fe(2+). MS and NMR measurements indicate that the two oxygen atoms on the terminal cyclohexene ring of astaxanthin chelate the metal to form 1:1 complexes with Ca(2+) and Zn(2+) at low salt concentrations <0.2 mM. The stability constants of these complexes increased by a factor of 85 upon changing the solvent from acetonitrile to ethanol for Ca(2+) and by a factor of 7 for Zn(2+) as a consequence of acetonitrile being a part of the complex. Optical studies showed that at high concentrations (>0.2 mM) of salt, 2:1 metal/astaxanthin complexes were formed in ethanol. In the presence of Ca(2+) and Zn(2+), salts the lifetime of the radical cation and dication formed electrochemically decreased relative to those formed from the uncomplexed carotenoid. DFT calculations showed that the deprotonation of the radical cation at the carbon C3 position resulted in the lowest energy neutral radical, while proton loss at the C5, C9, or C13 methyl groups was less favorable. Pulsed EPR measurements were carried out on UV-produced radicals of astaxanthin supported on silica-alumina, MCM-41, or Ti-MCM-41. The pulsed EPR measurements detected the radical cation and neutral radicals formed by proton loss at 77 K from the C3, C5, C9, and C13 methyl groups and a radical anion formed by deprotonation of the neutral radical at C3. There was more than an order of magnitude increase in the concentration of radicals on Ti-MCM-41 relative to MCM-41, and the radical cation concentration exceeded that of the neutral radicals.

A theoretical study of the reaction of beta-carotene with the nitrogen dioxide radical in solution

A theoretical study of the reaction of beta-carotene (BC) with the nitrogen dioxide radical (NO2*) in solution is carried out using the density functional theory (DFT) at the B3LYP/6-31G(d) level, to optimize the molecular geometries, and the polarizable continuum model (PCM), to account for solvent effects. The three most important reaction mechanisms--electron transfer from beta-carotene to the radical, hydrogen abstraction by the radical, and radical addition to form an adduct--are studied in detail. Three solvents with different polarities--heptane, methanol, and water--are employed to investigate the effect of the environment on the reaction mechanisms. Our results show that electron transfer is thermodynamically favored only in the polar solvents, the abstraction reactions are spontaneous in the three solvents, although faster in the polar ones, and the addition reactions are all endergonic and, therefore, unlikely to occur in any of the solvents. In both the abstraction and addition mechanisms, the attack of the radical takes place preferentially at the beta-ionone rings, in particular at positions H4 and C5, respectively. The higher stability of the reaction products in these cases is explained in terms of their molecular geometries and electronic structures.

Density functional theory study of the stability and vibrational spectra of the beta-carotene isomers

A density functional theory analysis of the stability and vibrational spectra of the beta-carotene isomers is carried out. The study includes the 7-, 9-, 11-, 13-, and 15-monocis isomers and the 7,13'-, 9,13-, 9,13'-, 9,15-, 11,11'-, and 13,15-dicis isomers. The optimized geometries needed to study the stability of the isomers are calculated at the B3LYP/6-31G(d) level of theory, and their energies are further recalculated at the higher B3LYP/6-311+G(2d,2p) level. In addition, the Wiberg bond orders and the natural bond orbital charges of the isomers are computed study the effect of the torsion of the beta-ionone rings on the conjugation degree of the polyene chain. The infrared and Raman spectra of the beta-carotene isomers are then calculated at the B3LYP/6-31G(d) level, scaling the calculated frequencies with an overall factor to account for the anharmonicity effects. The calculated frequencies are shown to compare quite well with the experiment, and the normal modes of the key bands are theoretically interpreted.

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.

Donator-acceptor map and work function for linear polyene-conjugated molecules. A density functional approximation study

Carotenoids are one particular type of conjugated chromophores with a great capacity for accepting electrons. The question posed here is how the capacity to accept electrons is related to extension of the conjugation. If there is a connection, any chromophore should represent a good antiradical, a point of interest for those investigating the biological effects of antioxidants. In order to analyze the relationship between the extension of the conjugation and the absorbance and electron-donor properties described in this paper, full geometry optimizations at the BPW91/D5DVZ level of theory are reported for a number of linear conjugated polyene systems. Maximum wavelengths were obtained using the TDDFT methodology. From these results, it is possible to conclude that large conjugated chromophores have a great capacity for accepting electrons but diminished power for donating electrons. Apparently, any chromophore should be a good antiradical, but various mechanisms exist for scavenging free radicals. In the case of linear polyene-conjugated molecules, indigo, blue, and green chromophores represent good antiradicals because they are also good antioxidants (effective electron donors). Yellow and red chromophores represent good antiradicals because they are good antireductants (effective electron acceptors). In the case of the molecules reported in this paper, the ionization energy and the electron affinity come close to the work function of graphite. This may be important for future applications, where the movement of the electrons is crucial.

Measuring Ti(III)-carotenoid radical interspin distances in TiMCM-41 by pulsed EPR relaxation enhancement method

Interspin distances between the Ti(3+) ions and the carotenoid radicals produced inside TiMCM-41 pores by photoinduced electron transfer from 7'-apo-7'-(4-carboxyphenyl)-beta-carotene (coordinated to Ti(3+)), canthaxanthin (formed as a random distribution of isomers), and beta-ionone (model for a short-chain polyene) to Ti(3+) framework sites were determined using the pulsed EPR relaxation enhancement method. To estimate the electron transfer distances, the temperature dependence of relaxation rates was analyzed in both siliceous and metal-substituted siliceous materials. The phase memory times, T(M), of the carotenoid radicals were determined from the best fits of two-pulse ESEEM curves. The spin-lattice relaxation times, T(1), of the Ti(3+) ion were obtained from the inversion recovery experiment with echo detection on a logarithmic time scale in the temperature range of 10-150 K. The relaxation enhancement for the carotenoid radicals in TiMCM-41 as compared to that in MCM-41 is consistent with an interaction between the radical and the fast relaxing Ti(3+) ion. For canthaxanthin and beta-ionone, a dramatic effect on the carotenoid relaxation rate, 1/T(M), occurs at 125 and 40 K, respectively, whereas for carboxy-beta-carotene 1/T(M) increases monotonically with increasing temperature. The interspin distances for canthaxanthin and beta-ionone were estimated from the 1/T(M) - 1/T(M0) difference, which corresponds to the Ti(3+) contribution at the temperature where the maximum enhancement in the relaxation rate occurs. Determination of the interspin distances is based on calculations of the dipolar interaction, taking into consideration the unpaired spin density distribution along the 20-carbon polyene chain, which makes it possible to obtain a fit over a wider temperature interval. A distribution of the interspin distances between the carotenoid radical and the Ti(3+) ion was obtained with the best fit at approximately 10 A for canthaxanthin and beta-ionone and approximately 9 A for 7'-apo-7'-(4-carboxyphenyl)-beta-carotene with an estimated error of +/-3 A. The interspin distances do not depend on 1/T(M) - 1/T(M0) for carboxy-beta-carotene which shows no prominent peak in the relaxation rate over the temperature range measured.

Structure and properties of 9'-cis neoxanthin carotenoid radicals by electron paramagnetic resonance measurements and density functional theory calculations: present in LHC II?

The radical intermediates formed upon catalytic or photooxidation of the carotenoid 9'-cis neoxanthin inside MCM-41 molecular sieves were detected by pulsed Mims and Davies electron nuclear double resonance (ENDOR) spectroscopies and characterized by density functional theory (DFT) calculations. Mims ENDOR spectra (20 K) were simulated using the hyperfine coupling constants predicted by DFT, which showed that a mixture of carotenoid radical cations (Car(+)) and neutral radicals (#Car) is formed. The DFT relative energies of the neutral radicals formed by proton loss from the C5, C5', C9, C9', C13, and C13'-methyl groups of Car(+) showed that #Car(9') is energetically most favorable, while #Car(9), #Car(13), #Car(13'), #Car(5'), and #Car(5) are less favorable for formation by 2.6, 5.0, 5.1, 22.5, and 25.6 kcal/mol. No evidence for formation of #Car(5') and #Car(5) was observed in the EPR spectra, consistent with DFT calculations. The epoxy group at the prime end and the allene bond at the unprime end prevent protons loss at the C5 and C5'-methyl groups by reducing the conjugation so crucial for the neutral radical stability. Previous CV measurements for allene-substituted carotenoids show that once the radical cations are formed, proton loss is rapid. These examined properties and the known crystal structure of the light harvesting complex II (LHC II) suggest the absence of the neutral radicals of 9'-cis neoxanthin available for quenching the excited states of Chl, consistent with its observed nonquenching properties.

Molecular Heterogeneous Catalysis: A Single-Site Zeolite-Supported Rhodium Complex for Acetylene Cyclotrimerization

By anchoring metal complexes to supports, researchers have attempted to combine the high activity and selectivity of molecular homogeneous catalysis with the ease of separation and lack of corrosion of heterogeneous catalysis. However, the intrinsic nonuniformity of supports has limited attempts to make supported catalysts truly uniform. We report the synthesis and performance of such a catalyst, made from [Rh(C(2)H(4))(2)(CH(3)COCHCOCH(3))] and a crystalline support, dealuminated Y zeolite, giving {Rh(C(2)H(4))(2)} groups anchored by bonds to two zeolite oxygen ions, with the structure determined by extended X-ray absorption fine structure (EXAFS) spectroscopy and the uniformity of the supported complex demonstrated by (13)C NMR spectroscopy. When the ethylene ligands are replaced by acetylene, catalytic cyclotrimerization to benzene ensues. Characterizing the working catalyst, we observed evidence of intermediates in the catalytic cycle by NMR spectroscopy. Calculations at the level of density functional theory confirmed the structure of the as-synthesized supported metal complex determined by EXAFS spectroscopy. With this structure as an anchor, we used the computational results to elucidate the catalytic cycle (including transition states), finding results in agreement with the NMR spectra.