Structure Of Carotenoids Research Articles

Antioxidant and Prooxidant Properties of Carotenoids

The ability of dietary carotenoids such as β-carotene and lycopene to act as antioxidants in biological systems is dependent upon a number of factors. While the structure of carotenoids, especially the conjugated double bond system, gives rise to many of the fundamental properties of these molecules, it also affects how these molecules are incorporated into biological membranes. This, in turn, alters the way these molecules interact with reactive oxygen species, so that the in vivo behavior may be quite different from that seen in solution. The effectiveness of carotenoids as antioxidants is also dependent upon their interaction with other coantioxidants, especially vitamins E and C. Carotenoids may, however, lose their effectiveness as antioxidants at high concentrations or at high partial pressures of oxygen. It is unlikely that carotenoids actually act as prooxidants in biological systems; rather they exhibit a tendency to lose their effectiveness as antioxidants.

Expression of a Ketolase Gene Mediates the Synthesis of Canthaxanthin in Synechococcus Leading to Tolerance Against Photoinhibition, Pigment Degradation and UV-B Sensitivity of Photosynthesis¶

The potential of ketocarotenoids to protect the photosynthetic apparatus from damage caused by excess light and UV-B radiation was assessed. Therefore, the cyanobacterium Synechococcus was transformed with a foreign beta-carotene ketolase gene under a strong promoter leading to the accumulation of canthaxanthin. This diketo carotenoid is absent in the original strain. Most of the newly formed canthaxanthin was located in the thylakoid membranes. The endogenous beta-carotene hydroxylase was unable to interact with the ketolase. Therefore, only traces of astaxanthin were found. The transformant was treated with strong light (500 or 1200 mumol m-2 s-1) and with UV-B radiation. In contrast to a nontransformed strain the overall photosynthesis, measured as oxygen evolution, was protected from inhibition by light of 500 mumol m-2 s-1 and UV-B radiation of 6.8 W m-2. Furthermore, degradation in the light of chlorophyll and carotenoids at an irradiance of 1200 mumol m-2 s-1, which was substantial in the nontransformed control, was prevented. These results indicate that in situ canthaxanthin, which is formed at the expense of zeaxanthin and replaces this hydroxy carotenoid within the photosynthetic apparatus, is a better protectant against solar radiation. These findings are discussed on the basis of the in vitro properties such as inactivating peroxyl radicals, quenching of singlet oxygen and oxidation stability of these different carotenoid structures.

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.

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.

Total synthesis of crassostreaxanthin B directed toward the biomimetic synthesis of carotenoids

Marine carotenoids halocynthiaxanthin 2, mytiloxanthin 3 and crassostreaxanthin B 4 have characteristic structures, commonly possessing a monoacetylenic end group. The cyclopentyl end group of 3 is believed to be formed in nature from the epoxide end group of 5,6-epoxy carotenoids such as 2 (Chart 1, route a). It is also conceivable that 4 including a novel tetrasubstituted olefinic end group arises from epoxy carotenoids by the opening of the C-6-oxygen bond of the oxirane ring and the subsequent migration of the methyl group at the C-1 position (route b). We found that treatment of the epoxide 6a having a partial structure of epoxy carotenoids with Lewis acids gave the cyclopentyl ethyl ketone 9 possessing the same configuration as 3, and the acyclic tetrasubstituted olefinic methyl ketone 11 including a partial structure of 4 (Chart 2). It supported the proposed metabolic pathway of 5,6-epoxy carotenoids. Toward the biomimetic synthesis of 4, we examined the reaction of epoxides having several substituents at C-6 position with Lewis acids. Among these epoxides, an epoxide 60 was found to provide a tetrasubstituted compound 61 as a major product. This could be converted into an aldehyde 73 in 8 steps which was transformed into a compound 75 through the coupling reaction with vinyllithium 63. Then, the first total synthesis of 4 was accomplished via the double Wittig condensation of 75 with phosphonium salts 76 and 77.

Supramolecular Structure of Precipitated Nanosizeβ-Carotene Particles

A combination of analytical methods and molecular modeling calculations has provided a detailed picture of the supramolecular and microscopic structure of precipitated lipophilic carotenoids. The nanoparticles have a core/shell structure (see schematic representation) in which the particle core (120 nm) consists of a variety of molecular aggregates of different sizes, and the shell (40 nm) consists of an adsorbed gelatin layer.

Supramolecular Structure of Precipitated Nanosize beta-Carotene Particles.

A combination of analytical methods and molecular modeling calculations has provided a detailed picture of the supramolecular and microscopic structure of precipitated lipophilic carotenoids. The nanoparticles have a core/shell structure (see schematic representation) in which the particle core (120 nm) consists of a variety of molecular aggregates of different sizes, and the shell (40 nm) consists of an adsorbed gelatin layer.

Apoptosis-Inducing Effect of Fucoxanthin on Human Leukemia Cell Line HL-60.

The apoptosis-inducing effect of fucoxanthin on human leukemic HL-60 cells was investigated. Fucoxanthin, which was obtained from the brown alga Undaria pinnatifida, inhibited the proliferation of HL-60 cells and induced DNA fragmentation on agarose gel electrophoresis, a typical characteristic feature of apoptotic cells. The results of sandwich ELISA using anti-biotin-antibody and anti-DNA-antibody also demonstrated DNA fragmentation in accordance with the increase in fucoxanthin concentration and incubation time. Since the level of DNA fragmentation did not increase after 24 h incubation, fucoxanthin appeared to have some effect on cell cycle. In contrast, β-carotene did not show an apoptosis-inducing effect on HL-60 cells. These results suggested that a carotenoid structure might be crucial for inducing apoptosis.

New carotenoids: Recent progress

Abstract

Functional integration of non-native carotenoids into chloroplasts by viral-derived expression of capsanthin-capsorubin synthase in Nicotiana benthamiana.

The biosynthesis of leaf carotenoids in Nicotiana benthamiana was altered by forced re-routing of the pathway to the synthesis of capsanthin, a non-native chromoplast-specific xanthophyll, using an RNA viral vector containing capsanthin-capsorubin synthase (Ccs) cDNA. The cDNA encoding Ccs was placed under the transcriptional control of a tobamovirus subgenomic promoter. Leaves from transfected plants expressing Ccs developed an orange phenotype and accumulated high levels of capsanthin (up to 36% of total carotenoids). This phenomenon was associated with thylakoid membrane distortion and reduction of grana stacking. In contrast to the situation prevailing in chromoplasts, capsanthin was not esterified and its increased level was balanced by a concomitant decrease of the major leaf xanthophylls, suggesting an autoregulatory control of chloroplast carotenoid composition. Capsanthin was exclusively recruited into the trimeric and monomeric light-harvesting complexes of photosystem II (PSII) and shown to significantly contribute to the light-harvesting capacity. On a chlorophyll basis, the concentrations of PSI and PSII reaction centres were not modified. This demonstration that higher plant antenna complexes can accommodate non-native carotenoids provides compelling evidence for functional remodelling of photosynthetic membranes toward a better photoreactivity by rational design of the incorporated carotenoid structures.

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.

Carotenoids and protection of phospholipids in solution or in liposomes against oxidation by peroxyl radicals: Relationship between carotenoid structure and protective ability

The ability of carotenoids to protect egg-yolk phosphatidylcholine (EYPC) lipids against oxidation by peroxyl radicals generated from azo-initiators was studied. In homogeneous organic solution, all the carotenoids tested ameliorated lipid peroxidation by AMVN, but none was as effective as α-tocopherol. β-Ring carotenoids showed a correlation between protective effect and rate of carotenoid destruction. β,β-Carotene and zeaxanthin, which react with peroxyl radicals at similar rates, gave a similar degree of protection in organic solution. The reactivity and protective ability of the 4,4′-diketocarotenoids, astaxanthin and canthaxanthin was less. Carotenoids incorporated into ordered membrane systems (EYPC liposomes) displayed different protective efficacies. Zeaxanthin and β-cryptoxanthin were more effective than β,β-carotene against oxidation initiated in the aqueous and lipid phases. Astaxanthin and canthaxanthin afforded less protection to the liposomal lipids. Lycopene was destroyed most rapidly but was least effective as an antioxidant. Located in the hydrophobic inner core of the bilayer, the hydrocarbons lycopene and β,β-carotene would not be in a position to readily intercept free-radicals entering the membrane from the aqueous phase. Carotenoids with polar end groups span the bilayer with their end groups located near the hydrophobic–hydrophillic interface where free-radical attack from AAPH first occurs. Hydrogen abstraction from C-4 may be one of the mechanisms of carotenoid antioxidant activity in this system. The chemical reactivity of a carotenoid is not the only factor that determines its ability to protect membranes against oxidation. The position and orientation of the carotenoid in the bilayer is also of importance.

Singlet Oxygen Quenching Abilities of Carotenoids

AbstractThe bimolecular rate constants kq for quenching of singlet oxygen (1Δg state) by 26 different natural and novel synthetic carotenoids were determined at 37 °C in a mixture of chloroform and ethanol. The steady‐state technique used involves the generation of 1O2 by thermal decomposition of disodium 3,3′‐naphtalene‐1,4‐diyl‐dipropionate endoperoxide (NDPO2) and the detection of its luminescence intensity at 1270 nm. Excitation energies (π,π*, 11Ag → 11Bu) and absorption maxima (430–590 nm) vary in the broadest range. Deeply coloured blue carotenoids are also included in the studies for the first time. An empirical correlation between the π,π* (11Ag → 11Bu) excitation energy and carotenoid structure (effective chain length Neff) was found: E(S) = 12642 cm−1 + 92027 cm−1 × 1/Neff. The quenching ability of the investigated carotenoids depends on the excitation energy of their transition at long wavelengths in a characteristic way showing as limiting factors either the thermal Arrhenius activation or the diffusion‐controlled rate. This dependence and the suspected relationship between singlet E(S) and triplet E(T) energies, respectively, are discussed.

Bacterial carotenoids 55. C50-carotenoids 25.† revised structures of carotenoids associated with membranes in psychrotrophic Micrococcus roseus

The carotenoid composition of a psychrotrophic bacterium Micrococcus roseus strain MTCC 678 has been re-examined qualitatively and quantitatively by chromatographic (TLC, HPLC) and spectroscopic (VIS, MS, 1H NMR and CD) methods, partly in direct comparison with authentic carotenoids. The carotenoid content of 0.13% of the dry weight consisted of (2S,2′S)-bactedorubedn (68% of total), bacterioruberin monoglycoside (21%), monoanhydrobacterioruberin (6%), an unidentified polar tridecasne carotenoid (3%), haloxanthin (2%) and β,β-carotene (0.4%): a carotenoid complement associated with that of halophilic bacteria. Rapid analysis demonstrated that bacterioruberin occurred as the all-E isomer (75% of total), together with the 5Z-(5%), 9Z-(5%) and 13Z-(15%) geometrical isomers. The established interaction of the major carotenoid with membrane vesicles, compatible with cold adaptation, is rationalized.

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...

Triplet energy transfer between the primary donor and carotenoids in Rhodobacter sphaeroides R-26.1 reaction centers incorporated with spheroidene analogs having different extents of pi-electron conjugation.

Three carotenoids, spheroidene, 3,4-dihydrospheroidene and 3,4,5,6-tetrahydrospheroidene, having 8, 9 and 10 conjugated carbon-carbon double bonds, respectively, were incorporated into Rhodobacter (Rb.) sphaeroides R-26.1 reaction centers. The extents of binding were found to be 95 +/- 5% for spheroidene, 65 +/- 5% for 3,4-dihydrospheroidene and 60 +/- 10% for 3,4,5,6-tetrahydrospheroidene. The dynamics of the triplet states of the primary donor and carotenoid were measured at room temperature by flash absorption spectroscopy. The carotenoid, spheroidene, was observed to quench the primary donor triplet state. The triplet state of spheroidene that was formed subsequently decayed to the ground state with a lifetime of 7.0 +/- 0.5 microseconds. The primary donor triplet lifetime in the Rb. sphaeroides R-26.1 reaction centers lacking carotenoids was 60 +/- 5 microseconds. Quenching of the primary donor triplet state by the carotenoid was not observed in the Rb. sphaeroides R-26.1 reaction centers containing 3,4-dihydrospheroidene nor in the R-26.1 reaction centers containing 3,4,5,6-tetrahydrospheroidene. Triplet-state electron paramagnetic resonance was also carried out on the samples. The experiments revealed carotenoid triple-state signals in the Rb. sphaeroides R-26.1 reaction centers incorporated with spheroidene, indicating that the primary donor triplet is quenched by the carotenoid. No carotenoid signals were observed from Rb. sphaeroides R-26.1 reaction centers incorporating 3,4-dihydrospheroidene nor in reaction centers incorporating 3,4,5,6-tetrahydrospheroidene. Circular dichroism, steady-state absorbance band shifts accompanying the primary photochemistry in the reaction center and singlet energy transfer from the carotenoid to the primary donor confirm that the carotenoids are bound in the reaction centers and interacting with the primary donor. These studies provide a systematic approach to exploring the effects of carotenoid structure and excited-state energy on triplet transfer between the primary donor and carotenoids in reaction centers from photosynthetic bacteria.

Quenching of chlorophyll fluorescence in the major light-harvesting complex of photosystem II: a systematic study of the effect of carotenoid structure.

The role of carotenoids in quenching of chlorophyll fluorescence in the major light-harvesting complex of photosystem II has been studied with a view to understanding the molecular basis of the control of photoprotective nonradiative energy dissipation by the xanthophyll cycle in vivo. The control of chlorophyll fluorescence quenching in the isolated complex has been investigated in terms of the number of the conjugated double bonds for a series of carotenoids ranging from n = 5-19, giving an estimated first excited singlet state energy from 20,700 cm-1 to 10,120 cm-1. At pH 7.8 the addition of exogenous carotenoids with >=10 conjugated double bonds (including zeaxanthin) stimulated fluorescence quenching relative to the control with no added carotenoid, whereas those with n </= 9 conjugated double bonds (e.g., violaxanthin) had no effect on fluorescence. When quenching in the light-harvesting complex of photosystem II was induced by a lowering of pH to 5.5, carotenoids with n </= 9 conjugated double bonds (including violaxanthin) caused a noticeable inhibition of fluorescence quenching relative to the control. Of the 10 carotenoids tested, quenching induced by the addition of the tertiary amine compound, dibucaine, to isolated light-harvesting complex of photosystem II could only be reversed by violaxanthin. These results are discussed in terms of the two theories developed to explain the role of zeaxanthin and violaxanthin in nonphotochemical quenching of chlorophyll fluorescence.

Structure and properties of carotenoids in relation to function.

The basic principles of structure, stereochemistry, and nomenclature of carotenoids are described and the relationships between structure and the chemical and physical properties on which all the varied biological functions and actions of carotenoids depend are discussed. The conjugated polyene chromophore determines not only the light absorption properties, and hence color, but also the photochemical properties of the molecule and consequent light-harvesting and photoprotective action. The polyene chain is also the feature mainly responsible for the chemical reactivity of carotenoids toward oxidizing agents and free radicals, and hence for any antioxidant role. In vivo, carotenoids are found in precise locations and orientations in subcellular structures, and their chemical and physical properties are strongly influenced by other molecules in their vicinity, especially proteins and membrane lipids. In turn, the carotenoids influence the properties of these subcellular structures. Structural features such as size, shape, and polarity are essential determinants of the ability of a carotenoid to fit correctly into its molecular environment to allow it to function. A role for carotenoids in modifying structure, properties, and stability of cell membranes, and thus affecting molecular processes associated with these membranes, may be an important aspect of their possible beneficial effects on human health.--Britton, G. Structure and properties of carotenoids in relation to function.

ChemInform Abstract: Carotenoids and Related Polyenes. Part 2. Photoisomerization of an Allenic Carotenoid, Peridinin, and Allenic Model Compounds.

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Carotenoids and related polyenes. Part 2. Photoisomerization of an allenic carotenoid, peridinin, and allenic model compounds

Iodine-catalysed photoisomerization of peridinin 1a gave the novel (6S)-allenic isomer 1b whose structure was confirmed by chemical synthesis. In addition, the photochemical behaviour of several model compounds having a part structure of allenic carotenoids was investigated. From these results, photoisomerization of the allenic double bond in allenic carotenoids was discussed.