Theoretical study on the chemical fate of adducts formed through free radical addition reactions to carotenoids

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.

Melatonin and its metabolites vs oxidative stress: From individual actions to collective protection

Oxidative stress (OS) represents a threat to the chemical integrity of biomolecules including lipids, proteins, and DNA. The associated molecular damage frequently results in serious health issues, which justifies our concern about this phenomenon. In addition to enzymatic defense mechanisms, there are compounds (usually referred to as antioxidants) that offer chemical protection against oxidative events. Among them, melatonin and its metabolites constitute a particularly efficient chemical family. They offer protection against OS as individual chemical entities through a wide variety of mechanisms including electron transfer, hydrogen transfer, radical adduct formation, and metal chelation, and by repairing biological targets. In fact, many of them including melatonin can be classified as multipurpose antioxidants. However, what seems to be unique to the melatonin's family is their collective effects. Because the members of this family are metabolically related, most of them are expected to be present in living organisms wherever melatonin is produced. Therefore, the protection exerted by melatonin against OS may be viewed as a result of the combined antioxidant effects of the parent molecule and its metabolites. Melatonin's family is rather exceptional in this regard, offering versatile and collective antioxidant protection against OS. It certainly seems that melatonin is one of the best nature's defenses against oxidative damage.

Carotenoids: Experimental Ionization Energies and Capacity at Inhibiting Lipid Peroxidation in a Chemical Model of Dietary Oxidative Stress

Carotenoids are important natural pigments and micronutrients contributing to health prevention by several mechanisms, including their electron-donating (antioxidant) activity. In this work, a large series of carotenoids, including 11 carotenes and 14 xanthophylls, have been investigated by wavelength-resolved atmospheric pressure photoionization mass spectrometry (DISCO line of SOLEIL synchrotron), thus allowing the experimental determination of their ionization energy (IE) for the first time. On the other hand, the carotenoids have been also investigated for their ability to inhibit the heme iron-induced peroxidation of linoleic acid in mildly acidic micelles, a simple but relevant chemical model of oxidative stress in the gastric compartment. Thus, the carotenoids can be easily classified from IC₅₀ concentrations deduced from the time dependence of the lipid hydroperoxide concentration. With a selection of two carotenes and three xanthophylls a quantitative analysis is also provided to extract stoichio-kinetic parameters. The influence of the carotenoid structure (number of conjugated carbon-carbon double bonds, presence of terminal six-membered rings, hydroxyl, keto, and/or epoxy groups) on the IE, IC₅₀, and stoichio-kinetic parameters is discussed in details. The data show that the antioxidant activity of carotenes is well correlated to their electron-donating capacity, which itself largely depends on the length of the conjugated polyene chain. By contrast, the IE of xanthophylls is poorly correlated to the polyene chain length because of the strong, and sometimes unexpected, electronic effects of the O-atoms. Although IE remains an approximate predictor of the antioxidant activity of xanthophylls, other factors (interaction with the aqueous phase, competing radical-scavenging mechanisms, the residual activity of the antioxidant's oxidation products) probably play a significant role.

Singlet Fission Reaction of Light-Exposed β-Carotene Bound to Bovine Serum Albumin. A Novel Mechanism in Protection of Light-Exposed Tissue by Dietary Carotenoids

We have attempted to investigate the role of carotenoids (Car) in protecting pigment-protein complexes against light-induced degradation. Upon direct photoexcitation of β-carotene (β-Car), nanosecond flash photolysis and femtosecond time-resolved spectroscopy detected a substantial population of triplet states for β-Car aggregates associated with bovine serum albumin (BSA) or dispersed in aqueous phase with 10% tetrahydrofuran (THF), but none were observed for monomeric β-Car in neat THF. The direct photogeneration of triplet states was on the time scale of <1 ps, indicating that the underlying reaction mechanism was singlet fission (SF). Efficient triplet-triplet annihilation in the time regime from picoseconds to microseconds resulted in a <1 μs triplet lifetime for β-Car aggregates, in contrast to a 20 μs lifetime for monomeric β-Car as determined by anthracene-sensitized flash photolysis. The short-lived triplet excitations of β-Car aggregates associated with BSA or dispersed in aqueous phase were found to be insensitive to the presence of oxygen, which are considered to be important for the protection of both protein and carotenoid against light-induced degradation via reaction with oxidative species.

Phenolic Melatonin-Related Compounds: Their Role as Chemical Protectors against Oxidative Stress

There is currently no doubt about the serious threat that oxidative stress (OS) poses to human health. Therefore, a crucial strategy to maintain a good health status is to identify molecules capable of offering protection against OS through chemical routes. Based on the known efficiency of the phenolic and melatonin (MLT) families of compounds as antioxidants, it is logical to assume that phenolic MLT-related compounds should be (at least) equally efficient. Unfortunately, they have been less investigated than phenols, MLT and its non-phenolic metabolites in this context. The evidence reviewed here strongly suggests that MLT phenolic derivatives can act as both primary and secondary antioxidants, exerting their protection through diverse chemical routes. They all seem to be better free radical scavengers than MLT and Trolox, while some of them also surpass ascorbic acid and resveratrol. However, there are still many aspects that deserve further investigations for this kind of compounds.

Binding to Bovine Serum Albumin Protects β-Carotene against Oxidative Degradation

Binding to bovine serum albumin (BSA) was found to protect β-carotene (β-Car) dissolved in air-saturated phosphate buffer solution/tetrahydrofuran (9:1, v/v) efficiently against photobleaching resulting from laser flash excitation at 532 nm. From dependence of the relative photobleaching yield upon the BSA concentration, an association constant of Ka = 4.67 × 10(5) L mol(-1) for β-Car binding to BSA was determined at 25 °C. Transient absorption spectroscopy confirmed less bleaching of β-Car on the microsecond time scale in the presence of BSA, while kinetics of triplet-state β-Car was unaffected by the presence of oxygen. The protection of β-Car against this type of reaction seems accordingly to depend upon dissipation of excitation energy from an excited state into the protein matrix. Static quenching of BSA fluorescence by β-Car had a Stern-Volmer constant of Ksv = 2.67 × 10(4) L mol(-1), with ΔH = 17 kJ mol(-1) and ΔS = 142 J mol(-1) K(-1) at 25 °C. Quenching of tryptophan (Trp) fluorescence by β-Car suggests involvement of Trp in binding of β-Car to BSA through hydrophobic interaction, while the lower value for the Stern-Volmer constant Ksv compared to the binding constant, Ka, may indicate involvement of β-Car aggregates. Bound β-Car increased the random coil fraction of BSA at the expense of α-helix, as shown by circular dichroism, affecting the β-Car configuration, as shown by Raman spectroscopy.

Lipoic acid and dihydrolipoic acid. A comprehensive theoretical study of their antioxidant activity supported by available experimental kinetic data

The free radical scavenging activity of lipoic acid (LA) and dihydrolipoic acid (DHLA) has been studied in nonpolar and aqueous solutions, using the density functional theory and several oxygen centered radicals. It was found that lipoic acid is capable of scavenging only very reactive radicals, while the dehydrogenated form is an excellent scavenger via a hydrogen transfer mechanism. The environment plays an important role in the free radical scavenging activity of DHLA because in water it is deprotonated, and this enhances its activity. In particular, the reaction rate constant of DHLA in water with an HOO(•) radical is close to the diffusion limit. This has been explained on the basis of the strong H-bonding interactions found in the transition state, which involve the carboxylate moiety, and it might have implications for other biological systems in which this group is present.

Ellagic acid: an unusually versatile protector against oxidative stress

Several aspects related to the antioxidant activity of ellagic acid were investigated using the density functional theory. It was found that this compound is unusually versatile for protecting against the toxic effects caused by oxidative stress. Ellagic acid, in aqueous solution at physiological pH, is able of deactivating a wide variety of free radicals, which is a desirable capability since in biological systems, these species are diverse. Under such conditions, the ellagic acid anion is proposed as the key species for its protective effects. It is predicted to be efficiently and continuously regenerated after scavenging two free radicals per cycle. This is an advantageous and unusual behavior that contributes to increase its antioxidant activity at low concentrations. In addition, the ellagic acid metabolites are also capable of efficiently scavenging a wide variety of free radicals. Accordingly, it is proposed that the ellagic acid efficiency for that purpose is not reduced after being metabolized. On the contrary, it provides continuous protection against oxidative stress through a free radical scavenging cascade. This is an uncommon and beneficial behavior, which makes ellagic acid particularly valuable to that purpose. After deprotonation, ellagic acid is also capable of chelating copper, in a concentration dependent way, decreasing the free radical production. In summary, ellagic acid is proposed to be an efficient multiple-function protector against oxidative stress.

Electron transfer from plant phenolates to carotenoid radical cations. Antioxidant interaction entering the Marcus theory inverted region

β-Carotene, lycopene, and zeaxanthin are maximally regenerated by plant phenolates from their radical cations formed during laser flash photolysis in 9:1 (v/v) chloroform/methanol for a driving force corresponding to the reorganization energy according to the Marcus theory. For β-carotene, the reorganization energy has values of 0.41 ± 0.04 and 0.40 ± 0.04 eV for the plant phenols in the presence of 1 and 2 equiv of base, respectively, at 23 °C. For a driving force lower than the reorganization energy, regeneration of the carotenoids is less efficient as is seen for m-hydroxybenzoic acid, vanillic acid, and p-coumaric acid. For a driving force above the maximum rate as determined to have kET = 6.3 × 10(9) L·mol(-1)·s(-1) for syringic acid and β-carotene, the reaction becomes gradually slower and regeneration less efficient as is seen for the more reducing caffeic acid, rutin, and quercetin corresponding to an inverted region for the rate of electron transfer. Lycopene and zeaxanthin show a similar behavior for the same series of plant phenols with slightly lower reorganization energy, in agreement with the lower reduction potential of their radical cations, while, for the ketocarotenoids astaxanthin and canthaxanthin, fast reactions with a solvent of radical cations inhibit regeneration from being detected. Intermediate reducing plant phenols accordingly yield maximal protection of carotenoids against photobleaching in foods and beverages.

On the peroxyl scavenging activity of hydroxycinnamic acid derivatives: mechanisms, kinetics, and importance of the acid-base equilibrium

The peroxyl radical scavenging activity of four hydroxycinnamic acid derivatives (HCAD) has been studied in non-polar and aqueous solutions, using the density functional theory. The studied HCAD are: ferulic acid (4-hydroxy-3-methoxycinnamic acid), p-coumaric acid (trans-4-hydroxycinnamic acid), caffeic acid (3,4-dihydroxycinnamic acid), and dihydrocaffeic acid (3-(3,4-dihydroxyphenyl)-2-propionic acid). It was found that the polarity of the environment plays an important role in the relative efficiency of these compounds as peroxyl scavengers. It was also found that in aqueous solution the pH is a key factor for the overall reactivity of HCAD towards peroxyl radicals, for their relative antioxidant capacity, and for the relative importance of the different mechanisms of reaction. The H transfer from the phenolic OH has been identified as the main mechanism of reaction in non-polar media and in aqueous solution at acid pHs. On the other hand, the single electron transfer mechanism from the phenoxide anion is proposed to be the one contributing the most to the overall peroxyl scavenging activity of HCAD in aqueous solution at physiological pH (7.4). This process is also predicted to be a key factor in the reactivity of these compounds towards a large variety of free radicals.

Carotenoids in antioxidant networks. Colorants or radical scavengers

Optical and electronic properties of carotenoids as also reflected in their colors have been fine-tuned through evolution, resulting in a structural diversity important for carotenoid properties as radical scavengers and as quenchers of electronically excited states. Carotenoids form antioxidant networks based on one-electron transfer with other carotenoids depending on the balance between ionization energy and electron affinity of the individual carotenoids as has been demonstrated by real-time kinetic studies and later confirmed by quantum mechanical calculations. The more hydrophilic xanthophylls serve as molecular wiring across membranes in these networks through anchoring in water/lipid interfaces resulting in synergism with more lipophilic carotenoids. Radical scavenging of such networks seems to be thermodynamically controlled according to a two-dimensional classification of potential antioxidants. Carotenoids in birds' plumage, as reflected by their color and color intensity, seem to be indicators of good antioxidant status and health of the bird, and such antioxidant networks appear to be in "equilibrium". Carotenoids are under other conditions involved in networks with other types of antioxidants as in egg yolk and in some fish. For the more hydrophilic (iso)flavonoids and their glycosides, antioxidant synergism through regeneration of the lipophilic carotenoids active as radical scavengers becomes kinetically controlled at interfaces. Carotenoids appear accordingly, and also in food, as antioxidants under two types of conditions: (i) in "equilibrium" with other antioxidants in thermodynamically controlled networks serving as color indicators of good antioxidant status and (ii) as antioxidants active through radical scavenging in networks with kinetically controlled regeneration.

Melatonin as a natural ally against oxidative stress: a physicochemical examination

Oxidative stress has been proven to be related to the onset of a large number of health disorders. This chemical stress is triggered by an excess of free radicals, which are generated in cells because of a wide variety of exogenous and endogenous processes. Therefore, finding strategies for efficiently detoxifying free radicals has become a subject of a great interest, from both an academic and practical points of view. Melatonin is a ubiquitous and versatile molecule that exhibits most of the desirable characteristics of a good antioxidant. The amount of data gathered so far regarding the protective action of melatonin against oxidative stress is overwhelming. However, rather little is known concerning the chemical mechanisms involved in this activity. This review summarizes the current progress in understanding the physicochemical insights related to the free radical-scavenging activity of melatonin. Thus far, there is a general agreement that electron transfer and hydrogen transfer are the main mechanisms involved in the reactions of melatonin with free radicals. However, the relative importance of other mechanisms is also analyzed. The chemical nature of the reacting free radical also has an influence on the relative importance of the different mechanisms of these reactions. Therefore, this point has also been discussed in detail in the current review. Based on the available data, it is concluded that melatonin efficiently protects against oxidative stress by a variety of mechanisms. Moreover, it is proposed that even though it has been referred to as the chemical expression of darkness, perhaps it could also be referred to as the chemical light of health.

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.