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Reactive oxygen species comprise oxygen-based free radicals and non-radical species such as peroxynitrite and electronically excited (singlet) oxygen. These reactive species often have short lifetimes, and much of our understanding of their formation and reactivity in biological and especially medical environments has come from complimentary fast reaction methods involving pulsed lasers and high-energy radiation techniques. These and related methods, such as EPR, are discussed with particular reference to singlet oxygen, hydroxy radicals, the superoxide radical anion, and their roles in medical aspects, such as cancer, vision and skin disorders, and especially pro- and anti-oxidative processes.

The interactions of dietary carotenoids, and particularly the xanthophylls in the macula, with singlet oxygen and three different oxy-radicals, (hydroxyl radical, nitrogen dioxide and the superoxide radical anion) are compared using pulsed laser and γ-techniques. The results give possible molecular mechanisms for the switch from anti-oxidant (protection) by carotenoids to pro-oxidant (damage) by carotenoids. The participation of oxygen in radical mechanisms in the presence of different carotenoids is compared for the different radicals. It is shown that the mechanistic role of oxygen differs very significantly for anti-/pro-oxidation by hydroxyl radicals when compared to nitrogen dioxide. Lutein was found to be an extremely good cell protector against hydroxyl radicals at all oxygen concentrations, including under physiological conditions.

Carotenoid pigments, particularly β-carotene and lycopene, are consumed in human foodstuffs and play a vital role in maintaining health. β-carotene is known to quench singlet oxygen and can have strong antioxidant activity. As such, it was proposed that β-carotene might reduce the risk of cancer. Epidemiological studies found inverse relationships between cancer risk and β-carotene intake or blood levels. However, clinical trials failed to support those findings and β-carotene supplementation actually increased lung cancer incidence in male smokers. Early experimental animal studies found dietary β-carotene inhibited UV-induced skin cancers. Later studies found that β-carotene supplementation exacerbated UV-carcinogenic expression. The discrepancies of these results were related to the type of diet the animals consumed. Lycopene has been associated with reduced risk of lethal stage prostate cancer. Other carotenoids, e.g., lutein and zeaxanthin, play a vital role in visual health. Numerous studies of molecular mechanisms to explain the carotenoids’ mode of action have centered on singlet oxygen, as well as radical reactions. In cellular systems, singlet oxygen quenching by carotenoids has been reported but is more complex than in organic solvents. In dietary β-carotene supplement studies, damaging pro-oxidant reactivity can also arise. Reasons for this switch are likely due to the properties of the carotenoid radicals themselves. Understanding singlet oxygen reactions and the anti-/pro-oxidant roles of carotenoids are of importance to photosynthesis, vision and cancer.

This work investigates the evolution of the tensile and structural properties of fused filament fabrication (FFF), formed polymers under gamma irradiation. Commercial off-the-shelf print filaments of Poly(lactic acid) (PLA), Thermoplastic polyurethane (TPU), Chlorinated polyethylene elastomer (CPE), Nylon, Acrylonitrile butadiene styrene (ABS) and Polycarbonate (PC) were exposed to gamma-ray doses of up to 5.3 MGy. The suitability of FFF-formed components made from these materials for use in radiation environments is evaluated by considering their structural properties. We identify clear trends in the structural properties of all the materials tested and correlate them with changes in the chemical structure. We find that Nylon shows the best performance under these conditions, with no change in ultimate tensile strength and an increase in stiffness. However, some of our findings suggest that the effect of additives to this type of filament may result in potentially undesirable adhesive properties. The organic polymer PLA was notably more radiation-sensitive than the other materials tested, showing 50% decrease in Young’s Modulus and ultimate tensile strength at order of magnitude lower radiation dose. A mechanism is proposed whereby FFF-processed components would have substantially different radiation tolerances than bulk material.

Reducing radiation damage is important and dietary antioxidants that can protect cells from such damage are of value. Dietary lycopene, a carotenoid found in tomatoes, protects human lymphoid cell membranes from damage by γ-radiation. We report that such protective effects are remarkably reduced as the oxygen concentration increases - near zero at 100% oxygen from fivefold protection at 20% oxygen and, dramatically, from 50-fold protection at 0% oxygen. Such huge differences imply that under higher oxygen concentrations lycopene could lead to improved cancer therapy using γ-radiation. The cells are not efficiently protected from the superoxide radical by lycopene. Noncellular studies suggest molecular mechanisms for the oxygen effect.

We report on studies of reactions of singlet oxygen with carotenoids and retinoids and a range of free radical studies on carotenoids and retinoids with emphasis on recent work, dietary carotenoids and the role of oxygen in biological processes. Many previous reviews are cited and updated together with new data not previously reviewed. The review does not deal with computational studies but the emphasis is on laboratory-based results. We contrast the ease of study of both singlet oxygen and polyene radical cations compared to neutral radicals. Of particular interest is the switch from anti- to pro-oxidant behavior of a carotenoid with change of oxygen concentration: results for lycopene in a cellular model system show total protection of the human cells studied at zero oxygen concentration, but zero protection at 100% oxygen concentration.

Molecular mechanisms associated with the anti-/pro-oxidative properties of carotenoids (CARs) are described in organic solvents, micro-heterogeneous environments and model lipid membranes and in cellular suspensions. Singlet oxygen is important in the skin and eye and CARs are efficient singlet oxygen (SO) quenchers with corresponding rate constants near diffusion controlled (typically app. 10¹⁰ M⁻¹ s⁻¹) with lycopene (LYC) exhibiting the most efficient quenching in organic solvents. However, in membrane environments there is little or no difference in the quenching efficiency between the dietary CARs. Furthermore, aggregation of CARs, particularly those in the macula (lutein and zeaxanthin), markedly reduces SO quenching efficiency. Free radical interactions with CARs leads to at least three processes, electron and hydrogen atom transfer and adduct formation. The most studied is electron transfer where the CAR loses an electron to become a radical cation. The reactivity/lifetime of such CAR radicals may lead to a switch from anti- to pro-oxidant behaviour of CARs. These reactions are related to CAR redox potentials with LYC being the lowest (most easily oxidised) allowing LYC to reduce/repair all other CAR radical cations and LYC 'sacrificed' where mixtures of CARs are present in oxidative environments. Such redox-controlled reactions may lead to deleterious as well as beneficial health effects.

Time-resolved pulse radiolysis investigations reported herein show that the carotenoids β-carotene, lycopene, zeaxanthin and astaxanthin (the last two are xanthophylls – oxygen containing carotenoids) are capable of both reducing oxidized guanosine as well as minimizing its formation. The reaction of the carotenoid with the oxidized guanosine produces the radical cation of the carotenoid. This behavior contrasts with the reactions between the amino acids and dietary carotenoids where the carotenoid radical cations oxidized the amino acids (tryptophan, cysteine and tyrosine) at physiological pH.

International audience; Reaction of [PdMe2(tmeda)] with pyridyl-N-di(tert-butyl)phosphinoimine spontaneously affords an unusual bimetallic palladium(I) ligand-based biradical complex, which behaves as a "masked" form of Pd0 in its reactions with neutral ligands and chlorobenzene.

Spectroscopic and chemical properties of γ-CD radicals, resulting from the abstraction by HO radicals of hydrogen atoms, have been investigated using pulse radiolysis. The reactions of γ-CD radicals with C60 in 2:1 γ-CD/C60 inclusion complexes have been studied in aqueous solutions. It has been demonstrated that the γ-CD radicals are reducing species producing C60- monoanion radicals, as well as doubly reduced C602-, well characterised by their absorption spectra in the near IR. The oxidation potential of γ-CD radical is estimated to be more negative than −390 mV vs. NHE. The kinetics of the C60 reduction by γ-CD radicals have been determined and compared with kinetics by other reducing species including the solvated electron (eaq-) and CO2- radicals. It was observed that the method of preparation of the 2:1 γ-CD/C60 inclusion complexes modifies the C60 reduction mechanism.

The essential thiol of the enzyme papain has been caged by linking to an aromatic thiol. The resulting caged protein is inactive but enzymatic activity is fully restored upon chemical cleavage of the protective disulfide bond. We have exploited the chemistry of this disulfide bond to uncage papain by pulse radiolysis. We have shown that up to 10% of the enzyme activity can be restored by reductive pulse radiolysis. This approach has been tested on a small-molecule model system, and experiments on this model compound show that pulse radiolysis of the mixed cysteine-aromatic disulfide results in selective reduction of the disulfide bond to generate a thiol in 10-20% yield, consistent with the radiolytically restored activity of the caged papain quantified by the biochemical assay.

The carbonate radical (CO 3 (*-)) is of importance in biology and chemistry. We used pulse radiolysis to generate the CO 3 (*-) radical and show there is no reaction with oxygen. However, in the presence of ammonia the CO 3 (*-) radical is removed by NO (*), which itself arises from the scavenging of NH 2 (*) by oxygen, and the mechanism of this process is reported. The CO 3 (*-) radical shows complex decay patterns in the presence of ammonia, which can be understood as a balance between the radical-radical reaction CO 3 (*-) + CO 3 (*-) and CO 3 (*-) + NH 2 (*) (the amino radical). Also, we report reactivity with glycine and alanine and with melanin models. The CO 3 (*-) reacts with both dopa-melanin (DM, a model of black eumelanin) and with cysteinyl-dopa-melanin (CDM, a model of red/blond phaeomelanin). However, the reaction rate constant is much higher with CDM than with DM.

Redox active metal ions, carbon-centered radicals, and oxygen-centered radicals are important to oxidative stress. A radical detector combining a nitrone spin trap, a phenol, and a cyclopropane radical clocklike unit was prepared and used with EPR spectroscopy to detect and distinguish between hydroxyl radicals, methyl radicals, and iron(III) ions. Iron(III) reacts with the phenol unit inducing opening of the cyclopropane ring and cyclization to generate a stable nitroxyl radical.

In this study, we report the protonation reactions for astaxanthin and canthaxanthin radical anions in methanol, alkaline methanol, and aqueous 2% Triton X-100 at different pH values. The pKa values for the corresponding alpha-hydroxy radical derivatives of astaxanthin, canthaxanthin, and beta-apo-8'-carotenal were estimated in 2% Triton X-100. Also, the effects of the microenvironment and the structure of the carotenoids on the protonation rate constant are discussed.

In search of a simple internal complexation of C 60 in cyclodextrins, six 2:1 cyclodextrin:[60]fullerene conjugates involving different types of linkers have been synthesised. Using spectrophotometry, laser flash photolysis and pulse radiolysis, we have investigated spectroscopic and photophysical properties of these 2:1 cyclodextrin [60]fullerene monoadduct conjugates ( CDF ), including ground and triplet state absorption spectra, triplet molar absorption coefficients ( e T ), quantum yields of triplet ( Φ T ) and singlet oxygen formation ( Φ Δ ) in water and toluene, in order to probe the possibility of 2:1 inclusion complexes of C 60 between two cyclodextrins. In water, the CDF spectroscopic and photophysical properties indicate hydrophobically driven self-assemblies of the amphiphilic conjugates into spherical micelles with no evidence for the presence of a 2:1 inclusion complex, while in toluene they show no sign of aggregation. It is observed that formation of aggregates in water does not prevent scavenging of HO radical by the C 60 moiety of the CD–C 60 conjugates.

The presence of an acetyl tryptophan ester gives rise to enhanced yields of carotenoid radical cations in chloroform following 355 nm laser excitation of the carotenoid, even though the tryptophan does not absorb at this wavelength. The increase is attributed to positive charge transfer from semi-oxidized tryptophan itself generated by light absorbed by the carotenoid. The mechanism of these radical processes has been elucidated by pulse radiolysis studies.

The radical cations of five dietary carotenoids have been shown to react with dopamelanin (a model of the black eumelanin) and cysteinyldopamelanin (a model of the red/yellow pheomelanin) in both micellar and homogeneous environments. It is suggested that such repair reactions show previously unidentified protective roles for melanin/carotenoid combinations, especially where the maintenance of the (small) carotenoid concentration is vital to avoid deleterious effects, such as in the retina of the eye.

Antioxidants like β-carotene, α-tocopherol and ascorbic acid should be able to protect human cells against damage due to ultraviolet light. Cultured human fibroblasts have been irradiated with UVA or UVB light after incubation with the antioxidants or combinations of them. The efficiency of the protection by the antioxidants in dietary concentrations is estimated by cell counting following cell culture. In the case of UVA irradiation we find synergistic effects of combinations with β-carotene as the main protector. On the other hand, only additive effects of the tested combinations are observed in the experiments with UVB light. Our experiments show a protective effect of dietary antioxidants against human tissue cell damage by ultraviolet light.

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

This chapter discusses a variety of free radicals and other reactive oxygen species that are biologically and medically relevant. Radiolytic and/or photochemical methods of production for each reactive oxygen species are shown and for each type of reactive oxygen species some antioxidant and/or biomolecule interactions are discussed. Additionally, the techniques of laser flash photolysis and pulse radiolysis are described in detail and a comparison of the two techniques is made.

Carotenoids are known to act as dietary antioxidants and so are of wide interest for their health benefits. Dietary lycopene, the carotenoid pigment in tomatoes, has been shown to protect against human lymphoid cell membrane damage from free radicals produced by gamma radiation and also by the nitrogen dioxide radical, generated photolytically. This protective effect is dramatically reduced as the oxygen concentration increases, particularly for damage due to gamma radiolysis – becoming near zero at 100% oxygen from 5-fold protection at 20% oxygen and 50-fold protection at zero per-cent oxygen. The effect is less pronounced for nitrogen dioxide-induced cell killing falling from 17-fold protection in the absence of oxygen to 9-fold at 100% oxygen. Non-cellular gamma radiation and laser studies were also carried out to support the molecular mechanisms suggested for the effect of oxygen. The remarkable reduction in protection by lycopene against gamma radiation at high oxygen concentrations could be exploited to enhance radiation procedures for therapy and preliminary studies have also been undertaken for irradiations with high energy protons.

The dietary carotenoids provide photoprotection to photosynthetic organisms, the eye and the skin. The protection mechanisms involve both quenching of singlet oxygen and of damaging free radicals. The mechanisms for singlet oxygen quenching and protection against free radicals are quite different - indeed, under some conditions, quenching of free radicals can lead to a switch from a beneficial anti-oxidant process to damaging pro-oxidative situation. Furthermore, while skin protection involves β-carotene or lycopene from a tomato-rich diet, protection of the macula involves the hydroxyl-carotenoids (xanthophylls) zeaxanthin and lutein. Time resolved studies of singlet oxygen and free radicals and their interaction with carotenoids via pulsed laser and fast electron spectroscopy (pulse radiolysis) and the possible involvement of amino acids are discussed and used to (1) speculate on the anti- and pro-oxidative mechanisms, (2) determine the most efficient singlet oxygen quencher and (3) demonstrate the benefits to photoprotection of the eye from the xanthophylls rather than from hydrocarbon carotenoids such as β-carotene.

There is current interest in the health benefits of dietary carotenoids and the possible deleterious effects on certain sub-populations such as smokers. Here we report in vivo protection of human lymphocytes, conferred by dietary supplementation of lycopene rich foods against the reactive oxygen species, NO(2)(*) radical (by electron transfer) and 1(O)(2) (by energy transfer). It was found that a lycopene rich diet, maintained for 14 days, increased the serum lycopene level 10 fold compared to serum obtained after the same period, where a typical western European diet had been consumed. Relative lymphocyte protection factors of 17.6 and 6.3 against NO(2)(*) radical and 1(O)(2), respectively, were obtained, which re-enforce epidemiological data, showing protection against several chronic diseases by tomato lycopene.

There is a resurgence of interest in the role of electron transfer reactions involving beta-carotene in photosynthesis. There is also current debate on the health benefits of dietary carotenoids and the possible deleterious effects on certain sub-populations such as smokers. The impact of dietary carotenoids on health may well be also related to radical reactions. A key parameter in biological systems is therefore the one-electron reduction potential of the carotenoid radical cation, now reported for the first time in a model biological aqueous environment. The value obtained is 1.06+/-0. 01 V and is sufficiently high to oxidise cell membrane proteins, but is low enough to repair P(680).+ in the photosynthetic reaction centre.

In situ 31P NMR spectroscopic studies of the reaction of the primary diphosphine 1,2-(PH2)2-C6H4 with the mixed-metal base system nBuLi/Sb(NMe2)3, combined with X-ray structural investigations, strongly support a mechanism involving a series of deprotonation steps followed by antimony-mediated reductive C–P bond cleavage. The central intermediate in this reaction is the tetraphosphide dianion [C6H4P2]22− ([4]) from which the final products, the 1,2,3-triphospholide anion [C6H4P3]− (3) and [PhPHLi] (8·Li), are evolved. An EPR spectrocopic study suggests that homolytic C–P bond cleavage is likely to be involved in this final step.

We report the direct observation of the quenching of the weakly absorbing transient due to the amino radical by oxygen and, hence determine, by a totally direct method, the corresponding rate constant (k = (1.1 +/- 0.1) x 10(9) dm3 mol(-1) s(-1)). We also report the rate constants for the reactions of the amino radical with several amino acids and models of black eumelanin and blond/red phaeomelanin. These reactions lead to a mechanism, based on free radicals, that can explain why ammonia is useful in commercial hair (melanin) bleaching, avoiding excessive amino acid (hair protein) damage.

The relative reduction potentials of a variety of carotenoids have been established by monitoring the reaction of carotenoid radical anion (CAR1(*-)) with another carotenoid (CAR2) in hexane and benzene. This order is consistent with the reactivities of the carotenoid radical anions with porphyrins and oxygen in hexane. In addition, investigation of the reactions of carotenoids with reducing radicals in aqueous 2% Triton-X 100, such as carbon dioxide radical anion (CO2(*-)), acetone ketyl radical (AC(*-)) and the corresponding neutral radical (ACH(*)), reveals that the reduction potentials for beta-carotene and zeaxanthin lie in the range -1950 to -2100 mV and those for astaxanthin, canthaxanthin and beta-apo-8'-carotenal are more positive than -1450 mV. This illustrates that the presence of a carbonyl group causes the reducing ability to decrease. The radical cations have been previously shown to be strong oxidising agents and we now show that the radical anions are very strong reducing agents.

Carotenoid radicals are generated from the interaction of a wide range of carotenoids with several oxy-radicals, such as CCl3O 2 • , RSO 2 • , NO 2 • and various aryl peroxyl radicals, while less strongly oxidizing radicals, such as alkyl peroxyl radicals, can lead to hydrogen atom transfer, thereby generating the neutral carotene radical. Comparison of the relative abilities of many pairs of carotenoids to donate/accept electrons: CAR1•++CAR2→CAR1+CAR2•+, has allowed the relative oxidation potentials to be established, showing that lycopene is the easiest carotenoid to oxidize to its cation radical and astaxanthin is the most difficult.

Pulse radiolysis was used to generate the radical cations of beta-carotene and two xanthophylls, zeaxanthin and lutein, in unilamellar vesicles of dipalmitoylphosphatidyl choline. The rate constants for the reaction (repair) of these carotenoid radical cations with the water-soluble vitamin C were found to be similar (approximately 1x10(7) M(-1) s(-1)) for beta-carotene and zeaxanthin and somewhat lower (approximately 0.5x10(7) M(-1) s(-1)) for lutein. The results are discussed in terms of the microenvironment of the carotenoids and suggest that for beta-carotene, a hydrocarbon carotenoid, the radical cation is able to interact with a water-soluble species even though the parent hydrocarbon carotenoid is probably entirely in the non-polar region of the liposome.

Carotenoids are abundant in many fruits and vegetables and they play diverse roles in photobiology, photochemistry and medicine. This review concerns the reactivity of carotenoids with singlet oxygen and the interaction of carotenoids with a range of free radicals. Mechanisms associated with the anti- and pro-oxidant behaviour of carotenoids are discussed including carotenoid interactions with other anti-oxidants.

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