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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.

Managing certain by-products of the nuclear fuel cycle, such as the radioactive isotopes of caesium: 134Cs, 135Cs and 137Cs is challenging due to their environmental mobility and radioactivity. While a great many materials can isolate Cs+ ions from neutral or basic aqueous solutions via ion exchange, few of these, with the exception of ammonium phosphomolybdate (AMP), function effectively in acidic media. The use of AMP, and its porous composite in polyacrylonitrile (PAN) for management of Cs radioisotopes in various nuclear wastes have been known for decades and are well studied, yet the effects of radiation on the physiochemical properties of such composites have only received limited attention to date. In a previous publication, we demonstrated that a 100 kGy gamma irradiation dose has negligible effect on the ion exchange performance of AMP and AMP&ndash, PAN with respect to capacity or kinetics under the Cs+ concentrations and acidity found in spent nuclear fuel (SNF) recycling. As a continuation of this prior study, in this publication we explore the effects of gamma irradiation on the physiochemical properties of AMP and AMP&ndash, PAN using a range of characterisation methods. The effects of the same gamma dose on the oxidation state of Mo in AMP and AMP&ndash, PAN, the thermal degradation of both AMP and AMP&ndash, PAN, combined with a first study into the high-temperature degradation AMP, are reported. The implications of irradiation, its possible mechanism, the conditions present in SNF recycling, and for the end-of-life disposal or recycling of these materials are also discussed.

Pathogenic bacteria found in low levels in natural water systems can present a hazard to human health, such as through contraction of Legionnaires disease, if conditions are favourable for growth and there is potential to create an aerosol. In this study, gamma radiation using a60Co source was applied to Legionella pneumophila and Pseudomonas aeruginosa bacteria for 60 min at varying dose rates. All L. pneumophila bacteria were eliminated by irradiation at a dose of 1.86 kGy, and all P. aeruginosa at a dose of 0.70 kGy. Approximate D10 values for L. pneumophila and P. aeruginosa bacteria were 166.3 Gy and 70.9 Gy respectively.

Gamma irradiation-induced defects at doses of 0.5 and 5 MGy were studied in lithium sodium-borosilicate (LiNaBSi) and sodium barium-borosilicate (NaBaBSi) glasses, used for high-level radioactive waste immobilisation in the UK and India, respectively. X-band electron paramagnetic resonance (EPR), Raman and UV-Vis-nIR spectroscopies were used to characterise the glasses before and after irradiation. EPR and UV-Vis-nIR absorption spectroscopies revealed the formation of boron-oxygen hole centres (BOHC), electrons trapped at alkali cations or ET centres and peroxy-radicals (PORs) as defects common to both glasses. In addition, E− or polaron centres were observed in NaBaBSi glasses, possibly related to formation of elemental sodium colloids. Time-dependent thermal annealing at a range of temperatures, including those relevant to canister centreline cooling (CCC), which may be of relevance to geological disposal in future technical assessments, was carried out to study thermal stability of these radiation-induced defects. It was observed that PORs are the most thermally-stable defects in both glasses. The influence of glass composition on the segregation of sodium and the possible formation of metal colloids upon irradiation have been discussed.

Redox active metalloenzymes catalyse a range of biochemical processes essential for life. However, due to their complex reaction mechanisms, and often, their poor optical signals, detailed mechanistic understandings of them are limited. Here, we develop a cryoreduction approach coupled to electron paramagnetic resonance measurements to study electron transfer between the copper centers in the copper nitrite reductase (CuNiR) family of enzymes. Unlike alternative methods used to study electron transfer reactions, the cryoreduction approach presented here allows observation of the redox state of both metal centers, a direct read‐out of electron transfer, determines the presence of the substrate/product in the active site and shows the importance of protein motion in inter‐copper electron transfer catalyzed by CuNiRs. Cryoreduction‐EPR is broadly applicable for the study of electron transfer in other redox enzymes and paves the way to explore transient states in multiple redox‐center containing proteins (homo and hetero metal ions)., Metalloenzyme catalysis: Through the use of cryoreduction and annealing, a method of studying electron transfer in redox active metalloenzymes is developed. Combined with electron paramagnetic resonance spectroscopy, active states in the catalytic cycle of the copper containing nitrite reductases are probed.

The effects of γ-radiation sterilization on the parenteral excipient L-histidine were analysed by means of EPR spectroscopy. The irradiation process was found to induce the formation of a deamination radical which was persistent in the solid state. The nature and reactivity of the radicals following dissolution in water was evaluated using spin-trapping EPR experiments. The deamination radical was found to regenerate in solution in the presence of trace metals, potentially leading to radical induced degradation reactions occurring up to an hour after the dissolution process. Understanding this process is significant for the improved design of parental pharmaceutical formulations in which unwanted radical reactions after γ‐radiation sterilization could lead to degradation of active ingredients.

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.

Anthraquinones have been the subject of numerous photochemical studies and their photopolymerization activities have been examined under various conditions to improve more efficient photochemical systems. This article involves further detailed investigations into the photophysical, photochemistry and photopolymerisation properties of 4 commercial derivatives of 2-substituted anthraquinone, namely, 2-Bromomethylanthraquinone (2BA), 2 Chloromethylanthraquinone (2CA), 2 Ethylanthraquinone (2EA), 2 Hydroxymethylanthraquinone (2HA) and one novel synthesized anthraquinone, 2 Acryloxymethylanthraquinone (2AA). 2AA is synthesized from 2HA. The results from both spectroscopic and analysis studies proved the 2AA to having the ester link. Absorption spectroscopy and solvent shift data are used to characterise their spectral activity. Luminescence studies involving fluorescence and phosphorescence analysis indicates efficient intersystem crossing to triplet state and n-π* nature of the lowest excited triplet state. The polymerisation activity was studied using methyl methacrylate (MMA) and analysis of the cure rate was measured using the gravimetric method. All the compounds are shown to be highly dependent on the structure. However, the rate of polymerisation (Rp) was reduced in the presence of amine. This is consistent with other results, proving the behaviour of derivatives with n-π* configuration. Hardness tests for all compounds took place using a different formula of acrylated resin/monomer systems. The excited state characteristics of the methyl derivatives have also been examined using micro and nanosecond flash photolysis. Triplet absorption spectra of all the anthraquinone derivatives show a significant red shift in the region of 340–370 nm with increasing solvent polarity due to stabilisation of the lowest triplet state by solvent reorganization. Hydrogen atom abstraction takes place in 2 propanol, forming a semiquinone radical. In the presence of the tertiary amine, triethylamine, all anthraquinone derivatives show the formation of intermediary species related to either the exciplex or the radical ion pair. Under aerobic conditions, the first decay rate for all anthraquinone derivatives increases and showed oxygen to be a good quencher with a bimolecular rate constant of around 2 × 10 mol. dm s. Relative to benzophenone, the molar absorption coefficient, ε, and quantum yield of intersystem crossing, Φisc were calculated, and it is summarised that the value for Φisc for all compound is less than 1.00 and controls to a major extent their photochemical activities., The authors thank the University of Sains Malaysia for funding Hazira Mazah.

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.

The mechanism responsible for the remarkable photomutagenicity of fluoroquinolone (FQ) antibiotics remains unknown. For this reason, it was considered worthwhile to study in detail the interactions between DNA and a dihalogenated FQ such as lomefloxacin (LFX; one of the most photomutagenic FQs) and its N-acetyl derivative ALFX. Studies of photosensitized DNA damage by (A)LFX, such as formation of DNA single-strand breaks (SSBs), together with pulse radiolysis, laser flash photolysis, and absorption and fluorescence measurements, have shown the important effects of the cationic character of the piperazinyl ring on the affinity of this type of drug for DNA. Hence, the formation of SSBs was detected for LFX, whereas ALFX and ciprofloxacin (a monofluorated FQ) needed a considerably larger dose of light to produce some damage. In this context, it was determined that the association constant (K-a) for the binding of LFX to DNA is ca. 2 x 10(3) M-1, whereas in the case of ALFX it is only ca. 0.5 x 10(3) M-1. This important difference is attributed to an association between the cationic peripheral ring of LFX and the phosphate moieties of DNA and justifies the DNA SSB results. The analysis of the transient species detected and the photomixtures has allowed us to establish the intermolecular processes involved in the photolysis of FQ in the presence of DNA and 2'-deoxyguanosine (dGuo). Interestingly, although a covalent binding of the dihalogenated FQ to dGuo occurs, the photodegradation of FQ center dot center dot center dot DNA complexes did not reveal any significant covalent attachment. Another remarkable outcome of this study was that (A)LFX radical anions, intermediates required for the onset of DNA damage, were detected by pulse radiolysis but not by laser flash photolysis. (C) 2013 Elsevier Inc. All, rights reserved., We thank Professor Suppiah Navaratnam for his help. We acknowledge the Spanish government for Grants CTQ2010-19909 and CTQ2012-32621 and the Generalitat Valenciana for Grants PROMETEOII/2013/005.

An effort has been made to bridge the gap between academic knowledge of polymeric seal degradation and industrial practices. A series of physical and mechanical properties that can be related to the sealing behavior of three commercial samples of nitrile rubber have been studied for their degradation when exposed to gamma radiation. For all samples the glass transition temperature (Tg) and retention factor (RF) were found to increase with total dose whilst percentage change in mass (ΔM%) was found to decrease. The ultimate uptake of carbon dioxide (CO2) did not appear to change with radiation dose but the kinetics of the absorption process were found to decrease, suggesting the formation of crosslinks. The crosslinks formed appear to be dependent on the original material composition and comparison against degradation of material properties supports the theory behind butadiene, BDN, content being linked to a propensity for crosslink clustering. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 44618.

The yield of singlet oxygen sensitized by chemically modified, carotenoidless bacterial photosynthetic reaction centres and the ensuing oxidative damage are both shown to be magnetic field-dependent.

The synthesis of metal(II) complexes of a bis(thiosemicarbazonato) ligand derived from 1,2-cyclohexanedione are reported. The compounds have been characterised by a range of techniques including reverse-phase HPLC, cyclic voltammetry, NMR, UV/Vis, IR, Raman and EPR spectroscopy. Xray crystal structures of the proligand and two copper(II) complexes have been obtained, and the electronic structures have been analysed by using DFT calculations. DFT calculations have also been used to map the potential energy surface of a related bis(thiosemicarbdzone) proligand and predict solution-phase one-electron reduction potentials of the copper(H) complex by using three polarisable continuum Solvation models. Electrochemistry experiments show that the copper(II) complex undergoes quasi-reversible one-electron reduction at biologically accessible potentials and is within the range proposed for the complex to be hypoxia-selective. In addition, UV/Vis spectroelectrochemistry experiments have been used to characterise the reduced copper(l) anion, and the reaction between the anion and dioxygen has been characterised by experiments and theory, Observation of this oxidation reaction has important implications for the proposed mechanisms of hypoxia selectivity of bis(thiosemicarbazonato)copper(H) complexes, The copper-64 radiolabelled complex has been prepared in aqueous solution which demonstrates the potential of rise of these complexes as medical imaging agents. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

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 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.

The synthesis of zinc(II) and copper(II) complexes of an unsymmetrical bis(thiosemicarbazonato) ligand containing a reactive styrene group are reported. The compounds have been characterised by a range of techniques including reverse-phase HPLC, cyclic voltammetry, NMR, UV/Vis, electron paramagnetic resonance and fluorescence emission spectroscopy. Time-dependent density functional theory calculations have been used to assign the electronic absorption spectrum of [ZnIIATSM] and probe the nature of the fluorescent excited state. Electrochemistry experiments show that the copper(II) complex undergoes quasi-reversible one-electron reduction at biologically accessible potentials and is within the range proposed for the complex to be hypoxia-selective. The copper-64 radiolabelled complex has been prepared in aqueous solution and characterised by reverse-phase radio-HPLC. Cellular uptake in HeLa cells has been observed using confocal fluorescence microscopy.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

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.

Hypochlorous acid and its conjugate base, hypochlorite ions, produced under inflammatory conditions, may produce chloramides of glycosaminoglycans, these being significant components of the extracellular matrix (ECM). This may occur through the binding of myeloperoxidase directly to the glycosaminoglycans. The N–Cl group in the chloramides is a potential selective target for both reducing and oxidizing radicals, leading possibly to more efficient and damaging fragmentation of these biopolymers relative to the parent glycosaminoglycans. To investigate the effect of the N–Cl group, we used ionizing radiation to produce quantifiable concentrations of the reducing radicals, hydrated electron and superoxide radical, and also of the oxidizing radicals, hydroxyl, carbonate, and nitrogen dioxide, all of which were reacted with hyaluronan and heparin and their chloramides in this study. PAGE gels calibrated for molecular weight allowed the consequent fragmentation efficiencies of these radicals to be calculated. Hydrated electrons were shown to produce fragmentation efficiencies of 100 and 25% for hyaluronan chloramide (HACl) and heparin chloramide (HepCl), respectively. The role of the sulfate group in heparin in the reduction of fragmentation can be rationalized using mechanisms proposed by M.D. Rees et al. (J. Am. Chem. Soc. 125:13719–13733; 2003), in which the initial formation of an amidyl radical leads rapidly to a C-2 radical on the glucosamine moiety. This is 100% efficient at causing glycosidic bond breakage in HACl but only 25% efficient in HepCl, the role of the sulfate group being to favor the nonfragmentary routes for the C-2 radical. The weaker reducing agent, the superoxide radical, did not cause fragmentation of either HACl or HepCl although kinetic reactivity had been demonstrated in earlier studies. Experiments using the oxidizing radicals, hydroxyl and carbonate, both potential in vivo species, showed significant increases in fragmentation efficiencies for both HACl and HepCl, relative to the parent molecules. The carbonate radical was shown to be involved in site-specific reactions at the N–Cl groups, reacting via abstraction of Cl, to produce the same amidyl radical produced by one-electron reductants such as the hydrated electron. As for the hydrated electrons, the data support fragmentation efficiencies of 100 and 29% for reaction of carbonate radicals at N–Cl for HACl and HepCl, respectively. For the weaker oxidant, nitrogen dioxide, no fragmentation was observed, probably because of a low kinetic reactivity and low reduction potential. It seems likely therefore that the N–Cl group can direct damage to extracellular matrix glycosaminoglycan chloramides, which may be produced under inflammatory conditions. The in vivo species, the carbonate radical, is also much more likely to be site-specific in its reactions with such components of the ECM than the hydroxyl radical.

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.

Three 3,6-difluoro-1,2,4,5-tetrakis(amino)benzene compounds, bearing dimethylamino (1), piperidin-1-yl (3), or morpholin-1-yl (5) substituents, have been synthesized and subsequently defluorinated to give the corresponding 1,2,4,5-tetrakis(amino)benzene compounds 2, 4, and 6; the crystal structures of compounds 1, 4, and 6 have been obtained. Cyclic voltammetry shows that all six compounds will lose two electrons to form dications, and the use of suitable oxidizing agents has allowed isolation and crystallographic characterization of the dications 2(2+) and 6(2+) (as [PF(6)](2) salts) and 4(2+) (as a [I(5)][I(3)] salt). The separation DeltaE between the loss of the first electron and the second varies between compounds, from 0.23 V in 1 to 0.01 V in 6. Electrochemical studies involving the use of the noncoordinating electrolyte [Bu(4)N][B{C(6)H(3)(CF(3))(2)}(4)] show that it is possible to increase this separation, stabilizing the intermediate monocationic phase, and this has allowed the isolation and crystallographic characterization of the radical salts 2[B{C(6)H(3)(CF(3))(2)}(4)] and 4[B{C(6)H(3)(CF(3))(2)}(4)], the first radical cations of this family to be isolated. DFT studies of the ion pairing between oxidized forms of 1 and 2 and anions imply that the location of the ion pairing is different in the two species.

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.

The formation and reactivity of excited states and free radicals from primaquine, a drug used in the treatment of malaria, was studied in order to evaluate the primary photochemical reaction mechanisms. The excited primaquine triplet was not detected, but is likely to be formed with a short lifetime (50 ns) and with a triplet energy250 kJ/mol as the drug is an efficient quencher of the fenbufen triplet and the biphenyl triplet, and forms (1)O(2) by laser flash photolysis ((PQ)Phi(Delta)=0.025). Primaquine (PQ) exists as the monocation (PQH(+)) in aqueous solution at physiological pH. PQH(+) photoionises by a biphotonic process and also forms the monoprotonated cation radical (PQH(2+)*) by one electron oxidation by HO* (k(q)=6.6 x 10(9) M(-1) s(-1)) and Br*(2)(-) (k(q)=4.7 x 10(9) M(-1) s(-1)) at physiological pH, detected as a long-lived transient decaying essentially by a second order process (k(2)=7.4 x 10(8) M(-1) s(-1)). PQH(2+)* is scavenged by O(2), although at a limited rate (k(q)=1.0 x 10(6) M(-1) s(-1)). The reduction potential (E degrees) of PQH(2+)*/PQH(+) is+1015 mV, as measured versus tryptophan (TRP*/TRPH). Primaquine also forms PQH(2+)* at pH 2.4, by one electron oxidation by Br*(2)(-) and proton loss (k(q)=2.7 x 10(9) M(-1) s(-1)). The non-protonated cation radical (PQ(+)*) is formed during one electron oxidation with Br*(2)(-) at alkaline conditions (k(q)=4.2 x 10(9) M(-1) s(-1) at pH 10.8). The estimated pK(a)-value of PQH(2+)*/PQ(+)* is pK(a) approximately 7-8. Primaquine is not a scavenger of O*(2)(-) at physiological pH. Thus self-sensitization by O*(2)(-) is eliminated as a degradation pathway in the photochemical reactions. Impurities in the raw material and photochemical degradation products initiate photosensitized degradation of primaquine in deuterium oxide, prevented by addition of the (1)O(2) quencher sodium azide. Photosensitized degradation by formation of (1)O(2) is thus important for the initial photochemical decomposition of primaquine, which also proceeds by free radical reactions. Formation of PQH(2+)* is expected to play an essential part in the photochemical degradation process in a neutral, aqueous medium.

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.

Detailed chemical, spectroelectrochemical and computational studies have been used to investigate the mechanism of hypoxia selectivity of a range of copper radiopharmaceuticals. A revised mechanism involving a delicate balance between cellular uptake, intracellular reduction, reoxidation, protonation and ligand dissociation is proposed. This mechanism accounts for observed differences in the reported cellular uptake and washout of related copper bis(thiosemicarbazonato) complexes. Three copper and zinc complexes have been characterised by X-ray crystallography and the redox chemistry of a series of copper complexes has been investigated by using electronic absorption and EPR spectroelectrochemistry. Time-dependent density functional theory (TDDFT) calculations have also been used to probe the electronic structures of intermediate species and assign the electronic absorption spectra. DFT calculations also show that one-electron oxidation is ligand-based, leading to the formation of cationic triplet species. In the absence of protons, metal-centred one-electron reduction gives the reduced anionic copper(I) species, [Cu(I)ATSM](-), and for the first time it is shown that molecular oxygen can reoxidise this anion to give the neutral, lipophilic parent complexes, which can wash out of cells. The electrochemistry is pH dependent and in the presence of stronger acids both chemical and electrochemical reduction leads to quantitative and rapid dissociation of copper(I) ions from the mono- or diprotonated complexes, [Cu(I)ATSMH] and [Cu(I)ATSMH(2)](+). In addition, a range of protonated intermediate species have been identified at lower acid concentrations. The one-electron reduction potential, rate of reoxidation of the copper(I) anionic species and ease of protonation are dependent on the structure of the ligand, which also governs their observed behaviour in vivo.

Heptarhodium wagon wheel [Rh7(PiPr3) 6H18][BArF4]2 (shown), which resembles a planar Rh(111) surface with 18 hydride ligands, was obtained together with [Rh8(PiPr3)6H16] [BArF4]2 from the reaction of [Rh(PiPr 3)2(nbd)][BArF4] and [Rh(nbd) 2][BArF4] with hydrogen (ArF = C6H3(CF3)2, nbd = norbornadiene). Some of the H ligands are located in threefold hollows between Rh centers and thus mimic the orientation of atomic hydrogen adsorbed on Rh(111). (Figure Presented). © 2007 Wiley-VCH Verlag GmbH and Co. KGaA.

The spectral and photophysical properties of indigo derivatives with di-, tetra-, and hexa-substitution in their neutral (keto) form are investigated in solution. The study comprises absorption and emission spectra, together with quantitative measurements of quantum yields of fluorescence (phi(F)) and singlet oxygen formation (phi(Delta)) and fluorescence lifetimes. The energy difference between the HOMO and LUMO orbitals is dependent on the degree (number of groups) and relative position of substitution. The phi(F) and phi(Delta) values were found to be very lowor=10(-3). Because of the absence of transient triplet-triplet signal, the intersystem crossing yields (phi(T)) were estimated by assuming that all the triplet states formed give rise to singlet oxygen formation, that is, phi(Delta) approximately phi(T) . It was then possible from phi(IC)=1-phi(F)-phi(T) to estimate the S(1) approximately approximately --S(0) internal conversion yields and thus, with the other data, to determine the rate constants for all decay processes. From these, several conclusions are drawn. Firstly, the radiationless rate constants, k(NR) , clearly dominate over the radiative rate constants, k(F) , (and processes). Secondly, the main deactivation channel for the compounds in their keto form is the radiationless S(1) approximately approximately --S(0) internal conversion process. Finally, although the changes are relatively small, internal conversion yield seems to be independent of the overall pattern of substitution. A more detailed investigation of the decay profiles with collection at the blue and red emission of the fluorescence band of indigo and one di-substituted indigo reveals the decays to be bi-exponential and that at longer emission wavelengths these appear to be associated with both rise and decay times indicating that two excited species exist, which is consistent with a keto-excited form giving rise (by fast proton transfer) to the enol-form of indigo. Evidence is presented which supports the idea that intramolecular (and possibly some intermolecular) proton transfer can explain the high efficiency of internal conversion in indigo.

A pulse radiolytic investigation has been conducted to establish whether a redox reaction takes place between dopaquinone and 5,6-dihydroxyindole (DHI) and its 2-carboxylic acid (DHICA) and to measure the rate constants of the interactions. To obviate possible confounding reactions, such as nucleophilic addition, the method employed to generate dopaquinone used the dibromide radical anion acting on dopa to form the semiquinone which rapidly disproportionates to dopaquinone. In the presence of DHI the corresponding indole-5,6-quinone (and/or tautomers) was also formed directly but, by judicious selection of suitable relative concentrations of initial reactants, we were able to detect the formation of additional indolequinone from the redox exchange reaction of DHI with dopaquinone which exhibited a linear dependency on the concentration of DHI. Computer simulation of the experimental time profiles of the absorption changes showed that, under the conditions chosen, redox exchange does proceed but not quite to completion, a forward rate constant of 1.4 x 10(6)/M/s being obtained. This is in the same range as the rate constants previously established for reactions of dopaquinone with cyclodopa and cysteinyldopa. In similar experiments carried out with DHICA, the reaction more obviously does not go to completion and is much slower, k (forward) =1.6 x 10(5)/M/s. We conclude that, in the eumelanogenic pathway, DHI oxidation may take place by redox exchange with dopaquinone, although such a reaction is likely to be less efficient for DHICA.

Vitamin A (retinol) and its derivatives-retinal and retinoic acid-are known for their ability to inhibit lipid peroxidation. Antioxidant actions of retinoids have been attributed to chain-breaking by scavenging of peroxyl radicals. Based on chemical analysis of retinoic acid degradation products formed during microsomal lipid peroxidation, it was previously suggested that retinoids interact with peroxyl radicals forming free carbon-centered radical adducts. However, it can be argued that such a mode of antioxidant action of retinoids is not sufficient to fully explain their effectiveness at inhibiting lipid peroxidation, which in many systems is comparable to, or even exceeds, that of alpha-tocopherol. In order to elucidate the mechanism of interaction of retinoids with peroxyl radicals, (trichloromethyl)peroxyl radical was generated by pulse radiolysis, and its interactions with retinoids solubilized in Triton X-100 micelles were followed by kinetic absorption spectroscopy. All retinoids--retinol, retinal, and retinoic acid--interacted with the peroxyl radical, and at least two transient products were detected. One of these products, absorbing at 590 nm, was identified as retinoid cation radical. Therefore, we postulate that, apart from formation of radical adducts, retinoids may also scavenge peroxyl radicals by electron transfer.

A pyridinium bisretinoid (A2E) is the only identified blue-absorbing chromophore of retinal lipofuscin that has been linked to its aerobic photoreactivity and phototoxicity. Pulse radiolysis has been used to study both the one-electron oxidation and the one-electron reduction of A2E in aqueous micellar solutions. The reduction to the semireduced A2E (lambda(max) broad and between 500 and 540 nm) was achieved with formate radicals and the subsequent decay of A2E* was slow (over hundreds of milliseconds) via complex kinetics. The long lifetime of the A2E* should facilitate its reactions with other biomolecules. For example, with oxygen, the A2E* produced the superoxide radical anion with a rate constant of 3 x 10(8) M(-1) s(-1). The A2E was also reduced by the NAD radical, the corresponding rate constant being 2.3 x 10(8) M(-1) s(-1). Other experiments showed that the one-electron reduction potential of A2E lies in the range -640 to -940 mV. The semioxidized form of A2E (lambda(max) 590 nm) was formed via oxidation with the Br2*- radical and had a much shorter lifetime than the semireduced form. With strongly oxidizing peroxyl radicals (CCl3O2*) our kinetic data suggest the formation of a radical adduct followed by dissociation to the semioxidized A2E. With milder oxidizing peroxyl radicals such as that from methanol, our results were inconclusive. In benzene we observed an efficient oxidation of zeaxanthin to its radical cation by the A2E radical cation; this may be relevant to a detrimental effect of A2E in vision.

Copper porphyrin dissolved in CH2Cl2:toluene as fluid and frozen solution was studied as a function of temperature using X-band electron paramagnetic resonance (EPR). Quantitative interpretation was obtained using a recently developed Stochastic Liouville simulation method. For the first time we address the large spin system that translates into a 400,000 dimensional Liouville equation solved under slow-motion conditions. Using a simple three parameter microscopic model, the temperature dependence of porphyrin rotational correlation time is determined to be in the range 1-10 ns and a fast local motion is in the subpico-second regime with an amplitude increasing with temperature. The methodology provides an important tool for arriving at an accurate set of spin Hamiltonian parameters since determining a unique set of parameters from a frozen solution EPR experiment is often difficult. Thus, the proposed method discriminates between parameters proposed from frozen solution EPR experiments or quantum chemistry calculations. The methodology presented is expected to be valuable in obtaining a molecular dynamics picture of metal proteins using EPR as well as in the study of artificial photosynthetic systems.

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.

The peroxynitrite anion and the nitrogen dioxide (radical) are important toxic species which can arise in vivo from nitric oxide. Both in vivo and in vitro cell protection is demonstrated for β-carotene in the presence of vitamin E and vitamin C. A synergistic protection is observed compared to the individual anti-oxidants and this is explained in terms of an electron transfer reaction in which the β-carotene radical is repaired by vitamin C.