Search

We report the use of the novel spin trap DEPMPO in conjunction with EPR spectroscopy to provide evidence for free-radical mediated oxidation of some proteins in aqueous solution, with systems including metal ions, peroxides or both. In addition we have used a novel approach involving detailed spectral simulation (to reproduce anisotropic features of phosphorus, hydrogen and nitrogen splittings) to confirm structural assignments. Oxidation of bovine serum albumin (BSA) with either Ce4+ or IrCl62− leads to the predominant formation of spin-trapped oxygen-centred free-radicals, attributed to tyrosyl-based adducts, a reaction also observed for sulfur-free analogues (lysozyme and tyrosine-containing models). Similar spectra were obtained from reactions involving oxidation with HRP/H2O2 and methaemoglobin/H2O2 couples, through intermolecular oxidation of the proteins via oxo–iron intermediates. Oxidation of BSA with peroxynitrite leads to an anisotropic spectrum which is assigned to the trapping of a sulfur-centred radical. This assignment has been confirmed via denaturation of the spin-adduct (and simulation of the resulting less immobilized spectrum). Reaction of the hydroxyl radical with BSA and lysozyme, in Fenton-type reactions, leads to the detection of anisotropic spectra derived from the spin-trapping of carbon-centred radicals, whose assignment is discussed. more...

We report the synthesis and characterisation of some novel hydrazine-derived monoradicals incorporating nitroxide 14, nitronyl-nitroxide15 and imino-nitroxide 16 functions, as well as the corresponding diradicals assigned the hydrazyl-nitroxide 17, hydrazyl-nitronyl-nitroxide 18 and hydrazyl-imino-nitroxide 19 structures. EPR, UV-Vis and electrochemical (CV) characteristics are reported, along with MS and NMR data where appropriate; pKa and BDE values are calculated as well as the average interelectronic separation in the triplet diradicals 18 and 19 obtained via solid-state EPR measurement and calculations. The reactions of these compounds with short-lived radicals generated by photolysis, as well with NO and NO2, are also reported. more...

The formation of azide spin-adducts of a number of spin-traps, including PBN, DMPO, DEPMPO and TBNB, in biphasic and organic media has been studied by EPR spectroscopy. A series of hydrazyl radicals has been employed to oxidise the azide anion to the corresponding short-lived azido radical, which is subsequently trapped. The approach has typically involved the presence of the ether 18-crown-6 in order to facilitate the transport of the azide anion from water into the organic media, with further experiments in non-aqueous systems. The results can largely be rationalised in terms of a “conventional” spin-trapping mechanism, with an efficiency that depends on the relevant redox potentials, which are themselves solvent dependent; however, conditions that favour direct oxidation of the trap have also been identified. It is also established that the azide spin-adducts of some nitrones can be transformed, under certain conditions, into secondary spin-adducts by nucleophilic substitution reaction of the first-formed nitroxides. more...

Radical species generated on photolysis of 2-nitrobenzyl compounds, including derivatives of 1-(2-nitrophenyl)ethyl phosphate (caged ATP 1 and caged monomethyl phosphate 3) have been studied by EPR spectroscopy and their identity confirmed by independent chemical generation of the appropriate nitrogen-centred radicals. The reactions which occur include photo-isomerisation, photo-fragmentation, photo-assisted electron-transfer, intramolecular addition and spin-trapping reactions of the nitroso compounds produced via molecular rearrangement. In particular, a mechanistic scheme is advanced to explain the co-formation of a radical-anion and a cyclic aryl alkoxy aminoxyl (2 and 30, respectively, from caged ATP). Which of these species is observed in the EPR spectrum depends upon the particular experimental conditions. more...

EPR experiments involving the attack of oxygen-centred free radicals on polysaccharides in aqueous solution reveal two sources of regioselectivity in C–H abstraction. For SO4−˙ (derived from one-electron reduction of S2O82−), hydrogen-atom abstraction is encouraged by SOMO–σ* interactions involving β-C–O bonds which eclipse the orbital of the unpaired electron at the developing radical centre. In addition, for a series of cyclodextrins, the reactivity observed with SO4−˙ points to the selective reaction of the latter within the cyclodextrin cavity; these findings are in contrast to those obtained with ˙OH (from H2O2). more...

Both free-radical and non-radical routes for peroxide oxidation of CuI can be identified for copper complexes obtained by the reduction of CuII by thiols, including glutathione. Copper(I)–thiolate complexes are obtained, except in the presence of e.g. 1,10-phenanthroline, and these undergo ready reaction with H2O2 and tBuOOH. EPR spin-trapping studies establish a free-radical reaction mechanism (to give tBuO˙) with the latter, and the formation of HO˙ from the former occurs only at low concentrations of copper. Kinetic studies (using UV-vis and EPR spectroscopies), together with NMR analysis, lead to the proposal that CuI aggregates react via non-radical pathways in contrast to monomeric CuI. more...

Attack by SO4–˙ (generated by photolysis and one-electron transfer) on a number of carbohydrates and some model compounds has been followed directly by EPR spectroscopy.SO4–˙ is a reactive, electrophilic species but is more selective (less reactive) than HO˙. We present evidence for the importance of polar effects and radical stabilisation, especially in determining trends in selectivity. Examples include the preference for C–H abstraction α to a ring oxygen atom in a five-membered ring (e.g. THF, sucrose, β-D-fructofuranose) and abstraction of the axial C1 hydrogen atom from β-D-glucose.The reaction of SO4–˙ with α-D-glucose is selective towards the C2, C5 and C6 positions. This reflects the activating effect of a β-oxygen substituent where the radical orbital can eclipse the β-C–O bond providing a SOMO–σ* interaction which stabilises the developing radical centre. For myo-inositol, abstraction is also observed from axial C–H bonds for which the resulting radicals are stabilised by an axial β-OH substituent but, due to the absence of a ring oxygen atom, more rapid abstraction occurs from the equatorial C–H bond. more...

EPR spectroscopy provides direct evidence for the intercalation of copper(II) complexes of 1,10-phenanthroline and 2,2′-bipyridine with DNA. Reduction of these complexes to copper(I) by glutathione is evidently facile, whereas ascorbate reacts more slowly. Reoxidation of copper(I) with tBuOOH (to give tBuO˙) is rapid in both systems, as judged by EPR spin-trapping results. Reaction of copper(I)–DNA with H2O2 in the presence of ascorbate leads to the generation of HO˙ and to the trapping of DNA-derived radical adducts. The role of 1,10-phenanthroline and ascorbate, especially, and the relevance to DNA scission, are discussed. more...

EPR spectroscopy has been employed for the direct detection of a variety of free radicals formed from reaction of Fe(II)–oxalate and H2O2 in the presence of carbohydrates and related compounds: this system has been designed to model the proposed mode of action of brown rot fungi. The observed hyperfine splittings allow characterization of individual radicals formed at different positions in the carbohydrate rings. Relative signal intensities in steady-state spectra indicate the rapid generation of the hydroxyl radical, followed by relatively unselective attack of ˙OH on the substrates’ C–H bonds: the rapidity of oxidation by Fe(III) of oxygen-conjugated carbon-centred radicals (typically k 108 dm3 mol–1 s–1) is significantly reduced if there is an eclipsing β-oxygen substituent. The relevance of these findings to cellulose-cleaving reactions of certain fungi is discussed. more...

The spin-trap 5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide (DEPMPO) is shown to provide a simple method to distinguish free-radical reactions which lead to the production of ˙OH from those which proceed via one-electron transfer followed by hydration or nucleophilic substitution of another adduct. In the first case, only an ˙OH adduct is detected; in the latter, signals from the hydroxyl adduct (labelled with 17O when H217O is employed) are accompanied by a second signal believed to result via internal trapping of a radical formed from an intermediate radical cation. This provides a potentially useful mechanistic probe, for example for use in biochemical systems. more...

Electrospray mass spectrometry has been used to identify the formation of oxo-manganese(V) species in the oxidation of 4-methoxyphenol by H2O2 catalysed by manganese 1,4,7-triazacyclononane complexes in aqueous solution. more...

EPR spectroscopy has been employed directly to monitor the initial stages in the free-radical copolymerisation of carbohydrates and methacrylic acid, initiated by a metal–peroxide couple. For the addition of a variety of oxygen-conjugated substrate-derived radicals (including those from myo-inositol, α- and β-D-glucose, D-fructose and sucrose) the rate constants are in the range (0.3–3.8) 106 dm3 mol–1 s–1; in contrast ˙CH2OH has a value of 6 106 and α,α-dioxygen-substituted radicals have values of ca. 107 dm3 mol–1 s–1.The reaction is clearly assisted by the +M electronic effect of the α-oxygen and by bending at the radical centre (for α-dioxygen-substituted species). The reaction rate is, however, reduced by β-oxygen substituents, especially through a SOMO–σ* (β-C–O) interaction, giving a substantial stereoelectronic retardation for axial β-oxygen substituents and for acyclic β-OH groups where eclipsing geometry (with respect to the radical centre) is favoured. more...

The reaction of [L2Mn2IV(µ-O)3](PF6)2, L = 1,4,7-trimethyl-1,4,7-triazacyclononane (1) with a range of phenols (2–5) in aqueous solution at pH 10.5 has been investigated. At least for electron-rich substrates, the reaction proceeds via a rapid overall one-electron process from the phenolate ion to the MnIV/MnIV species (1) to give, initially, a MnIII/MnIV species (detected via its characteristic 16-line EPR spectrum in frozen solution at 77 K) and the corresponding phenoxyl radical, detected directly for Trolox (3) by EPR spectroscopy in aqueous solution. The dimeric MnIII/MnIV species is ultimately converted to monomeric MnII. In the presence of H2O2, reoxidation of the manganese species is accompanied by an increase in the rate of formation of the phenoxyl radicals. In similar reactions, 4-methoxyphenol (4) and 2,6-dimethoxyphenol (5) are converted into polyphenols (with the formation of phenoxyl radicals trapped in a polymer matrix). Kinetic EPR and UV–VIS studies provide additional evidence of the reaction mechanism in the presence and absence of H2O2. The mechanism of phenolate oxidation by 1 in the presence and absence of H2O2 is discussed. more...

Irradiation (λ > 390 nm) of acidic anaerobic aqueous solutions of iron(III) tetrakis(2-N-methylpyridyl)porphyrin, in the presence of mono- or di-basic amino acids or their N-acylated derivatives, generates the iron(II) porphyrin and the corresponding acyloxyl radical; subsequent decarboxylation of the latter gives the corresponding ammonioalkyl or amidoalkyl radical. The rate and course of each reaction have been monitored by UV–VIS spectroscopy and EPR spin-trapping techniques. The large differences in the observed rates of iron(II) porphyrin formation are controlled by two factors: the binding affinity of the carboxy group for the iron(III) porphyrin to form the photoactive complex and the competitive reactions of the acyloxyl radical [decarboxylation and regeneration of the iron(III) porphyrin complex], following photolysis. With cationic carboxylate ligands, such as glycine or L-alanine, charge repulsion with the cationic porphyrin results in the former effect predominating, whereas with neutral ligands, for example N-acetylglycine, it is the latter that determines the overall rate of reaction.In aqueous base, the amino acids ligate to the iron(III) porphyrin via the amino rather than the carboxy group. Subsequent irradiation brings about an electron transfer from the ligand to give the iron(II) porphyrin and the amino acid cation–radical which reacts further to give an α-amino radical either by decarboxylation or proton loss. more...

Nucleobase, nucleoside, RNA and DNA hydroperoxides have been generated by exposure of the parent compounds to high energy electrons in the presence of oxygen; EPR spin-trapping experiments using 2-methyl-2-nitrosopropane (MNP) and 5,5-dimethyl-4,5-dihydro-3H-pyrrole N-oxide (DMPO) have been employed to study the reactions of alkoxyl radicals generated from their reaction with Fe2+. Alkoxyl radicals generated from the pyrimidine hydroperoxides (nucleobases and nucleosides) are shown to be capable of reacting with a variety of substrates, which include the pyrimidine nucleobases and nucleosides themselves and histone proteins. Attack on the parent pyrimidine compounds involves addition to the C(5) and C(6) atoms of the pyrimidine ring; reaction with the histone proteins, amino acids and peptides gives carbon-centred species, providing direct evidence for transfer of damage via hydrogen-atom abstraction. Rapid reactions with antioxidants are also demonstrated. more...

The complexes of iron(III) tetra(2-N-methylpyridyl)porphyrin with vinyl acetic acid and a selection of α-halo and α-hydroxycarboxylic acids, α-,β- and γ-ketoacids, β-dicarboxylic acids and polyacrylic acid have been photolysed (λ > 390 nm) in anaerobic aqueous solution. UV–VIS spectroscopy shows that the primary reaction is photoreduction of the iron(III) atom by the axial carboxylate ligand to give a solvent-caged carboxyl radical and iron(II) porphyrin.The rate of iron(II) formation is determined by the competitive reactions of the carboxyl radical in the solvent cage: recombination with the iron(II) porphyrin to regenerate the starting complex versus decarboxylation. EPR spin-trapping methods have been used to detect the carbon radicals arising from decarboxylation.Vinylacetic and α-hydroxycarboxylic acids lead to a rapid rate of iron(II) porphyrin formation; with the latter this is partly due to the one-electron reduction of the iron(III) porphyrin by the α-hydroxyalkyl radicals from the photo-decarboxylation. By contrast the build-up of iron(II) porphyrin from the photoreactions of the α-haloacids is slow and does not go to completion since the iron(III) porphyrin is regenerated by reaction of the halosubstrate with the iron(II) compound.Ketoacids show an unusual variety of reactions, depending on structure. The α-ketoacids give acyl and, by decarboxylation, alkyl radicals and the iron(II) porphyrin CO complex, whereas with β-ketoacids the photoreaction is catalytic, since, following decarboxylation of the carboxyl radical, the iron(III) porphyrin is rapidly regenerated by oxidation of the iron(II) porphyrin by the α-keto radical. γ-Ketoacids behave in a similar fashion to alkylcarboxylic acids.The photoreactions of β-dicarboxylic acids resemble those of their monocarboxylic analogues. With polyacrylic acid the rate of iron(II) formation is slow and this is attributed to the polyacid encouraging recombination of iron(II) porphyrin and carboxyl radical. more...

Anaerobic aqueous solutions of the complexes of iron(III) tetra(2-N-methylpyridyl)porphyrin with carboxylate anions, benzoate, phthalate and 2- and 4-methylbenzoate, have been photolysed with visible light (λ > 390 nm) to give solvent-caged iron(II) porphyrin and aroyloxyl radicals. Evidence is presented that the rate of build-up of the iron(II) porphyrin in the reaction is dependent on the partitioning of the caged aroyloxyl radicals between product formation (decarboxylation and hydrogen abstraction from the methyl group) and regeneration of the starting complex. The fastest reaction occurs with 2-methylbenzoate for which intramolecular hydrogen abstraction converts the initially formed 2-methylbenzoyloxyl into 2-carboxybenzyl radicals. The photolysis of the iron(III) porphyrin complexes of acrylic, methacrylic and dimethylacrylic acids shows analogous behaviour, with the extra possibility that the carboxyl radicals may also add to the alkenic group of the substrate molecules in the solvent cage. The rapid decarboxylation of phenylacetoxyl radicals results in the fast photoreduction of the iron(III) porphyrin by phenylacetic acid to iron(II) porphyrin and to benzyl radicals. Under aerobic conditions the reoxidation of the iron(II) to iron(III) allows the system to become photocatalytic in iron porphyrin. The stability of the catalyst to oxidative destruction is moderate but can be dramatically improved by anchoring the charged iron porphyrin by electrostatic interaction to a silica support. more...

Rate constants have been determined by steady-state EPR spectroscopy experiments for hydrogen-atom abstraction by ButO˙ from a variety of esters. The values, which range over two orders of magnitude ( 5 × 103–5 × 105 dm3 mol–1 s–1 at ca. 300 K) are significantly lower than those for related ketones and ethers and reflect, at least in part, the importance of the appropriate C–H bond dissociation energies and the stabilization of carbonyl-conjugated radicals. The relative lack of reactivity of C–H bonds in unsubstituted five-membered rings is believed to reflect ring-strain in the derived radicals; possible stereoelectronic effects are discussed. more...

EPR spectroscopy has been employed to monitor the reactions of CuII and the formation of organic radicals in the copper-catalysed oxidation of thiols to disulfides in the presence of oxygen and peroxides. Reaction of CuII with RSH gives CuI and disulfide (a process whose stoichiometry is 1:1 in certain solvents), which is followed by electron transfer to the peroxide to give CuII and, for example, HO˙(from H2O2) and ButO˙(from ButO2H) which convert more thiol into disulfide. EPR and UV–VIS spectroscopy allow the kinetic parameters of these reactions to be determined in N,N-dimethylformamide (DMF). more...

EPR spectroscopy has been employed, in conjunction with spin-trapping [using 2-methyl-2-nitrosopropane dimer (MNP)] to study the attack of ˙OH (generated by the Fe2+–EDTA/H2O2 couple) on deoxyribonucleic acid (DNA) and its components. The results confirm that for pyrimidine deoxyribonucleosides and deoxyribonucleotides the alkenic C5–C6 double bond of the base moiety is the predominant site of attack (as with the pyrimidine bases, nucleosides and nucleotides) though weak signals from sugar–derived radicals are also observed. Both sugar and base radicals have also been detected for purines (both ribose and deoxyribose derivatives), providing evidence that rapid transfer occurs from the initially formed base radicals to the sugarphosphate backbone.The spectra of the adducts from double-stranded (ds) and single-stranded (ss) DNA are found to vary with pH and ˙OH flux. With dsDNA at pH 7, an anisotropic spectrum due to a slowly tumbling adduct of a high molecular weight polymer radical-adduct is accompanied by a more isotropic signal due to a mobile, low molecular weight radical-adduct, the relative yield of which increases as the pH is lowered. Close similarities are observed between the spectra of dsDNA at pH 2 and ssDNA at pH 7, with a greater occurrence of strand-breakage under these conditions. Treatment of the partially-immobilised DNA adducts formed at pH 6–7 either with the enzyme DNase or with acid releases more mobile adducts enabling information to be obtained about sites of initial attack (at pyrimidine bases) and mechanisms of transfer of damage. more...

Aryl radicals may be responsible for the DNA damage observed in both cellular systems and isolated DNA exposed to a number of systems (such as benzoyl peroxide or arenediazonium ion/metal ion couples) which are believed to be capable of generating such species, though it is unclear how this damage arises. EPR spectroscopy in conjunction with spin trapping [using 2-methyl-2-nitrosopropane (MNP)] has therefore been utilised to study the mechanism and sites of attack of aryl radicals (generated by treatment of the corresponding diazonium ions with Fe2 +–EDTA or Ti3+) on DNA, RNA and their components. The results obtained suggest that, for the pyrimidine nucleobases, nucleosides and nucleotides, the major mode of reaction is addition to the alkenic C5–C6 double bond of the base moiety, though significant yields of other radicals, believed to arise from abstraction of hydrogen at the sugar moiety, are also observed with some of the nucleosides and nucleotides. Radicals arising from attack on adenosine 5′-triphosphate have also been detected. The increased yield of sugar-derived radicals in these reactions, when compared with those previously reported for (electrophilic) HO˙ and alkoxyl radicals, is in accord with the known nucleophilic nature of aryl radicals.Studies with the polyU, polyA·polyU, polyC, RNA and DNA suggest that aryl radicals also damage these macromolecules, though the broad nature of the spectral lines and interference from the signal of the arylradical adduct to the spin trap prevent detailed identification of the site(s) of attack. For DNA and RNA the signals obtained are pH dependent. At pH 7.4 both slowly tumbling and rapidly tumbling spin adducts are observed with tRNA, which is consistent with the spin trapping of both large, substrate-derived, radicals and low-molecular-mass fragments, possibly from the sugar moieties. With DNA only spectra from rapidly tumbling species are seen at pH 7.4; these are again believed to be due to the presence of low-molecular-mass material. The formation of these small fragments suggests that aryl radicals are capable of generating strand breaks in nucleic acids, and therefore that such species may be responsible for the genetic damage observed in cells exposed to aryl-radical-generating systems. more...

Absolute rate constants have been estimated for the transfer of chloride ligands from CuCl(OH2)5+, CuCl2(OH2)4 and CuCl3(OH2)3– to 2-benzoylphenyl radical in aqueous solution at ionic strength 5 mol dm–3 by comparison with its rate of cyclisation. The radical reacts with the monochlorocopper(II) complex with k=(8.8 ± 1.3)× 106 dm3 mol–1 s–1, but it does not discriminate between the di- and tri-chloro complexes, k=(1.0 ± 0.15)× 108 dm3 mol–1 s–1. Relative rate constants for the transfer of chloride and water ligands to a series of 2-, 3- and 4-substituted phenyl radicals have also been obtained; three copper complexes are implicated in chloride transfer for the conditions examined. Radicals with 3-and 4-substituents discriminate the monochlorocopper(II) complex from the two more highly chlorinated complexes, the discrimination increasing the more nucleophilic and reactive the radical. Radicals with strongly electron-withdrawing 2-substituents (CN, F) do not discriminate between any of the chlorocopper(II) complexes. A mechanistic rationale of the observations is proposed. more...

Exposure of dilute solutions of 2, 2′-bithienyl in 2-methyltetrahydrofuran glasses at 77 K to ionizing radiation gave the corresponding radical-anion, as indicated by the EPR spectra. Similar treatment of thiophene solutions failed to give the expected it radical-anion, although electron capture clearly occurred: for CD3OH and CD3OD solutions the primary products were identified as the H- and D-atom-addition products, formed by protonation of the anions.Under similar conditions, 2-bromothiophene gave clear EPR signals with features characteristic of the C(2)–˙Br–σ* radical-anion (with a spin density of ∼ 37% on bromine). A σ* anion, with the unpaired electron localized in one of the C–Cl σ* orbitals, was detected following electron-addition to 2,5-dichlorothiophene, whereas for 2-chlorothiophene the corresponding σ* radical-anion, if formed, evidently decomposes rapidly to the 2-thienyl radical, whose EPR spectrum is reported. Only for 2,2′-bithienyl was the radical-anion's EPR spectrum detected in fluid solution following electron attachment.These findings and the results of MO calculations are discussed in terms of the structures of the radicalanions and the alternative routes available for reaction following electron-addition. more...

Spin-trapping experiments with the trap Me3CNO have been employed to identify the radicals formed by reaction of ·OH (generated from reaction of H2O2 with a variety of transition-metal ions) with pyrimidine derivatives relevant to nucleic acids. Results for the nucleobases, nucleosides and nucleotides confirm the predominance of attack at the C5 and C6 carbon atoms in the pyrimidine ring. Evidence for subsequent transfer is derived from experiments with poly(U) and other polynucleotides as well as RNA, in which anisotropic spectra from immobilized nitroxide adducts can be distinguished from sharper, isotropic spectra which result from further radical damage [pH dependent for poly(U)] or enzymatic cleavage. more...

It has been established by continuous-flow studies in conjunction with EPR spectroscopy that the aminophosphonic acids 1–4 accelerate significantly the Fenton reaction between FeII and H2O2 in aqueous solution via complexation of the metal ion (with values of the rate constant k for the generation of the hydroxyl radical up to 2 × 105 dm3 mol–1 s–1 at room temperature). To a certain extent this behaviour parallels that of EDTA and some structurally-related aminocarboxylic acids. It is also shown that the N-oxides of the aminophosphonic acids 1–3 react readily with the hydroxyl radical to give long-lived nitroxides viaβ-scission of first-formed carbon-centred radicals.Neither of these findings is believed to correspond to the major chemistry which underlies the efficacy of these ligands as peroxide stabilizers. It is suggested instead that the crucial role of these compounds depends upon their ability to stabilize the higher valence state of iron, and hence not only to encourage oxidation of FeII by O2˙– and H2O2 but also to prevent effective reduction of FeIII by O2˙–, HO2˙ and H2O2. However, radical scavenging by N-oxides may be a secondary, contributory factor in this stabilizing function, especially in peroxide systems when the sequestrant is added before storage, when slow N-oxidation is to be expected. more...

EPR spectroscopy has been employed, in conjunction with a rapid-flow system and the TiIII–H2O2 and TiIII–HOOSO3– redox couples, to study the reactions of ˙OH and SO4˙– with pyrimidine nucleosides. From experiments which complement radiolytic and photolytic studies it is concluded that the formation of base-centred radical-cations (from SO4˙– and from ˙OH at low pH) is followed both by hydration and by loss of the C2′-H in the ribose (but not 2-deoxyribose) ring: further fragmentation follows the latter reaction. The crucial role of the C2′-OH group in the transfer of damage is discussed. more...

Radical adducts to nitrone spin traps have been shown, in aqueous solution, to undergo conversion to the hydroxyl radical adduct via nucleophilic substitution reactions; these observations have important consequences for the interpretation of data from EPR/spin-trapping experiments. more...

Rate constants have been estimated for the transfer to aryl radicals of Cl ligands from CuCl+ and CuCl2 relative to that for the transfer of water ligands from Cu2+: kOH:k1Cl:k2Cl= 1 : (59.3 ± 3.4): (205 ± 9) The rate data indicate that aryl radicals exhibit nucleophilic character during these ligand transfers and that, in the competition between chlorination and hydroxylation, the reactivity–selectivity principle is not obeyed. In competition between CuBr+ and CuCl+ in transferring halide ligands to 4-chlorophenyl radicals a value of k1Br/k1Cl= 2.3 is estimated; there is no selectivity between bromination and chlorination across a range of aryl radicals. The reactivities of the different copper complexes in ligand-transfer broadly parallel their reduction potentials. The nature of the transition state for ligand-transfer is discussed. more...

Continuous-flow and spin-trapping experiments have been employed in conjunction with EPR spectroscopy to investigate the metal-ion induced decomposition of magnesium monoperoxy-phthalate (MMPP) in aqueous solution (at pH ca. 2 and 6). As with the corresponding reactions of peroxymonosulphate (HOOSO3–) it is shown that the behaviour of CuI(which leads to the formation of ˙OH and ArCO2–, Ar = 2-C6H4CO2H) is in complete contrast with that of TiIII and FeII, both of which bring about electron transfer to give ArCO2˙ and OH–. Some subsequent reactions of ArCO2˙ and Ar˙, formed by rapid decarboxylation, have been studied. more...

E.s.r. spectra are described for a variety of radicals obtained from the reactions of α-D-glucose, β-D-glucose, and some related substrates with the hydroxyl radical (generated from the TiIII–H2O2 reaction in a flow system). Evidence is presented that attack of ˙OH is largely unselective; for both glucose anomers, all six radicals which can be formed via C–H abstraction are identified. A study has also been made of the relative ease with which these αβ-dioxygen-substituted radicals undergo acid-catalysed fragmentation; this appears to be governed not only by the nature of the leaving group (OH–, OR–) but also by the geometry of the radical (in particular, an axial β-OH group is much more readily lost than an equatorial group). Potential routes for carbohydrate degradation via glycosidic cleavage of first-formed radicals have been identified. more...

E.s.r. spectroscopy has been employed to characterize radical pathways in the thermal and photolytic decomposition of a variety of diaryl sulphoxides and aryl arenethiolsulphonates (ArSO2SAr). For the sulphoxides, S–C bond cleavage leads to the formation of the delocalized and relatively unreactive sulphinyl radicals (ArSO·); their subsequent reaction evidently involves disproportionation to sulphonyl (ArSO2·) and thiyl (ArS·) radicals. This pair of radicals is also formed directly in the homolysis of the thiolsulphonates, and the subsequent formation of sulphinyl radicals has been investigated. The results are analysed in terms of the products of the reactions and the efficacy of some of the parent substrates as antioxidants. more...

E.s.r. observations have been made on the rearrangements of radicals formed from the reaction between ·OH and a series of simple sugars at pH > 7. Two types of semidione have been identified [for example D-glucose gives geometric isomers of both HOCH2·CHOH·CHOH·CHOH·C(O·)C(O–)H and HOCH2·C(O·)C(O–)H]: their formation provides two routes to radical-induced base-catalysed degradation of carbohydrates, involving ring opening at the O–C(1) bond and, in the second case. C–C cleavage. We suggest that the latter reaction involves a Grob-type fragmentation and that this may account for the formation of malondialdehyde in these systems. more...

E.s.r. spectra detected when ·OH reacts with the aldopentoses xylose and ribose are attributed to pyranose ring radicals, formed in essentially unselective attack; in contrast, in the corresponding reactions of the model compound tetrahydrofurfuryl alcohol, as well as some isopropylidene derivatives of glucofuranose, the hydrogen atoms on the carbon atoms adjacent to oxygen in a furanose ring are activated towards abstraction. A similar activation, evidently stereoelectronic in origin, is also proposed to account for the enhanced reactivity of C(5′)–H in the furanose ring of sucrose, a finding which may have particular relevance to the mechanism of radiation damage in DNA. more...

E.s.r. spectroscopy has been employed to characterize a series of copper(II)–dithiolate complexes formed in aqueous solution at pH ca. 10. Some examples, e.g. that from D-penicillamine [Cu(D-pen)2], are long-lived whereas rapid-flow and fast-freezing methods are required for the study of more transient species (e.g. the analogous complex from L-cysteine). Spectra have been described for related copper complexes of S2N2, S2O2, and S4 type and both isotropic and anisotropic parameters are interpreted in terms of significant spin-delocalization onto S (especially) and N ligands. Some transient ternary complexes [e.g. from L-cysteine and mercaptoethanoic acid)] have also been identified. Stopped-flow experiments on the kinetics of the (second-order) decay of the transient complexes reveal the importance of steric and electronic effects in governing their lifetimes. more...

E.s.r. spectra of some relatively stable phenoxy-radicals with para thioaryl substituents show line-width effects which indicate that there is restricted rotation around the C(phenoxy)–S bond. The barriers to rotation which have been measured (>50 kJ mol–1) imply that in these radicals there is considerable delocalization of the unpaired electron onto sulphur. more...

A spin-trapping method, in conjunction with e.s.r. spectroscopy, has been employed to demonstrate that thiyl radicals are formed in the reactions of the hydroxyl radical with a variety of sulphur-containing compounds: radicals trapped with the nitromethane aci-anion have been characterised on the basis of the g values and β-proton splittings of the resulting adducts. The approach has been extended to the detection of radicals formed in the oxidation of some dialkyl phosphonates and in the photolytic decomposition of sulphur- and phosphorus-containing substrates; the magnitude of a(31P) in the resulting adducts is particularly informative. Complicating features which arise from nucleophilic attack by substrates on the traps have been noted. Finally the spin trapping–e.s.r. technique has been applied to the detection of radicals formed from the antioxidant zinc bis-(OO-di-isopropyl phosphorodithioate) by electrochemical oxidation and by reaction with t-butyl hydroperoxide. more...

E.s.r. spectra are reported for a range of alkylsulphonyl and carbon-centred radicals detected during the oxidation of sulphoxides with the TiIII–H2O2 couple in aqueous solution. Alkylsulphonyl radicals are also formed in some cases by reaction of substituted alkyl radicals with SO2 in aqueous solution; however, hydroxy-conjugated radicals under these conditions behave as one-electron reducing agents and yield SO2– and carbonyl- and carboxy-conjugated radicals do not appear to react. Evidence is adduced for the ready desulphonylation of HO2C·CH2·SO2˙. more...

Radical-cations have been detected during the oxidation of RSCHCH2(R = Et, Pri) with Cl2˙– or, ˙OH at low pH; e.s.r. parameters indicate that these possess a cyclic sulphonium structure ˙[graphic ommitted]R. Their formation from the hydroxy-adducts may involve anchimeric assistance from the sulphur atom. more...

E.s.r. spectroscopy has been employed with the radiomimetic TiIII–H2O2 couple and an aqueous flow system to study the reactions between the hydroxyl radical and some sugar phosphates. For the furanose compounds hydrogen-atom abstraction from the carbon adjacent to the alicyclic oxygen is preferred: subsequent phosphate loss occurs when the radical centre is adjacent to the carbon bound to phosphate, and, as judged by steady-state e.s.r. and time-resolved pulse-radiolysis experiments, is faster for radicals from ribose 5-phosphate, α-D-glucose 1-phosphate, and glycerol phosphates (k ca. 104s–1) than for fructose phosphates (k ca. 102–103 s–1). more...

Line-width alternation in the e.s.r. spectra of some 4-substituted phenoxyl radicals ·OC6H4X [X = C(O)R, SR, SAr] reveals restricted rotation about the C(4)–X bond. Though the barrier heights are similar for the carbonyl- and thio-substituted phenoxyl radicals, the considerably higher values of ring proton splittings for the former reflect the inductive effect of the CO substituent in redistributing spin density between (phenoxyl) oxygen and the ring. There is no evidence that sulphinyl and sulphonyl substituents are effective in spin-withdrawal, though their inductive effects on spin distribution are marked. A general model for the analysis of inductive and mesomeric effects in phenoxyl radicals is proposed. Barriers to rotation in the acetyl- and thio-substituted phenoxyl radicals are compared with those of their phenolic precursors. more...

Results are reported of an e.s.r. investigation of the reactions of the methyl radical, generated from dimethyl sulphoxide with the TiIII–H2O2 couple, with a variety of aliphatic compounds in aqueous solution. For example, with carboxylic acids and nitriles, the spectra of radicals formed by hydrogen-atom abstraction are detected: it is shown that the methyl radical is more selective than hydroxy, having a relatively greater propensity for abstracting hydrogen from the carbon atom adjacent to the functional group, and rate constants for these reactions, which are in the range ca. 102–104 l mol–1 s–1, have been determined. Reaction with nitromethane is more complex. The spectra observed are those of the nitroethane radical-anion or its conjugate acid, depending on the pH, and it is concluded that abstraction by the methyl radical to give ·CH2NO2 is followed by rapid one-electron reduction by TiIII to give the aci-compound to which methyl then adds. Nitroethane behaves analogously, and both EtNO2H and PriNO2H radicals are estimated to have pKa 4·4. more...

Evidence is presented that, when complexed with edta in acid solution, titanium(IV) is an effective oxidant for radicals in which tervalent carbon is bonded to a substituent of +M type. A lower limit for k(TiIV–edta +·CHMeOH) is estimated as ca. 108 dm3 mol–1 s–1. When the organic radicals are generated from the TiIII–RO2H couple (R = H or alkyl), a chain reaction operates; but stoicheiometric studies show that the chains are relatively short. more...

The e.s.r. spectra are described of a variety of arenesulphonyl radicals (ArSO2·) which have been generated photolytically over a wide range of temperature. It is shown that the spin distribution in these σ-radicals resembles that in the benzoyl radical; radicals without ortho-substituents, which undergo relatively rapid rotation about the C–S bond at all accessible temperatures, have a(m-H) > a(o-H), a(p-H). For some di-ortho-substituted radicals, rotation around C–S is slow at low temperature (barriers to rotation have been determined for some examples) and some mono-ortho-substituted radicals demonstrate marked conformational preferences; analysis shows that the largest proton-splitting (ca. 0.2 mT) derives from the meta-proton anti to the orbital of the unpaired electron on sulphur. INDO calculations yield results in good agreement with these findings. more...

For a series of lactones derived from sugars it is shown that reaction of ˙OH (from the TiIII–H2O2 couple) takes place preferentially at C(2) to give the appropriate carboxy-conjugated radical. Radicals formed by abstraction of C(3)–H in the γ-lactones are shown to react readily both by ring-opening [via fission of the C(β)–O(acyloxy) bond] and by rapid oxidation with H2O2, evidently to give enediols related to ascorbic acid. Oxidation of the latter with ˙OH is suggested to proceed both via direct electron transfer (at pH >5) and, at lower pH, addition to the enolic double bond followed by either acid-or base-catalysed elimination of water. more...

A conformational analysis is presented of a variety of radicals generated by reaction of the hydroxyl radical with A number of cyclopentane and cyclohexane derivatives. The β-hydrogen splittings in their e.s.r. spectra are interpreted in terms of a Bcos2θ dependence, where θ is the dihedral angle subtended by the β-C–H bond with the orbital of the unpaired electron. The results suggest that the cyclohexane-derived radical prefer a twisted-chair geometry except for 2-oxocyclohexyl which exists as a half-chair. The cyclopentyl-type radicals exist in a half-chair conformation except for the planar 2-oxocyclopentyl. A kinetic analysis for the conformational interconversion of 4-ammoniocyclohexyl and 2-oxocyclohexyl is reported. more...

E.s.r. spectroscopy has been employed to show that the reaction of ·OH with the model substrate myoinositol is unselective; at pH 4, e.s.r. signals of all possible radicals produced by C–H abstraction are detected. A conformational analysis based on the hyperfine splittings is presented. The rates of acid-catalysed rearrangement of the radicals have been measured; the radical with an axial β-hydroxy-group is shown to lose water with a rate constant (ca. 3 × 106 dm3 mol–1 s–1) which is considerably greater than those of the radicals with only equatorial β-hydroxy-groups. This is interpreted in terms of a significant stereoelectronic requirement for rearrangement. more...

E.s.r. spectra are described of radicals formed by the reactions of some alkynes both with hydroxyl and also with radicals formed from a variety of ethers in the presence of hydroxyl. Vinyl radicals, though not normally detected, are apparently involved, and evidence is presented for their intermolecular addition and intramolecular abstraction reactions. For example, the vinyl radical produced by addition of ·CHMeOEt to HO2CCCCO2H reacts to form the allyl radical ·CHMeC(CO2H)CHCO2H, evidently via a 1,5 carbon-to-carbon hydrogen shift and fragmentation. Other instances of ready 1,5-shifts (including an oxygen-to-carbon example) are described. more...

The complementary techniques of pulse-radiolysis and e.s.r. spectroscopy have been employed in a kinetic study of the dehydration of a variety of α,β-dihydroxyalkyl radicals [˙CR1(OH)CR2R3OH] into the appropriate carbonyl-conjugated radicals [˙CR2R3C(O)R1]. The overall rates of proton-catalysed dehydration, as revealed by steady-state (e.s.r.) and time-resolved (pulse-radiolysis) experiments, indicate the importance of the electronic effects of substituents (contrast values of 1.2 × 109 and 9.8 × 108 dm3 mol–1 s–1 for the radicals from cyclohexane-1,2-diol and butane-2,3-diol, respectively, with that for the radicals from erythritol of 4.2 × 106 dm3 mol–1 s–1). Time-resolved experiments allow information to be obtained about the generation of the protonated species [˙CR1(OH)CR2R3OH2+] and for the loss of water from this intermediate.For ˙CR(OH)CMe2OH (R = H and Me) evidence is obtained for a rapid uncatalysed dehydration reaction, (k 1–2 × 104 s–1), which is believed to be assisted by the steric effect of the gem-dimethyl group as well as the polarity of the solvent. For the latter substrate, the reaction is characterized by a strongly negative activation entropy (–93 J mol–1 K–1).α,β-Dihydroxyalkyl radicals are shown to reduce methyl viologen with rate constants that depend strongly on the nature of the substituents; there is a correlation between the reducing power of the radical and its rate of dehydration. more...

An examination of the e.s.r. spectra of >50 cation radicals derived from 10-arylphenoxazines and 10-arylphenothiazines shows that the spin distribution is influenced essentially inductive by the aryl substituent for substituents other than p-alkylamino-groups. The variations in spin distribution are correlated using established substituent constants and it is concluded that the radicals do not deviate significantly from the geometry previously adduced for the cation radicals of 10-phenylphenoxazine and 10-phenylphenothiazine. When the aryl substituents are p-alkylamino-groups, the spin distribution and probably also the geometry of the radicals are markedly perturbed from that in the respective 10-phenylazine cation radicals. more...