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Cross-linking of ethylene propylene diene monomer (EPDM) rubbers containing different amounts of dicyclopentadiene (DCPD), 5-ethylidene-2-norbornene (ENB) and 5-vinyl-2-norbornene (VNB) dienes was examined by EPR spectroscopy. The cross-linking was initiated by thermal decomposition of dicumyl peroxide at 440 K. The concentration of free radicals increased towards the end of the cross-linking process before reaching a maximum and decaying to zero. This is explained by the spatial confinement of the radicals in the cross-linked rubber, which leads to increased life time and, hence, higher radical concentration at a time when most peroxide has decomposed. The EPR spectra showed the presence of two components: a well-resolved spectrum overlapping a broad line. Both components are assigned to allyl radicals possessing different mobility. The more mobile component is assigned to allyl radicals along the EPDM chains, whereas the immobilised allyl radicals are formed in the cross-links. The stability of the allyl radicals decreases in the order DCPD > ENB > VNB. EPDMs containing two dienes show more persistent radicals than their single-diene counterparts. The most persistent radicals are observed for highly cross-linked (e.g., 28% ENB) or mixed diene EPDMs (e.g., 2.2% DCPD–4.4% ENB); the EPR spectra of free radicals in these systems can be observed for several hours.

The structure and bonding of the azo dye Orange II (Acid Orange 7) in parent and reduced forms have been studied using NMR, infrared, Raman, UV-visible, and electron paramagnetic resonance (EPR) spectroscopy, allied with density functional theory (DFT) calculations on three hydrazone models (no sulfonate, anionic sulfonate, and protonated sulfonate) and one azo model (protonated sulfonate). The calculated structures of the three hydrazone models are similar to each other and that of the model without a sulfonate group (Solvent Yellow 14) closely matches its reported crystal structure. The 1H and 13C NMR resonances of Orange II, assigned directly from 1D and 2D experimental data, indicate that it is present asor = 95% hydrazone in aqueous solution, and as a ca. 70:30 hydrazone:azo mixture in dimethyl sulfoxide at 300 K. Overall, the experimental data from Orange II are matched well by calculations on the hydrazone model with a protonated sulfonate group; the IR, Raman, and UV-visible spectra of Orange II are assigned to specific vibrational modes and electronic transitions calculated for this model. The EPR spectrum obtained on one-electron reduction of Orange II by the 2-hydroxy-2-propyl radical (*CMe2OH) at pH 4 is attributed to the hydrazyl radical produced on protonation of the radical anion. Calculations on reduced forms of the model dyes support this assignment, with electron spin density on the two nitrogen atoms and the naphthyl ring; in addition, they provide estimates of the structures, vibrational spectra, and electronic transitions of the radicals.

A series of Au nanoparticles modified with a nitroxide-functionalized ligand was prepared with a range of spin-label coverage. The X-band EPR spectra of frozen solutions of these nanoparticles showed coverage-dependent line-broadening due to dipole-dipole interactions between spin labels. We developed a methodology to analyze such spectra in terms of geometrical features of the nanoparticles (e.g., Au core size and the length of the spin-labeled ligand). Our method is based on the assumption that the spectral line shape is determined by the average distance between nearest-neighboring spin labels adsorbed on the Au particle. Geometrical and statistical analysis then relates this distance to the line shape parameter d1/d, which was calibrated using a model system. Application of this methodology to the experimental spectra provided information about the conformation of ligands on the Au surface. We found that, if the spin-labeled ligand is substantially longer than the surrounding protecting layer, it does not adopt a fully stretched conformation but wraps around the particle immediately above the layer of surrounding ligand. Our results also show that the ligands do not adsorb cooperatively on the Au surface.

The mechanism of a place-exchange reaction of ligand-protected Au nanoparticles was investigated using diradical disulfide spin labels. Analysis of reaction mixtures using a combination of GPC and EPR allowed us to determine concentration profile and propose a kinetic model for the reaction. In this model, only one branch of the disulfide ligand is adsorbed on the Au surface during exchange; the other branch forms mixed disulfide with the outgoing ligand. The two branches of the disulfide ligand therefore do not adsorb in adjacent positions on the surface of Au nanoparticles; this was ultimately proven by the powder EPR spectra of frozen exchange reaction mixtures. Our data also suggest the presence of different binding sites with different reactivity in the exchange reaction. The most-active sites are likely to be nanoparticle surface defects.

Using three readily-prepared free-radicals of DPPH type, with different oxidation potential, it is possible to generate, via one electron transfer or hydrogen-atom abstraction, short-lived radicals, characterised by the EPR spin- trapping technique. PBN, DMPO and DEPMPO were employed as spin-traps. The results show that DPPH and its congeners can be successfully employed for generation of short-lived oxygen-, sulfur-, carbon-, nitrogen-, and phosphorus-centered radicals.

The structure of dodecylamine (DDA) aggregates acting as templates in the synthesis of mesoporous silica (in the DDA/water/ethanol/tetraethoxysilane (TEOS) system) has been studied via the EPR spectra of a variety of spin probes and of a spin-labeled silica precursor. Two recipes have been examined: one, starting from a water-rich DDA solution, A0, reported to produce a silica with framework mesopores and textural pores comparable in volume, and another, starting from an ethanol-rich solution, B0 which yields a solid having practically only framework pores. For both compositions the relative positions of the spin probes in the aggregates have been determined, the local polarities expressed in terms of the Kosower scale and their motions examined. The aggregates formed have been characterized as emulsion droplets for A0 and micelles with radii

A kinetic and mechanistic study is reported of the oxidation of a number of azonaphthol dyes with hydrogen peroxide in aqueous solution, catalysed by some mono and dinuclear manganese(IV) complexes of 1,4,7-trimethyl-1,4,7-triazacyclononane (Me3TACN). The results of UV-Vis investigations, augmented by EPR and ESI-MS studies, are described for a series of experiments in which concentrations, pH and ionic strength have been varied. The reactions are characterised by an induction period followed by a relatively rapid oxidation. For the dinuclear manganese complex 2, these are consistent with an initial perhydrolysis of the manganese complex involving both the dye anion and HO2-, to give mononuclear manganese species and the operation of a catalytic cycle incorporating MnIIIL(OH)3, O = MnVL(OH)2 and MnIVL(OH)3 (L = Me3TACN) (cf. the reactions of peroxidase enzymes). ESI-MS results provide evidence for the formation and reaction (with the dye) of MnIVL(OH)3. With the mononuclear manganese complex MnIVL(OMe)3, there is a short lag-phase attributed to perhydrolysis by HO2- followed by the same catalytic cycle.

EPR spectroscopy has been employed to study directly the selectivity of hydrogen-atom abstraction by some alkoxyl radicals from a variety of linear and branched alkanes, as well as linear alkenes, chosen as models for low molecular-weight polyolefin cross-linking systems. In situ thermal and photolytic approaches, as well as spin-trapping, have been employed to provide information relating to an accessible temperature range of 233–453 K, in part to mimic conditions relevant to melt processing of polyolefins. Rate constants (in the range 3 × 103 − 3.7 × 105 dm3 mol−1 s−1 per hydrogen) have been determined for C–H abstraction at room temperature. Radical selectivity is largely governed by enthalpic effects (modelled via bond dissociation energy calculations and kinetic analysis). Direct evidence has been obtained for lack of reactivity, as a result of unfavourable steric interactions, for the secondary and tertiary C–H bonds in 2,4-dimethylpentane and 2,4,6-trimethylheptane, models for polypropylene. This has been rationalized via free-energy calculations using DFT.

Electron paramagnetic resonance (EPR) spin-trapping experiments, employing the novel spin-trap DEPMPO, provide evidence for the formation of protein-peroxyl radicals from the reaction of bovine serum albumin (BSA) or lysozyme with HO · in the presence of O 2 . Spin-trapping leads to the detection of anisotropic spectra of partially immobilized protein-peroxyl spin-adducts; positive identification is based on a novel spectrum simulation approach (through which broadened anisotropic spectra are simulated and compared with experiment) and by comparison of results with those obtained when MeO 2 · is trapped and the adduct frozen in a solid matrix.

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.

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.

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.

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.

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).

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.

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.

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.

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.

EPR experiments confirm that reaction of qinghaosu and some related endoperoxides with Fe2+ in aqueous acetonitrile leads to the production of carbon-centred radicals derived by rapid rearrangement of first-formed cyclic alkoxyl radicals. Signals obtained from qinghaosu itself with spin-traps DMPO and DBNBS are assigned to the adducts (15) and (16), a finding which accounts for the formation of the major products (11) and (14).

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.

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.

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.

Analysis and simulation of the EPR spectra of α-chymotrypsin spin-labelled at two sites (methionine-192 and serine-195) in water and sodium bis(2-ethylhexyl) sulfosuccinate (AOT)–isooctane reverse micelles has provided information on the rate and nature of label motion in these media. The correlation time of methionine-labelled chymotrypsin, and the value of A∥ for serine-labelled chymotrypsin in reverse micelles have been studied as functions of surfactant charge [AOT, negative, and cetyltrimethylammonium bromide (CTAB), positive], of the net protein charge above and below its isoelectric point, and of the addition of neutral co-surfactants. The results obtained are consistent with the ‘water-shell’ model of protein solvation in these systems, with no evidence for any ionic significant interactions between protein and surfactant headgroups.

EPR spectroscopy, in conjunction with spin-trapping and spin-scavenging techniques, has been employed together with UV–VIS spectroscopy to establish that iron(III) tetra(N-methylpyridyl)porphyrins ligated to carboxylate anions undergo ready photodissociation with light in the visible region of the spectrum. Studies conducted as a function of pH and for a variety of carboxylic acids provide evidence for initial photodecomposition to give FeII and acyloxyl radicals, for which recombination competes with decarboxylation to give alkyl radicals. The roles of oxygen and the structure of the initial complex have been explored.

A continuous-flow system approach with EPR detection and steady-state analysis has been employed to determine the rate constants for addition of a range of aliphatic radicals to acrylic acid and related alkenes in aqueous solution at room temperature. For acrylic and methacrylic acids the rate constants (in the range 2 × 105–ca. 108 dm3 mol–1 s–1) are governed largely by polar effects in the transition state, though the deviation from planarity of the attacking species may be important for ·CHMeOH and related radicals. As expected, steric effects play a prominent role in causing retardation in the reactions of crotonic and 3,3-dimethylacrylic acids, and in addition of ·CMe2OH.

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.

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.

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.

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.

EPR spin-trapping experiments using MNP (2-methyl-2-nitrosopropane) have been employed to study radicals formed by reaction of HO˙ (generated from reaction of H2O2 with Fe2+) with uridine as a model nucleoside; studies with [5-2H]uridine, [5,6-2H2]uridine and [1,3-15N2] have allowed us to obtain a more detailed analysis of the spectra following HO˙ attack on uridine, for which EPR spectra cannot at present be unambiguously assigned. These studies provide detailed information concerning the analysis of hyperfine splittings and hence the quantification of the different amounts of the C(5)- and C(6)-hydroxyl-radical adducts formed following HO˙ attack, which are comparable to results obtained from pulse radiolysis studies. Studies have also been extended to investigate the reactions of both SO4˙– and ButO˙ with uridine.

The use of spin-trapping techniques (with DMPO and MNP) in conjunction with EPR and NMR spectroscopic studies has allowed mechanistic details to be obtained about the process whereby thiols (particularly liquid polysulfide polymers) can be linked via reaction with inorganic oxidants (especially activated MnO2). The roles of activation, basic additives, and oxygen are interpreted in terms of the involvement of thiyl radicals which can undergo both dimerization and reaction with thiolate anions, evidently to give disulfide radical-anions as important reaction intermediates.

An EPR approach has been developed and applied to the determination of the rate constants for the decomposition of some arenediazonium ions by FeII complexes to give aryl radicals. In continuousflow experiments, steady-state concentrations have been measured both for the aryl radical-adducts to the fumarate anion and for ˙CH2CO2–(obtained by reaction of Ar˙ with the iodoacetate anion): analysis of the dependence of these upon [FeII] and [ArN2+] leads to values of the rate constants for electron transfer.For the reaction between FeII–EDTA and a series of diazonium ions, for which values of k are in the range 2 × 104–4 × 105 dm3 mol–1 s–1, it is shown that there is a good dual-parameter Hammett–Taft correlation (involving σI and σR+, and with ρ= 1.74), consistent with the occurrence of an outer-sphere electron-transfer process.

Continuous-flow EPR studies of the reaction between CeIV and the peroxymonosulphate anion HOOSO3– at low pH enable the isotropic EPR spectrum of SO5˙– to be characterized, and provide kinetic information on the formation and reaction of this radical. Alkylperoxyl radicals (RO2˙) detected when the reaction is carried out in the presence of alkenes (e.g. methyl methacrylate) are shown to arise from reactions of SO4˙–(derived from self-reaction of SO5˙–) with the alkene, and subsequent addition of oxygen (also formed from SO5˙–).

Reaction of Cu(BF(4))(2).6H(2)O with the N(3)O(2) donor ligand H(2)L (where H(2)L = N-benzyl-N',N''-di-tert-butyl-disalicyl-triaminocyclohexane) results in the formation of a novel Cu(II)L complex, 1. X-Ray crystallography of it shows the Cu(II) centre coordinated by two phenolate oxygens and two imine nitrogens in a distorted square plane with an elongated bond to the amine nitrogen (2.512 A) in the axial position. EPR spectroscopy gives g values of g(1) = 2.277, g(2) = 2.100, g(3) = 2.025, and A(1) = 15.6 mT which are consistent with the distorted square pyramidal coordination environment determined from the X-ray structure. UV/visible and electrochemical analysis of shows that it undergoes two reversible processes assigned to the successive oxidation of the phenolate oxygens to phenoxyl radicals, the first at E((1/2)) = 0.89 V (DeltaE = 81 mV, vs. Ag/AgCl) and the second at E((1/2)) = 1.13V (DeltaE = 84 mV, vs. Ag/AgCl). Chemical oxidation results in the formation of a species, assigned as [1](+)(.) which is EPR silent due to antiferromagnetic coupling between the Cu(II) centre and the bound phenoxyl radical. The oxidised species catalyses the oxidation of benzyl alcohol to benzaldehyde.

Spin-trapping experiments show the formation of sulfur-centered radicals during a ligand-exchange reaction between Ph3P-protected Au nanoparticles and alkanethiols in air (see picture). Oxidation of the alkanethiols by molecular O2 adsorbed on the nanoparticles is proposed as the key step. The feasibility of such a process is demonstrated by the Au-nanoparticle-catalyzed oxidation of BH4- and tBuOOH with air.

A series of Au nanoparticles functionalised with nitroxide spin labels has been prepared and studied by EPR spectroscopy. Samples with low coverage of the spin label were used to investigate the dynamics of the surface-attached labels at different distances from the Au surface. The rotational correlation times of spin labels vary from 10(-10) s to more than 3 x 10(-9) s, depending on the chain length of the label and the surrounding ligand. The samples with higher coverage of the spin label show an increasing contribution of the exchange interaction between nitroxides adsorbed in a close proximity to each other on the same nanoparticle. Quantitative analysis of the EPR spectra of these samples suggests the presence of non-equivalent binding sites on the surface of Au nanoparticles. Additionally, EPR signals of isolated radical pairs were observed at intermediate coverage.

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.

An EPR study of a place-exchange reaction of a diradical disulfide with butanethiol-protected Au nanoparticles showed that the two branches of the disulfide molecule do not adsorb adjacent to each other on the Au surface. The most likely mechanism includes adsorption of only one branch of the disulfide molecule in the exchange process. The exchange reaction was found to be zeroth-order with respect to the diradical, indicative of a dissociative "SN1"-type mechanism.

The reactions of cerium(IV) and the hydroxyl radical [generated from iron(ii)/H2O2] with bovine serum albumin (BSA) have been investigated by EPR spin trapping. With the former reagent a protein-derived thiyl radical is selectively generated; this has been characterized via the anisotropic EPR spectra observed on reaction of this radical with the spin trap DMPO. Blocking of the thiol group results in the loss of this species and the detection of a peroxyl radical, believed to be formed by reaction of oxygen with initially-generated, but undetected, carbon-centred radicals from aromatic amino acids. Experiments with a second spin trap (DBNBS) confirm the formation of these carbon-centred species and suggest that damage can be transferred from the thiol group to carbon sites in the protein. A similar transfer pathway can be observed when hydroxyl radicals react with BSA. Further experiments demonstrate that the reverse process can also occur: when hydroxyl radicals react with BSA, the thiol group appears to act as a radical sink and protects the protein from denaturation and fragmentation through the transfer of damage from a carbon site to the thiol group. Thiol-blocked BSA is shown to be more susceptible to damage than the native protein in both direct EPR experiments and enzyme digestion studies. Oxygen has a similar effect, with more rapid fragmentation detected in its presence than its absence.

The reactions of hydroxyl radicals generated from FeII/H2O2 and CuII/H2O2 redox couples with a variety of proteins (BSA, histones, cytochrome c, lysozyme and protamine) have been investigated by e.s.r. spin trapping. The signals obtained, which are generally anisotropic in nature, characterize the formation of partially-immobilized spin-adducts resulting from attack of the HO. radicals on the protein and subsequent reaction of the protein-derived radicals with the spin trap. Similar spin adducts are observed on incubation of two haem-proteins (haemoglobin and myoglobin) with H2O2 in the absence of added metal ions implying a reaction at the haem centre followed by internal electron transfer reactions. Two strategies have been employed to obtain information about the site(s) of radical damage in these proteins. The first involves the use of a variety of spin traps and in particular DMPO: with this particular trap the broad spectra from largely immobilized radicals show characteristic a(beta-H) values which enable carbon-, oxygen- and sulphur-centred radicals to be distinguished. The second involves the use of enzymatic cleavage of first-formed adducts to release smaller nitroxides, with isotropic spectra, which allow the recognition of beta-proton splittings and hence information about the sites of radical damage to be obtained. These results, which allows backbone and side-chain attack to be distinguished, are in agreement with random attack of the HO. radical on the protein and are in accord with studies carried out on model peptides. In contrast the use of less reactive attacking radicals [N3., .CH(CH3)OH] and oxidising agents (Ce4+) provides evidence for selective attack on these proteins at particular residues.

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.

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).