Showing papers on "Hydrogen atom abstraction published in 1995"

Spontaneous autoxidation of dopamine

Abstract: A detailed kinetic study has been carried out of the reaction of dopamine, 2-(3,4-dihydroxyphenyl)ethylamine, with dioxygen over the pH range 7–9 where it reacts spontaneously without the necessity of metal ion catalysis. The reaction was found to be accurately first-order in [O2] and in [dopamine] and first-order in [H+]–1 and, furthermore, stoichiometric amounts of H2O2 were shown to be produced. The other product of oxidation is, initially, the pink dopaminochrome which, however, is not stable and reacts further (without the consumption of dioxygen) to form the insoluble polymeric material known as ‘melanine’. The rate-determining step is assumed to be hydrogen atom abstraction from the monodeprotonated species by O2(as with many other catecholamines, dopamine is stable towards oxidation in acidic media in the complete absence of metal ions) with a second-order rate constant of k1= 0.47 ± 0.05 dm3 mol–1 s–1 at 25 °C in a solution of ionic strength 0.1 mol–1 dm–3(KCl).

Free radical generation at the solid/liquid interface in iron containing minerals

Abstract: The potential for free radical release has been measured by means of the spin trapping technique on three kinds of iron containing particulate: two asbestos fibers (chrysotile and crocidolite); an iron-exchanged zeolite and two iron oxides (magnetite and haematite). DMPO (5,5'-dimethyl-1-pirroline-N-oxide), used as spin trap in aqueous suspensions of the solids, reveals the presence of the hydroxyl and carboxylate radicals giving rise respectively to the two adducts [DMPO-OH] and [DMPO-CO2], each characterized by a well-defined EPR spectrum. Two target molecules have been considered: the formate ion to evidence potential for hydrogen abstraction in any biological compartment and hydrogen peroxide, always present in the phagosome during phagocytosis. The kinetics of decomposition of hydrogen peroxide has also been measured on all solids. Ferrozine and desferrioxamine, specific chelators of Fe(II) and Fe(III) respectively, have been used to remove selectively iron ions. Iron is implicated in free radical release but the amount of iron at the surface is unrelated to the amount of radicals formed. Only few surface ions in a particular redox and coordination state are active. Three different kinds of sites have been evidenced: one acting as H abstracter, the other as a heterogeneous catalyst for hydroxyl radical release, the third one related to catalysis of hydrogen peroxide disproportionation. In both mechanisms of free radical release, the Fe-exchanged zeolite mimics the behaviour of asbestos whereas the two oxides are mostly inert. Conversely magnetite turns out to be an excellent catalyst for hydrogen peroxide disproportionation while haematite is inactive also in this reaction. The results agree with the implication of a radicalic mechanism in the in vitro DNA damage and in the in vivo toxicity of asbestos.

Hydrogen atom bond increments for calculation of thermodynamic properties of hydrocarbon radical species

Abstract: Hydrogen atom bond increments (HBI) are defined and a data base is derived for accurately estimating AH°29s, S°298, and CP(T) (300 < 77K < 1500) on generic classes of hydrocarbon (HC) radical species relevant to combustion and atmospheric chemistry, using these thermodynamic property increments. The HBI group technique is based on known thermodynamic properties of the parent molecule and calculated changes that occur upon formation of a radical via loss of a H atom. The HBI approach incorporates (i) evaluated literature bond energies, (ii) calculated entropy and heat capacity increments resulting from loss and/or change in vibrational frequencies including frequencies corresponding to inversion of the radical center, (iii) increments from changes in barriers to internal rotation, and (iv) spin degeneracy. Twenty five HBI groups corresponding to alkyl (primary, secondary, and tertiary), vinyl, allenic, allylic, benzyl, acetylenic, and other conjugated hydrocarbon radicals are defined, and their group values are calculated. The HBI groups, when coupled with thermodynamic properties of the appropriate “parent” molecule, yield accurate thermodynamic properties for the respective radicals.

A hydrogen-atom abstraction model for the function of YZ in photosynthetic oxygen evolution.

Abstract: Recent magnetic-resonance work on YŻ suggests that this species exhibits considerable motional flexibility in its functional site and that its phenol oxygen is not involved in a well-ordered hydrogen-bond interaction (Tang et al., submitted; Tommos et al., in press). Both of these observations are inconsistent with a simple electron-transfer function for this radical in photosynthetic water oxidation. By considering the roles of catalytically active amino acid radicals in other enzymes and recent data on the water-oxidation process in Photosystem II, we rationalize these observations by suggesting that YŻ functions to abstract hydrogen atoms from aquo- and hydroxy-bound managanese ions in the (Mn)4 cluster on each S-state transition. The hydrogen-atom abstraction process may occur either by sequential or concerted kinetic pathways. Within this model, the (Mn)4/YZ center forms a single catalytic center that comprises the Oxygen Evolving Complex in Photosystem II.

Photochemistry of dibenzoyl methane UVA filters Part 1

Abstract: Two dibenzoyl methane derivatives, 4-isopropyldibenzoyl methane (I-DBM) and 4-t-butyl-3′-methoxydibenzoyl methand (BM-DBM), have been found to be photolabile towards UVA light in non-polar solvents, whereas in polar solvents photodegradation was very low. In each case about a dozen photoproducts were identified: their formation involves primary α bond cleavages of carbonyl groups, followed by hydrogen abstraction and/or oxidation or radical recombination. These photoproducts can be classified as substituted benzaldehydes, benzoic acids, acetophenones, phenglglyoxals, benzils, dibenzoyl methanes or dibenzoyl ethanes.

Nature of Rate-Limiting Steps in the Soybean Lipoxygenase-1 Reaction

Abstract: A series of kinetic isotope effect experiments were performed with the goal of understanding the nature of rate-limiting steps in the soybean lipoxygenase-1 (SBL-1) reaction. SBL-1 was reacted with linoleic acid (LA) and deuterated linoleic acid (D-LA) under a variety of experimental conditions involving changes in temperature, pH, viscosity, and replacement of H2O with D2O. The extrapolated intrinsic primary H/D isotope effect can be estimated to be possibly as large as 80. This value is probably the largest isotope effect published for an enzymatic reaction, and much larger than that predicted from semiclassical models. Due to this large primary isotope effect, the C-D bond cleavage fully limits the rate of reaction under all conditions tested. In the case of protonated linoleic acid, a number of steps are partially rate-limiting at room temperature; three distinct mechanistic steps which include substrate binding, an H2O/D2O sensitive step, and C-H bond cleavage have been characterized. Use of glucose as a solvent viscosogen demonstrates that substrate binding is approximately 48% rate-limiting for LA at 20 degrees C. SBL-1 is one of the few enzymes that fit the definition of a "perfect enzyme", in the sense that further optimization of any single step at room temperature will not significantly increase the overall rate. At lower temperatures, the step sensitive to solvent deuteration begins to dominate the reaction, whereas at higher temperatures, the hydrogen abstraction step is rate-limiting. The pH dependence of kcat/Km for SBL-1 can be explained as arising from two pKa's, one controlling substrate binding and the other substrate release. Below pH 7.8, the rate of substrate release increases, thus decreasing the commitment to catalysis and unmasking the large intrinsic isotope effect on the subsequent hydrogen abstraction. An abnormally high pKa, in the range of 7-8, has been determined for LA in the concentration range employed in these studies. We propose that the negatively charged form of LA, predominating above pH 7.8, is the preferred substrate with larger commitments to catalysis.

Uranium(VI)-Catalyzed Photooxidation of Hydrocarbons with Molecular Oxygen

Abstract: Uranium(VI) catalyzes the photooxidation of alkanes, alkenes, alcohols, and aldehydes by molecular oxygen in aqueous solution. Despite the mechanistic complexities, each of there actions investigated yielded a single organic product. On the basis of the quenching kinetics, the deuterium isotope effects, the nature of the products, and linear free energy relationships, all the reactions appear to take place by hydrogen atom abstraction from C-H bonds, followed by uranium-mediated product formation.

Enhancement of lipid peroxidation by indole-3-acetic acid and derivatives : substituent effects

Abstract: The peroxidation of liposomes by a haem peroxidase and hydrogen peroxide in the presence of indole-3-acetic acid and derivatives was investigated. It was found that these compounds can accelerate the lipid peroxidation up to 65 fold and this is attributed to the formation of peroxyl radicals that may react with the lipids, possibly by hydrogen abstraction. The peroxyl radicals are formed by peroxidase-catalyzed oxidation of the enhancers to radical cations which undergo cleavage of the carbon-carbon bond on the side-chain to yield CO2 and carbon-centred radicals that rapidly add oxygen. In competition with decarboxylation, the radical cations deprotonate reversibly from the Nl position. Rates of decarboxylation,pKa values and rate of reaction with the peroxidase compound I indicate consistent substituent effects which, however, can not be quantitatively related to the usual Hammett or Brown parameters. Assuming that the rate of decarboxylation of the radical cations taken is a measure of the electron dens...

Translational diffusion of transient radicals created by the photoinduced hydrogen abstraction reaction in solution : anomalous size dependence in the radical diffusion

Abstract: Diffusion coefficients (D) of various radicals created by the photoinduced hydrogen abstraction reactions from alcohols (ethanol and 2‐propanol) are investigated by using the transient grating (TG) method. In all the reaction systems, D’s of the transient radicals, as well as those of the parent molecules, can be measured simultaneously. The results clearly show slower diffusive motions of the radicals, at least of the π radicals in the hydrogen abstraction reaction systems, compared with those of the parent molecules. D’s of the parent molecules usually agree well with the calculated values based on the Spernol and Wirtz modification of the Stokes–Einstein (SE) relation. Although the measured D’s of the radicals are closer to the values predicted by the simple SE equation, the agreements and the ratio of D between the radicals and its parent molecules depend on the molecular size. The ratio becomes closer to unity as the molecular size becomes large. Possible origins of this dependence are discussed.

Rate coefficients for the reactions of OH radicals with Methylglyoxal and Acetaldehyde

Abstract: Rate coefficients have been measured for the reaction of OH radicals with methylglyoxal from 260 to 333 K using the discharge flow technique and laser-induced fluorescence detection of OH. The rate coefficient was found to be (1.32±0.30) × 10−11 cm3 molecule−1 s−1 at room temperature, with a distinct negative temperature dependence (E/R of −830 ± 300 K). These are the first measurements of the temperature dependence of this reaction. The reaction of OH with acetaldehyde was also investigated, and a rate coefficient of (1.45 ± 0.25) × 10−11 cm3 molecule−1 s−1 was found at room temperature, in accord with recent studies. Experiments in which O2 was added to the flow showed regeneration of OH following the reaction of CH3CO radicals with O2. However, chamber experiments at atmospheric pressure using FTIR detection showed no evidence for OH production. FTIR experiments have also been used to investigate the chemistry of the CH3COCO radical formed by hydrogen abstraction from methylglyoxal. © 1995 John Wiley & Sons, Inc.

Benz[a]anthracene photodegradation in the presence of known organic constituents of atmospheric aerosols

Abstract: Benz[a]anthracene photodegradation rates in toluene solutions containing co-solutes were compared using a photochemical turntable reactor. Each co-solute investigated was found at relatively high concentrations in atmospheric particulate matter. Benz-[a]anthracene photodegradation was accelerated by the presence of 9,10-anthraquinone, xanthone, 2-furaldehyde, 2,4-dimethylbenzaldehyde, 9,10-phenanthrenequinone, 2-acetylfuran, and furfuryl alcohol. Decay was inhibited by 7-benzanthrone. Other compounds had little or no effect on benz[a]-anthracene decay. Possible photochemical reaction mechanisms are discussed, and it is concluded that several competing mechanisms may be responsible, including electronic energytransferfollowed by reaction from the triplet state, singlet oxygen attack, and radical chain reactions initiated by hydrogen abstraction of aerosol constituents. The results suggest that particle organic composition can strongly influence polycyclic aromatic hydrocarbon photodegradation rates in atmospheric aerosols.

Photodissociation of ICN in liquid chloroform: Molecular dynamics of ground and excited state recombination, cage escape, and hydrogen abstraction reaction

Abstract: The photodissociation of ICN in liquid chloroform on different electronic states of the A band is studied using molecular dynamics simulations. By taking into account nonadiabatic transitions to the ICN ground state and by using a simple statistical model for the reaction between the CN radical and a chloroform molecule, the competition between recombination, cage escape and hydrogen abstraction reaction with the solvent is examined. Good agreement with the cage escape results of a recent experiment by Raftery et al. [J. Chem. Phys. 101, 8572 (1994)] is found. Simulations which did not include the nonadiabatic transitions to the ground state overestimated the probability for cage escape by about a factor of two. Very fast translational relaxation of the CN fragment and very low probability for achieving a transition state configuration for the abstraction reaction are calculated. This supports the suggestion that the abstraction reaction proceeds thermally and is controlled by a structural (entropic) barrier.

Laser flash photolysis studies on hydrogen atom abstraction from phenol by triplet naphthoquinones in acetonitrile

Abstract: By means of ns laser flash photolysis, the absorption spectra and molar absorption coefficients (Iµ) of the 1,4-naphthosemiquinone radical (NQH˙) and 2,3-dimethyl-1,4-naphthosemiquinone radical (DMNQH˙) have been determined to be 8200 dm3 mol–1 cm–1 at 365 nm and 7100 dm3 mol–1 cm–1 at 368 nm in acetonitrile (MeCN) and MeCN–H2O (4 : 1 v/v) at 295 K. On the basis of the determined absorption spectra and Iµ values, hydrogen atom abstraction (HA) by triplet 1,4-naphthoquinone (NQ) and 2,3-dimethyl-1,4-naphthoquinone (DMNQ) from phenol (PhOH) in MeCN was studied by laser photolysis techniques. It was found that HA by triplet NQ and DMNQ (3NQ* and 3DMNQ*) from PhOH occurred in a collision process with quenching rate constants, kq= 8.6 × 109 and 5.5 × 108 dm3 mol–1 s–1, respectively. On the basis of the obtained values of kq and quantum yields (ΦHA), the efficiencies (ψHA) for HA by both 3NQ* and 3DMNQ* from PhOH were obtained to be unity. The rate constants (kHA) for HA of 3NQ* and 3DMNA* were determined to be 8.6 × 109 and 5.5 × 108 dm3 mol–1 s–1, respectively. The difference in kHA may be derived from (1) the steric hindrance by the methyl groups of DMNQ and (2) the degree of 3(π,π*) character mixed in with the 3(n,π*) of NA and DMNQ. The triplet–triplet absorption spectra of NQ and DMNQ were identified, and their absolute molar absorption coefficients (IµT–Tλ) were determined to be 8200 dm3 mol–1 cm–1 at 365 nm and 5200 dm3 mol–1 cm–1 at 368 nm, respectively, in MeCN on the basis of the quantum yields of intersystem crossing (Φisc; 0.74 for NQ, 0.98 for DMNQ) evaluated by thermal lensing techniques. The deactivation mechanism of 3NQ* and 3DMNQ* in the absence of PhOH was found to be self-quenching by NQ and DMNQ in MeCN with the rate constants, kSQ= 9.7 × 108 and 1.2 × 108 dm3 mol–1 s–1, respectively.

Preparation of .alpha.,.omega.-Diisopropenyloligopropylene by Thermal Degradation of Isotactic Polypropylene

Abstract: It was found that α,ω-diisopropenyloligoprcpylene (M n = 3300-4800) is prepared as the nonvolatile oligomers isolated from the polymer residues resulting from the thermal degradation of isotactic polypropylene at 370 °C. Their structures were determined by 1 H and 13 C NMR spectroscopies in regard to the reactive end groups. Both the terminal vinylidene double bonds composed of an isopropenyl end group [CH 2 -C(CH 3 )-] and the saturated end group [CH 3 CH 2 CH 2 CH(CH 3 )-] were mainly detected with a molar ratio of about 9 :1. The average number of isopropenyl end groups per molecule f t ) is about 1.8, and therefrom it is indicated that about 80 mol % of the oligomer molecules is symmetric α,ω-diene-oligomer having two terminal isopropenyl groups. This oligopropylene retains highly the stereoregularity (microtacticity) of the original polymer and has a sharper dispersity of molecular weight (M w /M n : ca. 1.5) than the original polymer, in spite of lower molecular weights and T m (ca. 150 °C). This compound is useful as a new telechelic oligomer. These oligomers are considered to be formed by the intramolecular hydrogen abstraction (back-biting) of secondary terminal macroradicals followed by β scission at the end of the main chain (eq 3) and the intermolecular hydrogen abstraction of secondary terminal volatile radicals (eq 4), which are formed by the back-biting and other elementary reactions, followed by β scission.

Grafting of maleic anhydride to hydrocarbons below the ceiling temperature

Abstract: Maleic anhydride has been grafted to eicosane and squalane at 60–80°C using 1,2-dichlorobenzene as solvent and benzoyl peroxide as initiator. These hydrocarbons are low molecular weight models for hydrocarbon polymers containing secondary and tertiary hydrogen atoms. In the absence of the hydrocarbon and with monomer concentrations of the order of 1M, low molecular weight poly(maleic anhydride) is formed. On addition of the hydrocarbon, the main product is grafted material and very little homopolymer is formed. The grafts consist primarily of single succinic anhydride units but some of them are short poly(maleic anhydride) chains. Ceiling temperature considerations control the formation of homopolymer in the absence of hydrocarbon substrate. In the presence of eicosane or squalane, initiation of grafting proceeds by hydrogen abstraction from the hydrocarbon. The main factor controlling graft length is then the ratio of the rates of intramolecular hydrogen abstraction and of monomer addition to succinic anhydride radicals © 1995 John Wiley & Sons, Inc.

Determination of absolute rate constants for the reversible hydrogen-atom transfer between thiyl radicals and alcohols or ethers

Abstract: Absolute rate constants have been determined for the reversible hydrogen-transfer process R˙+ RSH ⇄ RH + RS˙ by pulse radiolysis, mainly through direct observation of the RS˙ radical formation kinetics in water–RH (1 : 1, v/v) mixtures. The thiols investigated were penicillamine and glutathione; the RH hydrogen donors were methanol, ethanol, propan-1-ol, propan-2-ol, ethylene glycol, tetrahydrofuran and 1,4-dioxane with the abstracted hydrogen being located α to the hydroxy or alkoxy function. Rate constants for the forward reaction of the above equilibrium (in radiation biology referred to as ‘repair’ reaction) were typically of the order of 107– 108 dm3 mol–1 s–1 while hydrogen abstraction from RH by thiyl radicals (reverse process) occurred with rate constants of the order of 103– 104 dm3 mol–1 s–1. This yields equilibrium constants of the order of 104. Based on these data, standard reduction potentials could be evaluated for the R′R″C˙OH/H+//R′R″CHOH, R′R″CO/H+//R′R″C˙(OH) and R′R″CO//R′R′C˙O– couples from methanol, ethanol and propan-2-ol. Effective hydrogen-atom abstraction by RS˙ required activation by neighbouring groups of the C—H bond to be cleaved in RH. No such process was observed for the RS˙ reaction with —CH3 groups, e.g. in 2-methylpropan-2-ol. Several halogenated hydrocarbons, including some anaesthetics (e.g. halothane) and Fe(CN)63– have been tested with respect to their ability to disturb the (CH3)2C˙OH + RSH ⇄(CH3)2CHOH + RS˙ equilibrium through an irreversible electron-transfer reaction with the reducing α-hydroxyl radical, thereby drawing the equilibrium to the lefthand side. The respective efficiencies are found to be related to the electronegativities of the electron acceptors. The results are briefly discussed in terms of their biological relevance.

Ab initio molecular orbital study of potential energy surface for the reaction of C2H3 with H2 and related reactions

Abstract: The potential energy surface of the reaction C2H3+H2→C2H4+H→C2H5 has been investigated using various theoretical methods including QCISD(T), CCSD(T), RCCSD(T), Gaussian‐2 (G2), and the density‐functional B3LYP approach. The reaction of the vinyl radical with molecular hydrogen is shown to take place through the hydrogen atom abstraction channel leading to the formation of C2H4+H with the activation energy of 10.4 kcal/mol at all the G2, QCISD(T)/6‐311+G(3df,2p), and CCSD(T)/6‐311+G(3df,2p) levels. The rate constant, calculated using the variational transition state theory with tunneling correction, k=3.68⋅10−20⋅T2.48⋅exp(−3587/T) cm3 molecule−1 s−1, is in good agreement with the experimental estimates. C2H5 cannot be formed directly by inserting C2H3 to H2, but can only be produced by addition of H to C2H4, with a barrier of 4.5–4.7 kcal/mol calculated at high levels of theory. In order to match the experimental rate constant, the activation energy needs to be adjusted to 2.8 kcal/mol. Generally, the B3LY...

On the mechanism of amine oxidations by P450

Abstract: 1. Experimental data previously used to support an electron/proton transfer mechanism for oxidative dealkylation of amines by P450 are critically analysed with the conclusion that the mechanistic evidence is indecisive. 2. A new mechanistic criterion recently proposed to distinguish between electron/proton transfer and hydrogen atom transfer mechanisms is discussed. It is based on isotope effect profiles determined for the deprotonation of a series of para-substituted N-methyl-N-trideuteriomethyl)aniline cation radicals by pyridine and for hydrogen atom abstraction from the corresponding neutral amines by the tert-butoxyl radical. These reactions model the steps proposed in the two P450 mechanisms. 3. Isotope effect profiles measured for the demethylation of substituted NN-bis(dideuteriomethyl)anilines by four different forms of P450 were found to be experimentally indistinguishable from the hydrogen atom transfer profile, and distinctly different from the cation radical deprotonation profile. This provides strong evidence that P450 oxidatively dealkylates the amines by a hydrogen atom transfer mechanism and not by an electron/proton transfer mechanism.

DNA Repair by Thiols in Air Shows Two Radicals Make a Double-Strand Break

Abstract: Using agarose gel electrophoresis, we have measured the yields of DNA single- and double-strand breaks (SSBs and DSBs) for plasmid DNA γ-irradiated in aerobic aqueous solution. The presence during irradiation of either of the thiols cysteamine or N-(2-thioethyl)-1,3-diaminopropane (WR-1065) resulted in a concentration-dependent decrease in the yield of SSBs and a much greater decrease in the yield of DSBs. This large differential protective effect was not produced by thioethers or an alcohol of structural similarity to the two thiols, suggesting that repair of DSB radical precursors by thiols is more efficient than for SSB precursors. These observations suggest the existence of a diradical intermediate in the formation of DSBs. The results argue against a major contribution by a single radical mechanism involving interstrand radical transfer via hydrogen abstraction by a peroxyl intermediate, since the half-life of this radical transfer reaction appears to be significantly greater than the lifetime of the...

Temperature dependence of diffusion of radical intermediates probed by the transient grating method

Abstract: Diffusion processes of intermediate radicals created by the photoinduced hydrogen abstraction reactions of ketones, quinones, and N‐hetero aromatic molecules in ethanol and 2‐propanol are studied at various temperatures by using the transient grating (TG) method. The temperature dependences of the translational diffusion coefficients (D’s) of both the radicals and the parent molecules can be expressed by the Arrhenius relationship. The activation energies (ED) for diffusion of the radicals are larger than those of the parent molecules and the difference in ED depends on the molecular size. The different ED is explained in terms of the molecular volume dependence of ED; that is, larger molecular volumes of the radicals could be the cause of the larger ED. The larger apparent molecular volumes of the radicals are consistent with a model of microscopic aggregation of the surrounding molecules around the radical.

Stabilization of reactants in a weakly bound complex: OH–H2 and OH–D2

Abstract: Binary complexes of OH X 2Π and H2/D2 have been stabilized in a shallow well below the activation barrier to the hydrogen abstraction reaction and identified by laser‐induced fluorescence in the OH A 2Σ+–X 2Π 1–0 spectral region. The characteristics of the OH (X 2Π, A 2Σ+)+H2/D2 potentials are in accord with ab initio predictions.

Direct evidence for hydroxyl radical-induced damage to nucleic acids by chromium(VI)-derived species: implications for chromium carcinogenesis.

Abstract: Reduction of Cr(VI) by NADH and NADPH has been shown to yield Cr(V) species, which have been detected by electron paramagnetic resonance (EPR) spectroscopy. The fine structure on the EPR signal of the Cr(V) species is consistent with the presence of two NAD(P)H ligands in a square-pyramidal arrangement with a single oxygen (oxo) group at the apex. Neither this species nor the initial Cr(VI) complex damage DNA components as evidenced by the lack of effect of these compounds on the optical and EPR signals of the Cr(VI) and Cr(V) species respectively. Addition of hydrogen peroxide to the Cr(V) species is shown to result in the formation of a further transient EPR signal, the parameters of which are consistent with an assignment to a Cr(V)-peroxide complex. Inclusion of the spin trap 5,5-dimethyl-1-pyrroline-N-oxide in this system demonstrates that hydroxyl radicals are also generated, possibly via the decomposition of the peroxide complex. Inclusion of DNA components in this system together with the spin trap 2-methyl-2-nitrosopropane results in the detection of base- and sugar-derived radicals; the characteristic EPR signals of these species have allowed both the identification of these species and their mechanism of formation to be determined. The signals from the former species are consistent with radical addition to the base, whereas the sugar-derived species are believed to be formed via hydrogen atom abstraction. In each case, this behaviour is consistent with hydroxyl radicals being the damaging species in systems where Cr(V) is generated in the presence of hydrogen peroxide. These results therefore suggest that it may be the hydroxyl radical that is the ultimate carcinogenic species in cells and systems exposed to Cr(VI).