Evaluation of the radical scavenging activity of some representative isoprenoid and aromatic cytokinin ribosides (N6-substituted adenosines) by in vitro chemical assays.

Cytokinins are naturally occurring adenine derivatives whose physiological role is that of growth regulators in plants and that show also many other activities either in plants and in mammalian cells. In plants, they can be found mainly as free bases ((N6-substituted adenines, CKs), but also as the corresponding N9- ribosides (N6-substituted adenosines, CKRs). In mammalian cells, CKRs are, in general, more active than CKs. In order to evaluate the intrinsic in vitro antioxidant capacity of some significant CKRs, their scavenging activity against synthetic radicals that are at the basis of well-established antioxidant assays (ORAC, TEAC, DPPH) has been evaluated. The results of the in vitro scavenging activity of biologically relevant radicals such as hydroxyl (HO•), superoxide (O2.-) and lipid peroxides (R-OO.) are reported and discussed.

Anti-angiogenic effect of quercetin and its 8-methyl pentamethyl ether derivative in human microvascular endothelial cells.

Angiogenesis is involved in many pathological states such as progression of tumours, retinopathy of prematurity and diabetic retinopathy. The latter is a more complex diabetic complication in which neurodegeneration plays a significant role and a leading cause of blindness. The vascular endothelial growth factor (VEGF) is a powerful pro-angiogenic factor that acts through three tyrosine kinase receptors (VEGFR-1, VEGFR-2 and VEGFR-3). In this work we studied the anti-angiogenic effect of quercetin (Q) and some of its derivates in human microvascular endothelial cells, as a blood retinal barrier model, after stimulation with VEGF-A. We found that a permethylated form of Q, namely 8MQPM, more than the simple Q, is a potent inhibitor of angiogenesis both in vitro and ex vivo. Our results showed that these compounds inhibited cell viability and migration and disrupted the formation of microvessels in rabbit aortic ring. The addition of Q and more significantly 8MQPM caused recoveries or completely re-establish the transendothelial electrical resistance (TEER) to the control values and suppressed the activation of VEGFR2 downstream signalling molecules such as AKT, extracellular signal-regulated kinase, and c-Jun N-terminal kinase. Taken together, these data suggest that 8MQPM might have an important role in the contrast of angiogenesis-related diseases.

Singlet oxygen quenching- and chain-breaking antioxidant-properties of a quercetin dimer able to prevent age-related macular degeneration.

A dimer of quercetin prepared through a Mannich reaction protects pyridinium bisretinoid A2E from photooxidation at 430 nm in aqueous medium at pH 7.4. In the presence of light and O2, A2E is quickly attacked by 1O2 produced in situ (by excited A2E) to give nonaoxirane and other oxygenated compounds which can be involved in diseases of the macula. Peroxyl radicals might have only a marginal role on the photooxidation of A2E. The dimer is actually a potent quencher of 1O2 with a rate constant kQ of 8.5 × 108 M-1 s-1 in methanol at room temperature. On the other hand, its antioxidant abilities against ROO· radicals are quite limited since kROO·â€¯= 7.3 × 105 M-1 s-1 (in buffer solution at pH 7.4), the value being essentially identical to that of quercetin. The quenching of 1O2 by the dimer is therefore the main reason for the A2E protection and prevention of age-related macular degeneration.

Quercetin derivatives as potent inducers of selective cytotoxicity in glioma cells.

Quercetin (Q) is a flavonoid widely distributed in the plant kingdom and well-known for its ability to exert antioxidant, prooxidant and anticarcinogenic activities in several tumor cells. Furthermore, quercetin plays an important role both in the regulation of key elements in cellular signal transduction pathways related to apoptotic cell death, and in cell cycle progression. Several studies have reported of toxic effects of Q against glioma cell lines. In this study, the effects of Q and of some Q-derivatives (acyl esters and bromo-derivatives) on U373-MG and 9L glioma cell lines survival are analyzed. The 24-hour treatment of glioma cells with several concentrations of Q (25, 50 and 100µM) did not cause any cytotoxic effects, while the administration of Q-derivatives, such as acylated and brominated quercetin, caused a sharp increase in cell death. Among all tested derivatives, 3-O-decanoylquercetin 10 manifested the strongest cytotoxic effect at a concentration as low as 25µM both in U373-MG (ca. 40% viability after 24h) and in 9L cells (ca. 20% viability after 24h). The cytotoxic effects of the Q-derivatives 3 and 10-13 were proven to be satisfactorily selective for glioma cells. When Q-derivatives were in fact administered to mouse primary astroglial or human fibroblast cell cultures, a higher cell survival rate (~90-70% and 55-45%, respectively) was observed relative to that detected in glioma cells. These results prove that selective esterification and bromination of Q increase to a great extent the toxicity of this polyphenol against glioma cells, thereby providing a possible new tool for cyto-specific glioma therapy.

Effect of organic co-solvents in the evaluation of the hydroxyl radical scavenging activity by the 2-deoxyribose degradation assay: The paradigmatic case of α-lipoic acid.

The 2-deoxyribose degradation assay (2-DR assay) is an in vitro method broadly used for evaluating the scavenging activity against the hydroxyl radical (HO). One of the major drawbacks of the assay, however, is that only water soluble compounds can be tested for their radical-scavenging activity. Lipoic acid (LA) is an excellent scavenger of HO but it exhibits a low solubility in the aqueous milieu of the 2-DR assay and a high tendency to polymerize under a variety of conditions. We used LA as a paradigmatic substrate to evaluate the effect of several organic co-solvents in increasing its solubility. Most of these solvents, however, demonstrated to be potent scavengers of HO making their use in the 2-DR assay improper. On the other hand, acetonitrile showed a remarkably low reactivity toward HO (rate constant ~8.7×106M-1s-1) which allowed us to use it as a co-solvent in the preparation of stock solutions of LA ~5mM. We therefore evaluated the radical-scavenging activity of LA by the 2-DR assay in a relatively large range of concentrations, 1-200µM. We found that the rate constant for LA+HO is diffusion-controlled (~1×1010M-1s-1 in water at 25°C) and uninfluenced by the presence of small quantities of acetonitrile. Therefore, the use of acetonitrile in the 2-DR assay does not interfere with the test and may increase the solubility of the radical scavengers.

Kinetics of curcumin oxidation by 2,2-diphenyl-1-picrylhydrazyl (DPPH˙): an interesting case of separated coupled proton-electron transfer.

The decay of dpph˙ in absolute ethanol at 25 °C and in the presence of curcumin (1), 4-methylcurcumin (3), 4,4-dimethylcurcumin (4) or curcumin 4'-methyl ether (5) follows bi-exponential kinetics. These unusual reaction kinetics are compatible with a two-step process in which an intermediate accumulates in a reversible first step followed by an irreversible process. As in other similar cases (Foti et al., Org. Lett., 2011, 13, 4826-4829), we have hypothesised that the intermediate is a π-stacked complex, formed between one curcumin anion (in the case of 1, 3 and 5 the enolate anion) and the picryl moiety of dpph˙, in which an intra-complex electron transfer from the (enolate) anion takes place. By comparing the kinetics of curcumin 4',4''-dimethyl ether (2) (no phenolic OH), (5) (one phenolic OH) and (1) (two phenolic OHs), we have deduced that the electron transfer process must be accompanied by a simultaneous proton transfer from the phenolic OHs to the bulk solvent (separated coupled proton-electron transfer). The rate constants kα for the forward reaction of 2, 5 and 1 with dpph˙ are in fact ∼0, 7.5 × 10(3) and 1.8 × 10(4) M(-1) s(-1), respectively, in a clear dependence on the number of phenolic OHs.

Use and Abuse of the DPPH(•) Radical.

The 2,2-diphenyl-1-picrylhydrazyl (DPPH(•)) radical is approaching 100 years from its discovery in 1922 by Goldschmidt and Renn. This radical is colored and remarkably stable, two properties that have made it one of the most popular radicals in a wide range of studies. First, there is the evaluation of the antioxidant abilities of phenols and other natural compounds (A-H) through a "test" that-at a closer look-is utterly inappropriate. In fact, the test-derived EC50, that is, the concentration of A-H able to scavenge 50% of the initial DPPH(•), is not a kinetic parameter and hence its purported correlation with the antioxidant properties of chemicals is not justified. Kinetic measurements, such as the second-order rate constants for H-atom abstraction from A-H by DPPH(•), in apolar media, are the only useful parameters to predict the antioxidant ability of A-H. Other applications of DPPH(•) include kinetic and mechanistic studies, kinetic solvent effects, EPR spectroscopy, polymer chemistry, and many more. In this review these applications are evaluated in detail by showing the usefulness of some and the uselessness of others. The chemistry of DPPH(•) is also briefly reviewed.

Antioxidant activity of essential oils.

Essential oils (EOs) are liquid mixtures of volatile compounds obtained from aromatic plants. Many EOs have antioxidant properties, and the use of EOs as natural antioxidants is a field of growing interest because some synthetic antioxidants such as BHA and BHT are now suspected to be potentially harmful to human health. Addition of EOs to edible products, either by direct mixing or in active packaging and edible coatings, may therefore represent a valid alternative to prevent autoxidation and prolong shelf life. The evaluation of the antioxidant performance of EOs is, however, a crucial issue, because many commonly used "tests" are inappropriate and give contradictory results that may mislead future research. The chemistry explaining EO antioxidant activity is discussed along with an analysis of the potential in food protection. Literature methods to assess EOs' antioxidant performance are critically reviewed.

Proton-electron transfer pathways in the reactions of peroxyl and dpph˙ radicals with hydrogen-bonded phenols.

The kinetics of the reaction of peroxyl and dpph˙ radicals with phenols H-bonded to N-bases have been studied for the first time. Electron-transfer processes are observed in MeCN but only with the dpph˙ radical.

Kinetics of the oxidation of quercetin by 2,2-diphenyl-1-picrylhydrazyl (dpph•).

In methanol/water, dpph(•) bleaching (519 nm) by quercetin, QH(2), exhibits biphasic kinetics. The dpph(•) reacts completely with the quercetin anion within 100 ms. Subsequent slower bleaching involves solvent and QH(2) addition to quinoid products. The fast reaction is first-order in dpph(•) but only ca. 0.38 order in [QH(2)]. This extraordinary nonintegral order is attributed to reversible formation of π-stacked {QH(-)/dpph(•)} complexes in which electron transfer to products, {QH(•)/dpph(-)}, is slow (k(ET) ≈ 10(5) s(-1)).

Influence of "remote" intramolecular hydrogen bonds on the stabilities of phenoxyl radicals and benzyl cations.

Remote intramolecular hydrogen bonds (HBs) in phenols and benzylammonium cations influence the dissociation enthalpies of their O-H and C-N bonds, respectively. The direction of these intramolecular HBs, para --> meta or meta --> para, determines the sign of the variation with respect to molecules lacking remote intramolecular HBs. For example, the O-H bond dissociation enthalpy of 3-methoxy-4-hydroxyphenol, 4, is about 2.5 kcal/mol lower than that of its isomer 3-hydroxy-4-methoxyphenol, 5, although group additivity rules would predict nearly identical values. In the case of 3-methoxy-4-hydroxybenzylammonium and 3-hydroxy-4-methoxybenzylammonium ions, the CBS-QB3 level calculated C-N eterolytic dissociation enthalpy is about 3.7 kcal/mol lower in the former ion. These effects are caused by the strong electron-withdrawing character of the -O(*) and -CH(2)(+) groups in the phenoxyl radical and benzyl cation, respectively, which modulates the strength of the HB. An O-H group in the para position of ArO(*) or ArCH(2)(+) becomes more acidic than in the parent molecules and hence forms stronger HBs with hydrogen bond acceptors (HBAs) in the meta position. Conversely, HBAs, such as OCH(3), in the para position become weaker HBAs in phenoxyl radicals and benzyl cations than in the parent molecules. These product thermochemistries are reflected in the transition states for, and hence in the kinetics of, hydrogen atom abstraction from phenols by free radicals (dpph(*) and ROO(*)). For example, the 298 K rate constant for the 4 + dpph(*) reaction is 22 times greater than that for the 5 + dpph(*) reaction. Fragmentation of ring-substituted benzylammonium ions, generated by ESI-MS, to form the benzyl cations reflects similar remote intramolecular HB effects.

Non-phenolic radical-trapping antioxidants.

OBJECTIVES: The aim of this review article is to introduce the reader to the mechanisms, rates and thermodynamic aspects of the processes involving the most biologically relevant non-phenolic radical-trapping antioxidants. KEY FINDINGS: Antioxidant defences in living organisms rely on a complex interplay between small molecules and enzymes, which cooperate in regulating the concentrations of potentially harmful oxidizing species within physiological limits. The noxious effects of an uncontrolled production of oxygen- and nitrogen-centered radicals are amplified by chain reactions (autoxidations), sustained mainly by peroxyl radicals (ROO(*)), that oxidize and alter essential biomolecules such as lipids, lipoproteins, proteins and nucleic acids. SUMMARY: Non-phenolic antioxidants represent an important and abundant class of radical scavengers in living organisms. These compounds react with peroxyl radicals through various mechanisms: (i) formal H-atom donation from weak X-H bonds (X = O, N, S), as in the case of ascorbic acid (vitamin C), uric acid, bilirubin and thiols; (ii) addition reactions to polyunsaturated systems with formation of C-radicals poorly reactive towards O(2), for example beta-carotene and all carotenoids in general; (iii) co-oxidation processes characterized by fast cross-termination reactions, for example gamma-terpinene; and (iv) catalytic quenching of superoxide (O(2)(*-)) with a superoxide dismutase-like mechanism, for example di-alkyl nitroxides and FeCl(3). Kinetic data necessary to evaluate and rationalize the effects of these processes are reported. The mechanisms underlying the pro-oxidant effects of ascorbate and other reducing agents are also discussed.

Reaction of phenols with the 2,2-diphenyl-1-picrylhydrazyl radical. Kinetics and DFT calculations applied to determine ArO-H bond dissociation enthalpies and reaction mechanism.

The formal H-atom abstraction by the 2,2-diphenyl-1-picrylhydrazyl (dpph(*)) radical from 27 phenols and two unsaturated hydrocarbons has been investigated by a combination of kinetic measurements in apolar solvents and density functional theory (DFT). The computed minimum energy structure of dpph(*) shows that the access to its divalent N is strongly hindered by an ortho H atom on each of the phenyl rings and by the o-NO(2) groups of the picryl ring. Remarkably small Arrhenius pre-exponential factors for the phenols [range (1.3-19) x 10(5) M(-1) s(-1)] are attributed to steric effects. Indeed, the entropy barrier accounts for up to ca. 70% of the free-energy barrier to reaction. Nevertheless, rate differences for different phenols are largely due to differences in the activation energy, E(a,1) (range 2 to 10 kcal/mol). In phenols, electronic effects of the substituents and intramolecular H-bonds have a large influence on the activation energies and on the ArO-H BDEs. There is a linear Evans-Polanyi relationship between E(a,1) and the ArO-H BDEs: E(a,1)/kcal x mol(-1) = 0.918 BDE(ArO-H)/kcal x mol(-1) - 70.273. The proportionality constant, 0.918, is large and implies a "late" or "product-like" transition state (TS), a conclusion that is congruent with the small deuterium kinetic isotope effects (range 1.3-3.3). This Evans-Polanyi relationship, though questionable on theoretical grounds, has profitably been used to estimate several ArO-H BDEs. Experimental ArO-H BDEs are generally in good agreement with the DFT calculations. Significant deviations between experimental and DFT calculated ArO-H BDEs were found, however, when an intramolecular H-bond to the O(*) center was present in the phenoxyl radical, e.g., in ortho semiquinone radicals. In these cases, the coupled cluster with single and double excitations correlated wave function technique with complete basis set extrapolation gave excellent results. The TSs for the reactions of dpph(*) with phenol, 3- and 4-methoxyphenol, and 1,4-cyclohexadiene were also computed. Surprisingly, these TS structures for the phenols show that the reactions cannot be described as occurring exclusively by either a HAT or a PCET mechanism, while with 1,4-cyclohexadiene the PCET character in the reaction coordinate is much better defined and shows a strong pi-pi stacking interaction between the incipient cyclohexadienyl radical and a phenyl ring of the dpph(*) radical.

A meta effect in nonphotochemical processes: the homolytic chemistry of m-methoxyphenol.

The m-methoxy group is normally electron-withdrawing (EW), sigma(m) = +0.12, sigma(m+) = +0.05. The strong EW activity of a phenoxyl radical's O* atom causes the m-methoxy group to become electron-donating (ED), sigma(m)(+) = -0.14. In valence bond terms, this can be ascribed to the nonclassical resonance structures 1c-e. Although it has long been known that m-methoxy is ED in photoexcited states, it has now been found to be ED for homolytic O-H bond breaking in ground-state 3-methoxyphenol.

Antioxidant properties of phenols.

The current understanding of the antioxidant properties of phenols (in homogeneous solutions) is reviewed, with particular emphasis on the role of the solvent. Phenols (ArOH) are known to reduce the rates of oxidation of organic matter by transferring a H atom (from their OH groups) to the chain-carrying ROO* radicals, a mechanism that most likely involves a concerted transfer of the hydrogen as a proton and of one electron between the two oxygen atoms, O-H---O* (proton-coupled electron transfer mechanism). The antioxidant capabilities of phenols are strongly reduced by hydrogen-bond accepting solvents since the hydrogen-bonded molecules ArOH---S are virtually unreactive toward ROO* radicals. The magnitude of these kinetic solvent effects is determined by the solute acidity alpha(2)(H) of ArOH (range 0 to 1) and solvent basicity beta(2)(H) (range 0 to 1). Hydroxyl solvents (alcohols) have a double effect on ArOH. On the one hand, they act as hydrogen-bond accepting solvents and reduce the conventional rates of the ArOH + ROO* reaction. On the other hand, these solvents favour the ionization of ArOH into their phenoxide anions ArO(-), which may react with ROO* very rapidly by electron transfer (sequential proton loss electron transfer mechanism). The overall effect is therefore determined by the ionization degree of ArOH. Other aspects of the kinetics and thermodynamics of ArOH + ROO* are also discussed.

Kinetic and thermodynamic parameters for the equilibrium reactions of phenols with the dpph. radical.

The kinetics and energetics of the reversible reaction of phenols with the dpph. radical have been studied; steric shielding of the divalent N by the o-NO2 in dpph. seems to be the main cause of the entropic barriers of this reaction.

New insight into solvent effects on the formal HOO. + HOO. reaction.

The 2,2'-azobis(isobutyronitrile)(AIBN)-induced autoxidation of gamma-terpinene (TH) at 50 degrees C produces p-cymene and hydrogen peroxide in a radical-chain reaction having HOO* as one of the chain-carrying radicals. The kinetics of this reaction in cyclohexane and tert-butyl alcohol show that chain termination involves the formal HOO. + HOO. self-reaction over a wide range of gamma-terpinene, AIBN, and O2 concentrations. However, in acetonitrile this termination process is accompanied by termination via the cross-reaction of the terpinenyl radical, T., with the HOO. radical under conditions of relatively high [TH] (140-1000 mM) and low [O2] (2.0-5.5 mM). This is because the formal HOO. + HOO. reaction is comparatively slow in acetonitrile (2k approximately 8 x 10(7) M(-1) s(-1)), whereas, this reaction is almost diffusion-controlled in tert-butyl alcohol and cyclohexane, 2k approximately 6.5 x 10(8) and 1.3 x 10(9) M(-1) s(-1), respectively. Three mechanisms for the bimolecular self-reaction of HOO. radicals are considered: 1) a head-to-tail hydrogen-atom transfer from one radical to the other, 2) a head-to-head reaction to form an intermediate tetroxide, and 3) an electron-transfer between HOO. and its conjugate base, the superoxide radical anion, O2-.. The rate constant for reaction by mechanism (1) is shown to be dependent on the hydrogen bond (HB) accepting ability of the solvent; that by mechanism (2) is shown to be too slow for this process to be of any importance; and that by mechanism (3) is dependent on the pH of the solvent and its ability to support ionization. Mechanism (3) was found to be the main termination process in tert-butyl alcohol and acetonitrile. In the gas phase, the rate constant for the HOO. + HOO. reaction (mechanism (1)) is about 1.8 x 10(9) M(-1) s(-1) but in water at pH< or =2 where the ionization of HOO. is completely suppressed, this rate constant is only 8.6 x 10(5) M(-1) s(-1). The very large retarding effect of water on this reaction has not previously been explained. We find that it can be quantitatively accounted for by using Abraham's HB acceptor parameter, beta(2)(H), for water of 0.38 and an estimated HB donor parameter, alpha(2)(H), for HOO. of about 0.87. These Abraham parameters allow us to predict a rate constant for the HOO. + HOO. reaction in water at 25 degrees C of 1.2 x 10(6) M(-1) s(-1) in excellent agreement with experiment.

Electron-transfer reaction of cinnamic acids and their methyl esters with the DPPH(*) radical in alcoholic solutions.

The kinetic behavior of cinnamic acids, their methyl esters, and two catechols 1-10 (ArOH) in the reaction with DPPH(*) in methanol and ethanol is not compatible with a reaction mechanism that involves hydrogen atom abstraction from the hydroxyl group of 1-10 by DPPH(*). The rate of this reaction at 25 degrees C is, in fact, comparatively fast despite that the phenolic OH group of ArOH is hydrogen bonded to solvent molecules. The observed rate constants (k(1)) relative to DPPH(*) + ArOH are 3-5 times larger for the methyl esters than for the corresponding free acids and, for the latter, decrease as their concentration is increased according to the relation k(1) = B/[ArOH](0)(m), where k(1) is given in units of M(-1) s(-1), m is ca. 0.5, and B ranges from 0.02 (p-coumaric acid) to ca. 3.48 (caffeic acid) in methanol and from 0.04 (p-coumaric acid) to ca. 13 (sinapic acid) in ethanol. Apparently, the reaction mechanism of DPPH(*) + ArOH involves a fast electron-transfer process from the phenoxide anion of 1-10 to DPPH(*). Kinetic analysis of the reaction sequence for the free acids leads to an expression for the observed rate constant, k(1), proportional to [ArOH](0)(-1/2) in excellent agreement with the experimental behavior of these phenols. The experimental results are also interpreted in terms of the influence that adventitious acids or bases present in the solvent may have. These impurities dramatically influence the ionization equilibrium of phenols and cause a reduction or an enhancement, respectively, of the measured rate constants.

Overlooked difference between hydrogen bonds of equal strength formed between catechol and an oxygen or nitrogen base. Experiments and DFT calculations.

The IR spectrum of catechol in CCl(4) shows two fairly sharp O-H stretching bands of roughly equal absorbance at 3615.0 and 3569.6 cm(-1) due, respectively, to the "free" OH and the intramolecularly H-bonded OH groups. Intermolecular H-bond formation between the "free" OH and a hydrogen bond acceptor (HBA) decreases its stretching frequency by several hundred wavenumbers and simultaneously decreases the frequency of the intramolecularly H-bonded OH by a few tens of wavenumbers. The magnitude of these frequency shifts, Deltaupsilon(inter) and Deltaupsilon(intra), respectively, are very well reproduced by DFT calculations. As would be expected, the magnitudes of Deltaupsilon(inter) and Deltaupsilon(intra) increase as the HB accepting ability of the HBA increases as quantified, on a relative scale, by the HBA's values (Abraham et al. J. Chem. Soc. Perkin Trans. 2 1990, 521). However, plots of experimental, or calculated, frequency shifts versus reveal that Deltaupsilon(inter) and Deltaupsilon(intra) are ca. 40% larger for a nitrogen atom HBA than for an oxygen atom HBA having equal HBA activity. We hypothesize that for HBAs of equal strength, i.e., of equal, the H-bond in (O-H- - -O)(inter) is shorter and, hence, intrinsically stronger than the H-bond in the (O-H- - -N)(inter). However, we further hypothesize that there is more charge separation in the H-bond to N because N is a better proton acceptor than O. Hence, it is the greater Coulombic attraction in (O-H- - -N)(inter) which strengthens this H-bond and compensates for its greater length. Theoretical calculations lend support to these hypotheses.

Unexpected superoxide dismutase antioxidant activity of ferric chloride in acetonitrile.

The azobis(isobutyronitrile)-initiated autoxidation of gamma-terpinene in acetonitrile at 50 degrees C yields only p-cymene and hydrogen peroxide (1:1) in a chain reaction carried by the hydroperoxyl radical, HOO. (Foti, M. C.; Ingold, K. U. J. Agric. Food Chem. 2003, 51, 2758-2765). This reaction is retarded by very low (microM) concentrations of FeCl(3) and CuCl(2). The kinetics of the FeCl(3)-inhibited autoxidation are consistent with chain-termination via the following: Fe(3+) + HOO. <==>[Fe(IV)-OOH](3+) and [Fe(IV)-OOH](3+) + HOO. --> Fe(3+) + H2O2 + O2. Thus, FeCl(3) in acetonitrile can be regarded as a very effective (and very simple) superoxide dismutase. The kinetics of the CuCl(2)-inhibited autoxidation indicate that chain transfer occurs and becomes more and more important as the reaction proceeds, i.e., the inhibition is replaced by autocatalysis. These kinetics are consistent withreduction of Cu2+ to Cu+ by HOO. and then the reoxidation of Cu+ to Cu2+ by both HOO.and the H2O2 product. The latter reaction yields HO. radicals which continue the chain.