The reactivity of air-stable pyridine- and pyrimidine-containing diarylamine antioxidants

We recently reported a preliminary account of our efforts to develop novel diarylamine radical-trapping antioxidants (Hanthorn et al. J. Am. Chem. Soc.2012, 134, 8306-8309), wherein we demonstrated that the incorporation of ring nitrogens into diphenylamines affords compounds that display a compromise between H-atom transfer reactivity to peroxyl radicals and stability to one-electron oxidation. Herein, we report the results of thermochemical and kinetic experiments on an expanded set of diarylamines (see the accompanying paper, DOI: 10.1021/jo301013c ), which provide a more complete picture of the structure-reactivity relationships of these compounds as antioxidants. Nitrogen incoporation into a series of alkyl-, alkoxyl-, and dialkylamino-substituted diphenylamines raises their oxidation potentials systematically with the number of nitrogen atoms, resulting in overall increases of 0.3-0.5 V on going from the diphenylamines to the dipyrimidylamines. At the same time, the effect of nitrogen incorporation on their reactivity toward peroxyl radicals was comparatively small (a decrease of only 6-fold at most), which is also reflected in their N-H bond dissociation enthalpies. Rate constants for reactions of dialkylamino-substituted diarylamines with peroxyl radicals were found to be >10(7) M(-1) s(-1), which correspond to the pre-exponential factors that we obtained for a representative trio of compounds (log A ～ 7), indicating that the activation energies (E(a)) are negligible for these reactions. Comparison of our thermokinetic data for reactions of the diarylamines with peroxyl radicals with literature data for reactions of phenols with peroxyl radicals clearly reveals that diarylamines have higher inherent reactivities, which can be explained by a proton-coupled electron-transfer mechanism for these reactions, which is supported by theoretical calculations. A similar comparison of the reactivities of diarylamines and phenols with alkyl radicals, which must take place by a H-atom transfer mechanism, clearly reveals the importance of the polar effect in the reactions of the more acidic phenols, which makes phenols comparatively more reactive.     

Kinetics of ethylbenzene oxy-chemiluminescence in the presence of antioxidants from tissues of the marine invertebrate Eupentacta fraudatrix: Estimating the concentration and reactivity of the natural antioxidants

Extracts from tissues of the holothuria Eupentacta fraudatrix suppress the oxy-chemiluminescence of ethylbenzene in chlorobenzene initiated by azobisisobutyronitrile thermolysis and enhanced by energy transfer to 9,10-dibromoanthracene. The decrease of the chemiexcitation rate in the reaction between peroxyl radicals is caused by radical interception by the antioxidants contained in the extracts. The concentrations of the antioxidants in the extracts and the rate constants of their reactions with peroxyl radicals have been estimated from chemiluminescence kinetics.     

The unusual reaction of semiquinone radicals with molecular oxygen

Hydroquinones (benzene-1,4-diols) are naturally occurring chain-breaking antioxidants, whose reactions with peroxyl radicals yield 1,4-semiquinone radicals. Unlike the 1,2-semiquinone radicals derived from catechols (benzene-1,2-diols), the 1,4-semiquinone radicals do not always trap another peroxyl radical, and instead the stoichiometric factor of hydroquinones varies widely between 0 and 2 as a function of ring-substitution and reaction conditions. This variable antioxidant behavior has been attributed to the competing reaction of the 1,4-semiquinone radical with molecular oxygen. Herein we report the results of experiments and theoretical calculations focused on understanding this key reaction. Our experiments, which include detailed kinetic and mechanistic investigations by laser flash photolysis and inhibited autoxidation studies, and our theoretical calculations, which include detailed studies of the reactions of both 1,4-semiquinones and 1,2-semiquinones with O2, provide many important insights. They show that the reaction of O2 with 2,5-di-tert-butyl-1,4-semiquinone radical (used as model compound) has a rate constant of 2.4 +/- 0.9 x 10(5) M-1 s-1 in acetonitrile and as high as 2.0 +/- 0.9 x 10(6) M-1 s-1 in chlorobenzene, i.e., similar to that previously reported in water at pH approximately 7. These results, considered alongside our theoretical calculations, suggest that the reaction occurs by an unusual hydrogen atom abstraction mechanism, taking place in a two-step process consisting first of addition of O2 to the semiquinone radical and second an intramolecular H-atom transfer concerted with elimination of hydroperoxyl to yield the quinone. This reaction appears to be much more facile for 1,4-semiquinones than for their 1,2-isomers.     

Peroxyl radical clocks

A series of peroxyl radical clocks has been developed and calibrated based on the competition between the unimolecular beta-fragmentation (k(beta)) of a peroxyl radical and its bimolecular reaction with a hydrogen atom donor (k(H)). These clocks are based on either methyl linoleate or allylbenzene and were calibrated directly with alpha-tocopherol or methyl linoleate, which have well-established rate constants for reaction with peroxyl radicals (k(H-tocopherol) = 3.5 x 10(6) M(-1) s(-1), k(H-linoleate) = 62 M(-1) s(-1)). This peroxyl radical clock methodology has been successfully applied to determine inhibition and propagation rate constants ranging from 10(0) to 10(7) M(-1) s(-1).     

Preparation of highly reactive pyridine- and pyrimidine-containing diarylamine antioxidants

We recently reported a preliminary account of our efforts to develop novel diarylamine radical-trapping antioxidants ( Hanthorn , J. J. et al. J. Am. Chem. Soc. 2012 , 134 , 8306 - 8309 ) wherein we demonstrated that the incorporation of ring nitrogens into diphenylamines affords compounds which display a compromise between H-atom transfer reactivity to peroxyl radicals and stability to one-electron oxidation. Herein we provide the details of the synthetic efforts associated with that report, which have been substantially expanded to produce a library of substituted heterocyclic diarylamines that we have used to provide further insight into the structure-reactivity relationships of these compounds as antioxidants (see the accompanying paper, DOI: 10.1021/jo301012x ). The diarylamines were prepared in short, modular sequences from 2-aminopyridine and 2-aminopyrimidine wherein aminations of intermediate pyri(mi)dyl bromides and then Pd-catalyzed cross-coupling reactions of the amines and precursor bromides were the key steps to yield the diarylamines. The cross-coupling reactions were found to proceed best with Pd(η(3)-1-PhC(3)H(4))(η(5)-C(5)H(5)) as precatalyst, which gave higher yields than the conventional Pd source, Pd(2)(dba)(3).     

The effect of ring nitrogen atoms on the homolytic reactivity of phenolic compounds: understanding the radical-scavenging ability of 5-pyrimidinols

Six substituted 5-pyrimidinols were synthesized, and the thermochemistry and kinetics of their reactions with free radicals were studied and compared to those of equivalently substituted phenols. To assess their potential as hydrogen-atom donors to free radicals, we measured their O-H bond dissociation enthalpies (BDEs) using the radical equilibration electron paramagnetic resonance technique. This revealed that the O-H BDEs in 5-pyrimidinols are, on average, about 2.5 kcal mol(-1) higher than those in equivalently substituted phenols. The results are in good agreement with theoretical predictions, and confirm that substituent effects on the O-H BDE of 5-pyrimidinol are essentially the same as those on the Obond;H BDE in phenol. The kinetics of the reactions of these compounds with peroxyl radicals has been studied by their inhibition of the AIBN-initiated autoxidation of styrene, and with alkyl and alkoxyl radicals by competition kinetics. Despite their larger O-H BDEs, 5-pyrimidinols appear to transfer their phenolic hydrogen-atom to peroxyl radicals as quickly as equivalently substituted phenols, while their reactivity toward alkyl radicals far exceeds that of the corresponding phenols. We suggest that this rate enhancement, which is large in the case of alkyl radical reactions, small in the case of peroxyl radical reactions, and nonexistent in the case of alkoxyl radical reactions, is due to polar effects in the transition states of these atom-transfer reactions. This hypothesis is supported by additional experimental and theoretical results. Despite this higher reactivity of 5-pyrimidinols towards radicals compared to phenols, electrochemical measurements indicate that they are more stable to one-electron oxidation than equivalently substituted phenols. For example, the 5-pyrimidinol analogues of 2,4,6-trimethylphenol and butylated hydroxytoluene (BHT) were found to have oxidation potentials approximately 400 mV higher than their phenolic counterparts, but reacted roughly one order of magnitude faster with alkyl radicals and at about the same rate with peroxyl radicals. The 5-pyrimidinol structure should, therefore, serve as a useful template for the rational design of novel air-stable radical scavengers and chain-breaking antioxidants that are more effective than phenols.     

A novel type of free radical reactions: isomerization of a radical with concerted fragmentation

A comparison of experimental data and results of the rate constant calculations by the method of intersecting parabolas (MIP) showed that abstraction of H atom by a peroxyl radical from the C_α-H bond in hydroperoxide is accompanied by concerted fragmentation of the molecule. The complicated character of the elementary event is due to high exothermicity of the reaction. The kinetic parameters of isomerization with fragmentation of peroxyalkyl, peroxyalkoxyl, and peroxyperoxyl radicals were calculated within the framework of the MIP method. The enthalpies, activation energies, and rate constants for a series of isomerization reactions of peroxyl radicals with concerted fragmentation were also obtained from the MIP calculations. Factors influencing these reactions are analyzed.     

How lipid unsaturation, peroxyl radical partitioning, and chromanol lipophilic tail affect the antioxidant activity of α-tocopherol: direct visualization via high-throughput fluorescence studies conducted with fluorogenic α-tocopherol analogues

The preparation of two highly sensitive fluorogenic α-tocopherol (TOH) analogues which undergo >30-fold fluorescence intensity enhancement upon reaction with peroxyl radicals is reported. The probes consist of a chromanol moiety coupled to the meso position of a BODIPY fluorophore, where the use of a methylene linker (BODIPY-2,2,5,7,8-pentamethyl-6-hydroxy-chroman adduct, H(2)B-PMHC) vs an ester linker (meso-methanoyl BODIPY-6-hydroxy-2,5,7,8-tetramethylchromane-2-carboxylic acid, H(2)B-TOH) enables tuning their reactivity toward H-atom abstraction by peroxyl radicals. The development of a high-throughput fluorescence assay for monitoring kinetics of peroxyl radical reactions in liposomes is subsequently described where the evolution of the fluorescence intensity over time provides a rapid, facile method to conduct competitive kinetic studies in the presence of TOH and its analogues. A quantitative treatment is formulated for the temporal evolution of the intensity in terms of relative rate constants of H-atom abstraction (k(inh)) from the various tocopherol analogues. Combined, the new probes, the fluorescence assay, and the data analysis provide a new method to obtain, in a rapid, parallel format, relative antioxidant activities in phospholipid membranes. The method is exemplified with four chromanol-based antioxidant compounds differing in their aliphatic tails (TOH, PMHC, H(2)B-PMHC, and H(2)B-TOH). Studies were conducted in six different liposome solutions prepared from poly- and mono-unsaturated and saturated (fluid vs gel phase) lipids in the presence of either hydrophilic or lipophilic peroxyl radicals. A number of key insights into the chemistry of the TOH antioxidants in lipid membranes are provided: (1) The relative antioxidant activities of chromanols in homogeneous solution, arising from their inherent chemical reactivity, readily translate to the microheterogeneous environment at the water/lipid interface; thus similar values for k(inh)(H(2)B-PMHC)/k(inh)(H(2)B-TOH) in the range of 2-3 are recorded both in homogeneous solution and in liposome suspensions with hydrophilic or lipophilic peroxyl radicals. (2) The relative antioxidant activity between tocopherol analogues with the same inherent chemical reactivity but bearing short (PMHC) or long (TOH) aliphatic tails, k(inh)(PMHC)/k(inh)(TOH), is ~8 in the presence of hydrophilic peroxyl radicals, regardless of the nature of the lipid membrane into which they are embedded. (3) Antioxidants embedded in saturated lipids do not efficiently scavenge hydrophilic peroxyl radicals; under these conditions wastage reactions among peroxyl radicals become important, and this translates into larger times for antioxidant consumption. (4) Lipophilic peroxyl radicals show reduced discrimination between antioxidants bearing long and short aliphatic tails, with k(inh)(PMHC)/k(inh)(TOH) in the range of 3-4 for most lipid membranes. (5) Lipophilic peroxyl radicals are scavenged with the same efficiency by all four antioxidants studied, regardless of the nature of their aliphatic tail or the lipid membrane into which they are embedded. These data underpin the key role the lipid environment plays in modulating the rate of reaction of antioxidants characterized by similar inherent chemical reactivity (arising from a conserved chromanol moiety) but differing in their membrane mobility (structural differences in the lipophilic tail). Altogether, a novel, facile method of study, new insights, and a quantitative understanding on the critical role of lipid diversity in modulating antioxidant activity in the lipid milieu are reported.     

Elucidation of the mechanisms of peroxyl radical reactions in aqueous solutions using the pulse radiolysis technique

In the investigation of peroxyl radicals the pulse radiolysis technique can be used with some advantage to determine the rate of their unimolecular or bimolecular decay. If the identities of the products of the peroxyl radical reactions are known, pulse radiolysis often provides evidence for mechanistic details. The absorptions of the peroxyl radicals are neither very specific nor strong and optical detection is usually of little help. However, there are many peroxyl radical reactions which result in the formation of HO ₂ ^. /H+O ₂ ^. (pK_a(HO ₂ ^. )=4.7) or other acids. Thus in neutral and alkaline solutions such species can be monitored even quantitatively by the pulse conductometric method. Furthermore, O ₂ ^. can be detected by its rapid reaction with tetranitromethane which yields the strongly absorbing nitroform anion. Since O ₂ ^. is only a short-lived intermediate in neutral solutions, it can be distinguished from permanent acids which are often formed in peroxyl radical reactions. In alkaline solutions, where O ₂ ^. is more stable, superoxide dismutase might be used with advantage to reduce its lifetime and to determine the yield of permanent acids. Some details of the fate of the peroxyl radicals derived from acetate, the -hydroxyethyl-peroxyl radicals, and the cyclopentylperoxyl radicals will be reviewed.     

Pro- and antioxidant characteristics of natural thiols

In aqueous solutions at physiological temperature, the mechanism of antioxidative action of natural thiols (glutathione, cysteine, and homocysteine) mainly involves reactions with reactive oxygen species (ROS), peroxyl radicals and hydrogen peroxide. Reduction of hydrogen peroxide by thiols is accompanied by radical generation. The kinetic characteristics of these processes, including those for the reactions of hydrogen peroxide and glutathione immobilized on solid supports such as sodium montmorillonite (clay) and cellulose were determined. Prooxidative effects of thiols are related with the reactions of thiyl radicals formed in the exchange reactions of thiols with other radicals and in the reactions between thiols and hydroperoxides. Thiyl radicals are known to react easily with double bonds. Resveratrol and caffeic acid, phenolic antioxidants containing double bond in their molecules, were shown to be consumed when reacted with glutathione and the process accelerated in the presence of hydrogen peroxide.     

Antioxidant action of organic sulfites—II. Esters of sulfurous acid as primary antioxidants

The effect of esters of sulfurous acid as primary antioxidants was examined. Different aliphatic, aromatic, open-chain and cyclic sulfites were synthesized. The reactions of organic sulfites with RO₂ and RO radicals, the chain carriers of the autoxidation of hydrocarbons and polymers, were simulated by means of the thermal decomposition of azobisisobutyronitrile (AIBN) in the presence of oxygen and of di-tert-butylperoxalate (DTBPO). The reactivity of organic sulfites with 2-cyanoisopropylperoxyl radicals is low. Only aromatic sulfites are able to trap peroxyl radicals; however, they are not very effective primary antioxidants. The reactions of the organic sulfites with tert-butoxyl radicals generally lead to an increase in the rate of decomposition of DTBPO, as determined from rate constants measured at 50 °C. A decomposition of DTBPO induced by liberated tert-butyl radicals in the presence of alkyl sulfites is very probable. Alkyl sulfites and aromatic sulfites with aliphatic groups act mainly as hydrogen donors in reactions with alkoxyl and peroxyl radicals.     

Multi‐faceted Reactivity of Alkyltellurophenols Towards Peroxyl Radicals: Catalytic Antioxidant Versus Thiol‐Depletion Effect

Hydroxyaryl alkyl tellurides are effective antioxidants both in organic solution and aqueous biphasic systems. They react by an unconventional mechanism with ROO^. radicals with rate constants as high as 10⁷ M ^?1 s^?1 at 303 K, outperforming common phenols. The reactions proceed by oxygen atom transfer to tellurium followed by hydrogen atom transfer to the resulting RO^. radical from the phenolic OH. The reaction rates do not reflect the electronic properties of the ring substituents and, because the reactions occur in a solvent cage, quenching is more efficient when the OH and TeR groups have an ortho arrangement. In the presence of thiols, hydroxyaryl alkyl tellurides act as catalytic antioxidants towards both hydroperoxides (mimicking the glutathione peroxidases) and peroxyl radicals. The high efficiency of the quenching of the peroxyl radicals and hydroperoxides could be advantageous under normal cellular conditions, but pro‐oxidative (thiol depletion) when thiol concentrations are low.     

Thermal decomposition mechanisms of tert-alkyl peroxypivalates studied by the nitroxide radical trapping technique

The thermolysis of a series of tert-alkyl peroxypivalates 1 in cumene has been investigated by using the nitroxide radical-trapping technique. tert-Alkoxyl radicals generated from the thermolysis underwent the unimolecular reactions, beta-scission, and 1,5-H shift, competing with hydrogen abstraction from cumene. The absolute rate constants for beta-scission of tert-alkoxyl radicals, which vary over 4 orders of magnitude, indicate the vastly different behavior of alkoxyl radicals. However, the radical generation efficiencies of 1 varied only slightly, from 53 (R = Me) to 63% (R = Bu(t)()), supporting a mechanism involving concerted two-bond scission within the solvent cage to generate the tert-butyl radical, CO(2), and an alkoxyl radical. The thermolysis rate constants of tert-alkyl peroxypivalates 1 were influenced by both inductive and steric effects [Taft-Ingold equation, log(rel k(d)) = (0.97 +/- 0. 14)Sigmasigma - (0.31 +/- 0.04)SigmaE(s)(c), was obtained].     

Rate constants for reactions of perfluorobutylperoxyl radical with alkenes

Perfluorobutylperoxyl radicals were produced by radiolytic reduction of perfluorobutyl iodide in aerated methanol solutions. Rate constants for the reactions of this peroxyl radical with various organic compounds were determined by kinetic spectrophotometric pulse radiolysis. The rate constants for alkanes and alkenes were determined by competition kinetics using chlorpromazine as a reference. The results indicate that hydrogen abstraction from aliphatic compounds takes place with a rate constant that is too slow to measure in our system (<10⁵ M^?1 s^?1), and that abstraction of allylic and doubly allylic hydrogens is slow compared with addition. Addition to alkenes takes place with rate constants of the order of k = 10⁶ ? 10⁸ M^?1 s^?1. Good correlation was obtained between log k and the Taft substituent constants σ* for the various substituents on the double bond. Perfluorobutylperoxyl radical is found to be more reactive than trichloromethylperoxyl and other peroxyl radicals.     

Reactions of 2,2'-azinobis-(3-ethylbenzthiazoline-6-sulfonate) dianion, ABTS2-, with *OH, (SCN)2*-, and glycine or valine peroxyl radicals

ABTS2-, 2,2'-azinobis-(3-ethylbenzthiazoline-6-sulfonate) dianion, was used as a reference to compare the reactivity of peroxyl radicals of two amino acids, glycine and valine, in aqueous solutions at natural pH. Peroxyl radicals were produced by pulse radiolysis and the product of their reaction with ABTS2- the ABTS*- radical was observed spectrophotometrically. Experimental kinetic traces were fitted using chemical simulation. The rate constants of reactions of glycine and valine peroxyl radicals with ABTS2- were (6.0+/-0.2)x10(6) and (1.3+/-0.1)x10(5) M-1.s-1, respectively. Moreover, it was found that only 60% of glycine radicals formed upon its reaction with *OH radicals reacted with molecular oxygen to yield peroxyl radicals. Comparison of experimental data with simulations of chemical reactions in irradiated ABTS and ABTS/NaSCN solutions showed that ABTS*- forms in the reaction with *OH with a yield of 43% and rate constant of (5.4+/-0.2)x10(9) M-1.s-1 and in the reaction with (SCN)2*- with a yield of 57% and rate constant of (8.0+/-0.2)x10(8) M-1.s-1.     

Reactivity of natural phenols in radical reactions

Activation enthalpies and energies and the rate constants of reactions with peroxyl, alkyl, and thiyl radicals (76 reactions) were calculated for a group of natural antioxidants (19 monohydroxy and polyhydroxy phenols). The calculation was performed with the use of the model of a radical abstraction reaction as the intersection of two parabolic potential curves. The results of the calculation were compared with experimental data: the average discrepancy in the activation energies of the reactions RO ₂ ^? + ArOH was 0.8 kJ/mol. Interatomic distances in the reaction centers of the transition states of the test reactions were calculated. Factors affecting the reactivity of these compounds are discussed.     

Theoretical calculations of carbon-oxygen bond dissociation enthalpies of peroxyl radicals formed in the autoxidation of lipids

Theoretical calculations were carried out to provide a framework for understanding the free radical oxidation of unsaturated lipids. The carbon[bond]hydrogen bond dissociation enthalpies (BDEs) of organic model compounds and oxidizable lipids (R[bond]H) and the carbon[bond]oxygen bond dissociation enthalpies of peroxyl radical intermediates (R[bond]OO*) have been calculated. The carbon[bond]hydrogen BDEs correlate with the rate constant for propagation of free radical autoxidation, and the carbon[bond]oxygen BDEs of peroxyl radicals correlate with rate constants for beta-fragmentation of these intermediates. Oxygen addition to intermediate carbon radicals apparently occurs preferentially at centers having the highest spin density. The calculated spin distribution therefore provides guidance about the partitioning of oxygen to delocalized carbon radicals. Where the C[bond]H BDEs are a function of the extent of conjugation in the parent lipid and the stability of the carbon radical derived therefrom, C[bond]OO* BDEs are also affected by hyperconjugation. This gives way to different rates of beta-fragmentation of peroxyl radicals formed from oxygen addition at different sites along the same delocalized radical. We have also studied by both theory and experiment the propensity for benzylic radicals to undergo oxygen addition at their ortho and para carbons which, combined, possess an equivalent unpaired electron spin density as the benzylic position itself. We find that the intermediate peroxyl radicals in these cases have negative C[bond]OO* BDEs and, thus, have rate constants for beta-fragmentation that exceed the diffusion-controlled limit for the reaction of a carbon-centered radical with oxygen.     

The mechanism of radical-trapping antioxidant activity of plant-derived thiosulfinates

It has long been recognized that garlic and petiveria, two plants of the Allium genus--which also includes onions, leeks and shallots--possess great medicinal value. In recent times, the biological activities of extracts of these plants have been ascribed to the antioxidant properties of the thiosulfinate secondary metabolites allicin and S-benzyl phenylmethanethiosulfinate (BPT), respectively. Herein we describe our efforts to probe the mechanism of the radical-trapping antioxidant activity of these compounds, as well as S-propyl propanethiosulfinate (PPT), a saturated analog representative of the thiosulfinates that predominate in non-medicinal alliums. Our experimental results, which include thiosulfinate-inhibited autoxidations of the polyunsaturated fatty acid (ester) methyl linoleate, investigations of their decomposition kinetics, and radical clock experiments aimed at obtaining some quantitative insights into their reactions with peroxyl radicals, indicate that the radical-trapping activity of thiosulfinates is paralleled by their propensity to undergo Cope elimination to yield a sulfenic acid. Since sulfenic acids are transient species, we complement our experimental studies with the results of theoretical calculations aimed at understanding the radical-trapping behaviour of the sulfenic acids derived from allicin, BPT and PPT, and contrasting the predicted thermodynamics and kinetics of their reactions with those of the parent thiosulfinates. The calculations reveal that sulfenic acids have among the weakest O-H bonds known (ca. 70 kcal mol(-1)), and that their reactions with peroxyl radicals take place by a near diffusion-controlled proton-coupled electron transfer mechanism. As such, it is proposed that the abundance of a thiosulfinate in a given plant species, and the ease with which it undergoes Cope elimination to form a sulfenic acid, accounts for the differences in antioxidant activity, and perhaps medicinal value, of extracts of these plants. Interestingly, while the Cope elimination of 2-propenesulfenic acid from allicin is essentially irreversible, the analogous reaction of BPT is readily reversible. Thus, in the absence of chain-carrying peroxyl radicals (or other appropriately reactive trapping agent), BPT is reformed.     

Kinetics and mechanism of peroxyl radical reactions with nitroxides

Cyclic nitroxides (>NO*) are stable radicals of diverse size, charge, lipophilicility, and cell permeability, which provide protection against oxidative stress via various mechanisms including SOD-mimic activity, oxidation of reduced transition metals and detoxification of oxygen- and nitrogen-centered radicals. However, there is no agreement regarding the reaction of nitroxides with peroxyl radicals, and many controversies in the literature exist. The question of whether nitroxides can protect by scavenging peroxyl radicals is important because peroxyl radicals are formed in biological systems. To further elucidate the mechanism(s) underlying the antioxidative effects of nitroxides, we studied by pulse radiolysis the reaction kinetics of piperidine, pyrrolidine, and oxazolidine nitroxides with several alkyl peroxyl radicals. It is demonstrated that nitroxides mainly reduce alkyl peroxyl radicals forming the respective oxoammonium cations (>N+=O). The most efficient scavenger of peroxyl radicals is 2,2,6,6-tetramethylpiperidine-N-oxyl (TPO), which has the lowest oxidation potential among the nitroxides tested in the present study. The rate constants of peroxyl reduction are in the order CH2(OH)OO*>CH3OO*>t-BuOO*, which correlate with the oxidation potential of these peroxyl radicals. The rate constants for TPO vary between 2.8x10(7) and 1.0x10(8) M-1 s-1 and for 3-carbamoylproxyl (3-CP) between 8.1x10(5) and 9.0x10(6) M-1 s-1. The efficacy of protection of nitroxides against inactivation of glucose oxidase caused by peroxyl radicals was studied. The results demonstrate a clear correlation between the kinetic features of the nitroxides and their ability to inhibit biological damage inflicted by peroxyl radicals.     

One-electron redox reactions of trifluoperazine in aqueous solutions

Absolute rate constants have been measured for the reactions of the primary and specific one-electron oxidant radicals with the protonated form of trifluoperazine (TFP). The primary radicals, e^- _aq and OH·, react with TFP at diffusion controlled rates. The transients thus produced have been characterized. Halogenated aliphatic peroxyl radicals oxidize TFP with rate constants between 10⁷ and 10⁸ dm³ mol^-1 s^-1, depending on the structure of the peroxyl radical. The reactivity of peroxyl radicals has been found to vary with Taft's inductive parameter. Oxidation of TFP at acidic pH has been studied using stopped-flow technique. The reaction between TFP radical cation and ascorbic acid has also been examined using pulse radiolysis technique. The results indicate that TFP radical cation is repaired by ascorbate. One-electron reduction potential of TFP · + /TFP at pH 3.5 has been calculated to be 0.964 V vs. NHE.