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In biology, the reaction of ONOO - with CO 2 is the main sink for ONOO - . This reaction yields CO 3 •- , NO 2 • , NO 3 - , and CO 2 . There is a long-standing debate with respect to the yield of the radicals relative to ONOO - . The reaction of ONOO - with CO 2 results at first in ONOOCO 2 - . According to one hypothesis, ONOOCO 2 - is extremely short-lived and devolves into a solvent cage that contains CO 3 •- and NO 2 • . Of these solvent cages, approximately two/thirds result in NO 3 - and CO 2 , and approximately one/third release CO 3 •- and NO 2 • that oxidize the substrate. According to our hypothesis, ONOOCO 2 - is formed much faster, is relatively long-lived, and may also be an oxidant; the limited yield is the result of ONOOCO 2 - being scavenged by a second CO 2 under conditions of a high CO 2 concentration. We disagree with the first hypothesis for three reasons: First, it is based on an estimated K for the reaction of ONOO - with CO 2 to form ONOOCO 2 - of ∼1 M -1 , while experiments yield a value of 4.5 × 10 3 M -1 . Second, we argue that the solvent cage as proposed is physically not realistic. Given the less than diffusion-controlled rate constant of CO 3 •- with NO 2 • , all radicals would escape from the solvent cage. Third, the reported ∼33% radical is not supported by an experiment where mass balance was established. We propose here a hybrid mechanism. After formation of ONOOCO 2 - , it undergoes homolysis to yield CO 3 •- with NO 2 • , or, depending on [CO 2 ], it is scavenged by a second CO 2 ; CO 3 •- oxidizes ONOO - , if present. These reactions allow us to successfully simulate the reaction of ONOO - with CO 2 over a wide range of ONOO - /CO 2 ratios. At lower ratios, fewer radicals are formed, while at higher ratios, radical yields between 30% and 40% are predicted. The differences in radical yields reported may thus be traced to the experimental ONOO - /CO 2 ratios. Given a physiological [CO 2 ] of 1.3 mM, the yield of CO 3 •- and NO 2 • is 19%, and lower if ONOOCO 2 - has a significant reactivity of its own.

With CO 2 present in excess, ONOO - reacts to form an adduct in solution and in the solid state, most likely ONOOCO 2 - . In solution, the adduct appears within 2 ms and absorbs at 300 with an extinction coefficient, which is either 50% or 100% (preferred) of that of ONOO - , 1.70 × 10 3 M -1 cm -1 , and at 685 nm with an extinction coefficient of 85 M -1 cm -1 . When solid [(CH 3 ) 4 N][ONOO] is treated with CO 2 , these two maxima are red-shifted by 30-50 nm. The equilibrium constant for adduct formation in solution is (4.5 ± 0.5) × 10 3 M. The adduct reacts further with another CO 2 at a rate of (2.6 ± 0.8) × 10 4 M -1 s -1 and produces 2 CO 2 and NO 3 - . Thermochemical calculations show that ΟΝΟΟCO 2 - is a strong two-electron oxidizing agent, E°(ONOOCO 2 - , H + /NO 2 - , HCO 3 - ) = +1.28 V at pH 7 and an even stronger one-electron oxidizing agent E°'(ONOOCO 2 - , H + /NO 2 • , HCO 3 - ) = +1.51 V at pH 7. The extent of homolysis, that is formation of NO 2 • and CO 3 •- , is small, slightly less than 1% relative to ONOO - at the physiological concentration of CO 2 of 1.3 mM in plasma. Thus, ONOOCO 2 - is more relevant than CO 3 •- under in vivo conditions.

Iron in iron overload disease is present as non-transferrin-bound iron, consisting of iron, citrate, and albumin. We investigated the redox properties of iron citrate by electrochemistry, by the kinetics of its reaction with ascorbate, by ESR, and by analyzing the products of reactions of ascorbate with iron citrate complexes in the presence of H2O2 with 4-hydroxybenzoic acid as a reporter molecule for hydroxylation. We report -0.03 V < E°' > +0.01 V for the (Fe(3+)-cit/Fe(2+)-cit) couple. The first step in the reaction of iron citrate with ascorbate is the rapid formation of mixed complexes of iron with citrate and ascorbate, followed by slow reduction to Fe(2+)-citrate with k = ca. 3 M(-1) s(-1). The ascorbyl radical is formed by iron citrate oxidation of Hasc(-) with k = ca. 0.02 M(-1) s(-1); the majority of the ascorbyl radical formed is sequestered by complexation with iron and remains EPR silent. The hydroxylation of 4-hydroxybenzoic acid driven by the Fenton reduction of iron citrate by ascorbate in the presence of H2O2 proceeds in three phases: the first phase, which is independent of the presence of O2, is revealed as a nonzero intercept that reflects the rapid reaction of accumulated Fe(2+) with H2O2; the intermediate oxygen-dependent phase fits a first-order accumulation of product with k = 5 M(-1) s(-1) under aerobic and k = 13 M(-1) s(-1) under anaerobic conditions; the slope of the final linear phase is ca. k = 5 × 10(-2) M(-1) s(-1) under both aerobic and anaerobic conditions. Product yields under aerobic conditions are greater than predicted from the initial concentration of iron, but they are less than predicted for continuous redox cycling in the presence of excess ascorbate. The ongoing formation of hydroxylated product supports slow redox cycling by iron citrate. Thus, when H2O2 is available, iron-citrate complexes may contribute to pathophysiological manifestations of iron overload diseases.

Cysteine, cysteinyl-glycine, glutathione, phenylalanyl-cysteinyl-glycine, and histidyl-cysteinyl-glycine were dissolved in acidic and neutral D(2)O in the presence of the radical generator 2,2'-azobis(2-methylpropionamidine) dihydrochloride and radical mediator compounds (benzyl alcohol and 2-propanol). An exchange of H-atoms by D-atoms took place in these peptides due to intramolecular H-abstraction equilibria. NMR measurements allow one to follow the extent of H-D exchanges and to identify the sites where these exchanges take place. Significant exchanges occur in acidic media in GSH at positions Glu-β and Glu-γ, in Phe-Cys-Gly at positions Phe ortho, Phe-β, Cys-α, Cys-β, and Gly-α, and in His-Cys-Gly at positions His H1, His H2, His β, Cys β, and Gly α. In neutral media, exchanges occur in Cys-Gly at position Cys β and in GSH at position Cys α. Phe-Cys-Gly and His-Cys-Gly were not examined in neutral media. Sites participating in the radical exchange equilibria are highly dependent on structure and pH; the availability of electron density in the form of lone pairs appears to increase the extent of exchange. Interestingly, and unexpectedly, 2D NMR experiments show that GSH rearranges itself in acidic solution: the signals shift, but their patterns do not change. The formation of a thiolactone from Gly and Cys residues matches the changes observed.

The reduction of oxidized glutathione GSSG by hydrated electrons and hydrogen atoms to form GSSG•⁻ is quantitative. The radical anion dissociates into GS• and GS⁻, and the S-centered radical subsequently abstracts a hydrogen intramolecularly. We observe sequential development of UV absorbance signatures that indicate the formation of both α- and β-carbon-centered radicals. From experiments performed at pH 2 and pH 11.8, we determined forward and reverse rate constants for the overall equilibrium between sulfur-centered and carbon-centered radicals: k(forward) = 3·10⁵ s⁻¹, k(reverse) = 7·10⁵ s⁻¹, and K = 0.4. Furthermore, on the basis of the differences between the kinetics traces at 240 and 280 nm, we estimate that α- and β-carbon-centered radicals are formed at a surprising ratio of 1:3. The ratios found at pH 2 also apply to pH 7, with the conclusion that the equilibrium ratio of S-centered:β-centered:α-centered radicals is, very approximately, 8:3:1. The formation of carbon-centered radicals could lead to irreversible damage in proteins via the formation of carbon-carbon bonds or backbone fragmentation.

We describe the preparation of aqueous solutions of LiONOO and NaONOO from (Me4N)ONOO. An aqueous solution of analytically pure, commercially available (Me4N)ONOO is applied to an Amberlyst 15 column at 4 degrees C, and the Me4N+ is rapidly (in 20 min) exchanged against Li+ or Na+. The exchanged peroxynitrite is produced in yields of ca. 90% with a nitrite content of 5-6%.

The goal of iron-chelation therapy is to reduce the levels of labile plasma iron, and intravenously administered desferrioxamine is the gold standard of therapeutic agents. Hydroxypyridinones, e.g., CP20 (3-hydroxy-1,2-dimethylpyridin-4(1H)-one), are used or are under investigation as orally administered iron chelators. We determined electrode potentials of CP20, the related hydoxypyridones CP361, CP363, and CP502, and ICL670 (4-[3,5-bis(2-hydroxyphenyl)-1H-1,2,4-triazol-1-yl]benzoic acid) under physiologically relevant conditions to address the question of whether iron in the presence of these chelating agents can carry out Fenton chemistry in vivo. We found that iron(III) but not iron(II) binds tightly to both CP20 and ICL670 at pH 7 and higher, compared to nearly complete binding of 1 microM iron(II) to 10 microM desferrioxamine at pH 7.4 The electrode potentials of the hydroxypyridinones shift to more negative values with decreasing pK(a) values at lower concentrations of iron(III) (0.02 mM) and ligand (0.1 mM). The electrode potential of the iron-CP20 system decreases as a function of increasing pH, with a minimum near pH 10.5. We estimate an electrode potential for the ascorbyl radical/ascorbate couple under physiological conditions of +105 mV, which is higher than the electrode potential of the iron(III) complex of CP20 at all concentrations of iron. The rate of oxidation of iron(II) in the presence of CP20 by hydrogen peroxide increases with the concentrations of both ligand and peroxide. Although iron(II) is oxidized by hydrogen peroxide, the thus-formed Fe(III)(CP20)(3) complex cannot be reduced by ascorbate. Therefore, the tight binding of iron(III) by this class of chelators prevents redox cycling.

Carbon dioxide catalyzed oxidative coupling of phenol by peroxynitrite occurs by two pathways distinguished by the isomer ratio of 2,2'- to 4,4'-biphenols. As already established, at neutral pH and moderate phenol concentrations, both biphenols are formed in comparable yields by the coupling of two phenoxyl radicals. However, at high pH and phenol concentration, 2,2'-biphenol is the only identified coupled product, and its formation does not involve phenoxyl radicals. Instead, under these conditions, a previously unreported long-lived (t(1/2) approximately 10 s at pH 10 and 1 mM phenol) diamagnetic intermediate with an absorption maximum at 400 nm is observed. This intermediate is formed from phenolate concomitantly with the decay of peroxynitrite and disappears via reaction with phenol [k = (2.4 +/- 0.1) x 10 M(-)(1) s(-)(1) at pH 10.5] to form 2,2'-biphenol. We also find that para-benzoquinone, previously unreported, is formed in up to 5% yield relative to the initial peroxynitrite concentration. The appearance of an absorption band above 500 nm, which might be due to quinhydrone, indicates that hydroquinone is a likely para-benzoquinone precursor. The dependence of para-benzoquinone yields on pH and phenol concentration suggests that its formation is related to the nonradical pathway of 2,2'-biphenol formation. This novel nonradical pathway of 2,2'-biphenol formation might be relevant to the mechanisms of reaction of phenolic antioxidants with peroxynitrite. The existence of two distinct pathways of biphenol formation implies that, apart from a CO(3)(*)(-)/NO(2)(*) radical pair, another reactive intermediate is formed during the carbon dioxide catalyzed decay of peroxynitrite.

Laser flash photolysis at 266 nm of 3-nitrotyrosine and N-acetyl-3-nitrotyrosine ethyl ester generates an oxidizing species, which shows all of the characteristics of a hydroxyl radical. This species reacts with Br(-) to yield Br(2).-, via an intermediate, that is kinetically identified as HOBr.-. Moreover, the formation of Br(2).- can be suppressed by methanol; competition kinetics yield relative rate constants for the reaction of the reactive species with Br(-) and methanol that are similar to those for the hydroxyl radical. Parallel time-resolved UV/vis spectroscopy suggests the formation of phenoxyl radicals, consistent with the formation of hydroxyl radicals. Laser flash photolysis at 355 nm also generates reactive intermediates that oxidize Br(-) to Br(2).- but appear not to be hydroxyl radicals.

A standard Gibbs energy of formation of 16.6 kcal mol(-)(1) has been reported for peroxynitrite [Merényi, G., and Lind, J. (1998) Chem. Res. Toxicol. 11, 243-246]. This value is based on the rate constants for the forward and backward rate constants of the equilibrium O2*- + NO* if ONOO(-). A rate constant of 0.017 s(-)(1) for the backward rate constant was determined by observing the formation of C(NO(2))(3)(-) when peroxynitrite was mixed with C(NO(2))(4). However, a similar rate constant is also observed in the presence of NO(*), which indicates that formation of C(NO(2))(3)(-) is due to a process other than the reduction of C(NO(2))(4) by O2*-. Additionally, copper(II) nitrilotriacetate enhances the decay of ONOO(-) at pH 9.3, without reduction of copper(II). The preferred thermodynamic values are therefore as follows: Delta(f)H degrees (ONOO(-)) = -10 +/- 2 kcal mol(-)(1), Delta(f)G degrees (ONOO(-)) = 14 +/- 3 kcal mol(-)(1), S degrees (ONOO(-)) = 31 eu, and E degrees '(ONOOH/NO(2)(*), H(2)O) = 1.6 V at pH 7 [Koppenol, W. H., and Kissner, R. (1998) Chem. Res. Toxicol. 11, 87-90].

Peroxynitrite is an inorganic toxin of biological importance. It is formed in vivo from the diffusion-limited reaction of nitrogen monoxide with superoxide. Due to the partial double bond between the nitrogen and the first peroxide oxygen, peroxynitrite can occur in two conformations, cis and trans. The synthesis of tetramethylammonium peroxynitrite in ammonia [Bohle, D. S., et al. (1994) J. Am. Chem. Soc. 116, 7423-7424] yields small crystals if the ammonia is left to evaporate slowly. X-ray structure analysis shows that peroxynitrite crystallizes in the cis form, relative to the N-O bond. Crystal twinning or disorder prevents the determination of accurate bond lengths and bond angles. However, a nearly flat (torsion angle of 22 degrees ) molecule with O=N, N-O, and O-O bond lengths of 1.16, 1.35, and 1.41 A, respectively, would fit the observed electron density best. The space group of tetramethylammonium peroxynitrite is Pmmn (59), and it has the following unit cell dimensions: a = 7.1778(11) A, b = 8.6893(13) A, and c = 5.7266(9) A.

Glutathione peroxidases and their mimics, e.g., ebselen or diaryl tellurides, efficiently reduce peroxynitrite/peroxynitrous acid (ONOO-/ONOOH) to nitrite and protect against oxidation and nitration reactions. Here, we report the second-order rate constant for the reaction of the reduced form of glutathione peroxidase (GPx) with peroxynitrite as (8.0 +/- 0.8) x 10(6) M-1 s-1 (per GPx tetramer) at pH 7.4 and 25 degreesC. The rate constant for oxidized GPx is about 10 times lower, (0.7 +/- 0.2) x 10(6) M-1 s-1. On a selenium basis, the rate constant for reduced GPx is similar to that obtained previously for ebselen. The data support the conclusion that GPx can exhibit a biological function by acting as a peroxynitrite reductase.

Recent thermodynamic calculations of Merenyi and Lind [(1997) Chem. Res. Toxicol. 10, 1216-1220] suggest that O=NOOH can undergo homolysis to form the hydroxyl radical and nitrogen dioxide. This result is based in part on our statement that the enthalpy of ionization of O=NOOH is close to zero [Koppenol et al. (1992) Chem. Res. Toxicol. 5, 834-842]. As the ionization of O=NOOH is sensitive to the milieu and the rate of isomerization (to nitrate) to the total concentration of O=NOOH and O=NOO- [Kissner et al. (1997) Chem. Res. Toxicol. 10, 1285-1292], we reinvestigated the temperature dependence of the ionization constant and determined a deltaHo of 4+/-2 kcal mol(-1). This results in a standard Gibbs energy of homolysis of 16 kcal mol(-1) and a rate of homolysis of 1 x 10(-2) s[-1]. Given the uncertainty in the Gibbs energy of homolysis, upper and lower rates are 1 x 10(-4) and 0.6 s(-1), slower than the rate of isomerization, 1.2 s(-1) at 25 degrees C. The recombination of the homolysis products NO2. and HO. is known to lead to mainly peroxynitrous acid. If one assumes that a few percent of the recombinations lead to nitrate instead, then the rate of homolysis must be much higher than the rate of isomerization. We conclude therefore that homolysis is unlikely.

Flash photolysis of alkaline peroxynitrite solutions results in the formation of nitrogen monoxide and superoxide. From the rate of recombination it is concluded that the rate constant of the reaction of nitrogen monoxide with superoxide is (1.9 +/- 0.2) x 10(10) M-1 s-1. The pKa of hydrogen oxoperoxonitrate is dependent on the medium. With the stopped-flow technique a value of 6.5 is found at millimolar phosphate concentrations, while at 0.5 M phosphate the value is 7.5. The kinetics of decay do not follow first-order kinetics when the pH is larger than the pKa, combined with a total peroxynitrite and peroxynitrous acid concentration that exceeds 0.1 mM. An adduct between ONOO- and ONOOH is formed with a stability constant of (1.0 +/- 0.1) x 10(4) M. The kinetics of the decay of hydrogen oxoperoxonitrate are not very pressure-dependent: from stopped-flow experiments up to 152 MPa, an activation volume of 1.7 +/- 1.0 cm3 mol-1 was calculated. This small value is not compatible with homolysis of the O-O bond to yield free nitrogen dioxide and the hydroxyl radical. Pulse radiolysis of alkaline peroxynitrite solutions indicates that the hydroxyl radical reacts with ONOO- to form [(HO)ONOO].- with a rate constant of 5.8 x 10(9) M-1 s-1. This radical absorbs with a maximum at 420 nm (epsilon = 1.8 x 10(3) M-1 cm-1) and decays by second-order kinetics, k = 3.4 x 10(6) M-1 s-1. Improvements to the biomimetic synthesis of peroxynitrite with solid potassium superoxide and gaseous nitrogen monoxide result in higher peroxynitrite to nitrite yields than in most other syntheses.

The kinetics and products of the reaction of peroxynitrite with the phenolic compounds phenol, tyrosine, and salicylate were studied as a function of pH. All reactions are first-order in peroxynitrite and zero-order in the phenolic compound. Relative to the hydroxyl group, electrophilic substitution in the 2- and 4-positions (if available) leads to hydroxylated and nitrated products. The total yield of the products is proportional to the concentration of peroxynitrite. The sum of the rates of hydroxylation and nitration of phenol, determined by the stopped-flow technique, is approximately equal to the rate constant for the isomerization of peroxynitrite to nitrate. The rate vs pH profiles of the nitration and hydroxylation reactions parallel the yield vs pH profile with nitration maxima at pH 1.8 and 6.8, while hydroxylation is dominant between these two pH values. The activation energies for both hydroxylation and nitration are 18.8 +/- 0.3 kcal mol-1, identical to that of the isomerization of peroxynitrite to nitrate. Ethanol decreases the yield of hydroxylation, but has less effect on the nitration. The rate of reaction in the presence of metal complexes is first-order in metal complex and peroxynitrite and zero-order in the phenolic compound. The enhancement of the nitration of phenol by Fe(III)-edta and -nta is pH-dependent, with a maximum near pH 7, while Fe(III)-citrate, Cu(II)-edta, and CuSO4 affect the nitration much less. The second-order rate constants for Fe(III)-edta at pH 4.8 and 7.2 are 1.4 x 10(3) and 5.5 x 10(3) M-1 s-1, respectively, at 25 degrees C. The activation energies for the nitration reaction in the presence of Fe(III)-edta are 11.5 and 12.2 kcal mol-1 at pH 4.8 and 7.2, respectively. The nitration of tyrosine and salicylate by peroxynitrite is maximally enhanced by Fe(III)-edta.

Peroxynitrite [oxoperoxonitrate(1-), ONOO-] may be formed in vivo from superoxide and nitric oxide. The anion is stable, but the acid (pKa = 6.8) decays to nitrate with a rate of 1.3 s-1 at 25 degrees C. The experimental activation parameters of this process are delta H++ = +18 +/- 1 kcal/mol, delta S++ = +3 +/- 2 cal/(mol.K), and delta G++ = +17 +/- 1 kcal/mol. Peroxynitrite (or its protonated form) oxidizes some compounds such as thiols and thioethers in a biomolecular reaction. The reactions with glutathione and cysteine have activation enthalpies of 10.9 and 9.7 kcal/mol, respectively, which are lower than that of the isomerization reaction. Peroxynitrite reacts with other compounds such as dimethyl sulfoxide and deoxyribose in a unimolecular reaction for which the activation of peroxynitrite is rate-limiting. In theory, activation could involve (1) heterolysis to OH- and NO2+ (delta rxn Gzero' = 13 kcal/mol at pH 7) or (2) homolysis to .OH and .NO2 (delta rxn Gzero = 21 kcal/mol), and these processes also could be involved in the isomerization to nitrate. However, thermodynamic and kinetic considerations indicate that neither process is feasible, although binding to metal ions may reduce the large activation energy associated with heterolysis. An intermediate closely related to the transition state for isomerization of ONOOH to HONO2 may be the strongly oxidizing intermediate responsible for hydroxyl radical-like oxidations mediated by ONOOH. Thus, peroxynitrite reacts with different compounds by at least two distinct mechanisms, and the hydroxyl radical is not involved in either.