Gabor Merenyi

The chemistry of peroxynitrite: implications for biological activity.

In biological systems, nitric oxide (NO) combines rapidly with superoxide (O2-) to form peroxynitrite ion (ONOO-), a substance that has been implicated as a culprit in many diseases. Peroxynitrite ion is essentially stable, but its protonated form (ONOOH, pKa = 6.5 to 6.8) decomposes rapidly via homolysis of the O-O bond to form about 28% free NO2 and OH radicals. At physiological pH and in the presence of large amounts of bicarbonate, ONOO- reacts with CO2 to produce about 33% NO2 and carbonate ion radicals (CO3-) in the bulk of the solution. The quantitative role of OH/CO3(-) and NO2 radicals during the decomposition of peroxynitrite (ONOOH/ONOO-) under physiological conditions is described in detail. Specifically, the effect of the peroxynitrite dosage rate on the yield and distribution of the final products is demonstrated. By way of an example, the detailed mechanism of nitration of tyrosine, a vital aromatic amino acid, is delineated, showing the difference in the nitration yield between the addition of authentic peroxynitrite and its continuous generation by NO and O2- radicals.

On the acid-base equilibrium of the carbonate radical

Abstract Using pulse radiolysis and coupled optical detection the pKa of the HCO3 radical is shown to lie between 7.0 and 8.2. The rate constant of self-recombination of CO-3 has a negative apparent activation energy which indicates a composite process.

Reaction of Ozone with Hydrogen Peroxide (Peroxone Process): A Revision of Current Mechanistic Concepts Based on Thermokinetic and Quantum-Chemical Considerations

The reaction of ozone with the anion of H(2)O(2) (peroxone process) gives rise to (*)OH radicals (Staehelin, J.; Hoigne, J. Environ. Sci. Technol. 1982, 16, 676-681). Thermokinetic considerations now suggest that the electron transfer originally assumed as the first step has to be replaced by the formation of an adduct, HO(2)(-) + O(3) --> HO(5)(-) (DeltaG degrees = -39.8 kJ mol(-1)). This decomposes into HO(2)(*) and O(3)(*-) (DeltaG(0) = 13.2 kJ mol(-1)). HO(2)(*) is in equilibrium with O(2)(*-) + H(+), and O(2)(*-) undergoes electron transfer to O(3) giving rise to further O(3)(*-). The decay of O(3)(*-) into (*)OH is now discussed on the basis of the equilibria O(3)(*-) right arrow over left arrow O(2) + O(*-) and O(*-) + H(2)O right arrow over left arrow (*)OH + OH(-), excluding HO(3)(*) as the intermediate originally assumed. To account for the observation of the peroxone process being only 50% efficient, the decay of HO(5)(-) into 2 O(2) + OH(-) (DeltaG(0) = -197 kJ mol(-1)) is proposed to compete with the decay into HO(2)(*) and O(3)(*-).

Standard electrode potentials involving radicals in aqueous solution : inorganic radicals (IUPAC Technical Report)

Abstract Recommendations are made for standard potentials involving select inorganic radicals in aqueous solution at 25 °C. These recommendations are based on a critical and thorough literature review and also by performing derivations from various literature reports. The recommended data are summarized in tables of standard potentials, Gibbs energies of formation, radical pKa’s, and hemicolligation equilibrium constants. In all cases, current best estimates of the uncertainties are provided. An extensive set of Data Sheets is appended that provide original literature references, summarize the experimental results, and describe the decisions and procedures leading to each of the recommendations.

Pulse radiolysis study on the reactivity of Trolox C phenoxyl radical with superoxide anion

The reaction between the phenoxyl radical of Trolox C, a water‐soluble vitamin E analogue, and superoxide anion radical was examined by using the pulse radiolysis technique. The results indicate that the Trolox C phenoxyl radical may undergo a rapid one‐electron transfer from superoxide radical [k=(4.5±0.5) × 108 M−1·s−1] to its reduced form. This finding indicates that superoxide radical might play a role in the repair of vitamin E phenoxyl radical.

THE REACTIVITY OF SUPEROXIDE (O2‐) and ITS ABILITY TO INDUCE CHEMILUMINESCENCE WITH LUMINOL

Abstract— The quantum yield and the kinetics of O‐induced luminol chemiluminescence was investigated in a broad pH interval at varying luminol and concentrations. It is suggested that the weak chemiluminescence observed is mediated via a luminol‐superoxide‐adduct proposed to be an a‐hydroxyperoxyl radical. At pH 7 the maximum quantum yield of chemiluminescence per initial percent was determined to be 4 times 10‐8. The degree of involvement in phagocytosis and related processes should be viewed against this maximum limit.

One- and two-electron reduction potentials of peroxyl radicals and related species

Utilising gaseous and aqueous thermodynamic quantities, estimates (in water versus NHE) have been made of the one-electron reduction potentials of alkyl peroxyl radicals including CCl3OO˙, percarboxyl and carboxyl radicals, and alkoxyl radicals, including CCl3O˙ and two-electron reduction potentials of alkyl hydroperoxides including CCl3OOH, alkyl peroxyl radicals including CCl3OO˙ and of percarboxyl radicals.

The reaction of ozone with the hydroxide ion: mechanistic considerations based on thermokinetic and quantum chemical calculations and the role of HO4- in superoxide dismutation.

The reaction of OH(-) with O(3) eventually leads to the formation of *OH radicals. In the original mechanistic concept (J. Staehelin, J. Hoigné, Environ. Sci. Technol. 1982, 16, 676-681), it was suggested that the first step occurred by O transfer: OH(-)+O(3)-->HO(2)(-)+O(2) and that *OH was generated in the subsequent reaction(s) of HO(2)(-) with O(3) (the peroxone process). This mechanistic concept has now been revised on the basis of thermokinetic and quantum chemical calculations. A one-step O transfer such as that mentioned above would require the release of O(2) in its excited singlet state ((1)O(2), O(2)((1)Delta(g))); this state lies 95.5 kJ mol(-1) above the triplet ground state ((3)O(2), O(2)((3)Sigma(g)(-))). The low experimental rate constant of 70 M(-1) s(-1) is not incompatible with such a reaction. However, according to our calculations, the reaction of OH(-) with O(3) to form an adduct (OH(-)+O(3)-->HO(4)(-); DeltaG=3.5 kJ mol(-1)) is a much better candidate for the rate-determining step as compared with the significantly more endergonic O transfer (DeltaG=26.7 kJ mol(-1)). Hence, we favor this reaction; all the more so as numerous precedents of similar ozone adduct formation are known in the literature. Three potential decay routes of the adduct HO(4)(-) have been probed: HO(4)(-)-->HO(2)(-)+(1)O(2) is spin allowed, but markedly endergonic (DeltaG=23.2 kJ mol(-1)). HO(4)(-)-->HO(2)(-)+(3)O(2) is spin forbidden (DeltaG=-73.3 kJ mol(-1)). The decay into radicals, HO(4)(-)-->HO(2)*+O(2)(*-), is spin allowed and less endergonic (DeltaG=14.8 kJ mol(-1)) than HO(4)(-)-->HO(2)(-)+(1)O(2). It is thus HO(4)(-)-->HO(2)*+O(2)(*-) by which HO(4)(-) decays. It is noted that a large contribution of the reverse of this reaction, HO(2)*+O(2)(*-)-->HO(4)(-), followed by HO(4)(-)-->HO(2)(-)+(3)O(2), now explains why the measured rate of the bimolecular decay of HO(2)* and O(2)(*-) into HO(2)(-)+O(2) (k=1 x 10(8) M(-1) s(-1)) is below diffusion controlled. Because k for the process HO(4)(-)-->HO(2)*+O(2)(*-) is much larger than k for the reverse of OH(-)+O(3)-->HO(4)(-), the forward reaction OH(-)+O(3)-->HO(4)(-) is practically irreversible.

Significance of the intramolecular transformation of glutathione thiyl radicals to α-aminoalkyl radicals. Thermochemical and biological implications

Product studies have been undertaken on the OH˙ radical-induced oxidation of glutathione in N2O- saturated aqueous solutions. Ammonia has been found to be a prominent product with G values around 2.5–2.9 × 10-7 J mol-1 from pH 6 to 10.5. The ammonia is considered to be a product of the disproportionation reaction of the α-amino carbon-centred radicals, formed via the intramolecular transformation of glutathione thiyl radicals. At pH ca. 4–6, the ammonia yield decreases due to the fact that the transformation reaction slows down with decreasing pH and eventually comes into competition with bimolecular recombination. From the pH dependence of the ammonia yield curve, the equilibrium constant between the glutathione thiyl radical and the α-amino carbon-centred radical is deduced to be >104. The strength of the C–H bond α to the NH2 and CO2- groups is thus <343 kJ mol-1. The corresponding bond energy of the C–H bond α to the NH2 and CO2H groups is estimated to be <329 kJ mol-1. Based on the ammonia formation, consumption of free SH groups and the HPLC chromatograms obtained at different pH values after γ-irradiation of N2O-saturated glutathione solutions, the overall reaction mechanism concerning the fate of glutathione thiyl radicals is proposed. This mechanism and its kinetics indicate that the intramolecular transformation is one of the principal pathways of self-removal of glutathione thiyl radicals, which is formed in various repair processes, in both anaerobic and aerobic conditions.

N–H bond dissociation energies, reduction potentials and pKas of multisubstituted anilines and aniline radical cations

The one-electron reduction potential and the pKa of 10 ortho-, meta- and para-substituted aniline radical cations have been determined by means of pulse radiolysis. The N–H bond dissociation energies of the corresponding anilines were also determined using a thermodynamic cycle. All three properties of the aniline radical cations and the corresponding anilines were shown to be linearly dependent on the sum of the Brown substituent constants, Σσ+. A conditional scale for ortho substituents was also derived (σo+= 0.73σp+). The results from this work along with previously published results have been used to derive a linear free energy relationship between one-electron reduction potentials of benzene radical cations and the substituent pattern. In addition an equation for the calculation of X–Y bond dissociation energies of arbitrarily substituted molecules with the general formula Ph–X–Y is proposed.