Nitrogen is fundamental to all of life and many industrial processes. The interchange of nitrogen oxidation states in the industrial production of ammonia, nitric acid, and other commodity chemicals is largely powered by fossil fuels. A key goal of contemporary research in the field of nitrogen chemistry is to minimize the use of fossil fuels by developing more efficient heterogeneous, homogeneous, photo-, and electrocatalytic processes or by adapting the enzymatic processes underlying the natural nitrogen cycle. These approaches, as well as the challenges involved, are discussed in this Review.

The thermodynamic properties of aqueous nitroxyl (HNO) and its anion (NO ؊ ) have been revised to show that the ground state of NO ؊ is triplet and that HNO in its singlet ground state has much lower acidity, pKa( 1 HNO͞ 3 NO ؊ ) Ϸ 11.4, than previously believed. These conclusions are in accord with the observed large differences between 1 HNO and 3 NO ؊ in their reactivities toward O2 and NO. Laser flash photolysis was used to generate 1 HNO and 3 NO ؊ by photochemical cleavage of trioxodinitrate (Angeli's anion). The spin-allowed addition of 3 O2 to 3 NO ؊ produced peroxynitrite with nearly diffusion-controlled rate (k ‫؍‬ 2.7 ؋ 10 9 M ؊1 ⅐s ؊1 ). In contrast, the spin-forbidden addition of 3 O2 to 1 HNO was not detected (k Ͻ Ͻ 3 ؋ 10 5 M ؊1 ⅐s ؊1 N itroxyl (HNO, also known as nitrosyl hydride) and its anion, NO Ϫ , are the simplest molecules with nitrogen in the ϩ1 oxidation state and yet their aqueous chemistry is not well understood. Recent suggestions that these redox neighbors of the biologically important NO radical may play a role in cellular metabolism (1-4) and in aerobic environments may be precursors to cytotoxic peroxynitrite, ONOO Ϫ , (5, 6) have engendered considerable interest in the chemistry of HNO͞NO Ϫ . The characterization of these species is complicated by their instability with respect to formation of nitrous oxide (7,8). In most cases where nitroxyl has been invoked as an intermediate, the rate-determining step was its generation, a situation that allows little insight into the properties and reactivities of HNO͞NO Ϫ themselves. The NO Ϫ anion is isoelectronic with O 2 and, like O 2 , should have a triplet ground state, whereas the ground state of HNO should be a singlet. Indeed, these ground state assignments have been well established for HNO͞NO Ϫ in the gas phase (9, 10).A frequently used source for aqueous HNO͞NO Ϫ is trioxodinitrate (N 2 O 3 2Ϫ , also known as Angeli's anion), whose conjugate acid (H 2 N 2 O 3 ) has consecutive pKa values of 2.5 and 9.7 (11). It is widely accepted (7, 8) that slow decomposition of the monoprotonated anion occurs through heterolytic NON bond cleavageSubsequent addition of O 2 could yield peroxynitriteHowever, nitrate, which is the peroxynitrite decomposition product, was not detected among the end products of HN 2 O 3 Ϫ decay (12). This result was interpreted as evidence against the occurrence of reaction 2. On the other hand, the same researchers reported peroxynitrite formation during N 2 O 3 2Ϫ photolysis in alkaline solution (13). To reconcile the data, it was suggested that thermal reaction 1 followed by deprotonation of HNO produces singlet NO Ϫ , which is the ground state in water, and that 1 NO Ϫ is unreactive toward O 2 . In contrast, photochemical cleavage of N 2 O 3 2Ϫ was thought to generate the long-lived triplet excited state of NO Ϫ , which reacted with O 2 . However, it seems unlikely that hydration can reverse a gas-phase energy gap of about 70 kJ͞mol between the ground state 3 NO Ϫ and the excited state 1 NO Ϫ (10). Moreover, by ana...

Superoxide ion reacts rapidly with nitric oxide in aqueous solutions to form the peroxonitrite (oxoperoxonitrate( 1 -), ONO2-) ion.* 1 11This anion and its conjugate acid, hydrogen oxoperoxonitrate (ONO2H), are powerful oxidants2 which are reported to rapidly oxidize sulfhydryl groups3 and thioethers4 and to nitrate and hydroxylate aromatic compounds.5-7 In

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 pK a '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.

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Peroxynitrite ion (ONO2-) reacted rapidly with CO2 to form a short-lived intermediate provisionally identified as the ONO2CO2- adduct. This adduct was more reactive in tyrosine oxidation than ONO2- itself and produced 3-nitrotyrosine and 3,3'-dityrosine as the major oxidation products. With tyrosine in excess, the rate of 3-nitrotyrosine formation was independent of the tyrosine concentration and was determined by the rate of formation of the ONO2CO2- adduct. The overall yield of oxidation products was also independent of the concentration of tyrosine and medium acidity; approximately 19% of the added ONO2- was converted to products under all reaction conditions. However, the 3-nitrotyrosine/3,3'-dityrosine product ratio depended upon the pH, tyrosine concentration, and absolute reaction rate. These data are in quantitative agreement with a reaction mechanism in which the one-electron oxidation of tyrosine by ONO2CO2- generates tyrosyl and NO2 radicals as intermediary species, but are inconsistent with mechanisms that invoke direct electrophilic attack on the tyrosine aromatic ring by the adduct. Based upon its reactivity characteristics, ONO2CO2- has a lifetime shorter than 3 ms and a redox potential in excess of 1 V, and oxidizes tyrosine with a bimolecular rate constant greater than 2 x 10(5) M-1 s-1. In comparison, in CO2-free solutions, oxidation of tyrosine by peroxynitrite was much slower and gave significantly lower yields (approximately 8%) of the same products. When tyrosine was the limiting reactant, 3,5-dinitrotyrosine was found among the reaction products of the CO2-catalyzed reaction, but this compound was not detected in the uncatalyzed reaction.

Mucoid, mucA mutant Pseudomonas aeruginosa cause chronic lung infections in cystic fibrosis (CF) patients and are refractory to phagocytosis and antibiotics. Here we show that mucoid bacteria perish during anaerobic exposure to 15 mM nitrite (NO2) at pH 6.5, which mimics CF airway mucus. Killing required a pH lower than 7, implicating formation of nitrous acid (HNO2) and NO, that adds NO equivalents to cellular molecules. Eighty-seven percent of CF isolates possessed mucA mutations and were killed by HNO2 (3-log reduction in 4 days). Furthermore, antibiotic-resistant strains determined were also equally sensitive to HNO2. More importantly, HNO2 killed mucoid bacteria (a) in anaerobic biofilms; (b) in vitro in ultrasupernatants of airway secretions derived from explanted CF patient lungs; and (c) in mouse lungs in vivo in a pH-dependent fashion, with no organisms remaining after daily exposure to HNO2 for 16 days. HNO2 at these levels of acidity and NO2 also had no adverse effects on cultured human airway epithelia in vitro. In summary, selective killing by HNO2 may provide novel insights into the important clinical goal of eradicating mucoid P. aeruginosa from the CF airways.