Excess NO Research Articles

Repression of proinflammatory cytokine and inducible nitric oxide synthase (NOS2) gene expression in activated microglia by N-acetyl-O-methyldopamine: Protein kinase a-dependent mechanism

Excessive proinflammatory cytokine and NO production by activated microglia play a role in neurodegenerative disorders. To investigate whether the neuroprotectant N-acetyl-O-methyldopamine (NAMDA) downregulates genes associated with microglial activation, we measured gene expression of TNF-alpha, IL-1beta, inducible nitric oxide synthase (NOS2), and an associated cofactor synthesis gene, GTP cyclohydrolase I (GTPCH) in LPS-stimulated microglia cells in the presence or absence of NAMDA. The temporal pattern of cytokine gene expression showed that LPS (0.2 microg/ml) increased TNF-alpha and IL-1beta gene expression at 1 and 3 h, which was repressed by cotreatment of NAMDA. Similarly, LPS also induced GTPCH and NOS2 gene expression at 3 and 6 h, and cotreatment of NAMDA repressed the induction with parallel reduction of nitrite, an oxidative metabolite of nitric oxide. Since transcription factor NF-kappaB is involved in regulating expression of these genes, the effects of NAMDA on NF-kappaB nuclear translocation and DNA binding in immunostimulated microglia were investigated. We found that neither LPS-induced NF-kappaB translocation nor DNA binding activity was affected by cotreatment with NAMDA in BV-2 microglia. On the other hand, NAMDA increased intracellular cAMP levels and potentiated LPS-induced phosphorylated cAMP-responsive element binding protein (pCREB) expression. Treatment with adenosine 3'5'-cyclic monophosphothioate, a specific inhibitor of cAMP-dependent protein kinase (PKA), reversed not only NAMDA-induced pCREB upregulation but also NAMDA-induced repression of TNF-alpha and IL-1beta gene transcription. The data demonstrate that NAMDA represses LPS-induced proinflammatory cytokines gene expression via a cAMP-dependent protein kinase pathway. Thus, repressing proinflammatory cytokines and NOS2 gene expression in activated microglia by NAMDA may provide new therapeutic strategies for ischemic cerebral disease as well as other neurodegenerative diseases.

Photodissociation of Nitric Oxide from Nitrosyl Metalloporphyrins in Micellar Solutions

Laser-photolysis studies of nitrosyl metalloporphyrins (MP), (NO)FeIIHem (Hem = protoporphyrin IX), (NO)CoIIOEP (OEP = octaethylporphyrin), and (NO)MnIITPP (TPP = tetraphenylporphyrin), in aqueous ionic micellar solutions were carried out. The nitrosyl porphyrins in the micellar solutions readily photodissociate NO, leaving the MP in micelles: the quantum yields are 0.15 (±0.1) for (NO)FeIIHem, 0.55 (±0.05) for (NO)CoIIOEP, and 0.21 (±0.02) for (NO)MnIITPP. The MP thus produced recombine with NO to regenerate the parent nitrosyl porphyrins. The decay of MP in the absence of the excess NO follows second-order kinetics with the rate constant kMNO(second) (MNO = FeNO, CoNO, and MnNO). In the presence of excess NO, the decay of MP follows pseudo-first-order kinetics with the rate constant kM (M = Fe, Co, and Mn). The value of kM was measured as a function of [NO]. For FeIIHem and CoIIOEP, the plots of kCo vs [NO] and kFe vs [NO] gave straight lines. The slopes of the lines obtained with FeIIHem and CoIIOEP afford the bimolecular rate constants kFeNO(pseudo) and kCoNO(pseudo), respectively. It is found that kFeNO(pseudo) > kFeNO(second) and kCoNO(pseudo) > kCoNO(second). The differences between kMNO(pseudo) and kMNO(second) observed for FeIIHem and CoIIOEP are interpreted by assuming that (1) NO molecules in the micellar solutions are dissolved in both micelles and the aqueous phase and (2) NO molecules trapped in micelles hardly react with MP because of the electrostatic repulsion between ionic micelles. In the case of MnIITPP, the pseudo-first-order rate constant, kMn, is found to asymptotically increase with an increase in [NO] to a limiting value. The reaction mechanisms for the nitrosylation of MP in micellar solutions are discussed in detail on the basis of the kinetic studies.

Role of nitric oxide and superoxide in acute cardiac allograft rejection in rats.

The role of NO and superoxide (O(2)(-)) in tissue injury during cardiac allograft rejection was investigated by using a rat ex vivo organ perfusion system. Excessive NO production and inducible NO synthase (iNOS) expression were observed in cardiac allografts at 5 days after cardiac transplantation, but not in cardiac isografts, as identified by electron spin resonance spectroscopy and Northern blotting. Cardiac isografts or allografts obtained on Day 5 after transplantation were perfused with Krebs bicarbonate buffer with or without various antidotes for NO or O(2)-, including N(omega)-monomethyl-L-arginine (L-NMMA; 1 mM), 2-phenyl-4,4,5, 5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO; 100 microM), 4-amino-6-hydroxypyrazolo[3,4-d]pyrimidine (AHPP; a xanthine oxidase inhibitor; 100 microM), and superoxide dismutase (SOD; 100 units/ml). Treatment of the cardiac allografts with PTIO showed most remarkable improvement of the cardiac injury as revealed by significant reduction in aspartate transaminase, lactate dehydrogenase, and creatine phosphokinase concentrations in the perfusate. Similar but less potent protective effect on the allograft injury was observed by treatment with L-NMMA, AHPP, and SOD. Immunohistochemical analyses for iNOS and nitrotyrosine indicated that iNOS is mainly expressed by macrophages infiltrating the allograft tissues, and nitrotyrosine formation was demonstrated not only in macrophages but also in cardiac myocytes of the allografts, providing indirect evidence for the generation of peroxynitrite during allograft rejection. Our results suggest that tissue injury in rat cardiac allografts during acute rejection is mediated by both NO and O(2)(-), possibly through peroxynitrite formation.

Oxidation of a 4-substituted chiral oxazoline using MCPBA and NO2

Oxidation of the chiral oxazoline 1 using 1.6 equivalents of MCPBA gives mainly the corresponding ring-opened nitroso compound 4 isolated as its dimer 5 together with an E/Z mixture of the isomeric oximes 2 and 3. Upon treatment of 1 with an excess of NO2 the product is the alkyl nitrate 23.

Pyridoxalated hemoglobin polyoxyethylene: a nitric oxide scavenger with antioxidant activity for the treatment of nitric oxide-induced shock

Hemoglobins modified for therapeutic use as either hemoglobin-based oxygen carriers or scavengers of nitric oxide are currently being evaluated in clinical trials. One such product, pyridoxalated hemoglobin polyoxyethylene conjugate (PHP), is a human-derived and chemically modified hemoglobin that has yielded promising results in Phase II clinical trials, and is entering a pivotal Phase III clinical trial for the treatment of shock associated with systemic inflammatory response syndrome (SIRS). Shock associated with SIRS is a NO-induced shock. PHP, a new mechanism-based therapy, has been demonstrated in clinical trials to have the expected hemodynamic activity of raising blood pressure and reducing catecholamine use, consistent with its mechanism of action as a NO scavenger. PHP is conjugated with polyoxyethylene, which results in a surface-decorated molecule with enhanced circulation time and stability as well as in attachment of soluble red blood cell enzymes, including catalase and superoxide dismutase. PHP thus contains an antioxidant profile similar to the intact red blood cell and is therefore resistant to both initial oxidative modification by oxidants such as hydrogen peroxide and subsequent ferrylhemoglobin formation. These studies suggest both that the redox activity of modified hemoglobins can be attenuated and that modified hemoglobins containing endogenous antioxidants, such as PHP, may have reduced pro-oxidant potential. These antioxidant properties, in addition to the NO-scavenging properties, may allow the use of PHP in other indications in which excess NO, superoxide, or hydrogen peroxide is involved, including ischemia-reperfusion injury and hemorrhagic shock.

Reactivity of the iron porphyrin Fe(TPP)(NO) with excess NO. Formation of Fe(TPP)(NO)(NO2) occurs via reaction with trace NO2.

Fe(TPP)(NO) (TPP = tetraphenylporphyrinato dianion) does not react observably with excess NO in organic solvents at ambient pressure and temperature; however, adventitious NO2 rapidly leads to the nitrosyl nitro product Fe(TPP)(NO)(NO2), which is labile toward loss of NO.

A non-nucleotide–bridged DNA decoy inhibits renal epithelial nitric oxide synthase expression

A non-nucleotide-bridged DNA decoy inhibits renal epithelial nitric oxide synthase expression. The expression of inducible nitric oxide synthase (iNOS) is subject to strict tissue-specific transcriptional control. In mouse renal epithelium, an interferon-gamma (IFN-gamma)-induced signaling protein, IFN-gamma regulatory factor 1 (IRF-1), appears to mediate the induction of iNOS expression by cytokines and bacterial lipopolysaccharide (LPS). We used a novel technique, namely, blockade of cytosolic IRF-1 activity with a triethyleneglycol-bridged decoy DNA oligonucleotide (ODN) containing the IRF-1 consensus binding sequences present in the iNOS promoter to inhibit iNOS gene expression. Cultured mouse renal epithelial cells were treated with a combination of LPS (1 microg/mL) and IFN (100 U/mL) in the absence or presence of IRF-1 decoy ODN followed by determinations of NO production and iNOS protein and mRNA expression. Treatment with IRF-1 decoy ODN resulted in concentration-dependent inhibition of NO production and a marked reduction in iNOS protein and mRNA levels. A scrambled ODN failed to affect LPS/IFN-stimulated NO production or iNOS protein and mRNA levels. Transcriptional assays showed that the IRF-1 decoy ODN inhibited transcriptional activity of an iNOS promoter-CAT gene construct. Decoy ODN-based techniques effectively inhibit iNOS expression in renal epithelium and represent a potentially useful approach for selective blockade of this enzyme in pathologic conditions associated with excessive NO production.

Estimation of Nitrate Contamination of an Agro‐Ecosystem Outwash Aquifer Using a Nitrogen Mass‐Balance Budget

AbstractA mass‐balance budget of N cycling was developed for an intensive agricultural area in west‐central Minnesota to better understand NO−3 contamination of ground water in the Otter Tail outwash aquifer. Fertilizer, biological fixation, atmospheric deposition, and animal feed were the N sources, and crop harvests, animal product exports, volatilization from fertilizer and manure, and denitrification were the N sinks in the model. Excess N, calculated as the difference between the sources and sinks, was assumed to leach to ground water as NO−3. The budget was developed using ground water data collected throughout the 212‐km2 study area. Denitrification was estimated by adjusting its value so the predicted and measured concentrations of NO3 in ground water agreed. Although biological fixation was the largest single N source, most was removed when crops were harvested, indicating that inorganic fertilizer was the primary source of N reaching the water table. It was estimated that denitrification removed almost half of the excess NO−3 that leached below the root zone. Even after accounting for denitrification losses, however, it was concluded that the ground water system was receiving approximately three times as much N as would be expected under background conditions.

NO Disproportionation Reactivity of Fe Tropocoronand Complexes

The synthesis and characterization of divalent [Fe(TC-5,5)] (1) and trivalent [Fe(OTf)(TC-5,5)] (2) tropocoronand complexes are described. Compound 1 reacts with 1 equiv of NO to form the {FeNO}7 complex 3. A single-crystal X-ray structure determination of 3 reveals a trigonal bipyramidal geometry with a linearly coordinated nitrosyl (Fe−N−O = 174.3(4)°) having a short Fe−N distance of 1.670(4) A. EPR and Mossbauer spectroscopy, SQUID susceptometry, and normal coordinate analysis indicate 3 to be a low-spin {FeIII(NO-)}2+ species. In the presence of excess NO, 3 converts to a metastable nitrosyl−nitrito complex that decomposes by losing NO2, which subsequently nitrates the aromatic tropolone rings of the ligand. The final products of the NO disproportionation reaction are N2O and [Fe(NO)(TC-5,5-NO2)] (4). The νNO stretching band of 4 is increased to 1716 cm-1 from its value of 1692 cm-1 in 3, owing to the electron-withdrawing nitro groups on the ligand, and the compound no longer promotes the disproportio...

NITROGEN UTILIZATION AND METABOLISM RELATIVE TO PATTERNS OF N2 FIXATION IN CULTURES OF TRICHODESMIUM NIBB1067

We measured uptake kinetics for four combined N sources, ambient rates of N uptake and N2 fixation, glutamine synthetase activity (transferase and biosynthetic), and concentrations of intracellular pools of glutamate (glu) and glutamine (gln) in cultures of Trichodesmium NIBB1067. N dynamics and metabolism were examined to assess the relative importance of N2 fixation and N uptake to Trichodesmium nutrition. Comparisons were made between cultures grown on medium without added N, with excess NO, or with excess urea. Of the combined N sources tested, Trichodesmium NIBB1067 had the highest affinity for NH; high uptake capacities for NH, urea, and glu; and little capacity for NO uptake. In cultures grown on medium without added N, NH accumulated in the medium during growth, resulting in high NH uptake rates relative to rates of N2 fixation. Glu uptake rates were low but consistent throughout the diel period. In cultures grown on excess NO or urea, uptake of these compounds supplied the majority of the daily N demand, although some N2 fixation occurred during the light period. NO uptake rates were reduced when N2‐fixation rates were high. In all of the cultures, the highest gln/glu ratios and the lowest glutamine synthetase transferase/biosynthetic ratios were observed during the period when rates of total N uptake were highest. In cultures growing exponentially on medium without added N, N2 fixation accounted for 14%– 16% of the total daily N uptake. Uptake of NH and glu, presumably regenerated within the culture vessels, represented 84%–86% of the daily N uptake. Because these systems were closed, net growth was constrained by the rate at which N2 could be fixed into the system. However, total daily N turnover was greater than that necessary to accommodate the observed increase in culture biomass. The rapid N turnover rates observed in these cultures may support gross productivity and balance the high rates of C fixation observed in natural populations of Trichodesmium.

Temperature Dependence of the Product Branching Ratio of the CN + O2Reaction

The reaction of CN radicals with O2 was studied using infrared diode laser absorption spectroscopy. CO and CO2 products were detected directly, while the yield of NCO products was inferred by measuring the N2O yield upon addition of excess NO. Experiments and kinetic modeling calculations were performed to examine the extent of secondary chemistry in this system. The following branching ratios of the CN + O2 reaction at 296 K were determined: φ(N+CO2) = 0.02 ± 0.01 and φ(CO+NO) = 0.22 ± 0.02. The branching ratio into the CO + NO channel has a strong negative temperature dependence over the range 239−643 K.

A study of the reactions of Fe and FeO with NO2, and the structure and bond energy of FeO2

The reaction Fe+NO2→FeO+NO was studied by the pulsed two-photon dissociation at 248 nm of ferrocene vapour to produce Fe atoms in an excess of NO2 and He bath gas. Atomic Fe was then monitored by time-resolved laser-induced fluorescence spectroscopy at 248.3 nm [Fe(x5F05–a5D4)], yielding k(192<T/K<471)=(4.84-1.10+1.24)×10-10 exp[-(2.60±0.56) kJ mol-1/RT] cm3 molecule-1 s-1, where the quoted uncertainties are a combination of the 2σ standard errors in the kinetic data and the systematic experimental errors. For the reaction FeO+NO2→FeO2+NO, FeO was formed from the pulsed photolysis of a ferrocene–NO2 mixture in N2 bath gas, and then monitored by time-resolved LIF in the Orange system at 582.0 nm [FeO(D5Δ4–X5Δ4)]. This yielded k(196<T/K<527)=(5.04-1.08+1.20)10-10 exp[-(3.64±0.58) kJ mol-1/RT] cm3 molecule-1 s-1. The preexponential factors of both reactions are shown to be in good accord with the collision frequencies calculated by applying the orbiting criterion on their respective long-range potentials. The small activation energy of FeO+NO2 implies that the bond energy D0(Fe–O2)>198 kJ mol-1. Abinitio calculations on FeO2, performed with hybrid density functional/Hartree–Fock theory, show that the isomeric forms of the molecule that are stable enough to be produced in this reaction are the triplet and quintet states of dioxo (inserted) FeO2, with D0(Fe–O2)=284±38 kJ mol-1.

Nitric oxide and peroxynitrite. The ugly, the uglier and the not so good

Nitric oxide, a gaseous free radical, is poorly reactive with most biomolecules but highly reactive with other free radicals. Its ability to scavenge peroxyl and other damaging radicals may make it an important antioxidant in vivo, particular in the cardiovascular system, although this ability has been somewhat eclipsed in the literature by a focus on the toxicity of peroxynitrite, generated by reaction of O·-2 with NO· (or of NO- with O2). On balance, experimental and theoretical data support the view that ONOO- can lead to hydroxyl radical (OH·) generation at pH 7.4, but it seems unlikely that OH· contributes much to the cytotoxicity of ONOO-. The cytotoxicity of ONOO- may have been over-emphasized: its formation and rapid reaction with antioxidants may provide a mechanism of using NO· to dispose of excess O·-2, or even of using O·-2 to dispose of excess NO·, in order to maintain the correct balance between these radicals in vivo. Injection or instillation of “bolus” ONOO- into animals has produced tissue injury, however, although more experiments generating ONOO- at steady rates in vivo are required. The presence of 3-nitrotyrosine in tissues is still frequently taken as evidence of ONOO- generation in vivo, but abundant evidence now exists to support the view that it is a biomarker of several “reactive nitrogen species”. Another under-addressed problem is the reliability of assays used to detect and measure 3-nitrotyrosine in tissues and body fluids: immunostaining results vary between laboratories and simple HPLC methods are susceptible to artefacts. Exposure of biological material to low pH (e.g. during acidic hydrolysis to liberate nitrotyrosine from proteins) or to H2O2 might cause artefactual generation of nitrotyrosine from NO-2 in the samples. This may be the origin of some of the very large values for tissue nitrotyrosine levels quoted in the literature. Nitrous acid causes not only tyrosine nitration but also DNA base deamination at low pH: these events are relevant to the human stomach since saliva and many foods are rich in nitrite. Several plant phenolics inhibit nitration and deamination in vitro, an effect that could conceivably contribute to their protective effects against gastric cancer development.

Kinetic study of the reactions of CF3O2 radicals with Cl and NO

Kinetic studies of the reactions CF3O2+Cl and CF3O2+NO were performed at room temperature in the gas phase using the discharge flow mass spectrometric technique (DFMS). The reactions were investigated under pseudo-first-order conditions with Cl or NO in large excess with respect to the CF3O2 radicals. The rate constant for the reaction CF3O2+NO was measured at 298 K and the value of (1.6±0.3)×10-11 cm3 molecule-1 s-1 is in very good agreement with all previous values. For the reaction CF3O2+Cl, we obtain a rate constant at 298 K of (4.2±0.8)×10-11 cm3 molecule-1 s-1 in excellent agreement with the only published value. Product analysis shows that this reaction occurs via the major reaction pathway CF3O2+Cl→CF3O+ClO at room temperature. In addition, an abinitio theoretical study was performed to gain insights on the different postulated reaction pathways. There is a significant disagreement between experimental and abinitio values recommended for the formation enthalpies of CF2O, CF3O and related molecules produced in this system. Consequently, we provide self-consistent values of enthalpies based on isodesmic reactions for the CF3O2+Cl reaction system using the G2, G2(MP2) and CBS-Q methods. These values are also compared with BAC-MP4 heats of formation calculated in this work.

Reaction of Stabilised Phosphorus Ylides with Nitrogen Dioxide

The reaction of the stabilised ylides 14–23 with an excess of NO2 in CH2Cl2 at room temperature gives different results depending on the structure of the starting ylide. The monacyl ylides 14–16 give the corresponding α-oxo nitriles 4 together with Ph3PO · HNO3 (24) which has been fully characterised for the first time. Under the same conditions, the ylide 18 gives 2,4-dinitrobenzonitrile (26), Ph3PO, and benzoic acid. The other ylides examined all give 24 together with a variety of other products.

Nitric Oxide Binding to the Ferri- and Ferroheme States of Nitrophorin 1, a Reversible NO-Binding Heme Protein from the Saliva of the Blood-Sucking Insect, Rhodnius prolixus

The recombinant NO-binding heme protein, nitrophorin 1 (NP1) from the saliva of the blood-sucking insect, Rhodnius prolixus, has been studied by spectroelectrochemistry, EPR, NMR, and FTIR spectroscopies and X-ray crystallography. It is found that NP1 readily binds NO in solution and in the crystalline state, but the protein is not readily autoreduced by excess NO. Likewise, dithionite is not a very effective reductant of NP1. However, the protein can be photoreduced by illumination with visible light in the presence of excess NO, deazaflavin, and EDTA. Optical spectra of the FeIIINO and FeIINO complexes of NP1 are extremely similar, which makes it difficult to characterize the oxidation state of the NO complex by UV−visible spectroscopy. The reduction potential of NP1 in the absence of NO is ∼300 mV more negative than that of metmyoglobin (metMb). In the presence of NO, the reduction potential shifts ∼+430 mV for NP1−NO, but the reduction potential of metMb−NO cannot be measured for comparison. Based on estimated values of Kd for NP1III−NO, the Kd values for the FeII−NO complex are 20.8 and 80.6 fM at pH 5.5 and 7.5, respectively. The lower driving force for NP1 reduction is qualitatively consistent with the slower rate of autoreduction of NP1−NO; the negative charges surrounding the heme probably also play a role in determining the much slower rate of autoreduction. The N−O stretching frequencies of NP1III−NO and NP1II−NO were measured by FTIR spectroscopy. The values obtained are very typical of other heme−NO stretching frequencies in the two oxidation states: νNO = 1917 and 1904 cm-1 for two species of FeIIINO and 1611 cm-1 for FeIINO; the values of νNO are consistent with 6-coordinate “base-on” heme−NO centers for both oxidation states. The breadths of the IR bands are consistent with the large solvent accessibility of the bound NO of NP1 and also with the possibility of minor dissociation of the protein-provided histidine ligand on the IR time scale. The ratio of the two FeIII−NO species changes with pH and the nature of the buffer. The CO complex of the Fe(II) form of NP1 has νCO = 1960 and 1936 cm-1, again showing the presence of two species. Both NMR and X-ray crystallography show that the protohemin center of NP1 imidazole has a very high preference for a single orientation of the unsymmetrical protoheme moiety. The structure shows the Fe−N−O unit to be quite bent, which is consistent with its being the FeII−NO form of the protein, presumably formed by photoreduction in the X-ray beam. The proximal base, His-59, is clearly coordinated to the iron in the crystalline state and in solution at ambient temperatures, based on FTIR data, but EPR studies of dithionite-reduced samples show that a percentage of the protein has lost the histidine ligand from the FeIINO center in frozen solution.

Prostaglandins prevent inducible nitric oxide synthase protein expression by inhibiting nuclear factor-κB activation in J774 macrophages

We investigated the effect of PGE 2 and iloprost (a prostacyclin analogue) on inducible nitric oxide synthase (iNOS) protein expression and nuclear factor-κB (NF-κB) activation in lipopolysaccharide (LPS)-stimulated J774 macrophages. Incubation of J774 cells with LPS (10 μg/ml) caused an increase of iNOS protein expression which was prevented in a concentration-dependent fashion by PGE 2 (0.1, 1, 10 μM) and iloprost (0.01, 0.1, 1 μM). Electrophoretic mobility shift assay indicated that both prostanoids blocked the activation of NF-κB, a transcription factor necessary for NO synthase induction. PGE 2 and iloprost also blocked disappearance of IκB-α from cytosolic fraction and nuclear translocation of NF-κB subunits p50 and p65. These results show for the first time that PGE 2 and iloprost down-regulate iNOS protein expression by inhibiting NF-κB activation and suggest a negative feed-back mechanism that may be important for limiting excessive or prolonged NO production in pathological events.

Influences of Ligand Environment on the Spectroscopic Properties and Disproportionation Reactivity of Copper−Nitrosyl Complexes

In studies of the chemistry of new copper−nitrosyl complexes supported by tris(3-(trifluoromethyl)-5-methylpyrazol-1-yl)hydroborate (TpCF3,CH3) and tris(3-mesitylpyrazol-1-yl)hydroborate (TpMs,H), significant effects of the scorpionate ligand substituents on the properties of the {CuNO}11 unit were found that have implications for environmental influences on similar species in biological and catalytic systems. The copper(I) complexes TpMs,HCu(THF) and TpCF3,CH3Cu(CH3CN) were structurally characterized by X-ray crystallography, and their respective CO and NO adducts were studied by FTIR, EPR, NMR, and/or UV−vis spectroscopies in solution. Both nitrosyl complexes disproportionate in the presence of excess NO to N2O and TpR,R‘Cu(NO2); an X-ray structure of the latter product supported by TpCF3,CH3 was determined. Unlike previously studied paramagnetic [CuNO]11 compounds that exhibit EPR signals with g < 2.0 and large ANO values at temperatures below ∼40 K (Ruggiero, C. E.; Carrier, S. M.; Antholine, W. E.; Whittaker, J. W.; Cramer, C. J.; Tolman, W. B. J. Am. Chem. Soc. 1993, 115, 11285−11298), TpMs,HCu(NO) is EPR silent at 4.2 K and exhibits an NMR spectrum (238 K, toluene-d8) with sharp signals. Peak assignments for the NMR spectrum were deduced from integrated intensities, temperature-dependent isotropic shifts, and the nuclear relaxation rates. The unique NMR spectral behavior for the TpMs,H complex, which only differs from those of analogues with simple phenyl substituents by virtue of the shape of the substrate binding pocket enforced by the mesityl methyl groups, suggests that caution should be exercised in characterizing such adducts in proteins and heterogeneous systems; subtle environmental effects may determine the applicability of EPR versus NMR methods. The electron-withdrawing effects of the trifluoromethyl substituents in TpCF3,CH3Cu(NO) perturb ν(NO) and the Cu(I) → NO MLCT energy in the respective FTIR and UV−vis spectra and induce a significant slowing of its disproportionation rate. These results, in conjunction with those obtained from kinetic and spectroscopic studies on the TpMs,H system, support a mechanism for the disproportionation involving generation of the CuNO adduct from NO and the Cu(I) precursor in a preequilibrium step, followed by electrophilic attack of a second NO molecule on the adduct that is rate-controlling.

Kinetics of the Gas-Phase Reaction of BrNO2 with NO

BrNO2 was prepared in situ in a static reactor (v = 420 L) by photolyzing Br2/NO2/N2 mixtures in the wavelength range 500−700 nm at temperatures between 263 and 294 K. After the lights were switched off, the excess NO was added, and IR and UV spectra were monitored simultaneously as a function of time. From the pseudo-first-order decay of the IR absorption of BrNO2 in the presence of a large excess of NO, the second-order rate constant for reaction 4, BrNO2 + NO ⇒ BrNO + NO2, was determined to be k4 = 2.3 × 10-12 exp[(−17.8 ± 2.1) kJ mol-1/RT] cm3molecule-1s-1 (2σ). The measured yields of BrNO were close to 100% (98 ± 5%). These results suggest that reaction 4 is unimportant as a loss process of BrNO2 under most tropospheric conditions. Additional experiments on the thermal stability of BrNO2 led to an upper limit of 4.0 × 10-4 s-1 for its thermal gas-phase decomposition rate constant at 298 K in 1 atm of synthetic air. Finally, the mechanism of the Br + NO2 reaction and the thermochemistry of BrNO2 and BrONO are discussed in light of the results of the present experiments and of previous work from the literature.

98/03972 Experience with combined coal/gas low NO x burners in utility boiler retrofit applications

While recently substantial progress has been achieved in predicting properties of high-NOx (type 2) flames, the task of computing low-NOx flames (type 1) happens to be much more complex. This is due to difficulties in predicting the complex near-burner-zone flow field of type 1 flames which can be characterized by a nonswirling fuel jet penetrating either partially or fully through the swirl-induced reverse flow zone. The objective of this paper is to examine what improvements to the predicted near-burner-zone flow field, chemistry, and subsequent NOx emissions can be made by applying the Reynolds stress (transport) model and the renormalization group theory k-ε model rather than the standard k-ε model. To this end, the 2-MW, flame data collected at the IFRF have been utilized.The flame length and volume, the near-burner-zone temperatures, and oxygen concentrations are directly affected by the jet penetration. Whereas the RSM predictions are in good agreement with the measurements, both k-ε and RNG-k-ε models predict too short and too intensive flames. The physical size of the fuel-rich region and subsequent NOx predictions are strongly dependent on the calculated jet penetration. The failures in predicting properties of the fuel-rich zone while using the k-ε or the RNG-k-ε models results in excessive NO flue concentrations. Thus, contrary to the findings of Lockwood and Shen [12], we see a superiority of the RSM for predicting type I, low-NOx flames.