Copper(I) Complex Mediated Nitric Oxide Reductive Coupling: Ligand Hydrogen Bonding Derived Proton Transfer Promotes N2O(g) Release.

A cuprous chelate bearing a secondary sphere hydrogen bonding functionality, [(PV-tmpa)CuI]+, transforms •NO(g) to N2O(g) in high-yields in methanol. Ligand derived proton transfer facilitates N-O bond cleavage of a putative hyponitrite intermediate releasing N2O(g), underscoring the crucial balance between H-bonding capabilities and acidities in (bio)chemical •NO(g) coupling systems.

Non-Heme Diiron Model Complexes Can Mediate Direct NO Reduction: Mechanistic Insight into Flavodiiron NO Reductases.

Flavodiiron nitric oxide reductases (FNORs), a common enzyme family found in various types of pathogenic bacteria, are capable of reducing nitric oxide (NO) to nitrous oxide (N2O) as a protective detoxification mechanism. Utilization of FNORs in pathogenic bacteria helps them survive and proliferate in the human body, thus causing chronic infections. In this paper, we present a new diiron model complex, [Fe2((Py2PhO2)MP)(OPr)2](OTf), with bridging propionate ligands (OPr-) that is capable of directly reducing NO to N2O in quantitative yield without the need to (super)reduce the complex. We first prepared the diferric precursor and characterized it by UV-vis, IR, NMR and Mössbauer spectroscopies, cyclic voltammetry, and mass spectrometry. This complex can then conveniently be reduced to the diferrous complex using CoCp2. Even though this diferrous complex is highly reactive, we have successfully isolated and characterized this species using X-ray crystallography and various spectroscopic techniques. Most importantly, upon reacting this diferrous complex with NO gas, we observe quantitative formation of N2O via IR gas headspace analysis, the first demonstration of direct NO reduction by a non-heme diiron model complex. This finding directly supports recent mechanistic proposals for FNORs.

Lewis Acid Activation of the Ferrous Heme-NO Fragment toward the N-N Coupling Reaction with NO To Generate N2O.

Bacterial NO reductase (bacNOR) enzymes utilize a heme/non-heme active site to couple two NO molecules to N2O. We show that BF3 coordination to the nitrosyl O-atom in (OEP)Fe(NO) activates it toward N-N bond formation with NO to generate N2O. 15N-isotopic labeling reveals a reversible nitrosyl exchange reaction and follow-up N-O bond cleavage in the N2O formation step. Other Lewis acids (B(C6F5)3 and K+) also promote the NO coupling reaction with (OEP)Fe(NO). These results, complemented by DFT calculations, provide experimental support for the cis: b3 pathway in bacNOR.

Nitric Oxide Reductase Activity in Heme-Nonheme Binuclear Engineered Myoglobins through a One-Electron Reduction Cycle.

FeBMbs are structural and functional models of native bacterial nitric oxide reductases (NORs) generated through engineering of myoglobin. These biosynthetic models replicate the heme-nonheme diiron site of NORs and allow substitutions of metal centers and heme cofactors. Here, we provide evidence for multiple NOR turnover in monoformyl-heme-containing FeBMb1 proteins loaded with FeII, CoII, or ZnII metal ions at the FeB site (FeII/CoII/ZnII-FeBMb1(MF-heme)). FTIR detection of the ν(NNO) band of N2O at 2231 cm-1 provides a direct quantitative measurement of the product in solution. A maximum number of turnover is observed with FeII-FeBMb1(MF-heme), but the NOR activity is retained when the FeB site is loaded with ZnII. These data support the viability of a one-electron semireduced pathway for the reduction of NO at binuclear centers in reducing conditions.

Mechanism of N-N Bond Formation by Transition Metal-Nitrosyl Complexes: Modeling Flavodiiron Nitric Oxide Reductases.

Nitric oxide (NO) has a number of important biological functions, including nerve signaling transduction, blood pressure control, and, at higher concentrations, immune defense. A number of pathogenic bacteria have developed methods of degrading this toxic molecule through the use of flavodiiron nitric oxide reductases (FNORs), which utilize a nonheme diiron active site to reduce NO → N2O. The well-characterized diiron model complex [Fe2(BPMP)(OPr)(NO)2]2+ (BPMP- = 2,6-bis[(bis(2- pyridylmethyl)amino)methyl]-4-methylphenolate), which mimics both the active site structure and reactivity of these enzymes, offers key insight into the mechanism of FNORs. Presently, we have used computational methods to elucidate a coherent reaction mechanism that shows how one and two-electron reduction of this complex induces N-N bond formation and N2O generation, while the parent complex remains stable. The initial formation of a N-N bond to generate hyponitrite (N2O22-) follows a radical-type coupling mechanism, which requires strong Fe-NO π-interactions to be overcome to effectively oxidize the iron centers. Hyponitrite formation provides the largest activation barrier with Δ G‡ = 7-8 kcal/mol (average of several functionals) in the two-electron, super-reduced mechanism. This is followed by the formation of a N2O22- complex with a novel binding mode for nonheme diiron systems, allowing for the facile release of N2O with the assistance of a carboxylate shift. This provides sufficient thermodynamic driving force for the reaction to proceed via N2O formation alone. Surprisingly, the one-electron "semireduced" mechanism is predicted to be competitive with the super-reduced mechanism. This is due to the asymmetry imparted by the BPMP- ligand, allowing a one-electron reduction to overcome one of the primary Fe-NO π-interactions. Generally, mediation of N2O formation by high-spin [{M-NO-}]2 cores depends on the ease of oxidizing the M centers and breaking of the M-NO π-bonds to formally generate a "full" 3NO- unit, allowing for the critical step of N-N bond formation to proceed (via a radical-type coupling mechanism).

Frozen in Time: A History of the Synthesis of Nitrous Oxide and How the Process Remained Unchanged for Over 2 Centuries.

Three major factors have contributed to the unrivaled popularity of nitrous oxide (N2O) among anesthetists in the 20th century and beyond: its impressive safety profile, its affordability, and its rapid induction and emergence times. These 3 characteristics of N2O have been discussed and written about extensively throughout the medical literature. Nonetheless, the characteristic that contributed most to N2O's initial discovery-the elegance and simplicity of its synthesis-has received substantially less attention. Although N2O was first used as an anesthetic in Hartford, CT, in 1844, it had been identified and synthesized as a distinct gas in the late 18th century. In this article, we track the developments in the recognition and early synthesis of N2O, highlight the major players credited with its discovery, and examine its evolution from the late 1700s to today. The discovery and assimilation of N2O into common medical practice, alongside ether and chloroform, heralded a new paradigm in surgical medicine-one that no longer viewed pain as a fundamental component of surgical medicine. Its continued usage in modern medicine speaks to the brilliance and skill of the chemists and scientists involved in its initial discovery.

A Nonheme, High-Spin {FeNO}8 Complex that Spontaneously Generates N2O.

One-electron reduction of [Fe(NO)-(N3PyS)]BF4 (1) leads to the production of the metastable nonheme {FeNO}8 complex, [Fe(NO)(N3PyS)] (3). Complex 3 is a rare example of a high-spin (S = 1) {FeNO}8 and is the first example, to our knowledge, of a mononuclear nonheme {FeNO}8 species that generates N2O. A second, novel route to 3 involves addition of Piloty's acid, an HNO donor, to an FeII precursor. This work provides possible new insights regarding the mechanism of nitric oxide reductases.

New Insights into the N2O formation mechanism over Pt-BaO/Al2O3 model catalysts using H2 as a reductant.

The N2O formation mechanism was investigated over a Pt-BaO/Al2O3 catalyst applied on light-duty diesel vehicles using H2 as a reductant in the absence and presence of H2O. In the absence of H2O, N2O forms mainly at the initial phase of lean NOx trapping; while in the presence of H2O, N2O appears mainly at the beginning of the rich reduction phase. In the lean period, N2O is formed via the gaseous NO/O2 reacting with the adsorbed H and NH3 that are formed during the previous rich period. The N2O formation in the rich period is insignificant in the absence of H2O but is greatly enhanced by the presence of H2O. The amount of N2O formed is proportional to the H2O level in the feed, and its formation is favored at low temperatures. Our FTIR data show that H2O enhances the rate of nitrite/nitrate reduction during the rich regeneration, which increases the amount of released NOx, an oxygen source for N2O formation. Our temperature-programmed experiments indicate that H2O competes with NH3 for adsorption sites on Pt surface. This competitive adsorption may increase the NH3 desorption rate at low temperatures in the rich phase and make Pt surface more accessible to NO.

N2O production in the Fe(II)(EDTA)-NO reduction process: the effects of carbon source and pH.

Chemical absorption-biological reduction (BioDeNOx), which uses Fe(II)(EDTA) as a complexing agent for promoting the mass transfer efficiency of NO from gas to water, is a promising technology for removing nitric oxide (NO) from flue gases. The carbon source and pH are important parameters for Fe(II)(EDTA)-NO (the production of absorption) reduction and N2O emissions from BioDeNOx systems. Batch tests were performed to evaluate the effects of four different carbon sources (i.e., methanol, ethanol, sodium acetate, and glucose) on Fe(II)(EDTA)-NO reduction and N2O emissions at an initial pH of 7.2 ± 0.2. The removal efficiency of Fe(II)(EDTA)-NO was 93.9%, with a theoretical rate of 0.77 mmol L(-1) h(-1) after 24 h of operation. The highest N2O production was 0.025 mmol L(-1) after 3 h when glucose was used as the carbon source. The capacities of the carbon sources to enhance the activity of the Fe(II)(EDTA)-NO reductase enzyme decreased in the following order based on the C/N ratio: glucose > ethanol > sodium acetate > methanol. Over the investigated pH range of 5.5-8.5, the Fe(II)(EDTA)-NO removal efficiency was highest at a pH of 7.5, with a theoretical rate of 0.88 mmol L(-1) h(-1). However, the N2O production was lowest at a pH of 8.5. The primary effect of pH on denitrification resulted from the inhibition of nosZ in acidic conditions.

Light-induced N2O production from a non-heme iron-nitrosyl dimer.

Two non-heme iron-nitrosyl species, [Fe2(N-Et-HPTB)(O2CPh)(NO)2](BF4)2 (1a) and [Fe2(N-Et-HPTB)(DMF)2(NO)(OH)](BF4)3 (2a), are characterized by FTIR and resonance Raman spectroscopy. Binding of NO is reversible in both complexes, which are prone to NO photolysis under visible light illumination. Photoproduction of N2O occurs in high yield for 1a but not 2a. Low-temperature FTIR photolysis experiments with 1a in acetonitrile do not reveal any intermediate species, but in THF at room temperature, a new {FeNO}(7) species quickly forms under illumination and exhibits a ν(NO) vibration indicative of nitroxyl-like character. This metastable species reacts further under illumination to produce N2O. A reaction mechanism is proposed, and implications for NO reduction in flavodiiron proteins are discussed.

Abiotic mechanism for the formation of atmospheric nitrous oxide from ammonium nitrate.

Nitrous oxide (N2O) is an important greenhouse gas and a primary cause of stratospheric ozone destruction. Despite its importance, there remain missing sources in the N2O budget. Here we report the formation of atmospheric nitrous oxide from the decomposition of ammonium nitrate via an abiotic mechanism that is favorable in the presence of light, relative humidity and a surface. This source of N2O is not currently accounted for in the global N2O budget. Annual production of N2O from atmospheric aerosols and surface fertilizer application over the continental United States from this abiotic pathway is estimated from results of an annual chemical transport simulation with the Community Multiscale Air Quality model (CMAQ). This pathway is projected to produce 9.3(+0.7/-5.3) Gg N2O annually over North America. N2O production by this mechanism is expected globally from both megacities and agricultural areas and may become more important under future projected changes in anthropogenic emissions.

[Nitrous oxide production by the German Armed Forces in the 20th century : History of medicine and pharmacy in the Armed Forces]. / Lachgasproduktion der Bundeswehr im 20. Jahrhundert : Geschichte der Wehrmedizin und Wehrpharmazie in der Bundeswehr.

The nitrous oxide production unit of the German Armed Forces was a worldwide unique facility which was only employed in the former main medical depot at Euskirchen (nitrous oxide: medical gas which is now obsolete). The last unit was phased out in 2002 and brought to the main medical depot at Blankenburg. Unfortunately the unit is now no longer in the depot and seems to have disappeared. This article describes the nitrous oxide production process and the use of the production unit which was designed by the Socsil company of Switzerland.

James M. Osgood: An Inventor Behind Sprague's "Self-Watching" Apparatus for G.Q. Colton's Revival of Nitrous-Oxide Anesthesia.

Inventor J.M. Osgood enabled a fellow Massachusetts inventor, A.W. Sprague, to manufacture heat-regulated nitrous-oxide generators. These generators assisted New Yorker G.Q. Colton in opening exodontia franchises nationwide which revived the use of nitrous-oxide anesthesia.

Divine's CO2 Absorber of 1867.

Chemist and inventor Silas R. Divine (1838-1912) sold ammonium nitrate and other anesthesia supplies in New York City. He offered a carbon dioxide absorber for the purpose of rebreathing nitrous oxide. Like his colleague Gardner Q. Colton, he denied the need for nitrous oxide to be supplemented with O2 gas.

Effect of temperature on N2O emissions from a highly enriched nitrifying granular sludge performing partial nitritation of a low-strength wastewater.

In the race to achieve a sustainable urban wastewater treatment plant, not only the energy requirements have to be considered but also the environmental impact of the facility. Thus, nitrous oxide (N2O) emissions are a key-factor to pay attention to, since they can dominate the total greenhouse gases emissions from biological wastewater treatment. In this study, N2O production factors were calculated during the operation of a granular sludge airlift reactor performing partial nitritation treating a low-strength synthetic influent, and furthermore, the effect of temperature on N2O production was assessed. Average gas emission relative to conversion of ammonium was 1.5 ± 0.3% and 3.7 ± 0.5% while the effluent contained 0.5 ± 0.1% and 0.7 ± 0.1% (% N-oxidized) at 10 and 20 °C, respectively. Hence, temperature increase resulted in higher N2O production. The reasons why high temperature favoured N2O production remained unclear, but different theoretical hypotheses were suggested.

An on-line synthesis of [15O]N2O: new blood-flow tracer for PET imaging.

This paper describes a recently developed on-line synthesis of a new blood-flow tracer, O-15-labeled nitrous oxide. The tracer was produced by catalytic oxidation of anhydrous ammonia in a gas mixture containing O-15-labeled molecular oxygen. (The oxygen-15 was produced by a 14N(d,n)15O reaction.) Anhydrous ammonia was mixed with the gas containing [15O]O2, and after preheating to about 200 degrees C was carried through an oven containing a Pt catalyst kept at about 310 degrees C. Labeled gas was purified in H3PO4 and KOH traps. O-15-labeled nitrous oxide was identified by gas radiochromatography and by various chemical reactions. Radiochemical purity of the O-15-labeled nitrous oxide exceeded 98%, radiochemical yield corrected for radioactioactive decay was 15-20%, and specific activity at the end of synthesis was about 50 mCi/mmole.

Oxidation of N-hydroxyguanidine by nitric oxide and the possible generation of vasoactive species.

It has been reported previously that the N-hydroxyguanidine function of N-hydroxy-L-arginine can react with nitric oxide (NO) to generate other species that can act as potent vasodilators with different biological lifetimes than NO. The identities of these species have yet to be determined. Therefore, we have studied the reaction between NO and N-hydroxyguanidine and determined that N-hydroxyguanidine is capable of reducing NO to yield nitrous oxide (N2O) and possibly other nitroso species. It is likely that at least some of the N2O formation in these reactions is due to the initial generation of nitroxyl (HNO). Since HNO has been shown to be a potent vasorelaxant, it is possible that some of the non-NO-mediated biological activity alluded to in previous studies was due to HNO and that other nitroso-species generated in the reaction may also contribute to the overall pharmacological activity by release of either NO or HNO.

Methane and nitrous oxide emissions from an irrigated rice of North India.

Upland rice was grown in the kharif season (June-September) under irrigated condition in New Delhi, India (28 degree 40'N and 77 degree 12'E) to monitor CH4 and N2O emission, as influenced by fertilizer urea, ammonium sulphate and potassium nitrate alone (at 120 kg ha-1) and mixed with dicyandiamide (DCD), added at 10% of applied N. The experimental soil was a typic ustochrept (Inceptisol), clay loam, in which rice (Oryza sativa L., var. Pusa-169, duration: 120-125 days) was grown and CH4 and N2O was monitored for 105 days by closed chamber method, starting from the 5 days and 1 day after transplanting, respectively. Methane fluxes had a considerable temporal variation (CV=52-77%) and ranged from 0.05 (ammonium sulphate) to 3.77 mg m-2 h-1 (urea). There was a significant increase in the CH4 emission on the application of fertilizers while addition of DCD with fertilizers reduced emissions. Total CH4 emission (105 days) ranged from 24.5 to 37.2 kg ha-1. Nitrous oxide fluxes were much lower than CH4 fluxes and had ranged from 0.18 to 100.5 g m-2 h-1 with very high temporal variation (CV=69-143%). Total seasonal N2O emission from different treatments ranged from 0.037 to 0.186 kg ha-1 which was a N loss of 0.10-0.12% of applied N. All the fertilizers significantly increased seasonal N2O emission while application of DCD reduced N2O emissions significantly in the range of 10-53%.

Production of nitrogen oxides by lightning and coronae discharges in simulated early Earth, Venus and Mars environments.

We present measurements for the production of nitrogen oxides (NO and N2O) in CO2-N2 mixtures that simulate different stages of the evolution of the atmospheres of the Earth, Venus and Mars. The nitrogen fixation rates by two different types of electrical discharges, namely lightning and coronae, were studied over a wide range in CO2 and N2 mixing ratios. Nitric oxide (NO) is formed with a maximum energy yield estimated to be ~1.3 x 10(16) molecule J-1 at 80% CO2 and ~1.3 x 10(14) molecule J-1 at 50% CO2 for lightning and coronae discharges, respectively. Nitrous oxide (N2O) is only formed by coronae discharge with a maximum energy yield estimated to be ~1.2 x 10(13) molecule J-1 at 50% CO2. The pronounced difference in NO production in lightning and coronae discharges and the lack of formation of N2O in lightning indicate that the physics and chemistry involved in nitrogen fixation differs substantially in these two forms of electric energy.

Controlling denitrification in closed artificial ecosystems.

Denitrification, the dissimilatory reduction of NO3- to N2O and N2, is found in a wide variety of organisms. In closed artificial systems, especially closed plant growth chambers, a significant loss of fixed-N occurs through denitrification, thereby decreasing the efficiency of the system and fouling the atmosphere with N2O. Denitrification is a form of anaerobic respiration. Whenever available, however, denitrifiers preferentially use O2 as their terminal electron acceptor. As a result, rates of denitrification and growth are a function of O2. Typically, in closed systems O2 consumption is greater than the diffusion of O2 through the medium to the cell, decreasing the O2 level near the cell and denitrification occurs. Using Pseudomonas fluorescens (ATCC # 17400) as a model organism grown in a two L bioreactor under varying levels of O2 we studied its effects on population growth and its ability to mitigate denitrification in closed systems. The results indicate that denitrification occurs in a closed system even when it is considered aerobic, that is well mixed and sparged with either air, or sufficient pure O2 to cause a complete turnover in the gaseous atmosphere in the bioreactor vessel every five minutes.