Nitrogen Oxide Species Research Articles

A dual nitrite isotopic investigation of chemodenitrification by mineral-associated Fe(II) and its production of nitrous oxide

Under anaerobic conditions, the environmental reduction of nitrate (NO3−) and nitrite (NO2−) to more reduced forms is widely regarded as being microbially catalyzed. However, the chemical reduction of oxidized nitrogen species by reduced iron (Fe(II)), whether mineral-bound or surface-associated, may also occur under environmentally relevant conditions. Here we examine the nitrogen (N) and oxygen (O) stable isotope dynamics of the chemical reduction of NO2− by mineral associated Fe(II) (chemodenitrification) and its production of the potent greenhouse gas nitrous oxide (N2O). By shedding light on factors controlling kinetics of the reaction and its corresponding dual isotopic expression in the reactant NO2− and product N2O, this work contributes to a growing body of work aiming to improve our ability to identify chemodenitrification in the environment.Consistent with previous studies, we find that while homogenous reactions between aqueous NO2− and Fe(II) were kinetically slow, heterogeneous reactions involving Fe(II)-containing minerals often catalyzed considerable nitrite loss. In particular, rapid reduction of NO2− was catalyzed by the Fe-rich smectite clay mineral nontronite as well as the mixed Fe(II)-Fe(III) oxyhydroxide phase green rust. These minerals serve as both a source of reduced iron within the mineral structure as well as a surface for promoting the reactivity of Fe(II). However, even in the presence of aqueous Fe(II), experiments with low-Fe and non-Fe containing minerals showed little to no NO2− loss, perhaps suggesting a more dominant role for structural iron during chemodenitrification. When catalyzed by nontronite and green rust, N and O isotope effects for chemodenitrification (15εcDNF and 18εcDNF) ranged from 2 to 11‰ and 4 to 10‰, respectively, with lower values generally observed at higher reaction rates. Higher reaction rates were also linked to higher molar yields of N2O (up to 31%), highlighting a strong potential for chemodenitrification to produce N2O – especially relative to its production by microbial pathways, which typically exhibit yields <1%. The intramolecular 15N site preference (SP) of the linear N2O molecule (the difference in δ15N between the central and outer atoms), reflective of different production mechanisms, was also measured for N2O produced during green rust catalyzed chemodenitrification. Relative to values measured in other recent studies of chemodenitrification, SP values were consistently high (+26.5‰±0.8‰), especially relative to N2O produced via bacterial denitrification (SP∼0‰). Finally, the coupling of 18εcDNF and 15εcDNF at a ratio of ∼1 during green rust catalyzed chemodenitrification contrasts distinctly with recently characterized bacterial nitrite reduction, potentially permitting disentangling of both processes under well-constrained conditions. This study contributes to the broader understanding of the potential relevance for mineral-derived Fe(II) to promote the reduction of nitrite and consequent production of N2O, especially in iron-rich systems hosting dynamic redox oscillations, including hyporheic zones, estuarine sediments and groundwater aquifers.

Serum free sulfhydryl status is associated with patient and graft survival in renal transplant recipients

Oxidative stress contributes significantly to graft failure, morbidity and mortality in renal transplant recipients (RTR). In cells, free sulfhydryl groups (reduced thiols, R-SH) are the transducers of redox-regulated events; their oxidation status is modulated by interaction with reactive oxygen and nitrogen oxide species and thought to be in equilibrium with the circulating pool. We hypothesized that high levels of serum free thiols are a reflection of a favorable redox status and therefore positively associated with cardiovascular risk parameters, patient and graft survival in RTR.To test this, reactive free thiol groups (R-SH; corrected for total protein) were quantified in serum of 695 RTR (57% male, 53±13yr, functioning graft ≥1yr) using Ellman's Reagent, and R-SH determinants were evaluated with multivariable linear regression models. Associations between R-SH and mortality or graft failure were assessed using multivariable Cox regression analyses.In multivariable models, male gender, estimated glomerular filtration rate and serum thiosulfate positively associated with R-SH while BMI, HbA1c, corrected calcium and NT-pro-BNP inversely associated with R-SH (model R2=0.26). During follow-up (3.1 [2.7–3.9] yrs), 79 (11%) patients died and 45 (7%) patients developed graft failure. R-SH correlated inversely with all-cause mortality (HR 0.58 [95% CI 0.45–0.75] per SD increase) and graft failure (HR 0.42 [0.30–0.59]; both P<0.001), independent of parameters with which R-SH significantly associated in the multivariable regression analyses, except for NT-pro-BNP.Serum R-SH are associated with a beneficial cardiovascular risk profile and better patient and graft survival in RTR, suggesting potential usefulness as low-cost, high-throughput screening tool for whole-body redox status in translational studies. Whether R-SH modification improves long-term outcome of RTR warrants further exploration.

Quantifying manganese and nitrogen cycle coupling in manganese‐rich, organic carbon‐starved marine sediments: Examples from the Clarion‐Clipperton fracture zone

AbstractExtensive deep‐sea sedimentary areas are characterized by low organic carbon contents and thus harbor suboxic sedimentary environments where secondary (autotrophic) redox cycling becomes important for microbial metabolic processes. Simulation results for three stations in the Eastern Equatorial Pacific with low organic carbon content (&lt;0.5 dry wt %) and low sedimentation rates (10−1–100 mm ky−1) show that ammonium generated during organic matter degradation may act as a reducing agent for manganese oxides below the oxic zone. Likewise, at these sedimentary depths, dissolved reduced manganese may act as a reducing agent for oxidized nitrogen species. These manganese‐coupled transformations provide a suboxic conversion pathway of ammonium and nitrate to dinitrogen. These manganese‐nitrogen interactions further explain the presence and production of dissolved reduced manganese (up to tens of μM concentration) in sediments with high nitrate (&gt;20 μM) concentrations.

Photocatalytic Conversion of Nitric Oxide on Titanium Dioxide: Cryotrapping of Reaction Products for Online Monitoring by Mass Spectrometry

Details of coupling a catalytic reaction chamber to a liquid nitrogen-cooled cryofocuser/triple quadrupole mass spectrometer for online monitoring of nitric oxide (NO) photocatalytic reaction products are presented. Cryogenic trapping of catalytic reaction products, via cryofocusing prior to mass spectrometry analysis, allows unambiguous characterization of nitrous oxide (N2O) and nitrogen oxide species (i.e., NO and nitrogen dioxide (NO2)) at low concentrations. Results are presented, indicating that the major photocatalytic reaction product of NO in the presence of titanium dioxide (TiO2) P25 and pure anatase catalysts when exposed to ultraviolet (UV) light (at a wavelength of 365 nm) is N2O. However, in the presence of rutile-rich TiO2 catalyst and UV light, the conversion of NO to N2O was less than 5% of that observed with the P25 or pure anatase TiO2 catalysts.

Retinopathy of prematurity: an oxidative stress neonatal disease.

Proteomics is the global study of proteins in an organism or a tissue/fluid and is clinically relevant since most disease states are accompanied by specific alterations in an organism's proteome. This review focuses on the application of proteomics to neonatology with particular emphasis on retinopathy of prematurity (ROP), which is a disease in which oxidative stress plays a key pathophysiological role. Oxidative stress is a physiologically relevant redox imbalance caused by an excess of reactive oxygen (ROS) or reactive nitrogen oxide species (RNOS). A major conclusion of this review is that proteomics may be the optimal technology for studying neonatal diseases such as ROP, particularly in the setting of a neonatal intensive care unit (NICU). Proteomics has already identified a number of ROP serum biomarkers. This review will also suggest novel therapeutic approaches to ROP and other neonatal oxidative stress diseases (NOSDs) based on a systems medicine approach.

A Novel Bioreactor for High Density Cultivation of Diverse Microbial Communities.

A novel reactor design, coined a high density bioreactor (HDBR), is presented for the cultivation and study of high density microbial communities. Past studies have evaluated the performance of the reactor for the removal of COD(1) and nitrogen species(2-4) by heterotrophic and chemoautotrophic bacteria, respectively. The HDBR design eliminates the requirement for external flocculation/sedimentation processes while still yielding effluent containing low suspended solids. In this study, the HDBR is applied as a photobioreactor (PBR) in order to characterize the nitrogen removal characteristics of an algae-based photosynthetic microbial community. As previously reported for this HDBR design, a stable biomass zone was established with a clear delineation between the biologically active portion of the reactor and the recycling reactor fluid, which resulted in a low suspended solid effluent. The algal community in the HDBR was observed to remove 18.4% of total nitrogen species in the influent. Varying NH4(+) and NO3(-) concentrations in the feed did not have an effect on NH4(+) removal (n=44, p=0.993 and n=44, p=0.610 respectively) while NH4(+) feed concentration was found to be negatively related with NO3(-) removal (n=44, p=0.000) and NO3(-) feed concentration was found to be positively correlated with NO3(-) removal (n=44, p=0.000). Consistent removal of NH4(+), combined with the accumulation of oxidized nitrogen species at high NH4(+) fluxes indicates the presence of ammonia- and nitrite-oxidizing bacteria within the microbial community.

A Novel Bioreactor for High Density Cultivation of Diverse Microbial Communities

A novel reactor design, coined a high density bioreactor (HDBR), is presented for the cultivation and study of high density microbial communities. Past studies have evaluated the performance of the reactor for the removal of COD1 and nitrogen species2-4 by heterotrophic and chemoautotrophic bacteria, respectively. The HDBR design eliminates the requirement for external flocculation/sedimentation processes while still yielding effluent containing low suspended solids. In this study, the HDBR is applied as a photobioreactor (PBR) in order to characterize the nitrogen removal characteristics of an algae-based photosynthetic microbial community. As previously reported for this HDBR design, a stable biomass zone was established with a clear delineation between the biologically active portion of the reactor and the recycling reactor fluid, which resulted in a low suspended solid effluent. The algal community in the HDBR was observed to remove 18.4% of total nitrogen species in the influent. Varying NH4+ and NO3- concentrations in the feed did not have an effect on NH4+ removal (n=44, p=0.993 and n=44, p=0.610 respectively) while NH4+ feed concentration was found to be negatively related with NO3- removal (n=44, p=0.000) and NO3- feed concentration was found to be positively correlated with NO3- removal (n=44, p=0.000). Consistent removal of NH4+, combined with the accumulation of oxidized nitrogen species at high NH4+ fluxes indicates the presence of ammonia- and nitrite-oxidizing bacteria within the microbial community.

Single-Cell-Genomics-Facilitated Read Binning of Candidate Phylum EM19 Genomes from Geothermal Spring Metagenomes.

The vast majority of microbial life remains uncatalogued due to the inability to cultivate these organisms in the laboratory. This "microbial dark matter" represents a substantial portion of the tree of life and of the populations that contribute to chemical cycling in many ecosystems. In this work, we leveraged an existing single-cell genomic data set representing the candidate bacterial phylum "Calescamantes" (EM19) to calibrate machine learning algorithms and define metagenomic bins directly from pyrosequencing reads derived from Great Boiling Spring in the U.S. Great Basin. Compared to other assembly-based methods, taxonomic binning with a read-based machine learning approach yielded final assemblies with the highest predicted genome completeness of any method tested. Read-first binning subsequently was used to extract Calescamantes bins from all metagenomes with abundant Calescamantes populations, including metagenomes from Octopus Spring and Bison Pool in Yellowstone National Park and Gongxiaoshe Spring in Yunnan Province, China. Metabolic reconstruction suggests that Calescamantes are heterotrophic, facultative anaerobes, which can utilize oxidized nitrogen sources as terminal electron acceptors for respiration in the absence of oxygen and use proteins as their primary carbon source. Despite their phylogenetic divergence, the geographically separate Calescamantes populations were highly similar in their predicted metabolic capabilities and core gene content, respiring O2, or oxidized nitrogen species for energy conservation in distant but chemically similar hot springs.

Local-scale recovery of wastewater nitrogen for edible plant growth.

An anaerobic/ion exchange (AN-IX) system was developed for recovery and reuse of wastewater nitrogen at point-of-origin. AN-IX combines upflow solids blanket anaerobic treatment with ammonium ion adsorption onto granular natural zeolite. AN-IX operates passively and without energy input. A 57 L empty-bed prototype was operated for 355 days on wastewater primary effluent. Total nitrogen removal exceeded 95% over the first 214 days of operation and ammonia reduction exceeded 99%; accumulation of oxidized nitrogen species (NO3(-) + NO2(-)) was not observed. The wastewater flowrate was increased during the last 35 days of operation to deliberately exhaust the ion exchange media. Spent granular media was removed from the AN-IX prototype and deployed in plant chamber experiments for cultivation of Solanum lycopersicum (cherry tomato). Wastewater nitrogen captured on zeolite was capable of supplying the total growth requirement for nitrogen. Canopy volume and plant flowering and fruiting were higher for wastewater nitrogen than for artificial fertilizer. The AN-IX process is a passive, mechanically simple and reliable system for local-scale nitrogen recovery. AN-IX is modular, scalable, adaptable and can be applied in diverse treatment contexts and recycling scenarios. AN-IX benefits include appropriate technology for local-scale nitrogen recovery, low capital and energy costs, and protection of health and the environment.

Transport pathways of peroxyacetyl nitrate in the upper troposphere and lower stratosphere from different monsoon systems during the summer monsoon season

Abstract. The Asian summer monsoon involves complex transport patterns with large-scale redistribution of trace gases in the upper troposphere and lower stratosphere (UTLS). We employ the global chemistry–climate model ECHAM5–HAMMOZ in order to evaluate the transport pathways and the contributions of nitrogen oxide species peroxyacetyl nitrate (PAN), NOx and HNO3 from various monsoon regions, to the UTLS over southern Asia and vice versa. Simulated long-term seasonal mean mixing ratios are compared with trace gas retrievals from the Michelson Interferometer for Passive Atmospheric Sounding aboard ENVISAT(MIPAS-E) and aircraft campaigns during the monsoon season (June–September) in order to evaluate the model's ability to reproduce these transport patterns. The model simulations show that there are three regions which contribute substantial pollution to the South Asian UTLS: the Asian summer monsoon (ASM), the North American monsoon (NAM) and the West African monsoon (WAM). However, penetration due to ASM convection reaches deeper into the UTLS compared to NAM and WAM outflow. The circulation in all three monsoon regions distributes PAN into the tropical latitude belt in the upper troposphere (UT). Remote transport also occurs in the extratropical UT where westerly winds drive North American and European pollutants eastward where they can become part of the ASM convection and lifted into the lower stratosphere. In the lower stratosphere the injected pollutants are transported westward by easterly winds. Sensitivity experiments with ECHAM5–HAMMOZ for simultaneous NOx and non-methane volatile organic compounds (NMVOCs) emission change (−10 %) over ASM, NAM and WAM confirm similar transport. Our analysis shows that a 10 % change in Asian emissions transports ~ 5–30 ppt of PAN in the UTLS over Asia, ~ 1–10 ppt of PAN in the UTLS of northern subtropics and mid-latitudes, ~ 7–10 ppt of HNO3 and ~ 1–2 ppb of ozone in UT over Asia. Comparison of emission change over Asia, North America and Africa shows that the highest transport of HNO3 and ozone occurs in the UT over Asia and least over Africa. The intense convective activity in the monsoon regions is associated with lightning and thereby the formation of additional NOx. This also affects the distribution of PAN in the UTLS. Simulations with and without lightning show an increase in the concentrations of PAN (~ 40 %), HNO3 (75 %), NOx (70 %) and ozone (30 %) over the regions of convective transport. Lightning-induced production of these species is higher over equatorial Africa and America compared to the ASM region. This indicates that the contribution of anthropogenic emissions to PAN in the UTLS over the ASM is higher than that of lightning.

High-Potential Electrocatalytic O2Reduction with Nitroxyl/NOxMediators: Implications for Fuel Cells and Aerobic Oxidation Catalysis

Efficient reduction of O2 to water is a central challenge in energy conversion and many aerobic oxidation reactions. Here, we show that the electrochemical oxygen reduction reaction (ORR) can be achieved at high potentials by using soluble organic nitroxyl and nitrogen oxide (NOx) mediators. When used alone, neither organic nitroxyls, such as 2,2,6,6-tetramethyl-1-piperidinyl-N-oxyl (TEMPO), nor NOx species, such as sodium nitrite, are effective ORR mediators. The combination of nitroxyl/NOx species, however, mediates sustained O2 reduction with overpotentials as low as 300 mV in acetonitrile containing trifluoroacetic acid. Mechanistic analysis of the coupled redox reactions supports a process in which the nitrogen oxide catalyst drives aerobic oxidation of a nitroxyl mediator to an oxoammonium species, which then is reduced back to the nitroxyl at the cathode. The electrolysis potential is dictated by the oxoammonium/nitroxyl reduction potential. The overpotentials accessible with this ORR system are significantly lower than widely studied molecular metal-macrocycle ORR catalysts and benefit from the mechanism-based specificity for four-electron reduction of oxygen to water mediated by NOx species, together with kinetically efficient reduction of oxidized NOx species by TEMPO and other organic nitroxyls.

Experimental and modelling studies of microwave plasma afterglows in N2–O2 mixtures

This paper presents results from both experimental and modelling studies of a flowing afterglow in N2–O2 mixtures observed in a plasma-chemical reactor. The plasma is generated at a pressure of 440 Pa by a microwave cavity working at the excitation frequency of 433 MHz. The main nitrogen and nitrogen oxide species are experimentally studied, mainly in the late afterglow of both a N2–O2 discharge and a nitrogen discharge with a downstream O2 admixture. The molecules and relative densities were monitored by optical emission spectroscopy and the atoms by means of mass spectrometry. A temporal 0D model is built and coupled to a steady state model on and states, for comparison with the experimental results, in order to describe the kinetics of the studied afterglows. This calculation is supported by the simulation of gas flow in the reactor using COMSOL Multiphysics®. We present results concerning the temporal density evolutions of , , , , , and steady density evolutions of and species as functions of the amount and location of an oxygen injection. The principal mechanisms in the described system are identified allowing the proposal of a simplified kinetic scheme showing a strong coupling among , and the main channels of the formation and destruction of NO, NO2 and O3 molecules.

Evaluation of updated nitric acid chemistry on ozone precursors and radiative effects

Abstract. This study shows that revising the reaction rate of NO2 + HO· → HNO3 improves simulated nitrogen partitioning and changes the simulated radiative effects of several short-lived climate forcers (SLCF). Both laboratory and field study analyses have found that the reaction rate should be reduced by 13–30% from current recommendations. We evaluate the GEOS-Chem model over North America with and without the recommended update using observations from the Intercontinental Chemical Transport Experiment – North America (INTEX-NA) Phase A campaign. Revising the NO2 + HO· → HNO3 rate coefficient improves model performance of oxidized nitrogen partitioning by increasing NOx concentrations in the upper troposphere and decreasing HNO3 throughout the troposphere. The increase in NOx concentrations has a corresponding global increase in O3 concentrations and local increases in sulfate aerosols, causing a perturbation in simulated radiative effects. These findings demonstrate the positive influence the mechanism update has on the partitioning of oxidized nitrogen species, the benefits it provides when compared to aircraft observations, and the simulated radiative effects that the reduction induces.

Reactivity of HNO‐Induced Modifications and Its Relevance to Phospholamban Function

HNO, a potential heart failure therapeutic, is known to post-translationally modify cysteine residues. Among reactive nitrogen oxide species, the modification of cysteine residues to sulfinamides [RS(O)NH2] is unique to HNO. Because this modification can alter protein structure and function, we have examined the reactivity of sulfinamides in several systems, including small organic molecules, peptides, and a protein. At physiological pH and temperature, relevant reactions of sulfinamides involve reduction to free thiols in the presence of excess thiol and hydrolysis to form sulfinic acids [RS(O)OH]. In addition to utilizing ESI-MS and other spectroscopic methods to study sulfinamide reduction, we have applied 15N-edited 1H-NMR techniques to sulfinamide detection and used this method to explore sulfinamide hydrolysis. We have also investigated the effect of local environment on the reactivity of HNO with C-terminal cysteines. Our findings indicate that the nature of HNO-derived modifications of C-terminal cysteines is influenced by the C-terminal carboxylate. HNO has been shown to enhance cardiac sarcoplasmic reticulum Ca2+ cycling independent of the β-adrenergic pathway. In a collaborative project, the effects of HNO on the cardiac protein, phospholamban (PLN) are investigated using 15N-edited NMR methods and biochemistry protocols. This research was supported by the National Science Foundation (CHE-1213438) and Cardioxyl Pharmaceuticals.

Bradford Hill Criteria and the Case for γ‐Tocopherol as an Essential Dietary Nutrient in the Prevention of Nitrogen Oxide‐mediated Cellular Damage and Associated Chronic Diseases of Aging.

The nutrition field is awash with recommendations for the benefits of “antioxidants” for the prevention of chronic diseases of aging, e.g. cancer, yet evidence of efficacy in humans for individual antioxidants is limited to cell culture, animal studies, or to epidemiologic associations. Indeed, large clinical trials with relatively high doses of the most promising antioxidant agents, β‐carotene and α‐tocopherol, have paradoxically demonstrated pro‐carcinogenic effects in humans. γ‐Tocopherol is an ubiquitous dietary antioxidant which is typically found at micromolar levels in humans. It possesses unique antioxidant properties that specifically target nitrogen oxide species capable of damaging DNA in cells. Data from published studies of the chemistry of γ‐tocopherol, its biology, its epidemiology, its effects on animal models of disease and clinical effects of decreased levels in humans are assessed in light of the Bradford Hill criteria for causality in order to establish a prima facie case for the essentiallity of γ‐tocopherol in human nutrition. While the epidemiology of γ‐tocopherol is uniquely complex, its coherence with fundamental studies of γ‐tocopherol and the plausibility provided by our understanding of its chemical mechanism of action and unique biological effects add considerable weight to epidemiological and clinical observations of its associations with chronic diseases of aging. Taken together, the evidence supports a beneficial effect of and possible requirement for moderate intake of γ‐tocopherol in the diet, however, it is unlikely that significantly higher intake beyond levels found in the typical American diet would offer additional benefit.

Exploring the Environmental Photochemistry on the TiO2(110) Surface in Situ by Near Ambient Pressure X-ray Photoelectron Spectroscopy

Heterogeneous reactions of nitrogen oxides on metal oxide surfaces have been suggested to play a significant role in environmental chemistry, physics, and engineering. Many of the metal oxide compounds found among atmospheric mineral dust particles are inherently semiconducting substrates. Due to their low band gap, they are effective photoactive materials in the environmentally relevant ultraviolet (UVA) range of solar radiation. Here, we have studied nitrogen oxide species evolution and photochemistry on TiO2(110) surfaces in the context of atmospheric chemistry by means of near ambient pressure X-ray photoelectron spectroscopy (AP-XPS) coupled with a 375 nm UV-laser module. In the presence of molecular O2 only, changes in TiO2 surface potential under UV irradiation were observed, attributed to band flattening. Under humid conditions, a significant increase in the BE range attributed to surface hydroxyl groups was observed, which may be the basis for the light-induced superhydrophilicity observed elsewhere with titania-based nanomaterials. The formation of surface nitrite and nitrate was observed after exposure to NO2 in the dark. Core-level metal cation, O, and N XPS spectra were measured at elevated pressures of O2, NO2, and H2O. By selective UV irradiation of only the XPS measurement spot on the sample, we obtained differential information on the surface chemical state on the UV-irradiated compared to dark reference spots. Upon UV irradiation, increased oxidation of NO2 was observed, while in turn a substantial increase of a reduced nitrate species possibly from electron transfer to nitrate and of a further reduced nitrogen species was observed during exposure to UV-radiation. The effect of surface hydroxylation and the involvement of carbon-containing surface compounds in the formation of nitrogenated organic species are emphasized.

Potential role of glutathione in evolution of thiol-based redox signaling sites in proteins.

Cysteine is susceptible to a variety of modifications by reactive oxygen and nitrogen oxide species, including glutathionylation; and when two cysteines are involved, disulfide formation. Glutathione-cysteine adducts may be removed from proteins by glutaredoxin, whereas disulfides may be reduced by thioredoxin. Glutaredoxin is homologous to the disulfide-reducing thioredoxin and shares similar binding modes of the protein substrate. The evolution of these systems is not well characterized. When a single Cys is present in a protein, conjugation of the redox buffer glutathione may induce conformational changes, resulting in a simple redox switch that effects a signaling cascade. If a second cysteine is introduced into the sequence, the potential for disulfide formation exists. In favorable protein contexts, a bistable redox switch may be formed. Because of glutaredoxin's similarities to thioredoxin, the mutated protein may be immediately exapted into the thioredoxin-dependent redox cycle upon addition of the second cysteine. Here we searched for examples of protein substrates where the number of redox-active cysteine residues has changed throughout evolution. We focused on cross-strand disulfides (CSDs), the most common type of forbidden disulfide. We searched for proteins where the CSD is present, absent and also found as a single cysteine in protein orthologs. Three different proteins were selected for detailed study—CD4, ERO1, and AKT. We created phylogenetic trees, examining when the CSD residues were mutated during protein evolution. We posit that the primordial cysteine is likely to be the cysteine of the CSD which undergoes nucleophilic attack by thioredoxin. Thus, a redox-active disulfide may be introduced into a protein structure by stepwise mutation of two residues in the native sequence to Cys. By extension, evolutionary acquisition of structural disulfides in proteins can potentially occur via transition through a redox-active disulfide state.

Properties of nitrogen-oxygen surface compounds on ZrO2 samples with different phase compositions according to in situ IR spectroscopy data

The zirconium dioxide surface has a wide variety of adsorption sites differing in their nature. The proportions of these sites can be changed by varying the oxide preparation and pretreatment conditions. This fact shows itself as a wide diversity of surface structures resulting from NO and O2 adsorption. Under conditions of the selective catalytic reduction of NOx, the most stable nitrogen oxide species are nitrates that result from the interaction between NOx and the ZrO2 surface. The concentrations of the other nitrogen-oxygen surface compounds are two orders of magnitude lower. The routes of NO3− formation and decomposition on the ZrO2 surface are discussed. In these routes, monodentate nitrates (which show themselves at 1550–1555 cm−1) are considered as intermediates in the formation and decomposition of bidentate NO3−.

Nitrite: on the journey from toxin to therapy.

Nitrite: on the journey from toxin to therapy.

An update to the ambient ratio method for 1-h NO2 air quality standards dispersion modeling

Nitrogen oxide (NOX) gases are typically emitted by fuel combustion sources in the form of nitric oxide (NO), which then reacts with ozone and other oxidants in the atmosphere to convert a portion of the NO to nitrogen dioxide (NO2). EPA has promulgated a 1-h average National Ambient Air Quality Standard (NAAQS) for NO2, and major sources of NOX emissions must estimate their NO2 air quality impacts as part of EPA's air quality permitting programs. The AERMOD dispersion model has been developed by EPA for these air quality impact analyses, and AERMOD contains three different NO to NO2 conversion methods for estimating the ambient concentrations of NO2. This paper describes a refinement to one of the methods, the Ambient Ratio Method version 2 (ARM2). ARM2 is an empirical approach that uses a variable conversion factor, based on an analysis of ambient air measurements of NO and NO2, to estimate the portion of the AERMOD predicted air concentration of total NOX species that is in the form of NO2. The performance of ARM2 has been evaluated and found to compare well to actual ambient measurements and to other more complex EPA conversion methods. EPA has included ARM2 as a “beta-testing” option in AERMOD version 14134, and provided guidance on the use of ARM2 for regulatory modeling analyses in a September 2014 memorandum. This paper also discusses this recent EPA guidance.