How Biology Handles Nitrite

Enzymatic colouring for meat without nitrite: Exploration of bacterial nitric oxide synthase fused with YkuN-YumC

This study developed an innovative strategy for colouring meat products without nitrite addition, using nitric oxide synthase (NOS) fused with flavodoxin YkuN and flavodoxin reductase YumC derived from Bacillus subtilis. The results showed that the plasmids containing nos linked with ykuN and yumC genes by rigid linkers were constructed and chemically transformed into B. subtilis 168, and the enzyme fused with YkuN and YumC (NOS-YkuN-YumC) was successfully expressed and then purified. The activity of the fusion enzyme was approximately 12 times greater than that of NOS. In a model system, NOS-YkuN-YumC significantly increased the a*-value (redness) compared to those of the control and the sample treated with NOS (P < 0.05). UV-Vis spectral analysis indicated that metmyoglobin was converted to nitrosylmyoglobin (NO-Mb). In minced meat, the addition of NOS-YkuN-YumC significantly promoted the formation of NO-Mb and enhanced the a*-value (P < 0.05). The colour of the minced meat did not differ significantly between the NOS-YkuN-YumC group and the nitrite group (P > 0.05). This study provides a promising solution for enhancing colour formation in meat products without nitrite.

Synthesis of Iron Porphyrin with Pendant Thiol Groups: Progress Toward a Model of the Heme/Cu Active Site of Sulfite Reductase

The heme/Cu sulfite reductase bears a distal Cu metal coordinated to two cysteines. This enzyme catalyzes the reduction of sulfite to H2S. Very little is known about the electronic structure and reactivity of this enzyme. Synthetic modelling can allow access to the properties of the active site. However, installing thiolates on porphyrin rings is not adequately represented in the literature. In this paper, we describe a synthetic methodology to install multiple thiolates on porphyrin rings which can be used to bind a second metal. In addition, to the complexity of keeping several thiols adjacent to each other, this work also reports unprecedented demetallation of iron porphyrin in the presence of multiple thiol functional groups in the vicinity.

Probing the Reversible Binding of Anionic Reactive Sulfur and Nitrogen Species in Imidazolium Receptors with Directional C-H Hydrogen Bonds

H2S and NO are physiologically important signaling molecules with complex roles in biology and intermolecular crosstalk. Although these species are often referred to as neutral on paper, they are primarily found in anionic and/or oxidized forms in aerobic solutions as HS- or NO2-/NO3-, respectively. Despite the prominence of these anions in biology, particularly HS- and NO2-, few investigations have focused on the molecular recognition and reversible binding of these important species. Using a library of imidazolium receptors with C-H hydrogen bonding interactions, we investigate the influences on binding affinity through modulation of charge, multiplicity, and preorganization, while also investigating how anion volume impacts binding. These factors are probed by solution-state titration experiments and solid-state X-ray crystallographic data showing the specific molecular interactions involved in guest binding. Both solution-state NMR and solid-state X-ray crystallography support the importance and abundance of C-H···X- interactions in facilitating guest binding as well as conformational changes upon anion recognition.

Reversible Interconversion of Nitrate and Nitrite Catalyzed by Periplasmic Nitrate Reductase from Campylobacter jejuni

The periplasmic nitrate reductase catalytic subunit (NapA) belongs to the DMSO reductase (DMSOR) family of the molybdenum cofactor (Moco) containing enzymes and catalyzes nitrate reduction to nitrite. Some members of the DMSOR family (e.g., DMSO reductase, formate dehydrogenase, nitrite oxidoreductase, and arsenate reductase) catalyze the reversible interconversion of the substrate and product with varying efficiency. Here, we demonstrate that NapA not only can reduce nitrate to nitrite but also can also oxidize nitrite to nitrate. At least three mechanisms involving the oxygen atom transfer (OAT) process have been proposed for NapA-catalyzed nitrate reduction. NapA can also oxidize nitrite via an OAT process. However, there is no experimental evidence for any of these OAT processes using isolated pure enzymes so far. Here, we provide the first experimental evidence for OAT reactions in both forward (i.e., nitrate reduction) and backward (i.e., nitrite oxidation) reactions catalyzed by NapA using isotope labeling, mass spectrometry, and NMR experiments. Finally, we propose a mechanistic scheme supported by experiment and computation consistent with atom transfer as the rate-limiting step.

Functional Nonheme Diiron(II) Complexes Catalyze the Direct Reduction of Nitrite to Nitric Oxide in Relevance to the Diiron Protein YtfE

The present work reports the functional modeling chemistry of YtfE, which features a nonheme diiron active site and mediates the direct reduction of NO2- to NO. The model complex, [Fe2(HPTP)Cl2]1+ (1), reduces NO2- to NO in a 100% yield within 12 h and generates [Fe4(HPTP)2(μ-O)3(μ-OH)]3+ (2). Similar to YtfE, the reaction involves stepwise oxidation of two Fe(II) centers and product (NO) inhibition, of which the latter produces [Fe2(HPTP)(NO)2Cl2]1+ (3). Complex 3 could also be synthesized by the reaction of [Fe2(HPTP)(NO)2(ClO4)]2+ (4) and chloride. Complex 1 catalyzes the reduction of NO2- to NO in the presence of PhS-, albeit with a low TON of 5, due to the formation of an insoluble product, [Fe2(HPTP)(μ-SPh)Cl2] (5). Another model complex [Fe2(HPTP)(OPr)]1+ (6), reduced NO2- to NO in an 80% yield after 24 h, generated [Fe2(HPTP)(OPr)(NO)2]1+ (7), and offered a TON of 19. The third model complex, [Fe2(HPTP)(ClO4)2]1+ (8), could reduce NO2- to NO in a 100% yield but only after 48 h. A comparison of these results establishes that easy oxidation of the Fe(II) centers, easy accessibility of the Fe(II) centers for the coordination of NO2-, and easy release of NO from the in situ generated dinitrosyl diiron complex increase the efficiency of the functional model complexes of YtfE.

Surface-like NOx Reduction at an Atomically-Precise Tricopper Cluster

The combustion of nitrogen-containing fuels and increasing use of nitrogen-rich fertilizers is accumulating NOx pollutants in the environment. Copper is an attractive catalyst material for reductive NOx remediation, yet ambiguity persists regarding the elementary bond-making and bond-breaking steps occurring at the catalyst interface. Starting from a molecular tricuprous μ3-oxo complex (1), an unusually reduced and highly reactive surface-like cluster (2) has been prepared. Characterization data and electronic structure calculations are consistent with 2 featuring σ-aromaticity that primes the tricopper core for two-electron chemistry. Cluster 2 mediates catalytic reductive coupling of NO to N2, proceeding through N2O, via sequential oxygen atom transfer steps. Stoichiometric reduction of NO3 - and NO2 - is also disclosed, mapping the complete denitrification cycle at an atomically-precise molecular cluster.

Selective Stepwise Reduction of Nitrate and Nitrite to Dinitrogen or Ammonia

This study reports a method for the selective reduction of NO3- and NO2- to N2 or NH3, extending prior work in our lab where NO3- was reduced to NO by [N(afaCy)3Fe]OTf2 (N(afaCy)3 = tris(5-cyclohexyl-amineazafulvene-2-methyl)amine, OTf = triflate). The first pathway involves the reduction of NO2- to N2, where the NO generated in the initial step is transformed to N2O by PPh3 and further reduced to N2 by the [N(afaCy)3Fe]OTf2 complex. An alternative pathway showcases the reduction of the bound NO complex, [N(afaCy)3Fe(NO)]2+, to NH3 using chemical reductants, albeit with a modest yield of 29%. Confirmation of the nitrogen source as NO is established through 15N labeling studies. Hydroxylamine (NH2OH) is proposed as a plausible intermediate in the reduction of bound NO, supported by independent NH2OH reduction experiments and computational studies. Nature employs a well-orchestrated, stepwise process involving several enzymes to reduce N-containing oxyanions, and this approach provides valuable insights into the stepwise reduction mechanisms of nitrate and nitrite, yielding NH3 or N2 as the product.

Advances in Electrochemical Nitrite Reduction toward Nitric Oxide Synthesis for Biomedical Applications

Nitric oxide (NO) is an essential molecule in biomedicine, recognized for its antibacterial properties, neuronal modulation, and use in inhalation therapies. The effectiveness of NO-based treatments relies on precise control of NO concentrations tailored to specific therapeutic needs. Electrochemical generation of NO (E-NOgen) via nitrite (NO2 -) reduction offers a scalable and efficient route for controlled NO production, while also addressing environmental concerns by reducing NO2 - pollution and maintaining nitrogen cycle balance. Recent developments in catalysts and E-NOgen devices have propelled NO2 - conversion, enabling on-demand NO production. This review provides an overview of NO2 - reduction pathways, with a focus on cutting-edge Fe/Cu-based E-NOgen catalysts, and explores the development of E-NOgen devices for biomedical use. Challenges and future directions for advancing E-NOgen technologies are also discussed.

Surface-Enhanced Raman Spectroscopy for Nitrite Detection

Nitrite is an important chemical intermediate in the nitrogen cycle and is ubiquitously present in environmental and biological systems as a metabolite or additive in the agricultural and food industries. However, nitrite can also be toxic in excessive concentrations. As such, the development of quick, sensitive, and portable assays for its measurement is desirable. In this review, we summarize the working principles and applications of surface-enhanced Raman spectroscopy (SERS) as a rapid, portable, and ultrasensitive method for nitrite detection and showcase its applicability in various water, food, and biological samples. The challenges and opportunities for future developments are also discussed.

A terpyridine-based copper complex for electrochemical reduction of nitrite to nitric oxide

In biological systems, nitrite reductase enzymes (NIRs) are responsible for reduction of nitrite (NO2-) to nitric oxide (NO). These NIRs have mostly Cu- or Fe-containing active sites, surrounded by amine-containing ligands. Therefore, mononuclear Cu complexes with N-donor ligands are highly relevant in the development of NIR model systems and in the mechanistic investigation of the nitrite reduction reaction. Herein, we report on a terpyridine-based CuII complex with square planar geometry for H+-assisted electrochemical reduction of NO2-. Through electrochemical measurements, spectroscopic characterization and isotope-labelling experiments we propose a mechanistic reaction pathway involving an unstable HNO2 state. The CuI intermediate, formed electrochemically, was isolated and its molecular structure was deduced, showing linkage isomerism of the nitrite ligand. Moreover, qualitative and quantitative product analysis by GC-MS shows N2O formed as a side product along with the main product NO. Furthermore, by obtaining single crystals and conducting structural analysis we were able to determine the structural arrangement and redox state of the complex after electrochemical treatment.

Rational designing of ZIF-67-derived Co3O4 nanocomposite with hierarchical porous structure and extensive peroxidase mimetic activities for highly sensitive colorimetric detection of nitrite in drinking water

A simple, fast, and cost-effective colorimetric nitrite (NO2-) sensor based on ZIF-67-derived Co3O4 nanocomposite (ZCo-2 NC) structure has been developed. The prepared colorimetric sensor (ZCo-2 NC) was employed to sensitively detect NO2- in drinking water system by the exhibition of promising peroxidase-mimicking nanozyme-like features. The sensor manifest well-determined sensing response with excellent linear and wide range of NO2- sensitivity (0.001-0.810 μM). The lower detection-limit (LOD) and lower quantification-limit (LOQ) were 0.14 ± 0.05 nM and 0.72 ± 0.05 nM, respectively, which is far below the US-EPA limit (21.7 μM). Further, the sensor also provides strong selectivity response to NO2-, better reversibility (12 cycles), and commendable stability of 10 weeks. In addition, it also perceived astonishing practicality towards NO2- in real water samples. Thus, this study opens a new pathway for the sensitive detection of NO2- in drinking water for future endeavor.

Tetrapodal iron complexes invoke observable intermediates in nitrate and nitrite reduction

This study investigates the mechanistic pathways of nitrate and nitrite reduction by the tetrapodal iron complex [Py2Py(afamcyp)2Fe]OTf2, revealing key intermediates to elucidate the reaction process. Using UV-Vis, IR, mass and NMR spectroscopies, stable binding of oxyanions to the iron centre was observed, supporting the formation of the iron(iii)-hydroxide intermediate [Py2Py(afamcyp)2Fe(OH)]OTf2. This intermediate is less stable than in previous systems, providing insights into the behaviour of metalloenzymes. A bimetallic mechanism is proposed for nitrogen oxyanion reduction where additional iron is required to drive the complete reaction, resulting in the formation of the final nitrosyl complex, Py2Py(pimcyp)2Fe(NO), and water. Our findings enhance the understanding of iron-based reduction processes and contribute to the broader knowledge of oxyanion reduction mechanisms.

Titanium nitride sensor for selective NO2 detection

Efficient detection methods are needed to monitor nitrogen dioxide (NO2), a major NOx pollutant from fossil fuel combustion that poses significant threats to both ecology and human health. Current NO2 detection technologies face limitations in stability and selectivity. Here, we present a transition metal nitride sensor that exhibits exceptional selectivity for NO2, demonstrating a sensitivity 30 times greater than that of the strongest interfering gas, NO. The sensor maintains stability over 6 months and does not utilize platinum or other precious metals. This notable performance has been achieved through preparation of highly active titanium nitride (TiNx) nanoparticles with exceptionally large surface area and a high concentration of nitrogen vacancies. By contrast, a commercial sample of TiN shows no gas sensing activity. Such devices are potentially scalable for everyday NO2 detection and demonstrate that robust high-performance gas sensors based on inexpensive metal nitrides without precious metals are leading candidates for environmental monitoring technologies.

Inhibitory effects of nitrite and sulfite/peroxymonosulfate on bacteria are mediated respectively through respiration and intracellular GSH homeostasis

As nitrite, sulfite has been used in food preservation for centuries but how it inhibits bacterial growth remains underexplored. To address this issue, in this study, we set out to test if cytochrome (cyt) c proteins protect bacteria from the damage of certain reactive sulfur species (RSS) because they do so in the case of reactive nitrogen species (RNS). We show that some reactive sulfur species, such as sulfite and peroxymonosulfate (PMS), inhibit growth of bacterial strains devoid of cytochrome (cyt) c proteins. Subsequent investigations link the inhibition of sulfite/PMS to activity of cbb3-type heme-copper oxidase (cbb3-HCO). However, in vitro comparative analysis rules out that either cbb3-HCO or cyt bd oxidase is the primary target of sulfite/PMS. Instead, we found that sulfite/PMS and the cbb3-HCO loss regulate intracellular redox status in a similar manner, by affecting GSH/GSSG homeostasis. The link between the GSH/GSSG homeostasis and sulfite/PMS is further substantiated by using the mutants with enhanced GSSG generation. Furthermore, we present the data to show that inhibitory effects of nitrite and sulfite/PMS are additive although the overall effects may vary depending on species. Our results open an avenue to control bacteria by developing more robust agents that modulating intracellular redox status, which may be used in combination with nitrite as a promising antimicrobial strategy.

Influence of in situ biochar capping on microbial dynamics and ammonia nitrogen release in sediment

To study the influence of in situ biochar (BC) capping technique on the release of ammonia nitrogen (NH4+-N) from sediments, a field mesocosm experiment was conducted in Baiyangdian Lake (BYDL), a critical water body often referred to as the "kidney of North China" where sediment pollution poses a significant threat to water quality. This study also assessed the impact of BC on sediment microorganisms. The results showed that the NH4+-N concentration in the overlying water of the BC-treated mesocosms was the lowest among four treatments, decreasing to 0.051 mg L-1 by the 60th day. More importantly, the BC treatment showed the least increase in NH4+-N concentrations in sediments compared to other treatments. For sediments capped with a 4 cm layer of BC, the potential release flux of NH4+-N was reduced from 1.84 mg m-2 d-1 to -0.76 mg m-2 d-1. This reduction is likely due to the negatively charged surfaces of biochar, which enhance NH4+-N adsorption through electrostatic interactions. Additionally, BC modified the physical and chemical properties of the surface sediment, improving pH and increasing both organic content and the carbon/nitrogen (C/N) ratio. These changes influenced the microbial community structure within the sediments, enhancing NH4+-N removal. After 60 days, a significant alteration in the microbial community was observed in the BC-treated surface sediments. The addition of BC significantly increased the abundance of Proteobacteria and Firmicutes of the phyla in the sediments. Furthermore, BC enhanced the expression of functional genes including amoA, amoB, nirK, nirS, hzsB, nrfA and ureC, which are likely the primary microbial mechanisms promoting NH4+-N conversion in sediments for final removal.

Directional Electron Transfer in Enzymatic Nano-Bio Hybrids for Selective Photobiocatalytic Conversion of Nitrate

Semi-artificial photosynthetic system (SAPS) that combines enzymes or cellular organisms with light-absorbing semiconductors, has emerged as an attractive approach for nitrogen conversion, yet faces the challenge of reaction pathway regulation. Herein, we find that photoelectrons can transfer from the -C≡N groups at the edge of cyano-rich carbon nitride (g-C3N4-CN) to nitrate reductase (NarGH), while the direct electron transfer to nitrite reductase (cd1NiR) is inhibited due to the physiological distance limit of active sites (>14 Å). By means of the directional electron transfer between g-C3N4-CN and extracted biological enzymes, the product of the denitrification reaction was switched from inert N2 to usable nitrite with an unprecedented selectivity of up to 95.3 %. The converted nitrite could be further utilized by anammox microbiota and dissimilatory nitrate reduction to ammonia (DNRA) microorganisms, doubling the efficiency of total nitrogen removal (96.5±2.3 %) for biological nitrogen removal and ammonia generation (12.6 mg NH4 +-N L-1 h-1), respectively. Thus, our work paves an appealing way for the sustainable treatment and utilization of nitrate for ammonia fuel production by strategically regulating the electron transfer pathway across the biotic-abiotic interface.

Synthetically Reversible, Proton-Mediated Nitrite N-O Bond Cleavage at a Dicopper Site

A monocationic dicopper(I,I) nitrite complex [Cu2(μ-κ1:κ1-O2N)DPFN][NTf2] (2) (DPFN = 2,7-bis(fluoro-di(2-pyridyl)methyl)-1,8-naphthyridine, NTf2- = N(SO2CF3)2-), was synthesized by treatment of a dicopper acetonitrile complex, [Cu2(μ-MeCN)DPFN][NTf2]2 (1), with tetrabutylammonium nitrite ([nBu4N][NO2]). DFT calculations indicate that 2 is one of three linkage isomers that are close in energy and presumably accessible in solution. Reaction of the μ-κ1:κ1-O2N complex with p-TolSH produces nitrous acid (HONO) and the corresponding dicopper thiolate species via an acid-base exchange reaction. Notably, treatment of 2 with HNTf2 results in N-O bond cleavage in the putative, HONO-ligated complex to form the more thermodynamically favorable nitrosyl-bridged dicopper complex [Cu2(μ-NO)(μ-OH)DPFN][NTf2]2 (4). This scission can be reversed via deprotonation of the hydroxy ligand with KOtBu. X-ray diffraction studies confirmed the solid-state molecular structures of 2 and 4. DFT calculations were used to construct a reaction coordinate diagram detailing formation of the μ-NO complex and to describe its electronic structure. The nitrosyl ligand in 4 is chemically labile, as demonstrated by its ready displacement in reactions with CO or NO2-.

Tuning the Reactivity of Copper(II)-Nitrite Core Towards Nitric Oxide Generation

Insights into the molecular mechanism and factors affecting nitrite-to-NO transformation at transition metal sites are essential for developing sustainable technologies relevant to NO-based therapeutics, waste water treatment, and agriculture. A set of copper(II)-nitrite complexes 1-4 have been isolated employing tridentate pincer-type ligands (quL, pyL, ClArOL-, PhOL-) featuring systematically varied donors. Although the X-ray crystal structures of the copper(II)-nitrite cores in 1-4 are comparable, electrochemical studies on complexes 1-4 reveal that redox properties of these complexes differ due to the changes in the σ-donor abilities of the phenolate/N-heterocycle based donor sites. Reactivity of these nitrite complexes with oxygen-atom-transfer (OAT) reagent (e. g. triphenyl phosphine Ph3P) and H+/e- donor reagent (e. g. substituted phenols ArOH) show the reduction of nitrite to NO gas. Detailed kinetic investigations including kinetic isotope effect (KIE), Eyring analyses for determining the activation parameters unfold that reduction of nitrite at copper(II) by Ph3P or ArOH are influenced by the CuII/CuI redox potential. Finally, this study allows mechanism driven development of catalytic nitrite reduction by ArOH in the presence of 10 mol % copper complex (1).

Mechanistic studies of NOx reduction reactions involving copper complexes: encouragement of DFT calculations

The reduction of nitrogen oxides (NOx), which is mainly mediated by metalloenzymes and metal complexes, is a critical process in the nitrogen cycle and environmental remediation. This Frontier article highlights the importance of density functional theory (DFT) calculations to gain mechanistic insights into nitrite (NO2-) and nitric oxide (NO) reduction reactions facilitated by copper complexes by focusing on two key processes: the reduction of NO2- to NO by a monocopper complex, with special emphasis on the concerted proton-electron transfer, and the reduction of NO to N2O by a dicopper complex, which involves N-N bond formation, N2O2 isomerization, and N-O bond cleavage. These findings underscore the utility of DFT calculations in unraveling complicated reaction mechanisms and offer a foundation for future research aimed at improving the reactivity of transition metal complexes in NOx reduction reactions.

Reduction of Nitrite in an Iron(II)-Nitrito Compound by Thiols and Selenol Produces Dinitrosyl Iron Complexes via an {FeNO}7 Intermediate

Reaction of an Fe(II) complex, [Fe(6-COO--tpa)]1+ (1), with PhE- and NO2- produced [Fe(6-COO--tpa)(EPh)] (E = S, 2a; Se, 3) and [Fe(6-COO--tpa)(κ2-O,O'-NO2)] (4), respectively (6-COOH-tpa is bis(2-pyridylmethyl)(6-carboxyl-2-pyridylmethyl)amine). Treatment of 4 with 2 equiv of PhEH (E = S, Se) produced NO in ∼40% yields, respectively, along with 1 and the DNICs, [Fe(EPh)2(NO)2]1- (E = S, Se). Treatment of 4 with excess PhEH produced NO in similar yields, while 4 was converted to the same DNICs and 2a/3 (instead of 1). The DNICs have been proposed to be generated via the reaction of PhE- with an in situ generated, unstable {FeNO}7 intermediate, [Fe(6-COO--tpa)(NO)]1+ (6), which has also been synthesized separately. Compound 6 reacts with PhS- to generate [Fe(SPh)2(NO)2]1-, thus supporting the proposed reaction pathway. Finally, while the treatment of two unique compounds, featuring inbuilt proton sources, [Fe(6-COO--tpa)(S-C6H4-p-COOH)] (7) and [Fe(6-COO--tpa)(S-C6H4-o-OH)] (8), with 0.5 and 1 equiv of NO2- could produce NO only in 8-26% yields, treatment of 4 with HS-C6H4-p-COOH and HS-C6H4-o-OH produced NO in much higher yields (65-77%). The combined results delineated the importance of coordination of NO2- for the proton-assisted reduction of NO2- to generate NO.

Revealing the size effect mechanisms of micro(nano)plastics on nitrogen removal performance of constructed wetland

Micro(nano)plastics (MPs) in aquatic environments can disrupt wastewater treatment, particularly nitrogen removal in constructed wetlands (CWs). However, their broader effects on microbial and plant nitrogen metabolism remain unclear. This study investigated the effects of different-sized MPs (4 mm, 100 µm, and 100 nm) on nitrogen transformation in CWs. Results revealed that 4 mm- and 100 µm-MPs did not significantly affect total nitrogen (TN) removal, although 100 µm-MPs significantly increased leaf antioxidant enzyme activities and reduced plant uptake of nitrogen by 12.95 % (p < 0.05). In contrast, 100 nm-MPs decreased the TN removal efficiency by 7.97 % via inhibiting both nitrification and denitrification, since 100 nm-MPs penetrated cell membranes, disrupted reactive oxygen species balance, and reduced bacterial viability, thus suppressing microbial nitrogen degradation by 8.07 % (p < 0.05). Additionally, 100 nm-MPs significantly inhibited plant growth and reduced plant nitrogen uptake by 16.05 % (p < 0.05). Furthermore, 100 µm-MPs increased the abundance of nitrifiers but reduced denitrifiers and functional genes, whereas 100 nm-MPs reduced the abundance of both nitrifiers and denitrifiers along with their functional genes (p < 0.05). These findings highlight the need to improve waste management to mitigate the adverse effects of MPs on nitrogen removal.

Metagenomic Insights into the Enhancement of Bioavailable Nitrogen in Continuous Cropping Soil Through the Application of Traditional Chinese Medicine Residue Following Fumigation

Background/Objectives: Chemical fumigation can effectively inhibit the occurrence of soil-borne diseases; however, this approach can negatively affect the structure of the soil microbial community. The combination of soil fumigant and organic fertilizer application thus represents a widely adopted strategy in agricultural practice. Traditional Chinese medicine residue (TCMR) is a high-quality organic fertilizer; however, the impact of post-fumigation TCMR application on keystone taxa and their functional traits remains uncertain. Methods: This study examined the effects of five fertilization treatments on the diversity, key species, and related functional genes of microbial communities in rhizosphere soil of continuous cropping pepper. Results: Chemical fumigation followed by TCMR application markedly enhanced soil nutrient content in the rhizosphere and significantly influenced microbial community composition as well as functional gene patterns associated with microbial nitrogen cycling. It was also strongly correlated with soil bioavailable nitrogen content. The abundance of keystone bacterial species (Pseudomonadota, Actinomycetota, and Bacillota) substantially increased following TCMR application, alongside a notable rise in Ascomycota abundance within the fungal community. This shift contributed to an increase in beneficial bacterial abundance while reducing that of harmful bacteria. Additionally, TCMR addition affected the abundance of denitrification and DNRA genes involved in nitrogen cycling; specifically, nirB and nirK were strongly associated with soil organic nitrogen content. Conclusions: The combined application of chemical fumigants and TCMR modified the composition of keystone microbial community species by influencing rhizosphere soil TN and other nutrients, and these alterations were linked to multiple nitrogen-cycling functional genes.

Ammonia Synthesis with Visible Light and Quantum Dots

Light-assisted synthesis of ammonia from nitrate and nitrite sources is a sustainable approach to reduce the burden of the energy-intensive Haber-Bosch process. However, poor selectivity and the need for UV-active photocatalysts are the current bottlenecks in the synthesis of ammonia from nitrate and nitrite sources. Herein, we introduce selective visible-light-driven ammonia production from nitrate and nitrite ions with indium phosphide quantum dots (InP QDs) as the photocatalyst. The presence of catalytic indium sites and microenvironment modulation through an interplay of catalyst-reactant interactions resulted in efficient and selective ammonia formation under visible light. Ammonia was produced in an attractive yield of ∼94% in both aqueous and gaseous phases within 2 h of visible-light irradiation at room temperature. A decent formation of ammonia was observed under sunlight as well, strengthening the translational prospects of InP QD photocatalysts. Mechanistic investigations ascertained a negligible role of competing hydrogen evolution in direct nitrate reduction, confirming the active participation of photoexcited charge carriers from InP QDs in the ammonia synthesis. Kinetic studies revealed the energetically challenging nitrate-to-nitrite conversion as the rate-determining step, with subsequent reactions proceeding with ∼100% conversion to yield ammonia. A series of experiments concluded that water is the proton source in the InP QD-photocatalyzed synthesis of ammonia. Our study shows the impact of the rationally designed core and surface of InP QD-based photocatalysts in developing sustainable routes to produce ammonia beyond the Haber-Bosch process.

Mechanistic insights into nitric oxide generation from nitrite via O-atom transfer in the unsymmetrical β-diketiminato copper(II) nitrite complex

In this study, we employed DFT calculations to elucidate the mechanism of NO generation from nitrite via PPh3-induced oxygen atom transfer (OAT) in the unsymmetrical β-diketiminato copper(II), LCuIIONO. We discovered that the OAT process involves the cooperation of two PPh3 ligands and follows the mechanism distinct from that of the symmetrical β-diketiminato analogue. The ΔG‡ value, calculated to be 34.8 kcal mol-1, closely matches experimental data. The finding is further supported by analyzing the OAT product yields with varying equivalents of PPh3. The penta-coordinated species 5a, with PPh3 occupying the axial site, forms in the final stage of the OAT process. The isomerization of 5a and the decoordination of the hemilabile pyridyl arm synergistically reduce Cu(II) to Cu(I), facilitating NO release from the Cu(I) centre. These computational results provide valuable insights for the ligand design for PPh3-induced OAT reactions to produce NO in Cu(II) nitrite systems.

Evolutionarily conserved core microbiota as an extended trait in nitrogen acquisition strategy of herbaceous species

Microbiota have co-evolved with plants over millions of years and are intimately linked to plants, ranging from symbiosis to pathogenesis. However, our understanding of the existence of a shared core microbiota across phylogenetically diverse plants remains limited. A common garden field experiment was conducted to investigate the rhizosphere microbial communities of phylogenetically contrasting herbaceous families. Through a combination of metagenomic sequencing, analysis of plant economic traits, and soil biochemical properties, we aimed to elucidate the eco-evolutionary role of the core rhizosphere microbiota in light of plant economic strategies. We identified a conserved core microbiota consisting of 278 taxa that was closely associated with the phylogeny of the plants studied. This core microbiota actively participated in multiple nitrogen metabolic processes and showed a strong correlation with the functional potential of rhizosphere nitrogen cycling, thereby serving as an extended trait in the plant nitrogen acquisition. Furthermore, our examination of simulated species loss revealed the crucial role of the core microbiota in maintaining the rhizosphere community's network stability. Our study highlighted that the core microbiota, which exhibited a phylogenetically conserved association with plants, potentially represented an extension of the plant phenotype and played an important role in nitrogen acquisition. These findings held implications for the utilization of microbiota-mediated plant functions.

Discussing the Terms Biomimetic and Bioinspired within Bioinorganic Chemistry

The terms biomimetic and bioinspired are very relevant in the field of bioinorganic chemistry and have been widely applied. Although they were defined by the International Organization for Standardization in 2015, these terms have at times been used rather ambiguously in the literature. This may be due to the inherent complexity of bioinorganic systems where, for example, a structural model of an enzyme active site may not replicate its function. Conversely, the function of an enzyme may be reproduced in a system where the structure does not resemble the enzyme's active site. To address this, we suggest definitions for the terms biomimetic and bioinspired wherein structure and function have been decoupled. With the help of some representative case studies we have outlined the challenges that may arise and make suggestions on how to apply terminology with careful intention.

Electrokinetics of Nitrite to Ammonia Conversion in the Neutral Medium Over A Platinum Surface

Polycrystalline Pt electrode was employed to selectively convert nitrite ions (N O 2 - ${{{\rm N}{\rm O}}_{2}^{-}}$ ) into useful nitrogenous compound through electrochemical reduction reaction in neutral medium. According to adsorptive stripping analysis, the reduction process produced nitric oxide (NO) on the surface of Pt electrode. The spectroscopic test and gas chromatographic studies discovered the presence of ammonia (NH3) in the electrolyzed solution, suggesting the transformation of adsorbed NO into NH3 during the reverse scan. Scan rate dependent investigation was performed to elucidate kinetic information relating to this reaction on Pt surface. From Ep vs scan rate (υ) and jp vs υ (logarithmic plot), it was found that the conversion ofN O 2 - ${{{\rm N}{\rm O}}_{2}^{-}}$ ion into NO is an irreversible reaction which relies on the diffusion ofN O 2 - ${{{\rm N}{\rm O}}_{2}^{-}}$ ions to electrode surface. The Tafel analysis unveiled that the first electron transfer sets the overall reaction rate, having formal reduction potential, E0'=-0.46 V and standard heterogeneous rate constant, k0=1 . 07 × 10 - 2 ${1.07\times {10}^{-2}}$ cm s-1. Reductive transfer coefficient (α) is another kinetics parameter, which was found to be approximate 0.77 from the difference between Ep and Ep/2 of the voltammograms obtained over scan rate range 0.005 V s-1 to 0.250 V s-1, indicating a stepwise process. According to temperature-dependent voltammograms, the nitrite reduction reaction on Pt had a calculated activation energy of about 19.8 kJ mol-1 and a pre-exponential factor of about 8.39×103 mA cm-2.

Role of ancillary ligands in S-nitrosothiol and NO generation from nitrite-thiol interactions at mononuclear zinc(ii) sites

Generation of S-nitrosothiol (RSNO) and nitric oxide (NO) mediated by zinc(ii) coordination motifs is of prime importance for understanding the role of zinc(ii)-based cofactors in redox-signalling pathways. This study uniquely employs a set of mononuclear [L2ZnII]2+ cores (where L = Me4PzPz/Me2PzPy/Me2PzQu) for introducing subtle alterations of the primary coordination sphere and investigates the role of ligand tuning in the transformation of NO2 - in the presence of thiols. Single crystal X-ray diffraction (SCXRD) analyses on [L2ZnII-X](X) (where X = perchlorate/triflate) illustrate consistent changes in the bond distances, thereby showing variations of the metal-ligand interactions depending on the nature of the heterocyclic donor arms (pyrazole/pyridine/quinoline). Moreover, such tuning of the ligands affects the Lewis-acidity of the [L2ZnII]2+ cores as evaluated by 31P NMR and SCXRD studies on the 1 : 1 acid-base adducts [L2ZnII(OPEt3)]2+. Crystallographic and 15N NMR spectroscopic analyses on the nitrite complexes [L2ZnII(κ2-nitrite)](ClO4) reveal that the chemical environments of the nitrite anions in these complexes are nearly identical, despite the dissimilarity in the Lewis-acidity of the [L2ZnII]2+ cores. Interestingly, RSNO and NO generation from the reactions of [L2ZnII(κ2-nitrite)](ClO4) with 4-tert-butylbenzylthiol ( t BuBnSH) exhibits that the [(Me2PzQu)2ZnII]2+ core is the most efficient in promoting nitrite-thiol interactions due to the ease of available hemilabile coordination sites at the Lewis acidic [ZnII]. Detailed UV-vis studies in tandem with computational investigation, for the first time, provide an unambiguous demonstration of the nitrous acid (HNO2) intermediate generated through an intramolecular proton-transfer from thiol to nitrite at zinc(ii).

Insights into the individual and combined effects of Cu(Ⅱ) and Ni(Ⅱ) on anammox: Nitrogen removal performance, enzyme activity and microbial community

Anaerobic ammonium oxidation (anammox) is an efficient and economical nitrogen removal process for treating ammonium-rich industrial wastewaters. However, Cu(Ⅱ) and Ni(Ⅱ) present in industrial wastewaters are toxic to anaerobic ammonium-oxidizing bacteria (AnAOB). Unfortunately, the effects of Cu(Ⅱ) and Ni(Ⅱ) on anammox have not been thoroughly investigated, especially when Cu(Ⅱ) and Ni(Ⅱ) coexist. This work comprehensively investigated the individual and combined effects of Cu(Ⅱ) and Ni(Ⅱ) on anammox and revealed the inhibitory mechanisms. With the influent NH4+-N and NO2--N concentration of 230 and 250 mg L-1, the inhibition thresholds on anammox are 2.00 mg L-1 Cu(Ⅱ), 1.00 mg L-1 Ni(Ⅱ) and 1.00 mg L-1 Cu(Ⅱ) + 1.00 mg L-1 Ni(Ⅱ), and higher Cu(Ⅱ) or Ni(Ⅱ) concentrations resulted in sharp deteriorations of nitrogen removal performance. The inhibition of Ni(Ⅱ) on anammox was mainly attributed to the adverse effect on NiR activity, while the inhibition mechanism of Cu(Ⅱ) seemed to be unrelated to the four functional enzymes, but associated with disruption of cellular and organellar membranes. The behavior of extracellular polymeric substances (EPS) contributed to the antagonistic effect between Cu(Ⅱ) and Ni(Ⅱ) on anammox. In addition, the niche of Candidatus Brocadia and Candidatus Jettenia shifted under the Cu(II) and Ni(II) stress, and Candidatus Jettenia displayed greater tolerance to Cu(II) and Ni(II) stress. In conclusion, this research clarified the combined effect and the inhibitory mechanism of multiple heavy metals on anammox, and provide the guidances for anammox process application in treating high-ammonium industrial wastewaters containing heavy metals.

Electrochemical Synthesis of Urea: Co-Reduction of Nitrite and Carbon Dioxide on Binuclear Cobalt Phthalocyanine

Exploration of molecular catalysts with the atomic-level tunability of molecular structures offers promising avenues for developing high-performance catalysts for the electrochemical co-reduction reaction of carbon dioxide (CO2) and nitrite (NO2 -) into value-added urea. In this work, a binuclear cobalt phthalocyanine (biCoPc) catalyst is prepared through chemical synthesis and applied as a C─N coupling catalyst toward urea. Achieving a remarkable Faradaic efficiency of 47.4% for urea production at -0.5 V versus reversible hydrogen electrode (RHE), this biCoPc outperforms many known molecular catalysts in this specific application. Its unique planar macromolecular structure and the increased valence state of cobalt promote the adsorption of nitrogenous and carbonaceous species, a critical factor in facilitating the multi-electron C─N coupling. Combining highly sensitive in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) with density functional theory (DFT) calculations, the linear adsorbed CO (COL) and bridge adsorbed CO (COB) is captured on biCoPc catalyst during the co-reduction reaction. COB, a pivotal intermediate in the co-reduction from CO2 and nitrite to urea, is evidenced to be labile and may be attacked by nitrite, promoting urea production. This work demonstrates the importance of designing molecular catalysts for efficient co-reduction of CO2 and nitrite to urea.

Nitrate Upcycling Mediated by Organonickel Catalysis

Nitrogen oxides (NOx) are major environmental pollutants and to neutralize this long-term environmental threat, new catalytic methods are needed. Although there are biological denitrification processes involving four different enzymatic reactions to convert nitrate (NO3 -) into dinitrogen (N2), it is unfortunately difficult to apply in industry due to the complexity of the processes. In particular, nitrate is difficult to functionalize because of its chemical stability. Thus, there is no organometallic catalysis to convert nitrate into useful chemicals. Herein, we present a nickel pincer complex that is effective as a bifunctional catalyst to stepwise deoxygenate NO3 - by carbonylation and further through C-N coupling. By using this nickel catalysis, nitrate salts can be selectively transformed into various oximes (>20 substrates) with excellent conversion (>90 %). Here, we demonstrate for the first time that the highly inert nitrate ion can be functionalized to produce useful chemicals by a new organonickel catalysis. Our results show that the NOx conversion and utilization (NCU) technology is a successful pathway for environmental restoration coupled with value-added chemical generation.

Microbial response to the chronic toxicity effect of graphene and graphene oxide nanomaterials within aerobic granular sludge systems

Nanomaterials present in wastewater can pose a significant threat to aerobic granular sludge (AGS) systems. Herein, we found that compared to graphene nanomaterials (G-NMs), the long-term presence (95 days) of graphene oxide nanomaterials (GO-NMs) resulted in an increased proliferation of filamentous bacteria, poorer sedimentation performance (SVI30 of 74.1 mL/g) and smaller average particle size (1224.4 µm) of the AGS. In particular, the GO-NMs posed a more significant inhibitory effect to the total nitrogen removal efficiency of AGS (decreased by 14.3 %), especially for the denitrification process. The substantial accumulation of GO-NMs within the sludge matrix resulted in a higher level of reactive oxygen species in AGS compared to G-NMs, thereby inducing lactate dehydrogenase release, and enhancing superoxide oxidase and catalase activities. Such excessive oxidative stress could potentially result in a significant reduction in the activity of nitrogen metabolism enzymes (e.g., nitrate reductase and nitrite reductase) and the expression of key functional genes (e.g., nirS and nirK). Altogether, compared to G-NMs, prolonged exposure to GO-NMs had a more significant chronic toxicity effect on AGS systems. These findings implied that the presence of G-NMs and GO-NMs is a hidden danger to biological nitrogen removal and should receive more attention.

Do oral and gut microbiota communicate through redox pathways? A novel asset of the nitrate-nitrite-NO pathway

Nitrate may act as a regulator of •NO bioavailability via sequential reduction along the nitrate-nitrite-NO pathway with widespread health benefits, including a eubiotic effect on the oral and gut microbiota. Here, we discuss the molecular mechanisms of microbiota-host communication through redox pathways, via the production of •NO and oxidants by the family of NADPH oxidases, namely hydrogen peroxide (via Duox2), superoxide radical (via Nox1 and Nox2) and peroxynitrite, which leads to downstream activation of stress responses (Nrf2 and NFkB pathways) in the host mucosa. The activation of Nox2 by microbial metabolites is also discussed. Finally, we propose a new perspective in which both oral and gut microbiota communicate through redox pathways, with nitrate as the pivot linking both ecosystems.

Nitrite reductase-mimicking catalysis temporally regulating nitric oxide concentration gradient adaptive for antibacterial therapy

The unique bacterial infection microenvironment (IME) usually requires complicated design of nanomaterials to adapt to IME for enhancing antibacterial therapy. Here, an alternative IME adaptative nitrite reductase-mimicking nanozyme is constructed by in situ growth of ultrasmall copper sulfide clusters on the surface of a nanofibrillar lysozyme assembly (NFLA/CuS NHs), which can temporally regulate nitric oxide (NO) gradient concentration to kill bacteria initially and promote tissue regeneration subsequently. Benefiting from a copper nitrite reductase (CuNIR)-inspired structure with CuS cluster as active center and NFLA as skeleton, NFLA/CuS NHs efficiently boost the catalytic reduction of nitrite to NO. The inherent supramolecular fibrillar networks displays excellent bacterial capture capability, facilitating initial high-concentration NO attacks on the bacteria. The subsequent catalytic release of low-concentration NO by NFLA/CuS NHs-mediated nitrite reduction remarkably promotes cell migration and angiogenesis. This work paves the way for dynamically eliminating MDR bacterial infection and promoting tissue regeneration in a simple and smart way through CuNIR-mimicking catalysis.

Cascade Electrocatalytic Reduction of Nitrate to Ammonia Using a Heterobimetallic Covalent Organic Framework Composed of Cu-Porphyrin and Co-Bipyridine

The electrocatalytic reduction of nitrate (NO3-) to ammonia (NH3) not only offers an effective solution to environmental problems caused by the accumulation of NO3- but also provides a sustainable alternative to the Haber-Bosch process. However, the conversion of NO3- to NH3 is a complicated process involving multiple steps, leading to a low Faradaic efficiency (FE) for NH3 production. The structural designability of covalent organic frameworks (COFs) renders feasible and precise modulation at the molecular level, facilitating the incorporation of multiple well-defined catalytic sites with different reactivities into a cohesive entity. This promotes the efficiency of the overall reaction through the coupling of multistep reactions. Herein, heterobimetallic CuP-CoBpy was prepared by postmodification, involving the anchoring of cobalt ions to the CuP-Bpy structure. As a result of the cascade effect of the bimetallic sites, CuP-CoBpy achieved an outstanding NH3 yield of 13.9 mg h-1 mgcat.-1 with a high FE of 96.7% at -0.70 V versus the reversible hydrogen electrode and exhibited excellent stability during catalysis. A series of experimental and theoretical studies revealed that the CuP unit facilitates the conversion of NO3- to NO2-, while the CoBpy moiety significantly prompts the reduction of NO2- to NH3. This study demonstrates that tailoring the structural units for the construction of COFs based on each step in the multistep reaction can enhance both the catalytic activity and product selectivity of the overall process.

Design and synthesis of a nucleobase functionalized peptide hydrogel: in vitro assessment of anti-inflammatory and wound healing effects

Over the past several years, a significant increase in the expanding field of biomaterial sciences has been observed due to the development of biocompatible materials based on peptide derivatives that have intrinsic therapeutic potential. In this report, we synthesized nucleobase functionalized peptide derivatives (NPs). Hydrogelation in the synthesized NPs was induced by increasing their hydrophobicity with an aromatic moiety. The aggregation behavior of the NPs was analyzed by performing molecular dynamics simulations and DOSY NMR experiments. We performed circular dichroism (CD), thioflavin-T binding and PXRD to characterize the supramolecular aggregation in the NP1 hydrogel. The mechanical strength of the NP1 hydrogel was tested by performing rheological experiments. TEM and SEM experiments were performed to investigate the morphology of the NP1 hydrogel. The biocompatibility of the newly synthesized NP1 hydrogel was investigated using McCoy and A549 cell lines. The hemolytic activity of the NP1 hydrogel was examined in human blood cells. The stability of the newly formed NP1 hydrogel was examined using proteinase K and α-chymotrypsin. The NP1 hydrogel was used for in vitro wound healing. Western blotting, qRT-PCR and DCFDA assay were performed to determine the anti-inflammatory activity of the NP1 hydrogel. The synthesized NP1 hydrogel also exhibits antibacterial efficacy.

Nitrogen monoxide and calix[4]pyrrolato aluminate: structural constraint enabled NO dimerization

The dimerization of nitrogen monoxide (NO) is highly relevant in homo- and heterogeneous biochemical and environmental redox processes, but a broader understanding is challenged by the endergonic nature of this equilibrium. The present work describes NO-dimerization leveraged by structurally constrained aluminum and metal-ligand cooperativity at the anionic calix[4]pyrrolato aluminate(III). Quantum chemical calculations reveal the driving force for N-N bond formation, while reactivity tests shed light on subsequent redox chemistry and NO decomposition at metal surfaces. Inhibiting the dimerization pathway by saturating NO's unpaired electron with a phenyl group (nitrosobenzene) allows trapping the 1,2-adduct as a key intermediate. Elevated temperatures result in an unprecedented and high-yielding rearrangement of the calix[4]pyrrolato ligand scaffold. Kinetic and theoretical studies provide a comprehensive picture of the rearrangement mechanism and delineate systematics for ring modification of the prominent calix[4]pyrrole macrocycle.

Effects of Weak Electric Fields on the Denitrification Performance of Pseudomonas stutzeri: Insights into Enzymes and Metabolic Pathways

Enhanced denitrification has been reported under weak electric fields. However, it is difficult to investigate the mechanism of enhanced denitrification due to the complex interspecific interactions of mixed-culture systems. In this study, Pseudomonas stutzeri, capable of denitrification under anaerobic conditions, was selected for treating low COD/N (2.0, ratio between concentration of chemical oxygen demand and NO3--N) artificial wastewater under constant external voltages of 0.2, 0.4, and 0.6 V. The results revealed that P. stutzeri exhibited the highest efficiency in nitrate reduction at 0.2 V. Moreover, the maximum nitrate removal rate was 15.96 mg/(L·h) among the closed-circuit groups, 19.39% higher than that under the open-circuit group. Additionally, a notable reduction in nitrite accumulation was observed under weak electric fields. Enzyme activity analysis showed that the nitrate reductase activities were significantly increased among the closed-circuit groups, while nitrite reductase activities were inhibited. Transcriptomic analysis indicated that amino acid metabolism, carbohydrate metabolism, and energy metabolism were increased, enhancing the resistance of P. stutzeri to environmental stress and the efficiency of carbon source utilization for denitrification. The current study examined the impacts of weak electric fields on enzyme activities and microbial metabolic pathways and offers valuable insights into the mechanism by which denitrification is enhanced by weak electric fields.

Effects of bagasse as a carbon source on biofloc formation, water quality, and microbial community structure in shrimp culture system

As a widely available, low-cost agricultural byproduct, bagasse is a potential solid carbon source and provides microbial attachment as a biofilm carrier. In this study, the effects of bagasse as a carbon source on biofloc formation, water quality, microbial community structure, and nitrogen conversion in a shrimp culture system were explored, and the performance of bagasse bioflocs was assessed. No bagasse was added to the control group (CK), and three bagasse addition groups were set up, with the floc content of the water maintained at 5 mL/L (BF5 group), 10 mL/L (BF10 group), and 15 mL/L (BF15 group). The results showed that bagasse bioflocs formed in the fourth week when bagasse was placed in the culture water, and the surface of bagasse was covered with thick biofilm at that time. The DOC content of the BF15 group was significantly greater than that of the CK group, from 30.31 to 105.06% (P < 0.05), and the DOC increased with increasing bagasse biofloc content. The BF group rapidly converted TAN to NO2--N and then to NO3--N because the accumulation of nitrite nitrogen in the BF15 group occurred 1 week earlier than in the other groups; at the 8th week, the nitrite nitrogen conversion rate of each BF group was close to 100%, which was significantly greater than that of the CK group (P < 0.05). The relative abundances of genes encoding microbial glutamate dehydrogenase and glutamate synthase increased in the bagasse biofloc groups (P < 0.05). The relative abundances of genes from Rhodobacterales and Hyphomicrobiales in each group were greater, but bagasse bioflocs increased the proportion of Hyphomicrobiale. In summary, adding bagasse to the shrimp culture system can form a biofloc system, resulting in the formation of a rich bacterial biofilm on its surface. Bagasse addition not only affects the composition of microbial communities but also accelerates the nitrification process in water. As a result, ammonia and nitrite are converted into nitrate, which is essential for maintaining the stability of the ecosystem balance in aquaculture water.

Reactivity of Thiolate and Hydrosulfide with a Mononuclear {FeNO}7 Complex Featuring a Very High N-O Stretching Frequency

Synthesis, characterization, electronic structure, and redox reactions of a mononuclear {FeNO}7 complex with a very high N-O stretching frequency in solution are presented. Nitrosylation of [(LKP)Fe(DMF)]2+ (1) (LKP = tris((1-methyl-4,5-diphenyl-1H-imidazol-2-yl)methyl)amine) produced a five-coordinate {FeNO}7 complex, [(LKP)Fe(NO)]2+ (2). While complex 2 could accommodate an additional water molecule to generate a six-coordinate {FeNO}7 complex, [(LKP)Fe(NO)(H2O)]2+ (3), the coordinated H2O in 3 dissociates to generate 2 in solution. The molecular structure of 2 features a nearly linear Fe-N-O unit with an Fe-N distance of 1.744(4) Å, N-O distance of 1.162(5) Å, and Collapse

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Affiliation(s)

Soumik Karmakar School of Chemical Sciences, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India
Suman Patra School of Chemical Sciences, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India
Koushik Pramanik School of Chemical Sciences, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India
Amit Adhikary Department of Chemistry, Technology Campus, University of Calcutta, JD Block, Sector III, Salt Lake, Kolkata 700098, India
Abhishek Dey School of Chemical Sciences, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India
Amit Majumdar School of Chemical Sciences, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India

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A novel mechanism for dissimilatory nitrate reduction to ammonium in Acididesulfobacillus acetoxydans

The biological route of nitrate reduction has important implications for the bioavailability of nitrogen within ecosystems. Nitrate reduction via nitrite, either to ammonium (ammonification) or to nitrous oxide or dinitrogen (denitrification), determines whether nitrogen is retained within the system or lost as a gas. The acidophilic sulfate-reducing bacterium (aSRB) Acididesulfobacillus acetoxydans can perform dissimilatory nitrate reduction to ammonium (DNRA). While encoding a Nar-type nitrate reductase, A. acetoxydans lacks recognized nitrite reductase genes. In this study, A. acetoxydans was cultivated under conditions conducive to DNRA. During cultivations, we monitored the production of potential nitrogen intermediates (nitrate, nitrite, nitric oxide, hydroxylamine, and ammonium). Resting cell experiments were performed with nitrate, nitrite, and hydroxylamine to confirm their reduction to ammonium, and formed intermediates were tracked. To identify the enzymes involved in DNRA, comparative transcriptomics and proteomics were performed with A. acetoxydans growing under nitrate- and sulfate-reducing conditions. Nitrite is likely reduced to ammonia by the previously undescribed nitrite reductase activity of the NADH-linked sulfite reductase AsrABC, or by a putatively ferredoxin-dependent homolog of the nitrite reductase NirA (DEACI_1836), or both. We identified enzymes and intermediates not previously associated with DNRA and nitrosative stress in aSRB. This increases our knowledge about the metabolism of this type of bacteria and helps the interpretation of (meta)genome data from various ecosystems on their DNRA potential and the nitrogen cycle.IMPORTANCENitrogen is crucial to any ecosystem, and its bioavailability depends on microbial nitrogen-transforming reactions. Over the recent years, various new nitrogen-transforming reactions and pathways have been identified, expanding our view on the nitrogen cycle and metabolic versatility. In this study, we elucidate a novel mechanism employed by Acididesulfobacillus acetoxydans, an acidophilic sulfate-reducing bacterium, to reduce nitrate to ammonium. This finding underscores the diverse physiological nature of dissimilatory reduction to ammonium (DNRA). A. acetoxydans was isolated from acid mine drainage, an extremely acidic environment where nitrogen metabolism is poorly studied. Our findings will contribute to understanding DNRA potential and variations in extremely acidic environments.

Technical Evaluation of a Paper-Based Electrochemical Strip to Measure Nitrite Ions in the Forensic Field

Nitrite is a compound used as a food additive for its preservative action and coloring capability, as well as an industrial agent for its antifreezing action and for preventing corrosion, and it is also used as a pharmaceutical in cyanide detoxification therapy. However, even recently, because of its high toxicity, it has been used as a murder and suicidal agent due to its affordability and ready availability. In this technical report, we describe an electrochemical paper-based device for selectively determining nitrite in complex biofluids, such as blood, cadaveric blood, vitreous humor, serum, plasma, and urine. The approach was validated in terms of the linearity of response, selectivity, and sensitivity, and the accuracy of the determination was verified by comparing the results with a chromatographic instrumental method. A linear response was observed in the micromolar range; the sensitivity of the method expressed as the limit of detection was 0.4 μM in buffer measurements. The simplicity of use, the portability of the device, and the performance shown make the approach suitable for detecting nitrite in complex biofluids, including contexts of forensic interest, such as murders or suicides in which nitrite is used as a toxic agent. Limits of detection of ca. 1, 2, 4, 5, 3, and 4 μM were obtained in vitreous humor, urine, serum and plasma, blood, and cadaveric blood, also highlighting a satisfactory accuracy comprised between 91 and 112%.

Bioelectro-barriers prevent nitrate leaching into groundwater via nitrogen retention

Groundwater, the main freshwater resource for humans, has been widely contaminated with nitrate from fertilizers. Here, we report a new and chemical-free strategy to prevent nitrate leaching from soil based on the enrichment of electroactive bacteria, mainly of the genus Geobacter, with bioelectro-barriers, which leads to a nearly 100 % interception of nitrate and partly conserves reactive nitrogen in the form of weakly mobile ammonium by dissimilatory nitrate reduction to ammonium (DNRA). G. sulfurreducens was recognized to efficiently secrete nitrite reductase (NrfA) for rapid DNRA because it lacks nitrate reductase, which inhibits DNRA by competing with nitrite and producing toxic intracellular nitric oxide. With an increase in G. sulfurreducens abundance, near-zero nitrate leaching and 3-fold greater N retention was achieved. Periodic application of weak electricity to the bioelectro-barrier ensured the dominance of G. sulfurreducens in the microbial community and therefore its ability to consistently prevent nitrate leaching. The ability of G. sulfurreducens to intercept nitrate was further demonstrated in more diverse agricultural soils, providing a novel way to prevent nitrate leaching and conserve bioavailable nitrogen in the soil, which has broader implications for both sustainable agriculture and groundwater protection.

Phase-Regulated Active Hydrogen Behavior on Molybdenum Disulfide for Electrochemical Nitrate-to-Ammonia Conversion

Electrochemical reduction of nitrate waste is promising for environmental remediation and ammonia preparation. This process includes multiple hydrogenation steps, and thus the active hydrogen behavior on the surface of the catalyst is crucial. The crystal phase referred to the atomic arrangements in crystals has a great effect on active hydrogen, but the influence of the crystal phase on nitrate reduction is still unclear. Herein, enzyme-mimicking MoS2 in different crystal phases (1T and 2H) are used as models. The Faradaic efficiency of ammonia reaches ≈90 % over 1T-MoS2 , obviously outperforming that of 2H-MoS2 (27.31 %). In situ Raman spectra and theoretical calculations reveal that 1T-MoS2 produces more active hydrogen on edge S sites at a more positive potential and conducts an effortless pathway from nitrate to ammonia instead of multiple energetically demanding hydrogenation steps (such as *HNO to *HNOH) performed on 2H-MoS2 .

Reduction of nitrite to nitric oxide and generation of reactive chalcogen species by mononuclear Fe(II) and Zn(II) complexes of thiolate and selenolate

Comparative reactivity of a series of new Zn(II) and Fe(II) compounds, [(Py2ald)M(ER)] (E = S, R = Ph: M = Zn, 1aZn; M = Fe, 1aFe; E = S, R = 2,6-Me2-C6H3: M = Zn, 1bZn; M = Fe, 1bFe; E = Se, R = Ph: M = Zn, 2Zn; M = Fe, 2Fe), and [(Py2ald)M]22+ (M = Zn, 5Zn; M = Fe, 5Fe) is presented. Compound 1aZn could react with nitrite (NO2-) to produce [(Py2ald)Zn(ONO)] (3Zn), which, upon treatment with thiols and PhSeH (proton source), could regenerate either 1aZn/5Zn and 2Zn respectively, along with the production of nitric oxide (NO) where the yield of NO increases in the order tBuSH ≪ PhCH2SH < PhSH < PhSeH. In contrast to this, 1aFe, 2Fe and 5Fe could affect the direct reduction of NO2- in the absence of protons to generate NO and [{(Py2ald)(ONO)Fe}2-μ2-O] (8Fe). Moreover, 8Fe could regenerate 5Fe and 1aFe/2Fe upon treatment with 4 and 6 equiv. of PhEH (E = S/Se), respectively, along with the generation of NO. Finally, a comparative study of the mononuclear Zn(II) and Fe(II) compounds for the transfer of the coordinated thiolate/selenolate and the generation and transfer of reactive sulfur/selenium species (RES-, E = Se, S) to a series of organic substrates has been provided.

How to Provide Nitrite Robustly for Anaerobic Ammonium Oxidation in Mainstream Nitrogen Removal

Innovation in decarbonizing wastewater treatment is urgent in response to global climate change. The practical implementation of anaerobic ammonium oxidation (anammox) treating domestic wastewater is the key to reconciling carbon-neutral management of wastewater treatment with sustainable development. Nitrite availability is the prerequisite of the anammox reaction, but how to achieve robust nitrite supply and accumulation for mainstream systems remains elusive. This work presents a state-of-the-art review on the recent advances in nitrite supply for mainstream anammox, paying special attention to available pathways (forward-going (from ammonium to nitrite) and backward-going (from nitrate to nitrite)), key controlling strategies, and physiological and ecological characteristics of functional microorganisms involved in nitrite supply. First, we comprehensively assessed the mainstream nitrite-oxidizing bacteria control methods, outlining that these technologies are transitioning to technologies possessing multiple selective pressures (such as intermittent aeration and membrane-aerated biological reactor), integrating side stream treatment (such as free ammonia/free nitrous acid suppression in recirculated sludge treatment), and maintaining high activity of ammonia-oxidizing bacteria and anammox bacteria for competing oxygen and nitrite with nitrite-oxidizing bacteria. We then highlight emerging strategies of nitrite supply, including the nitrite production driven by novel ammonia-oxidizing microbes (ammonia-oxidizing archaea and complete ammonia oxidation bacteria) and nitrate reduction pathways (partial denitrification and nitrate-dependent anaerobic methane oxidation). The resources requirement of different mainstream nitrite supply pathways is analyzed, and a hybrid nitrite supply pathway by combining partial nitrification and nitrate reduction is encouraged. Moreover, data-driven modeling of a mainstream nitrite supply process as well as proactive microbiome management is proposed in the hope of achieving mainstream nitrite supply in practical application. Finally, the existing challenges and further perspectives are highlighted, i.e., investigation of nitrite-supplying bacteria, the scaling-up of hybrid nitrite supply technologies from laboratory to practical implementation under real conditions, and the data-driven management for the stable performance of mainstream nitrite supply. The fundamental insights in this review aim to inspire and advance our understanding about how to provide nitrite robustly for mainstream anammox and shed light on important obstacles warranting further settlement.

Visible-Light-Driven Iron-Catalyzed Decarboxylative C-N Coupling Reaction of Alkyl Carboxylic Acids with NaNO2

An efficient visible-light-driven iron-catalyzed decarboxylative C-N coupling reaction of alkyl carboxylic acids with NaNO2 under mild conditions was developed. The reaction proceeds under photosensitizer-free conditions and features good to excellent yields, broad functional group tolerance, and an easy operation procedure. Preliminary mechanistic investigations showed that visible-light-driven iron catalysis not only achieved oxidative decarboxylation of alkyl carboxylic acids to alkyl radicals but also promoted the reduction of NO2- to NO, thus leading to the C-N radical coupling reaction.

Nitrite and Nitric Oxide Interconversion at Mononuclear Copper(II): Insight into the Role of the Red Copper Site in Denitrification

Nitrite (NO2 - ) and nitric oxide (NO) interconversion is crucial for maintaining optimum NO flux in mammalian physiology. Herein we demonstrate that [L2 CuII (nitrite)]+ moieties (in 2 a and 2 b; where, L = Me2 PzPy and Me2 PzQu) with distorted octahedral geometry undergo facile reduction to provide tetrahedral [L2 CuI ]+ (in 3 a and 3 b) and NO in the presence of biologically relevant reductants, such as 4-methoxy-2,6-di-tert-butylphenol (4-MeO-2,6-DTBP, a tyrosine model) and N-benzyl-1,4-dihydronicotinamide (BNAH, a NAD(P)H model). Interestingly, the reaction of excess NO gas with [L2 CuII (MeCN)2 ]2+ (in 1 a) provides a putative {CuNO}10 species, which is effective in mediating the nitrosation of various nucleophiles, such as thiol and amine. Generation of the transient {CuNO}10 species in wet acetonitrile leads to NO2 - as assessed by Griess assay and 14 N/15 N-FTIR analyses. A detailed study reveals that the bidirectional NOx -reactivity, namely, nitrite reductase (NIR) and NO oxidase (NOO), at a common CuII site, is governed by the geometric-preference-driven facile CuII /CuI redox process. Of broader interest, this study not only highlights potential strategies for the design of copper-based catalysts for nitrite reduction, but also strengthens the previous postulates regarding the involvement of red copper proteins in denitrification.

Two-dimensional molybdenum disulfide nanosheets evoke nitric oxide-dependent antibacterial effects

Nanomaterials are currently being explored as novel antimicrobial agents. In this study, we first investigated the ability of two-dimensional (2D) molybdenum disulfide (MoS2) nanosheets to trigger neutrophil extracellular traps (NETs) using neutrophil-differentiated HL-60 cells as well as primary human peripheral blood neutrophils. We then addressed whether the MoS2 nanosheets themselves function as antibacterial agents. We found that MoS2 and Na2MoO4 both triggered NETs, as evidenced by the quantification of neutrophil elastase (NE) activity and immunofluorescence staining of extracellular NE, as well as scanning electron microscopy. The release of NETs was found to be nitric oxide (NO)-dependent. We also found that the MoS2 nanosheets but not the soluble salt prompted acellular NO production in the presence of NaNO2. The acellular generation of NO, suggestive of nanozyme properties of the MoS2 nanosheets, was demonstrated by electron paramagnetic resonance analysis. Electrochemical analysis using cyclic voltammetry confirmed the redox transition of the MoS2 nanosheets. Finally, MoS2 nanosheets inhibited the growth of Escherichia coli in the presence of sodium nitrate. Taken together, MoS2 nanosheets triggered cellular effects as well as acellular antibacterial effects, and we provided evidence for nitrite reductase-like properties of MoS2.