Nitrating Agent Research Articles

Bio-inspired nitrogen oxide (NOx) interconversion reactivities of synthetic heme Compound-I and Compound-II intermediates

Dioxygen activating heme enzymes have long predicted to be powerhouses for nitrogen oxide interconversion, especially for nitric oxide (NO) oxidation which has far-reaching biological and/or environmental impacts. Lending credence, reactivity of NO with high-valent heme‑oxygen intermediates of globin proteins has recently been implicated in the regulation of a variety of pivotal physiological events such as modulating catalytic activities of various heme enzymes, enhancing antioxidant activity to inhibit oxidative damage, controlling inflammatory and infectious properties within the local heme environments, and NO scavenging. To reveal insights into such crucial biological processes, we have investigated low temperature NO reactivities of two classes of synthetic high-valent heme intermediates, Compound-II and Compound-I. In that, Compound-II rapidly reacts with NO yielding the six-coordinate (NO bound) heme ferric nitrite complex, which upon warming to room temperature converts into the five-coordinate heme ferric nitrite species. These ferric nitrite complexes mediate efficient substrate oxidation reactions liberating NO; i.e., shuttling NO2− back to NO. In contrast, Compound-I and NO proceed through an oxygen-atom transfer process generating the strong nitrating agent NO2, along with the corresponding ferric nitrosyl species that converts to the naked heme ferric parent complex upon warmup. All reaction components have been fully characterized by UV–vis, 2H NMR and EPR spectroscopic methods, mass spectrometry, elemental analyses, and semi-quantitative determination of NO2− anions. The clean, efficient, potentially catalytic NOx interconversions driven by high-valent heme species presented herein illustrate the strong prospects of a heme enzyme/O2/NOx dependent unexplored territory that is central to human physiology, pathology, and therapeutics.

A Useful Method for the Conversion of Olefins to Nitro Olefins.

Triflyl nitrate is easily generated from tetra-n-butylammonium nitrate in CH2Cl2 solution and serves as an effective nitrating agent for a wide range of unsaturated substrates to form nitro olefins.

Cardiolipin interactions with cytochrome c increase tyrosine nitration yields and site-specificity

The interaction between cytochrome c and cardiolipin is a relevant process in the mitochondrial redox homeostasis, playing roles in the mechanism of electron transfer to cytochrome c oxidase and also modulating cytochrome c conformation, reactivity and function. Peroxynitrite is a widespread nitrating agent formed in mitochondria under oxidative stress conditions, and can result in the formation of tyrosine nitrated cytochrome c. Some of the nitro-cytochrome c species undergo conformational changes at physiological pH and increase its peroxidase activity. In this work we evaluated the influence of cardiolipin on peroxynitrite-mediated cytochrome c nitration yields and site-specificity. Our results show that cardiolipin enhances cytochrome c nitration by peroxynitrite and targets it to heme-adjacent Tyr67. Cytochrome c nitration also modifies the affinity of protein with cardiolipin. Using a combination of experimental techniques and computer modeling, it is concluded that structural modifications in the Tyr67 region are responsible for the observed changes in protein-derived radical and tyrosine nitration levels, distribution of nitrated proteoforms and affinity to cardiolipin. Increased nitration of cytochrome c in presence of cardiolipin within mitochondria and the gain of peroxidatic activity could then impact events such as the onset of apoptosis and other processes related to the disruption of mitochondrial redox homeostasis.

Process optimization and thermal risk analysis for 1,3,5-trihydroxy-2,4,6-trinitrobenzenen synthesis

1,3,5-Trihydroxy-2,4,6-trinitrobenzene (TNPG) is an important precursor material for high-performance explosive 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). In this work, TNPG was synthesized by nitration of phloroglucinol (PG) using the nitrating agent of ammonium nitrate (AN) and sulfuric acid (H2SO4) mixture. The reaction rate and thermodynamics of the nitration reaction was studied. On the basis of a series of reaction calorimetry experiments and thermal stability tests, the feeding sequence and reaction temperature were optimized. The feed sequence of dosing PG/H2SO4 solution into the AN/H2SO4 solution is better than the opposite feed sequence. The best reaction temperature is at 5°C. The highest yield of ˜93% TNPG was obtained. In addition, the thermal risk analysis revealed that the thermal risk at the optimal operating condition was low.

Identification and relative quantification of 3-nitrotyrosine residues in fibrinogen nitrated in vitro and fibrinogen from ischemic stroke patient plasma using LC-MS/MS

Ischemic stroke is one of the leading causes of death and disability worldwide. This acute vascular event interferes with blood supply to the brain and induces a burst of free radicals such as nitric oxide and superoxide, producing peroxynitrite, a precursor of strong nitrating agents. Fibrinogen is one of the most abundant plasma proteins; it plays a role in the hemostatic system, mediating clot formation, which can be affected by nitrotyrosine formation. We hypothesized that nitration of fibrinogen by ONOOH and ONOOCO2− radical products could be one of the early events of the ischemic stroke, and protein-bound 3-nitrotyrosine could be a potential biomarker for diagnosis and/or prognosis of this condition. A targeted mass spectrometry approach was developed to analyze the nitration of fibrinogen and its association with ischemic stroke. First, a comprehensive mapping of 3-nitrotyrosine locations and their relative quantification was performed by LC-MS/MS, using in vitro nitrated fibrinogen samples. Twenty different 3-nitrotyrosine residues were identified on fibrinogen nitrated in vitro, varying with the peroxynitrite tofibrinogen molar ratio used. Nine tyrosine residues that were consistently modified at different treatment ratios were chosen to perform a targeted LC-MS/MS analysis in clinical samples. Enriched fibrinogen fractions from clinical samples from 24 ischemic stroke and 12 patients with non-inflammatory conditions were analysed with this method. Three of the nine tyrosine residues analysed (βY452, βY475 and γY380) showed a significant difference between the ischemic stroke and non-inflammatory disease groups. ROC curve analysis suggested an association of these residues either individually or in combination with ischemic stroke. Different tyrosine nitration patterns were also observed in fibrinogen modified in vitro and in vivo, suggesting differences in the nitration process in these situations. This is the first study showing a putative association between the nitration profile of specific tyrosine residues in human fibrinogen and ischemic stroke.

Regioselective nitration of a biphenyl derivative to derive a fluorescent chloride sensor

Methyl 2′-aminobiphenyl-4-carboxylate, an optical chloride sensor has been synthesized via Suzuki-Miyaura cross-coupling followed by regioselective nitration. In general biphenyl systems are prone to undergo poly nitration whenever they are subjected to nitration via different nitrating agents. But in this present work, a highly selective nitration in the 2′ position of methyl biphenyl-4-carboxylate was achieved using 70% HNO3 and acetic anhydride to derive 3. Upon reduction, 3 produces 4 which shows bright blue emission in different organic solvents. In presence of chloride ion the blue emission of the sensor changes to enhanced bright green emission. The chloride sensing has also been explored using other spectroscopic techniques such as 1H NMR, UV–vis spectroscopy, and theoretical study.

Application of [PVI-SO3H]NO3 as a novel polymeric nitrating agent with ionic tags in preparation of high-energetic materials.

In this paper, poly(vinyl imidazole) sulfonic acid nitrate [PVI-SO3H]NO3 was synthesized and fully characterized. Then, [PVI-SO3H]NO3 was applied for the preparation of energetic materials such as 1,1-diamino-2,2-dinitroethene (FOX-7), pentaerythritol tetranitrate (PETN), 1,3,5-trinitro-1,3,5-triazinane (RDX) and trinitrotoluene (TNT). The major advantages of the presented methodology are mild, facile workup, high yields and short reaction times. [PVI-SO3H]NO3 is a suitable nitrating agent for in situ generation of NO2 and without using any co-catalysts of the described nitrating reagent.

Fe- and Mn-modified SO42−/ZrO2 conjoined O2–Ac2O as a composite catalytic system for highly selective nitration of 1-nitronaphthalene with NO2 to valuable 1,5-dinitronaphthalene

NO2 as benign nitrating agent and Fe–Mn bimetallic modified SZ as catalyst replaced traditional nitric–sulfuric mixed acid systems for 1-NN nitration. The synergism of Fe1.5Mn0.5–SZ and O2–Ac2O significantly improved the selectivity to 1,5-DNN.

Nitration of aromatics with dinitrogen pentoxide in a liquefied 1,1,1,2-tetrafluoroethane medium.

Regardless of the sustainable development path, today, there are highly demanded chemical productions still operating that bear environmental and technological risks inherited from the previous century. The fabrication of nitro compounds, and nitroarenes in particular, is traditionally associated with acidic wastes formed in nitration reactions exploiting mixed acids. However, nitroarenes are indispensable for industrial and military applications. We faced the challenge and developed a greener, safer, and yet effective method for the production of nitroaromatics. The proposed approach comprises the application of an eco-friendly nitrating agent, namely dinitrogen pentoxide (DNP), in the medium of liquefied 1,1,1,2-tetrafluoroethane (TFE) – one of the most non-hazardous Freons. Importantly, the used TFE is not emitted into the atmosphere but is effortlessly recondensed and returned into the process. DNP is obtained via the oxidation of dinitrogen tetroxide with ozone. The elaborated method is characterized by high yields of the targeted nitro arenes, mild reaction conditions, and minimal amount of easy-to-utilize wastes.

Нитрование гидролизного лигнина в водно-апротонных средах

Industrial lignins are formed from native lignins during chemical or biochemical processing of plant raw materials. Lignins can be modified to produce valuable products, including monomers, polymeric materials, and composites. The article presents the results of a study of hydrolysis lignin nitration under various conditions. The aim of the study was to obtain a nitrated hydrolysis lignin with a maximum yield and maximum nitrogen content. Therefore, the nitration was carried out using nitric acid in a water-aprotic solvent binary mixtures (1,4-dioxane, dimethyl sulfoxide, tetrahydrofuran, dimethylformamide, acetonitrile). Acetyl nitrate, which is a mixed anhydride of nitric and acetic acids, was also used as a nitrating agent. In this regard, the consumption of acetic anhydride in the synthesis of acetyl nitrate was used taking into account the water present in concentrated nitric acid. Acetyl nitrate was obtained by the reaction of acetic anhydride and concentrated nitric acid at room temperature for 30 min. Acetyl nitrate is a mild nitrating agent opposed to nitric acid. Nitration was carried out under reflux in a boiling water bath for 2–5 min (with nitric acid) or 1–60 min (with acetyl nitrate). Upon completion of the nitration reaction, the products were filtered, washed with distilled water and dried to constant weight without heating. When nitration was performed with nitric acid, the maximum yield of nitrated hydrolysis lignin (83–101 %) was achieved using 1,4-dioxane, acetonitrile, and tetrahydrofuran; and the maximum nitrogen content (4.3–4.5 %) was achieved using 1,4-dioxane or acetonitrile. The use of dimethyl sulfoxide and dimethylformamide leads to a decrease in the product yield to 23–35 %, to a lower nitrogen content of 1.3–3.9 % and an increased oxygen content, which indicates the occurrence of not only nitration, but also depolymerization and oxidative transformations. When nitration with acetyl nitrate, the reaction takes place for 1–3 min, herewith the product contains up to 4.7 % of nitrogen. On the IR spectra of nitrated hydrolysis lignins, new absorption bands appear at 1555 and 1710 cm–1 due to the appearance of carboxyl and nitro groups.

A New Family of Fused Azolo[1,5-a]pteridines and Azolo[5,1-b]purines.

The nitration of azolo[1,5-a]pyrimidin-7-amines with several nitration agents (such as acetic nitric anhydride, nitronium tetrafluoroborate, and a mixture of concentrated nitric acid and sulfuric acid) has been investigated. It has been shown that, depending on the conditions, the nitration of pyrazolopyrimidin-7-amines bearing electron-withdrawing groups in the pyrazole ring leads to nitration products in the pyrimidine and/or pyrazole ring. The nitration of triazolo[1,5-a]pyrimidin-7-amines with “nitrating mixture” has been optimized, thus allowing us to obtain a series of 6-nitro[1,2,4]triazolo[1,5-a]pyrimidin-7-amines, followed by their reduction into the corresponding [1,2,4]triazolo[1,5-a]pyrimidin-6,7-diamines (yields 86–89%). The latter have been subjected to heterocyclization by a variety of electrophilic compounds (such as CS2, glyoxal, triethyl orthoformate) with the formation of five- or six-membered annulated cycles.

Synthesis of 5-fluoro-2-nitrobenzotrifluoride in a continuous-flow millireactor with a safe and efficient protocol

A synthesis process of 5-fluoro-2-nitrobenzotrifluoride was achieved in a continuous-flow millireactor system through the nitration of 3-fluorobenzotrifluoride using the mixed acid as a nitrating agent. The process safety was evaluated based on the temperature rise with the use of Reaction Calorimeter and Differential Scanning Calorimetry. The mass transfer limitation for this nitration process in the millireactor was evaluated. Effects of the composition of the mixed acid, the molar ratio of nitric acid to the substrate, the residence time, and the temperature on the reaction performance in the millireactor were studied in order to obtain optimal operational conditions. A heat transfer assessment was made to discuss the effects of the residence time and the inner diameter on the temperature distribution and the substrate conversion. Compared with the traditional batch reactor, the flow reactor could achieve better control over impurity and higher process efficiency because of its enhanced mass and heat transfer rates under the optimized conditions. This continuous-flow synthesis strategy would be beneficial for the commercial manufacturing of fine chemicals and pharmaceutical intermediates with the involvement of nitration.

3-Nitrotyrosine and related derivatives in proteins: precursors, radical intermediates and impact in function.

Oxidative post-translational modification of proteins by molecular oxygen (O2)- and nitric oxide (•NO)-derived reactive species is a usual process that occurs in mammalian tissues under both physiological and pathological conditions and can exert either regulatory or cytotoxic effects. Although the side chain of several amino acids is prone to experience oxidative modifications, tyrosine residues are one of the preferred targets of one-electron oxidants, given the ability of their phenolic side chain to undergo reversible one-electron oxidation to the relatively stable tyrosyl radical. Naturally occurring as reversible catalytic intermediates at the active site of a variety of enzymes, tyrosyl radicals can also lead to the formation of several stable oxidative products through radical-radical reactions, as is the case of 3-nitrotyrosine (NO2Tyr). The formation of NO2Tyr mainly occurs through the fast reaction between the tyrosyl radical and nitrogen dioxide (•NO2). One of the key endogenous nitrating agents is peroxynitrite (ONOO-), the product of the reaction of superoxide radical (O2•-) with •NO, but ONOO--independent mechanisms of nitration have been also disclosed. This chemical modification notably affects the physicochemical properties of tyrosine residues and because of this, it can have a remarkable impact on protein structure and function, both in vitro and in vivo. Although low amounts of NO2Tyr are detected under basal conditions, significantly increased levels are found at pathological states related with an overproduction of reactive species, such as cardiovascular and neurodegenerative diseases, inflammation and aging. While NO2Tyr is a well-established stable oxidative stress biomarker and a good predictor of disease progression, its role as a pathogenic mediator has been laboriously defined for just a small number of nitrated proteins and awaits further studies.

Nitrated food proteins induce a regulatory immune response associated with allergy prevention after oral exposure in a Balb/c mouse food allergy model

BackgroundFood allergy is associated with a high personal health and economic burden. For immunomodulation toward tolerance, food compounds could be chemically modified, for example, by posttranslational protein nitration, which also occurs via diet‐derived nitrating agents in the gastrointestinal tract.ObjectiveWe sought to analyze the effect of pretreatment with nitrated food proteins on the immune response in a mouse food allergy model and on human monocyte‐derived dendritic cells (moDCs) and PBMCs.MethodsThe model allergen ovalbumin (OVA) was nitrated in different nitration degrees, and the secondary structures of proteins were determined by circular dichroism (CD). Allergy‐preventive treatment with OVA, nitrated OVA (nOVA), and maximally nitrated OVA (nOVAmax) were performed before mice were immunized with or without gastric acid‐suppression medication. Antibody levels, regulatory T‐cell (Treg) numbers, and cytokine levels were evaluated. Human moDCs or PBMCs were incubated with proteins and evaluated for expression of surface markers, cytokine production, and proliferation of Th2 as well as Tregs.ResultsIn contrast to OVA and nOVA, the conformation of nOVAmax was substantially changed. nOVAmax pretreated mice had decreased IgE as well as IgG1 and IgG2a levels and Treg numbers were significantly elevated, while cytokine levels remained at baseline level. nOVAmax induced a regulatory DC phenotype evidenced by a decrease of the activation marker CD86 and an increase in IL‐10 production and was associated with a higher proliferation of memory Tregs.ConclusionOral pretreatment with highly nitrated proteins induces a tolerogenic immune response in the food allergy model and in human immune cells.

A Serendipitous Synthesis: SiO2‐HNO3 Mediated Oxidative Aromatization and Regioselective Nitration of 1,3,5‐Trisubstituted‐4,5‐Dihydro‐1H‐Pyrazoles

AbstractResearch often leads to unprecedented results that open the gates for new methodologies. We report herein the oxidative aromatization and serendipitous regioselective nitration of 1,3,5‐trisubstituted‐4,5‐dihydro‐1H‐pyrazoles under mild reaction conditions using SiO2‐HNO3 as an oxidizing agent to afford 1,3,5‐trisubstituted pyrazoles with high regioselectivity. Here SiO2‐HNO3 acts as a nitrating agent along with oxidizing property. SiO2‐HNO3 is cheap, easy to prepare, eco‐friendly and practically applicable catalyst.

Recent Advances in Safe Synthesis of Energetic Materials: An Overview

The development of novel energetic materials with highest possible performance is of current interest. Synthesis of such materials is performed at various stages of pilot plant production all over the world. However, their synthesis involves hazardous production processes. This paper discusses relatively safe and eco-friendly approaches and techniques such as microwave technology and the use of ionic liquids for the synthesis of high-performance energetic materials that can be used as explosives and propellants. In addition, the use of dinitrogen pentoxide as an efficient nitrating agent for the synthesis of energetic materials is considered.

Formation of Nitrophenolic Byproducts during Heat-Activated Peroxydisulfate Oxidation in the Presence of Natural Organic Matter and Nitrite.

Sulfate radical (SO4•-)-based advanced oxidation is a viable in situ remediation technology for degrading organic contaminants in the subsurface. In this study, we demonstrated that SO4•- could induce the activation of nitrite, an anion commonly present in the subsurface environment, leading to the formation of nitrophenolic byproducts. Fourier-transform infrared spectroscope and 15N nuclear magnetic resonance analysis revealed that the inorganic nitrite was incorporated into natural organic matter (NOM) to form organic nitrogen upon SO4•- oxidation. Nitrophenolic byproducts, including 2-hydroxy-5-nitrobenzoic acid, 4-nitrophenol, and 2,4-dinitrophenol, were identified using high-resolution mass spectrometry in combination with a 15N labeling technique. Formation of nitrated byproducts was ascribed to the scavenging of SO4•- by nitrite, which not only generated the nitrating agent NO2• but also inhibited the degradation of organic compounds, making them more available to the reactions with NO2•. The phenolic moieties in NOM served as the main reactive sites for NO2• attack. The nitration begins with H abstraction on the phenoxy oxygen, followed by the addition of another NO2• to its ortho or para site. Decarboxylation followed by NO2• addition can also generate nitrophenolic byproducts. To the best of our knowledge, this is the first study reporting the nitration of NOM and formation of toxic nitrophenolic byproducts during SO4•--based oxidation. It sheds light on the potential risks of this technology in subsurface remediation practices.

Peroxynitrite-Mediated Dimerization of 3-Nitrotyrosine: Unique Chemistry along the Spectrum of Peroxynitrite-Mediated Nitration of Tyrosine.

Peroxynitrite (ONOO−, PN) has long been considered a potent nitrating agent implicated in numerous inflammation-mediated diseases. The current work highlights an unexplored oxidation chemistry initiated under conditions of sustained PN exposure. Impetus for this investigation developed from mass spectral results that suggested dimerization of a model peptide with a single tyrosine residue that was first nitrated following extended exposure to PN generated in situ. In attempts to substantiate this dimerization event and divulge the possible mode of linkage between the tyrosine derivatives of the peptide monomers, 3-nitrotyrosine (3-NT) was exposed to sustained fluxes of PN in a two-component PN-generating platform developed in this laboratory. Such exposure afforded products with tandem mass spectrometry and fluorescence spectroscopy profiles indicative of C–O coupling between 3-NT moieties. Synthesis and comparative analysis of the C–C coupled 3-NT isomer corroborated these findings. Most notably, the mass spectral data of the C–C coupled 3-NT dimer displayed a 226.80 m/z peak following exposure to high collision energy, corresponding to symmetric cleavage of the parent dimer peak (m/z = 453) along with a fragmentation product at m/z = 180.04 (–NO2 species). This fragmentation profile was distinct from the C–O coupled 3-NT dimer that exhibited a predominant 209.14 m/z peak with a small secondary 226.15 m/z peak indicative of asymmetric cleavage of the parent dimer. Results of this study indicate that formation of C–O coupled 3-NT dimer is promoted by elevated levels of 3-NT formed under high and sustained flux of PN.

The fibrils untwisting limits the rate of cellulose nitration process

The rate of cellulose nitration is lower compared to the low molecular weight substances. Theoretical estimates assess that the rate of the nitrating agents’ diffusion cannot provide for the characteristic time of cellulose nitration, as the densely packed regions are too small. However, the electrostatic barrier between the nitrating mixture and the microcrystallites makes the latter inaccessible for the nitronium ion. The cellulose nitration rate decreases and the transformation of elementary fibril structure occurs at the same degree of substitution corresponding to a complete nitration of the fibrils’ surface. The supercoiled macromolecules in the elementary fibrils cannot dissociate without untwisting. The fibrils’ untwisting as well as their swelling are very slow. Thus, we propose that the nanofibrils’ untwisting limits the rate of the nitronium ion transport into the cellulose nanofibrils and, thus, the rate of the nitration reaction as a whole.

Selective O- and N-nitration of steroids fused to the pyrazole ring

Chemoselective methods for O- and N-nitration of steroids fused to the pyrazole ring were developed. The nitrating agents were acetyl, propionyl, and trifluoroacetyl nitrates generated from carboxylic acid anhydrides and either copper nitrate or nitric acid. Depending on the reaction conditions, the developed methods allow both selective nitration at the pyrazole nitrogen atom leaving intact the hydroxy group of steroid framework and dinitration giving rise to the corresponding nitrates of steroidal nitropyrazoles.