HNO Reaction Research Articles

Mechanistic Origin of Favorable Substituent Effects in Excellent Cu Cyclam Based HNO Sensors

AbstractHNO has broad chemical and biomedical properties. Metal complexes and derivatives are widely used to make excellent HNO sensors. However, their favorable mechanistic origins are largely unknown. Cu cyclam is a useful platform to make excellent HNO sensors including imaging agents. A quantum chemical study of Cu cyclams with various substitutions was performed, which reproduced diverse experimental reactivities. Structural, electronic, and energetic profiles along reaction pathways show the importance of HNO binding and a proton‐coupled electron transfer mechanism for HNO reaction. Results reveal that steric effect is primary and electronic factor is secondary (if the redox potential is sufficient), but their interwoven effects can lead to unexpected reactivity, which looks mysterious experimentally but can be explained computationally. This work suggests rational substituent design ideas and recommends a theoretical study of a new design to save time and cost due to its subtle effect.

Mechanistic Origin of Favorable Substituent Effects in Excellent Cu Cyclam Based HNO Sensors.

HNO has broad chemical and biomedical properties. Metal complexes and derivatives are widely used to make excellent HNO sensors. However, their favorable mechanistic origins are largely unknown. Cu cyclam is a useful platform to make excellent HNO sensors including imaging agents. A quantum chemical study of Cu cyclams with various substitutions was performed, which reproduced diverse experimental reactivities. Structural, electronic, and energetic profiles along reaction pathways show the importance of HNO binding and a proton-coupled electron transfer mechanism for HNO reaction. Results reveal that steric effect is primary and electronic factor is secondary (if the redox potential is sufficient), but their interwoven effects can lead to unexpected reactivity, which looks mysterious experimentally but can be explained computationally. This work suggests rational substituent design ideas and recommends a theoretical study of a new design to save time and cost due to its subtle effect.

Kinetic Study on the Reactivity of Azanone (HNO) toward Cyclic C-Nucleophiles.

Azanone (HNO) is an elusive electrophilic reactive nitrogen species of growing pharmacological and biological significance. Here, we present a comparative kinetic study of HNO reactivity toward selected cyclic C-nucleophiles under aqueous conditions at pH 7.4. We applied the competition kinetics method, which is based on the use of a fluorescein-derived boronate probe FlBA and two parallel HNO reactions: with the studied scavenger or with O2 (k = 1.8 × 104 M−1s−1). We determined the second-order rate constants of HNO reactions with 13 structurally diverse C-nucleophiles (k = 33–20,000 M−1s−1). The results show that the reactivity of HNO toward C-nucleophiles depends strongly on the structure of the scavenger. The data are supported with quantum mechanical calculations. A comprehensive discussion of the HNO reaction with C-nucleophiles is provided.

Kinetics of Azanone (HNO) Reactions with Thiols: Effect of pH

HNO (nitroxyl, IUPAC name azanone) is an electrophilic reactive nitrogen species of growing pharmacological and biological significance. Here, we present data on the pH-dependent kinetics of azanone reactions with the low molecular thiols glutathione and N-acetylcysteine, as well as with important serum proteins: bovine serum albumin and human serum albumin. The competition kinetics method used is based on two parallel HNO reactions: with RSH/RS− or with O2. The results provide evidence that the reaction of azanone with the anionic form of thiols (RS−) is favored over reactions with the protonated form (RSH). The data are supported with quantum mechanical calculations. A comprehensive discussion of the HNO reaction with thiolates is provided.

34 - The Kinetic Study on the Reactivity of HNO (Azanone) Towards Nitrite Ions, Hydroxylamine, Selected Nitrosocompounds and Selected Phosphines

Nitroxyl (HNO, IUPAC name azanone), being formally the protonated product of one-electron reduction of nitric oxide (NO), is an elusive reactive nitrogen species possessing interesting biological chemistry and high pharmacological importance. 1,2 Contrary to nitric oxide, HNO is a strong electrophile highly reactive towards nucleophiles. Here, we present the study on the kinetics of HNO reaction with nitrate ions, hydroxylamine, selected nitrosocompounds and selected phosphines. In this study we applied previously described 4 competition kinetics method based on two parallel, competing HNO reactions – with thiol and with molecular oxygen. The latter, relatively fast reaction (k = (1.8 ± 0.3) ∙ 10 4 M -1 s -1 ), 3 results in the formation of peroxynitrite, 3 that can be easily detected fluorometrically with the use of fluorogenic boronate derivative of resorufin, PC1. PC1 reacts rapidly and directly with ONOO – (k = 1∙10 6 M -1 s -1 ) with the formation of fluorescent resorufin as the major product. In this study the formation of resorufin was monitored by Applied Photophysics SX20 stopped-flow spectrophotometer equipped with a fluorescence detector. HNO released from Angeli's salt (3 μM) reacts either with the azanone scavenger, or with molecular oxygen (225 μM) forming peroxynitrite detected by PC1 (25 μM). Table 1. The determined values of the second-order rate constant of HNO reaction. HNO scavenger k (M -1 s -1 ) Nitrite ions (4,8 ± 0,2) × 10 3 Hydroxyloamine (2,0 ± 0,1) × 10 4 2-Nitroso-1-naphtol (9,8 ± 0,2) × 10 5 Nitrosobenzene >1,1× 10 5 S-Nitrosoglutathione 2,9 × 10 5 Triphenylophosphine > 6,1 × 10 6 Triphenylphosphine-3,3′,3′′-trisulfonic acid trisodium salt (1,5 ± 0,1) × 10 7 References 1: J. M. Fukuto, M. D. Bartberger et al., Chem. Res. Toxicol., 2005, 18 , 790. 2: N. Paolocci, M. I. Jackson et al. , Pharmacol. Ther. , 2007, 113 , 442. 3: R. Smulik, D. Debski et al. , J. Biol. Chem., 2014, 289 , 35570.

Comparison of Reductive Ligation-Based Detection Strategies for Nitroxyl (HNO) and S-Nitrosothiols.

Invited for this month's cover picture is the group of Prof. S. Bruce King at the Department of Chemistry of Wake Forest University. The cover picture shows a prefluorescent phosphine‐based probe reacting with nitroxyl (HNO) and S ‐nitrosothiol (RSNO), nitrogen oxide‐derived biological signals. Both species react with the prefluorescent probe, but only the product from the HNO reaction can complete a further chemical ligation pathway that results in fluorescence, indicating the presence of HNO. The product of the probe with RSNO does not complete this ligation and does not generate a fluorescent species. These phosphine‐based probes thus demonstrate a selectivity for HNO over RSNO based on their chemical reactivity and can be used in biological systems to differentiate these species. For more details, see the Communication on p. 110 ff. Read the full text of the article at 10.1002/open.201500200

Quantum Diffusion-Controlled Chemistry: Reactions of Atomic Hydrogen with Nitric Oxide in Solid Parahydrogen

Our group has been working to develop parahydrogen (pH2) matrix isolation spectroscopy as a method to study low-temperature condensed-phase reactions of atomic hydrogen with various reaction partners. Guided by the well-defined studies of cold atom chemistry in rare-gas solids, the special properties of quantum hosts such as solid pH2 afford new opportunities to study the analogous chemical reactions under quantum diffusion conditions in hopes of discovering new types of chemical reaction mechanisms. In this study, we present Fourier transform infrared spectroscopic studies of the 193 nm photoinduced chemistry of nitric oxide (NO) isolated in solid pH2 over the 1.8 to 4.3 K temperature range. Upon short-term in situ irradiation the NO readily undergoes photolysis to yield HNO, NOH, NH, NH3, H2O, and H atoms. We map the postphotolysis reactions of mobile H atoms with NO and document first-order growth in HNO and NOH reaction products for up to 5 h after photolysis. We perform three experiments at 4.3 K and one at 1.8 K to permit the temperature dependence of the reaction kinetics to be quantified. We observe Arrhenius-type behavior with a pre-exponential factor of A = 0.036(2) min(-1) and Ea = 2.39(1) cm(-1). This is in sharp contrast to previous H atom reactions we have studied in solid pH2 that display definitively non-Arrhenius behavior. The contrasting temperature dependence measured for the H + NO reaction is likely related to the details of H atom quantum diffusion in solid pH2 and deserves further study.

Nitroxyl (HNO) Reacts with Molecular Oxygen and Forms Peroxynitrite at Physiological pH: BIOLOGICAL IMPLICATIONS

Nitroxyl (HNO), the protonated one-electron reduction product of NO, remains an enigmatic reactive nitrogen species. Its chemical reactivity and biological activity are still not completely understood. HNO donors show biological effects different from NO donors. Although HNO reactivity with molecular oxygen is described in the literature, the product of this reaction has not yet been unambiguously identified. Here we report that the decomposition of HNO donors under aerobic conditions in aqueous solutions at physiological pH leads to the formation of peroxynitrite (ONOO(-)) as a major intermediate. We have specifically detected and quantified ONOO(-) with the aid of boronate probes, e.g. coumarin-7-boronic acid or 4-boronobenzyl derivative of fluorescein methyl ester. In addition to the major phenolic products, peroxynitrite-specific minor products of oxidation of boronate probes were detected under these conditions. Using the competition kinetics method and a set of HNO scavengers, the value of the second order rate constant of the HNO reaction with oxygen (k = 1.8 × 10(4) m(-1) s(-1)) was determined. The rate constant (k = 2 × 10(4) m(-1) s(-1)) was also determined using kinetic simulations. The kinetic parameters of the reactions of HNO with selected thiols, including cysteine, dithiothreitol, N-acetylcysteine, captopril, bovine and human serum albumins, and hydrogen sulfide, are reported. Biological and cardiovascular implications of nitroxyl reactions are discussed.

Hydroxylamine acutely activates glucose uptake in L929 fibroblast cells

Nitroxyl (HNO) has a unique, but varied, set of biological properties including beneficial effects on cardiac contractility and stimulation of glucose uptake by GLUT1. These biological effects are largely initiated by HNO's reaction with cysteine residues of key proteins. The intracellular production of HNO has not yet been demonstrated, but the small molecule, hydroxylamine (HA), has been suggested as possible intracellular source. We examined the effects of this molecule on glucose uptake in L929 fibroblast cells. HA activates glucose uptake from 2 to 5-fold within two minutes. Prior treatment with thiol-active compounds, such as iodoacetamide (IA), cinnamaldehyde (CA), or phenylarsine oxide (PAO) blocks HA-activation of glucose uptake. Incubation of HA with the peroxidase inhibitor, sodium azide, also blocks the stimulatory effects of HA. This suggests that HA is oxidized to HNO by L929 fibroblast cells, which then reacts with cysteine residues to exert its stimulatory effects. The data suggest that GLUT1 is acutely activated in L929 cells by modification of cysteine residues, possibly the formation of a disulfide bond within GLUT1 itself.

Computational Studies on Metathetical and Redox Processes of HOCl in Gas Phase. III. Its Self-Reaction and Interactions with HNOx (x = 1−3)

The gas-phase redox reactions of HOCl with its self and HNO(x) (x = 1-3) have been studied theoretically by ab initio quantum chemical and statistical mechanical theories. The structures of reactants, intermediate complexes, products, and transition states were optimized at the MPW1PW91/6-311+G(3df,2p) level of theory. The potential energy surface of each reaction was refined at the CCSD(T)/6-311+G(3df,2p) level of theory. The most favorable products are predicted to be ClClO + H(2)O and ClOCl + H(2)O for the HOCl self-reaction (A), H(2)O + Cl + NO for the HOCl + HNO reaction (B), H(2)O + ClNO(2) for the HOCl + HONO-t reaction (C), H(2)O + cis-ClONO for the HOCl + HONO-c reaction (D). For the HOCl + HONO(2) reaction (E), only one dehydration reaction channel was considered to produce H(2)O + ClONO(2). The rate constants of all above five reactions have been predicted at 300-3000 K by the VTST/RRKM theory. The calculation shows that the theoretical rate constants are within the upper limits of experimental results. In addition, we calculated the equilibrium constant for the Cl(2)O + H(2)O --> HOCl + HOCl reaction, which is also in reasonable agreement with experimental data within the error of the available experimental enthalpy change.

The shy Angeli and his elusive creature: the HNO route to vasodilation

what we now call Angeli's salt (AS) was discovered more than a century ago by the Italian chemist Angelo Angeli (Fig. 1) . He found that “salts of nitroxylaminic acid are readily resolved into the corresponding nitrites and the unsaturated residue nitroxyl” (HNO), proposing their use as suppliers of HNO (3). However, by analogy with the genial but very reclusive personality of the discoverer (17), the physiological properties of HNO didn't draw too much attention until early 1990s. In their seminal work, Fukuto and colleagues (19) reported that HNO elicits vasorelaxation in both the rabbit aorta and bovine intrapulmonary artery by a soluble guanylate cyclase (sGC)- dependent pathway. Later, it was evident that the thiol-donating agent l-cysteine can discriminate the vasodilatative profile of HNO from that of nitric oxide (NO·) or nitrosothiols: AS/HNO action can be blocked by this agent, whereas the NO· effect is potentiated (36). In combination with the suggestion that NO· is the only species able to directly activate sGC (9), these data led to the conclusion that HNO-induced vasorelaxation is due to its intracellular conversion to NO· with subsequent activation of sGC. For several years, this idea has relegated HNO to a mere precursor of the presence and biological activity of NO·, de facto negating to HNO its own right to exist.

HNO trapping and assisted decomposition of nitroxyl donors by ferric hemes

The role of nitric oxide (NO) as a signalling molecule in biological systems has been thoroughly studied in the last decades. More recently, there has been an increasing interest in the one-electron reduction product of NO, namely nitroxyl (HNO/NO −). Some studies suggest that nitroxyl can be produced by nitric oxide synthases under certain conditions, and that distinct pharmacological effects are observed for NO and nitroxyl donors. HNO is capable of react with heme proteins, thiols, molecular oxygen, NO and HNO itself. However, only recently the different reactivity patterns are being thoroughly understood. Heme model compounds offer the opportunity to study the reaction kinetics without the complexity arising from ligand interactions with the protein matrix. In this study we analyzed the reaction between the commonly used nitroxyl donors sodium trioxodinitrate and toluene sulfohydroxamic acid, with the ferric model compounds microperoxidase-11 (MP11) and the cationic metalloporphyrin [Fe IIITEPyP] 5+ (Tetrakis N-ethylpyridinium-2yl porphyne). Our results show that there are two alternative modes of reactivity for nitroxyl donors towards heme in aqueous solutions. The first one comprises the heme assisted decomposition of the donor, enhancing its decomposition rate more than 100-fold. In the second, the donor produces HNO which subsequently reacts with the porphyrin. The observed rate constants (of about 10 5 M −1 s −1) are consistent with the estimated data for the HNO reaction with heme proteins, and may be controlled by the leaving water ligand. This rate constant probably represents an upper limit for the bimolecular rate constant of HNO towards these proteins.

Theoretical Study of the H2 + NO and Related Reactions of [H2NO] Isomers

Mechanisms of the reactions occurring on the multichannel lowest doublet potential energy surface of the [H2NO] system containing H2NO, HNOH, and NOH2 intermediates and including the H2 + NO, NH2 + O, NH + OH, and H + HNO entry channels have been probed using unrestricted coupled-cluster formalism. Also, the transition structure on the doublet energy surface for the direct hydrogen abstraction from H2 + NO yielding H + HNO has been identified. The energetic and molecular parameters derived from coupled cluster singles and doubles with triples correction (CCSD(T)) calculations using the 6-311++G(3df,3pd) basis set, based on their respective optimized geometries obtained with a 6-311++G(d,p) basis set, have then been utilized to compute the apparent rate constants of the different competitive channels in the [H2NO] system within the framework of a quantum version of Rice−Ramsperger−Kassel theory (QRRK). The H2 + NO reaction is found to be a rather slow reaction, and it occurs via bimolecular hydrogen abstraction. The calculated total rate constant for this reaction at 2000 K and 1 atm is 2.35 × 108 cm3 mol-1 s-1. Stabilization of NH2O or NHOH intermediates plays a minor role in the kinetics of NH2 + O reaction which is dominated by the dissociation products, HNO + H, NH + OH and H2 + NO. The H + HNO reaction kinetics has only 10% contribution from addition/isomerization/dissociation paths at 2000 K and 1 atm. H2 + NO and H + HNO are expected to be the competitive products in the reaction of OH with nitrene. The calculated standard heats of formation are ΔHf298(NH2O) = 17.5 ± 2 and ΔHf298(HNOH) = 23.7 ± 2 kcal/mol.

Theoretical aspects of H/N/O-chemistry relevant to the thermal reduction of NO by H 2

Kinetic modeling of existing data on the reduction of NO by H 2, initiated photolytically or thermally, with a comprehensive reaction mechanism established by means of ab initio quantum-chemical and statistical rate-constant calculations, allows us to identify several key elementary processes which are important in different temperature and NO-concentration regimes. At T < 900 K, the reduction of NO by H 2 and other hydrides induced photolytically occurs primarily by the bimolecular reaction HNO + HNO → cis/trans-(HNO) 2 under NO-lean conditions, and by the termolecular process HNO + 2NO → HN 2O + NO 2, followed by the fast redox reaction, HN 2O + NO → HN 2 + NO 2 and N 2 + HONO under NO-rich conditions. In the temperature range of 900 K < T < 1500 K, the reduction of NO by H 2, initiated by the H 2 + NO → H + HNO reaction, occurs readily and the global reduction rate is dominated by the HNO + NO → N 2O + OH reaction. At temperatures higher than 1500 K, commonly heated by shock waves, the rate of NO reduction is controlled almost exclusively by the H + NO → N + OH reaction. The N atom thus formed generates efficiently the O atom by the fast N + NO → N 2 + O process. These two NO reduction reactions are greatly enhanced in this temperature regime by the abundance of H atoms, produced by the fast chain processes: O + H 2 → H + OH and OH + H 2 → H + H 2O. The rate constants as well as the mechanisms of these key elementary processes involving H/N/O-species have been interpreted in terms of the theoretical results derived from ab initio quantum-chemical and statistical-theory calculations.

A detailed SACM study of the H+NO→HNO reaction based on a realistic potential energy surface

The full version of the statistical adiabatic channel model has been used for a detailed study of the title reaction at the limit of high pressure for the 100–1000 K temperature range. The calculations have been carried out on an analytic potential energy surface for the ground state of HNO developed from recent high-level ab initio data. The effects of quantum mechanical tunneling through the barriers on the adiabatic channel potentials are included in the calculations. The tunneling increases the rate of recombination by factors of 3.3, 1.7 and 1.08 at 100, 300 and 1000 K, respectively. The calculated rate constant at 300 K was found to have a difference of only 20% from the available experimental one. State-resolved rate constants are also presented. Calculations for the D+NO→DNO reaction between 100 and 400 K were also performed. The kinetic isotopic effect is calculated to decrease from 2.5 to 1.8 over this temperature range.