Recent advances in the chemical biology of nitroxyl (HNO) detection and generation

Nitroxyl donating and visualization with a coumarin-based fluorescence probe

Nitroxyl (HNO), the single-electron reduction product of nitric oxide (NO), has attracted great interest in the treatment of congestive heart failure in clinical trials. In this paper, we describe the first coumarin-based compound N-hydroxy-2-oxo-2H-chromene-6-sulfonamide (CD1) as a dualfunctional HNO donor, which can release both an HNO signaling molecule and a fluorescent reporter. Under physiological conditions (pH 7.4 and 37 °C), the CD1 HNO donor can readily decompose with a half-life of ∼90 min. The corresponding stoichiometry HNO from the CD1 donor was confirmed using both Vitamin B12 and phosphine compound traps. In addition to HNO releasing, specifically, the degradation product 2-oxo-2H-chromene-6-sulfinate (CS1) was generated as a fluorescent marker during the decomposition. Therefore, the HNO amount released in situ can be accurately monitored through fluorescence generation. As compared to the CD1 donor, the fluorescence intensity increased by about 4.9-fold. The concentration limit of detection of HNO releasing was determined to be ∼0.13 μM according to the fluorescence generation of CS1 at physiological conditions. Moreover, the bioimaging of the CD1 donor was demonstrated in the cell culture of HeLa cells, where the intracellular fluorescence signals were observed, inferring the site of HNO release. Finally, we anticipate that this novel coumarin-based CD1 donor opens a new platform for exploring the biology of HNO.

Azobenzene-based colorimetric and fluorometric chemosensor for nitroxyl releasing

The precise release and characterization of nitroxyl (HNO) gas signaling molecule remain a challenge due to its short lifetime to date. To solve this issue, an azobenzene-based HNO donor (Azo-D1) was proposed as a colorimetric and fluorometric chemosensor for HNO releasing, to release both HNO and an azobenzene fluorescent reporter together. Specifically, the Azo-D1 has an HNO release half-life of ∼68 min under physiological conditions. The characteristic color change from the original orange to the yellow color indicated the decomposition of the donor molecule. In addition, the stoichiometry release of HNO was qualitatively and quantitatively verified through the classical phosphine compound trap. As compared with the donor molecule by itself, the decomposed product demonstrates a maximum fluorescence emission at 424 nm, where the increase of fluorescence intensity by 6.8 times can be applied to infer the real-time concentration of HNO. Moreover, cellular imaging can also be achieved using this Azo-D1 HNO donor through photoexcitation at 405 and 488 nm, where the real-time monitoring of HNO release was achieved without consuming the HNO source. Finally, the Azo-D1 HNO donor would open a new platform in the exploration of the biochemistry and the biology of HNO.

Advances in fluorescent probe development for bioimaging of potential Parkinson's biomarkers

Parkinson's disease (PD) is a common neurodegenerative disease. The clinical symptoms of PD are usually related to motor symptoms, including postural instability, rigidity, bradykinesia, and resting tremors. At present, the pathology of PD is not yet clear. Therefore, revealing the underlying pathological mechanism of PD is of great significance. A variety of bioactive molecules are produced during the onset of Parkinson's, and these bioactive molecules may be a key factor in the development of Parkinson's. The emerging fluorescence imaging technology has good sensitivity and high signal-to-noise ratio, making it possible to deeply understand the pathogenesis of PD through these bioactive molecules. Currently, fluorescent probes targeting PD biomarkers are widely developed and applied. This article categorizes and summarizes fluorescent probes based on different PD biomarkers, systematically introduces their applications in the pathological process of PD, and finally briefly elaborates on the challenges and prospects of these probes. We hope that this review will provide in-depth reference insights for designing fluorescent probes, and contribute to study of the pathogenesis and clinical treatment of PD.

β-Galactosidase-activated nitroxyl (HNO) donors provide insights into redox cross-talk in senescent cells

The cross-talk among reductive and oxidative species (redox cross-talk), especially those derived from sulfur, nitrogen and oxygen, influence several physiological processes including aging. One major hallmark of aging is cellular senescence, which is associated with chronic systemic inflammation. Here, we report a chemical tool that generates nitoxyl (HNO) upon activation by β-galactosidase, an enzyme that is over-expressed in senescent cells. In a radiation-induced senescence model, the HNO donor suppressed reactive oxygen species (ROS) in a hydrogen sulfide (H2S)-dependent manner. Hence, the newly developed tool provides insights into redox cross-talk and establishes the foundation for new interventions that modulate levels of these species to mitigate oxidative stress and inflammation.

Sixth Ligand Induced HNO/NO- Release by a Five-Coordinated Cobalt(II) Nitrosyl Complex Having a {CoNO}8 Configuration

Nitroxyl (HNO) and nitroxide (NO-) anion, the one-electron-reduced form of nitric oxide (NO), have been shown to have distinct advantages over NO from pharmacological and therapeutic points of view. However, the role of nitroxyl in chemical biology has not yet been studied as extensively as that of NO. Consequently, only a few examples of HNO donors such as Angeli's salt, Piloty's acid, or acyl- and acyloxynitroso derivatives are known. However, the intrinsic limitations of all of these hinder their widespread utility. Metal nitrosyl complexes, although few examples, could serve as an efficient HNO donor. Here, a cobalt nitrosyl complex of the {CoNO}8 (1) configuration has been reported. This complex in the presence of a sixth ligand [BF4-, DTC- (diethyldithiocarbamate anion), or imidazole] releases/donates HNO/NO-. This has been confirmed using well-known HNO/NO- acceptors like [Fe(TPP)Cl] and [Fe(DTC)3]. The HNO release has been authenticated further by the detection and estimation of N2O using gas chromatography-mass spectroscopy as well as its reaction with PPh3.

Synthesis and nitroxyl (HNO) donating properties of benzoxadiazole-based Piloty's acids

Developing functional nitroxyl (HNO) donors play a significant role in the further exploration of endogenous HNO in biochemistry and pharmacology. In this work, two novel Piloty's acids (SBD-D1 and SBD-D2) were proposed by incorporating benzoxadiazole-based fluorophores, in order to achieve the dual-function of releasing both HNO and a fluorophore in situ. Under physiological conditions, both SBD-D1 and SBD-D2 efficiently donated HNO (t_1/2 = 10.96 and 8.18 min, respectively). The stoichiometric generation of HNO was determined by both vitamin B₁₂ and phosphine compound traps. Interestingly, due to the different substitution groups on the aromatic ring, SBD-D1 with the chlorine showed no fluorescence emission, but SBD-D2 was strongly fluorescent due to the presence of the dimethylamine group. Specifically, the fluorescent signal would decrease during the release process of HNO. Moreover, theoretical calculations were performed to understand the emission difference. A strong radiation derived from benzoxadiazole with dimethylamine group due to the large transition dipole moment (∼4.3 Debye), while the presence of intramolecular charge transfer process in the donor with chlorine group caused a small transition dipole moment (<0.1 Debye). Finally, these studies would contribute to the future design and application of novel functional HNO donors for the exploration of HNO biochemistry and pharmacology.

Discovery of endogenous nitroxyl as a new redox player in Arabidopsis thaliana

Nitroxyl (HNO) is the one-electron reduced and protonated congener of nitric oxide (•NO), owning a distinct chemical profile. Based on real-time detection, we demonstrate that HNO is endogenously formed in Arabidopsis. Senescence and hypoxia induce shifts in the redox balance, triggering HNO decay or formation mediated by non-enzymatic •NO/HNO interconversion with cellular reductants. The stimuli-dependent HNO generation supports or competes with •NO signalling, depending on the local redox environment.

Quantification of intracellular HNO delivery with capillary zone electrophoresis

Redox signaling, wherein reactive and diffusible small molecules are channeled into specific messenger functions, is a critical component of signal transduction. A central principle of redox signaling is that the redox modulators are produced in a highly controlled fashion to specifically modify biotargets. Thiols serve as primary mediators of redox signaling as a function of the rich variety of adducts, which allows initiation of distinct cellular effects. Coupling the inherent reactivity of thiols with highly sensitive and selective chemical analysis protocols can facilitate identification of redox signaling agents, both in solution and in cultured cells. Here, we describe use of capillary zone electrophoresis to both identify and quantify sulfinamides, which are specific markers of the reaction of thiols with nitroxyl (HNO), a putative biologically relevant reactive nitrogen species.

Can a Nitrosyl of a Mn(II)-Porphyrin Complex Release Nitroxyl/HNO?

In general, the nitrosyl complexes of Mn(II)-porphyrinate having the {Mn(NO)}⁶ configuration are not considered as HNO or nitroxyl (NO^-) donors because of [Mn^I-NO⁺] nature. A nitrosyl complex of Mn(II)-porphyrin, [Mn(TMPP^2-)(NO)], 1 [TMPPH₂ = 5,10,15,20-tetrakis-4-methoxyphenylporphyrin], is shown to release HNO in the presence of HBF₄. It is evidenced from the characteristic reaction of HNO with triphenylphosphine and isolation of the [(TMPP^2-)Mn^III(H₂O)₂](BF₄), 2. This is attributed to the fact that H⁺ from HBF₄ polarizes the NO group whereas the BF₄^- interacts with metal ion to stabilize the Mn(III) form. These two effects cooperatively result in the release of HNO from complex 1. In addition, complex 1 behaves as a nitroxyl (NO^-) donor in the presence of [Fe(dtc)₃] (dtc = diethyldithiocarbamate anion) and [Fe(TPP)(Cl)] (TPP = 5,10,15,20-tetraphenylporphyrinate) to result in [Fe(dtc)₂(NO)] and [Fe(TPP)(NO)], respectively.

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.

BODIPY derivatives as fluorescent reporters of molecular activities in living cells

Fluorescent compounds have become indispensable tools for imaging molecular activities in the living cell. 4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) is currently one of the most popular fluorescent reporters due to its unique photophysical properties. This review provides a general survey and presents a summary of recent advances in the development of new BODIPY-based cellular biomarkers and biosensors. The review starts with the consideration of the properties of BODIPY derivatives required for their application as cellular reporters. Then review provides examples of the design of sensors for different biologically important molecules, ions, membrane potential, temperature and viscosity defining the live cell status. Special attention is payed to BODPY-based phototransformable reporters.

The bibliography includes 339 references.

Sodium borohydride and thiol mediated nitrite release from nitroaromatic antibiotics

Nitroaromatic antibiotics are used to treat a variety of bacterial and parasitic infections. These prodrugs require reductive bioactivation for activity, which provides a pathway for the release of nitrogen oxide species such as nitric oxide, nitrite, and/or nitroxyl. Using sodium borohydride and 2-aminoethanol as model reductants, this work examines release of nitrogen oxide species from various nitroaromatic compounds through several characterization methods. Specifically, 4- and 5-nitroimidazoles reproducibly generate higher amounts of nitrite (not nitric oxide or nitroxyl) than 2-nitroimidazoles during the reaction of model hydride donors or thiols. Mass spectrometric analysis shows clean formation of products resulting from nucleophile addition and nitro group loss. 2-Nitrofurans generate nitrite upon addition of sodium borohydride or 2-aminoethanethiol, but these complex reactions do not produce clean organic products. A mechanism that includes nucleophile addition to the carbon βto the nitro group to generate a nitronate anion followed by protonation and nitrous acid elimination explains the observed products and labeling studies. These systematic studies give a better understanding of the release mechanisms of nitrogen oxide species from these compounds allowing for the design of more efficient therapeutics.

Structural and Photodynamic Studies on Nitrosylruthenium-Complexed Serum Albumin as a Delivery System for Controlled Nitric Oxide Release

How to deliver nitric oxide (NO) to a physiological target and control its release quantitatively is a key issue for biomedical applications. Here, a water-soluble nitrosylruthenium complex, [(CH₃)₄N][RuCl₃(5cqn)(NO)] (H5cqn = 5-chloro-8-quinoline), was synthesized, and its structure was confirmed with ¹H NMR and X-ray crystal diffraction. Photoinduced NO release was investigated with time-resolved Fourier transform infrared and electron paramagnetic resonance (EPR) spectroscopies. The binding constant of the [RuCl₃(5cqn)(NO)]^- complex with human serum albumin (HSA) was determined by fluorescence spectroscopy, and the binding mode was identified by X-ray crystallography of the HSA and Ru-NO complex adduct. The crystal structure reveals that two molecules of the Ru-NO complex are located in the subdomain IB, which is one of the major drug binding regions of HSA. The chemical structures of the Ru complexes were [RuCl₃(5cqn)(NO)]^- and [RuCl₃(Glycerin)NO]^-, in which the electron densities for all ligands to Ru are unambiguously identified. EPR spin-trapping data showed that photoirradiation triggered NO radical generation from the HSA complex adduct. Moreover, the near-infrared image of exogenous NO from the nitrosylruthenium complex in living cells was observed using a NO-selective fluorescent probe. This study provides a strategy to design an appropriate delivery system to transport NO and metallodrugs in vivo for potential applications.

Improvement in storage quality of postharvest tomato fruits by nitroxyl liposomes treatment

Nitroxyl (HNO) has attracted much attention due to its unique biological activity. To investigate the preservation effect of HNO on fruits, a nitroxyl liposome based on 1-nitrosocyclohexyl acetate was prepared and characterized by infrared spectroscopy and transmission electron microscopy. The optimal preparation conditions were explored, and then HNO liposomes were prepared under the optimal conditions to study the effect of HNO liposomes on postharvest quality of tomatoes. The tomato fruits were treated with different concentrations (0, 5, 10, 15 and 20 μmol L^-1) of HNO liposomes and stored at room temperature. The results indicated that treatment with HNO liposomes can more effectively delay the browning and slow down the decrease in lightness of tomatoes. Additionally, HNO liposomes can reduce the activity of PPO and POD, inhibit the increase of MDA and total phenol content. These results suggest that treatment with HNO liposomes can effectively preserve the quality of tomatoes.

Piloty's acid and its hydrazide analogue: Insights from the density functional theory and vibrational spectroscopy on the conformational stability and chemical reactivity

N-hydroxybenzenesulfonamide (commonly known as Piloty's acid) is considered a major source for nitroxyl (HNO) species which has potential biological and medicinal applications. In the present study, the conformational preferences and chemical reactivity of Piloty's acid (PA) and its hydrazide analogue (benzenesulfonylhydrazide, BSH) were studied using spectroscopic and computational tools. Six stable conformations of each molecule were theoretically identified, and their structures were fully optimized at the DFT-B3LYP and MP2 levels. Both molecules in their most stable forms adopt the anti configuration with the NH bond of the secondary amine pointing away from the terminal hydroxyl and amine moieties in the acid and hydrazide molecules, respectively. Three stable gauche states facilitated by weak intramolecular interactions of the SO⋯HO and SO⋯HN types arise due to the internal rotation about the SN linkage. Reliable assignments of the vibrational modes and the calculated reaction coordinates support a two-step mechanistic pathway of the Piloty's acid dissociation leading to the production of the nitroxyl (HNO) intermediate with moderate transition state barriers. Frontier molecular orbitals distributions, molecular electrostatic potential maps and condensed Fukui functions analysis of the molecules were employed to elucidate the agility of PA to dissociate to produce HNO and the absence of such a dissociation of BSH that would produce diazene (N₂H₂).

Interaction among Hydrogen Sulfide and Other Gasotransmitters in Mammalian Physiology and Pathophysiology

Hydrogen sulfide (H₂S), nitric oxide (NO), carbon monoxide (CO), and sulfur dioxide (SO₂) were previously considered as toxic gases, but now they are found to be members of mammalian gasotransmitters family. Both H₂S and SO₂ are endogenously produced in sulfur-containing amino acid metabolic pathway in vivo. The enzymes catalyzing the formation of H₂S are mainly CBS, CSE, and 3-MST, and the key enzymes for SO₂ production are AAT1 and AAT2. Endogenous NO is produced from L-arginine under catalysis of three isoforms of NOS (eNOS, iNOS, and nNOS). HO-mediated heme catabolism is the main source of endogenous CO. These four gasotransmitters play important physiological and pathophysiological roles in mammalian cardiovascular, nervous, gastrointestinal, respiratory, and immune systems. The similarity among these four gasotransmitters can be seen from the same and/or shared signals. With many studies on the biological effects of gasotransmitters on multiple systems, the interaction among H₂S and other gasotransmitters has been gradually explored. H₂S not only interacts with NO to form nitroxyl (HNO), but also regulates the HO/CO and AAT/SO₂ pathways. Here, we review the biosynthesis and metabolism of the gasotransmitters in mammals, as well as the known complicated interactions among H₂S and other gasotransmitters (NO, CO, and SO₂) and their effects on various aspects of cardiovascular physiology and pathophysiology, such as vascular tension, angiogenesis, heart contractility, and cardiac protection.

Updating NO•/HNO interconversion under physiological conditions: A biological implication overview

Azanone (HNO/NO^-), also called nitroxyl, is a highly reactive compound whose biological role is still a matter of debate. A key issue that remains to be clarified regarding HNO and its biological activity is that of its endogenous formation. Given the overlap of the molecular targets and reactivity of nitric oxide (NO•) and HNO, its chemical biology was perceived to be similar to that of NO• as a biological signaling agent. However, despite their closely related reactivity, NO• and HNO's biochemical pathways are quite different. Moreover, the reduction of nitric oxide to azanone is possible but necessarily coupled to other reactions, which drive the reaction forward, overcoming the unfavorable thermodynamic barrier. The mechanism of this NO•/HNO interplay and its downstream effects in different contexts were studied recently, showing that more than fifteen moderate reducing agents react with NO• producing HNO. Particularly, it is known that the reaction between nitric oxide and hydrogen sulfide (H₂S) produces HNO. However, this rate constant was not reported yet. In this work, firstly the NO•/H₂S effective rate constant was measured as a function of the pH. Then, the implications of these chemical (non-enzymatic), biologically compatible, routes to endogenous HNO formation was discussed. There is no doubt that HNO could be (is?) a new endogenously produced messenger that mediates specific physiological responses, many of which were attributed yet to direct NO• effects.

Mechanisms and consequences of protein cysteine oxidation: the role of the initial short-lived intermediates

Thiol groups in protein cysteine (Cys) residues can undergo one- and two-electron oxidation reactions leading to the formation of thiyl radicals or sulfenic acids, respectively. In this mini-review we summarize the mechanisms and kinetics of the formation of these species by biologically relevant oxidants. Most of the latter react with the deprotonated form of the thiol. Since the pKa of the thiols in protein cysteines are usually close to physiological pH, the thermodynamics and the kinetics of their oxidation in vivo are affected by the acidity of the thiol. Moreover, the protein microenvironment has pronounced effects on cysteine residue reactivity, which in the case of the oxidation mediated by hydroperoxides, is known to confer specificity to particular protein cysteines. Despite their elusive nature, both thiyl radicals and sulfenic acids are involved in the catalytic mechanism of several enzymes and in the redox regulation of protein function and/or signaling pathways. They are usually short-lived species that undergo further reactions that converge in the formation of different stable products, resulting in several post-translational modifications of the protein. Some of these can be reversed through the action of specific cellular reduction systems. Others damage the proteins irreversibly, and can make them more prone to aggregation or degradation.

Specific Reactions of RSNO, HSNO, and HNO and Their Applications in the Design of Fluorescent Probes

Nitric oxide (NO)-derived species play essential roles in regulating cellular responses. Among these species, S-nitrosothiols (including RSNO and HSNO) and nitroxyl (HNO) are especially interesting. Owing to their high reactivity and short survival time, the detection of these molecules in biological settings can be challenging. In this regard, much effort has been invested in exploring novel reactions of RSNO/HSNO/HNO and applying these reactions to develop fluorescence probes. Herein, reported specific reactions of RSNO/HSNO/HNO are summarized and strategies used in the design of fluorescent probes are illustrated. The properties and potential problems of representative probes are also discussed.

Tuning Inner-Sphere Electron Transfer in a Series of Copper/Nitrosoarene Adducts

A series of copper/nitrosoarene complexes was created that mimics several steps in biomimetic O₂ activation by copper(I). The reaction of the copper(I) complex of N,N,N',N'-tetramethypropylenediamine with a series of para-substituted nitrosobenzene derivatives leads to adducts in which the nitrosoarene (ArNO) is reduced by zero, one, or two electrons, akin to the isovalent species dioxygen, superoxide, and peroxide, respectively. The geometric and electronic structures of these adducts were characterized by means of X-ray diffraction, vibrational analysis, ultraviolet-visible spectroscopy, NMR, electrochemistry, and density functional theory (DFT) calculations. The bonding mode of the NO moiety depends on the oxidation state of the ArNO moiety: κN for ArNO, mononuclear η²-NO and dinuclear μ-η²:η¹ for ArNO^•-, and dinuclear μ-η²:η² for ArNO^2-. ¹⁵N isotopic labeling confirms the reduction state by measuring the NO stretching frequency (1392 cm^-1 for κN-ArNO, 1226 cm^-1 for η²-ArNO^•-, 1133 cm^-1 for dinuclear μ-η²:η¹-ArNO^•-, and 875 cm^-1 for dinuclear μ-η²:η² for ArNO^2-). The ¹⁵N NMR signal disappears for the ArNO^•- species, establishing a unique diagnostic for the radical state. Electrochemical studies indicate reduction waves that are consistent with one-electron reduction of the adducts and are compared with studies performed on Cu-O₂ analogues. DFT calculations were undertaken to confirm our experimental findings, notably to establish the nature of the charge-transfer transitions responsible for the intense green color of the complexes. In fine, this family of complexes is unique in that it walks through three redox states of the ArNO moiety while keeping the metal and its supporting ligand the same. This work provides snapshots of the reactivity of the toxic nitrosoarene molecules with the biologically relevant Cu(I) ion.

Nitroxyl as a Potential Theranostic in the Cancer Arena

Significance: As one-electron reduced molecule of nitric oxide (NO), nitroxyl (HNO) has gained enormous attention because of its novel physiological or pharmacological properties, ranging from cardiovascular protective actions to antitumoricidal effects. Recent Advances: HNO is emerging as a new entity with therapeutic advantages over its redox sibling, NO. The interests in the chemical, pharmacological, and biological characteristics of HNO have broadened our current understanding of its role in physiology and pathophysiology. Critical Issues: In particular, the experimental evidence suggests the therapeutic potential of HNO in tumor pharmacology, such as neuroblastoma, gastrointestinal tumor, ovarian, lung, and breast cancers. Indeed, HNO donors have been demonstrated to attenuate tumor proliferation and angiogenesis. Future Directions: In this review, the generation and detection of HNO are outlined, and the roles of HNO in cancer progression are further discussed. We anticipate that the completion of this review might give novel insights into the roles of HNO in cancer pharmacology and open up a novel field of cancer therapy based on HNO.

Bioinspired oxidation of oximes to nitric oxide with dioxygen by a nonheme iron(II) complex

The ability of two iron(II) complexes, [(Tp^Ph2)Fe^II(benzilate)] (1) and [(Tp^Ph2)(Fe^II)₂(NPP)₃] (2) (Tp^Ph2 = hydrotris(3,5-diphenylpyrazol-1-yl)borate, NPP-H = α-isonitrosopropiophenone), of a monoanionic facial N3 ligand in the O₂-dependent oxidation of oximes is reported. The mononuclear complex 1 reacts with dioxygen to decarboxylate the iron-coordinated benzilate. The oximate-bridged dinuclear complex (2), which contains a high-spin (Tp^Ph2)Fe^II unit and a low-spin iron(II)-oximate unit, activates dioxygen at the high-spin iron(II) center. Both the complexes exhibit the oxidative transformation of oximes to the corresponding carbonyl compounds with the incorporation of one oxygen atom from dioxygen. In the oxidation process, the oxime units are converted to nitric oxide (NO) or nitroxyl (HNO). The iron(II)-benzilate complex (1) reacts with oximes to afford HNO, whereas the iron(II)-oximate complex (2) generates NO. The results described here suggest that the oxidative transformation of oximes to NO/HNO follows different pathways depending upon the nature of co-ligand/reductant.Graphic abstract.

A Chemiluminescent Probe for HNO Quantification and Real-Time Monitoring in Living Cells

Azanone (HNO) is a reactive nitrogen species with pronounced biological activity and high therapeutic potential for cardiovascular dysfunction. A critical barrier to understanding the biology of HNO and furthering clinical development is the quantification and real-time monitoring of its delivery in living systems. Herein, we describe the design and synthesis of the first chemiluminescent probe for HNO, HNOCL-1, which can detect HNO generated from concentrations of Angeli's salt as low as 138 nm with high selectivity based on the reaction with a phosphine group to form a self-cleavable azaylide intermediate. We have capitalized on this high sensitivity to develop a generalizable kinetics-based approach, which provides real-time quantitative measurements of HNO concentration at the picomolar level. HNOCL-1 can monitor dynamics of HNO delivery in living cells and tissues, demonstrating the versatility of this method for tracking HNO in living systems.

Mechanisms of HNO Reactions with Ferric Heme Proteins

Many HNO-scavenging pathways exist to regulate its biological and pharmacological activities. Such reactions often involve ferric heme proteins and form an important basis for HNO probe development. However, mechanisms of HNO reactions with ferric heme proteins are largely unknown. We performed a computational investigation using metmyoglobin and catalase as representative ferric heme proteins with neutral and negatively charged axial ligands to provide the first detailed pathways. The results reproduced experimental barriers well with an average error of 0.11 kcal mol-1 . The rate-limiting step was found to be dissociation of the resting ligand or HNO coordination when there is no resting ligand. For both heme proteins, in contrast to the non-heme case, the reductive nitrosylation step was found to be barrierless proton-coupled electron transfer, which provides the major thermodynamic driving force for the overall reaction. The origin of the difference in reactivity between metmyoglobin and catalase was also revealed.

Fluorescent probes for the detection of nitroxyl (HNO)

Nitroxyl (HNO), which according to the IUPAC recommended nomenclature should be named azanone, is the protonated one-electron reduction product of nitric oxide. Recently, it has gained a considerable attention due to the interesting pharmacological effects of its donors. Although there has been great progress in the understanding of HNO chemistry and chemical biology, it still remains the most elusive reactive nitrogen species, and its selective detection is a real challenge. The development of reliable methodologies for the direct detection of azanone is essential for the understanding of important signaling properties of this reactive intermediate and its pharmacological potential. Over the last decade, there has been considerable progress in the development of low-molecular-weight fluorogenic probes for the detection of HNO, and therefore, in this review, we have focused on the challenges and limitations of and perspectives on nitroxyl detection based on the use of such probes.

Don't just say no: Differential pathways and pharmacological responses to diverse nitric oxide donors

Nitric oxide (NO) is a gaseous free radical molecule with a short half-life (∼1 s), which can gain or lose an electron into three interchangeable redox-dependent forms, the radical (NO), the nitrosonium cation (NO⁺), and nitroxyl anion (HNO). NO acts as an intra and extracellular signaling molecule regulating a wide range of functions in the cardiovascular, immune, and nervous system. NO donors are collectively known by their ability to release NOin vitro and in vivo, being proposed as therapeutic pharmacological tools for the treatment of several pathologies, such as cardiovascular disease. The highly reactive NO molecule is easily oxidized under physiological conditions to N-oxides, nitrate/nitrite and nitrogen dioxide. Different cellular responses are triggered depending on: 1) NO concentration [e.g., nanomolar for heme coordination in the allosteric site of guanylate cyclase (sGC) enzyme]; 2) the type of chemical bound to the nitrosated group (i.e., bound to nitrogen, N-nitro, or bound to sulphur atom, S-nitro) determining post-translational cysteine nitrosation; 3) the time-dependent availability of molecular targets. Classic NO donors are: organic nitrates (e.g., nitroglycerin, or glyceryl trinitrate, GTN; isosorbide mononitrate, ISMN), diazeniumdiolates having a diolate group [or NONOates, e.g., 2-(N,N-diethylamino)-diazenolate-2-oxide], S-nitrosothiols (e.g., S-nitroso glutathione, GSNO; S-nitroso-N-acetylpenicillamine, SNAP) or the organic salt sodium nitroprusside (SNP). In addition, nitroxyl (HNO) donors such as Piloty's acid and Angeli's salt can also be considered. The specific NO form released, as well as its differential reactivity to thiols, could act on different molecular targets and should be discussed in the context of: a) the type and amount of NO species determining the sensitivity of molecular targets (e.g., heme coordination, or S-nitrosation); b) the cellular redox state that could gate different effects. Experimental designs should take special care when choosing which NO donors to use, since different outcomes are to be expected. This article will comment recent findings regarding physiological responses involving NO species and their pharmacological modulation with donor drugs, especially in the context of the photic transduction pathways at the hypothalamic circadian clock.

Reaction-Based Fluorescent Probes for the Imaging of Nitroxyl (HNO) in Biological Systems

Nitroxyl (HNO) has been identified as an important signaling molecule in biological systems and plays critical roles in many physiological processes. Fluorescence imaging could provide a robust approach to explore the biological formation of HNO and its physiological functions. Herein, we summarize the organic reaction types for constructing HNO probes and specifically focus on review of the recent advances in the development of the reaction-based HNO probes and their imaging applications in living systems.

Anaerobic Bacterial Response to Nitrosative Stress

This chapter provides an overview of current knowledge of how anaerobic bacteria protect themselves against nitrosative stress. Nitric oxide (NO) is the primary source of this stress. Aerobically its removal is an oxidative process, whereas reduction is required anaerobically. Mechanisms required to protect aerobic and anaerobic bacteria are therefore different. Several themes recur in the review. First, how gene expression is regulated often provides clues to the physiological function of the gene products. Second, the physiological significance of reports based upon experiments under extreme conditions that bacteria do not encounter in their natural environment requires reassessment. Third, responses to the primary source of stress need to be distinguished from secondary consequences of chemical damage due to failure of repair mechanisms to cope with extreme conditions. NO is generated by many mechanisms, some of which remain undefined. An example is the recent demonstration that the hybrid cluster protein combines with YtfE (or RIC protein, for repair of iron centres damaged by nitrosative stress) in a new pathway to repair key iron-sulphur proteins damaged by nitrosative stress. The functions of many genes expressed in response to nitrosative stress remain either controversial or are completely unknown. The concentration of NO that accumulates in the bacterial cytoplasm is essentially unknown, so dogmatic statements cannot be made that damage to transcription factors (Fur, FNR, SoxRS, MelR, OxyR) occurs naturally as part of a physiologically relevant signalling mechanism. Such doubts can be resolved by simple experiments to meet six proposed criteria.

Over or under: hydride attack at the metal versus the coordinated nitrosyl ligand in ferric nitrosyl porphyrins

Hydride attack at a ferric heme-NO to give an Fe-HNO intermediate is a key step in the global N-cycle. We demonstrate differential reactivity when six- and five-coordinate ferric heme-NO models react with hydride. Although Fe-HNO formation is thermodynamically favored from this reaction, Fe-H formation is kinetically favored for the 5C case.

Development and cellular application of visible-light-controllable HNO releasers based on caged Piloty's acid

Novel visible-light-controllable HNO releasers was developed based on a caged form of Piloty's acid, and applied for cellular systems.

Ligand and electronic effects on copper–arylnitroso self-assembly

The topology and degree of electron transfer in self-assembled redox reactions between copper(i) species and nitrosoarenes are controlled by ligand properties.

NitroxylFluor: A Thiol-Based Fluorescent Probe for Live-Cell Imaging of Nitroxyl

Detection of nitroxyl (HNO), the transient one-electron reduced form of nitric oxide, is a significant challenge owing to its high reactivity with biological thiols (with rate constants as high as 10⁹ M^-1 s^-1). To address this, we report a new thiol-based HNO-responsive trigger that can compete against reactive thiols for HNO. This process forms a common N-hydroxysulfenamide intermediate that cyclizes to release a masked fluorophore leading to fluorescence enhancement. To ensure that the cyclization step is rapid, our design capitalizes on two established physical organic phenomena; the alpha-effect and the Thorpe-Ingold effect. Using this new trigger, we developed NitroxylFluor, a selective HNO-responsive fluorescent probe. Treatment of NitroxylFluor with an HNO donor results in a 16-fold turn-on. This probe also exhibits excellent selectivity over various reactive nitrogen, oxygen, and sulfur species and efficacy in the presence of thiols (e.g., glutathione in mM concentrations). Lastly, we successfully performed live cell imaging of HNO using NitroxylFluor.

NO and HNO donors, nitrones, and nitroxides: Past, present, and future

The biological effects attributed to nitric oxide (^• NO) and nitroxyl (HNO) have been extensively studied, propelling their array of putative clinical applications beyond cardiovascular disorders toward other age-related diseases, like cancer and neurodegenerative diseases. In this context, the unique properties and reactivity of the N-O bond enabled the development of several classes of compounds with potential clinical interest, among which ^• NO and HNO donors, nitrones, and nitroxides are of particular importance. Although primarily studied for their application as cardioprotective agents and/or molecular probes for radical detection, continuous efforts have unveiled a wide range of pharmacological activities and, ultimately, therapeutic applications. These efforts are of particular significance for diseases in which oxidative stress plays a key pathogenic role, as shown by a growing volume of in vitro and in vivo preclinical data. Although in its early stages, these efforts may provide valuable guidelines for the development of new and effective N-O-based drugs for age-related disorders. In this report, we review recent advances in the chemistry of NO and HNO donors, nitrones, and nitroxides and discuss its pharmacological significance and potential therapeutic application.

Synthesis of Nitric Oxide Donors Derived from Piloty's Acid and Study of Their Effects on Dopamine Secretion from PC12 Cells

This study investigated the mechanisms and kinetics of nitric oxide (NO) generation by derivatives of Piloty's acid (NO-donors) under physiological conditions. In order to qualitatively and quantitatively measure NO release, electron paramagnetic resonance (EPR) was carried out with NO spin trapping. In addition, voltammetric techniques, including cyclic voltammetry and constant potential amperometry, were used to confirm NO release from Piloty's acid and its derivatives. The resulting data showed that Piloty's acid derivatives are able to release NO under physiological conditions. In particular, electron-withdrawing substituents favoured NO generation, while electron-donor groups reduced NO generation. In vitro microdialysis, performed on PC12 cell cultures, was used to evaluate the dynamical secretion of dopamine induced by the Piloty's acid derivatives. Although all the studied molecules were able to induce DA secretion from PC12, only those with a slow release of NO have not determined an autoxidation of DA itself. These results confirm that the time-course of NO-donors decomposition and the amount of NO released play a key role in dopamine secretion and auto-oxidation. This information could drive the synthesis or the selection of compounds to use as potential drugs for the therapy of Parkinson's disease (PD).

Angeli's Salt, a nitroxyl anion donor, reverses endothelin-1 mediated vascular dysfunction in murine aorta

Nitroglycerin (Gtn) is a treatment for cardiovascular patients due to its vasodilatory actions, but induces tolerance when given chronically. A proposed mechanism is the superoxide (O₂^-)-oxidative stress hypothesis, which suggests that Gtn increases O₂^- production. Nitric oxide (NO) exists in three different redox states; the protonated, reduced state, nitroxyl anion (HNO) is an emerging candidate in vascular regulation. HNO is resistant to scavenging and of particular interest in conditions where high levels of reactive oxygen species (ROS) exist. We hypothesize that treatment with Gtn will exacerbate endothelin 1 (ET-1) induced vascular dysfunction via an increase in ROS, while treatment with Angeli's Salt (AS), an HNO donor, will not. Aorta from mice were isolated and divided into four groups: vehicle, ET-1 [0.1μM, 1μM], ET-1+Gtn [Gtn 1μM] and ET-1+AS [AS 1μM]. Concentration response curves (CRCs) to acetylcholine (ACh) and phenylephrine (Phe) were performed. Aorta incubated with ET-1 (for 20-22h) exhibited a decreased relaxation response to ACh and an increase in Phe-mediated contraction. Aorta incubated with AS exhibited a reversal in ET-1 induced vascular and endothelial dysfunction. ET-1 increased ROS in aortic vascular smooth muscle cells (VSMCs), visualized by dihydroethidium (DHE) staining. AS incubated reduced this ROS generation, yet maintained with Gtn treatment. These data suggest that aorta incubated with the HNO donor, AS, can reverse ET-1 mediated vascular dysfunction, which may be through a decrease or prevention of ROS generation. We propose that HNO may be vasoprotective and that HNO donors studied as a therapeutic option where other organic nitrates are contraindicative.