Discrimination of nitroxyl and nitric oxide by water-soluble Mn(III) porphyrins

Porphyrins functionalized by quaternary pyridinium units

In this review, free-base and metalloporphyrins, functionalized on meso-positions by quaternary pyridinium units, also referred to as cationic porphyrins, are presented. The article consists of five parts. In the first part free-base porphyrins are described, especially taking account on generation of singlet oxygen; next parts concern metalloporphyrins. The second and third parts deal with zinc and manganese porphyrins, respectively; in the fourth part copper, palladium, and platinum porphyrins are presented. In the fifth part, describing porphyrins with various metal ions an attention is paid to porphyrin metal-organic frameworks (MOFs) and metal-organic materials (MOMs) in which metalloporphyrins are immobilized; syntheses and characterization of obtained products are shown.

Quantitative detection of nitroxyl upon trapping with glutathione and labeling with a specific fluorogenic reagent

Donors of nitroxyl (HNO) have shown promise for treatment of stroke, heart failure, alcoholism and cancer. However, comparing the pharmacological capacities of various donors is difficult without first quantifying the amount of HNO released from each donor. Detection and quantitation of HNO has been complicated by the rapid self-consumption of HNO through irreversible dimerization, poor selectivity of trapping agents against other nitrogen oxides, and/or low sensitivity towards HNO. Here, an assay is described for the trapping of HNO by glutathione (GSH) followed by labeling of GSH with the fluorogenic agent, naphthalene-2,3-dicarboxaldehyde (NDA), and subsequent quantitation by fluorescence difference. The newly developed assay was used to validate the pH-dependence of HNO release from isopropylamine NONOate (IPA/NO), which is a dual donor of HNO and NO at physiological pH. Furthermore, varied assay conditions were utilized to suggest the ratios of the products of the reaction of GSH with HNO. At intracellular concentrations of GSH, the disulfide (GSSG) was the major product, but significant concentrations of glutathione sulfinamide (GS(O)NH₂) were also detected. This suggests that GS(O)NH₂, which is a selective biomarker of HNO, may be produced in concentrations that are amenable to in vivo analysis.

Discriminative EPR detection of NO and HNO by encapsulated nitronyl nitroxides

Nitric oxide, •NO, is one of the most important molecules in the biochemistry of living organisms. By contrast, nitroxyl, NO-, one-electron reduced analog of •NO which exists at physiological conditions in its protonated form, HNO, has been relatively overlooked. Recent data show that HNO might be produced endogenously and display unique biological effects. However, there is a lack of specific and quantitative methods of detection of endogenous HNO production. Here we present a new method for discriminative •NO and HNO detection by nitronyl nitroxides (NNs) using electron paramagnetic resonance (EPR). It was found that NNs react with •NO and HNO with similar rate constants of about 10(4) M(-1) s(-1) but yield different products: imino nitroxides and the hydroxylamine of imino nitroxides, correspondingly. An EPR approach for discriminative •NO and HNO detection using liposome-encapsulated NNs was developed. The membrane barrier of liposomes protects NNs against reduction in biological systems while is permeable to both analytes, •NO and HNO. The sensitivity of this approach for the detection of the rates of •NO/HNO generation is about 1 nM/s. The application of encapsulated NNs for real-time discriminative •NO/HNO detection might become a valuable tool in nitric oxide-related studies.

Detailed mechanism of the autoxidation of N-hydroxyurea catalyzed by a superoxide dismutase mimic Mn(III) porphyrin: formation of the nitrosylated Mn(II) porphyrin as an intermediate

The in vitro autoxidation of N-hydroxyurea (HU) is catalyzed by Mn(III)TTEG-2-PyP(5+), a synthetic water soluble Mn(III) porphyrin which is also a potent mimic of the enzyme superoxide dismutase. The detailed mechanism of the reaction is deduced from kinetic studies under basic conditions mostly based on data measured at pH = 11.7 but also including some pH-dependent observations in the pH range 9-13. The major intermediates were identified by UV-vis spectroscopy and electrospray ionization mass spectrometry. The reaction starts with a fast axial coordination of HU to the metal center of Mn(III)TTEG-2-PyP(5+), which is followed by a ligand-to-metal electron transfer to get Mn(II)TTEG-2-PyP(4+) and the free radical derived from HU (HU˙). Nitric oxide (NO) and nitroxyl (HNO) are minor intermediates. The major pathway for the formation of the most significant intermediate, the {MnNO} complex of Mn(II)TTEG-2-PyP(4+), is the reaction of Mn(II)TTEG-2-PyP(4+) with NO. We have confirmed that the autoxidation of the intermediates opens alternative reaction channels, and the process finally yields NO(2)(-) and the initial Mn(III)TTEG-2-PyP(5+). The photochemical release of NO from the {MnNO} intermediate was also studied. Kinetic simulations were performed to validate the deduced rate constants. The investigated reaction has medical implications: the accelerated production of NO and HNO from HU may be utilized for therapeutic purposes.

Computational investigations of HNO in biology

HNO (nitroxyl) has been found to have many physiological effects in numerous biological processes. Computational investigations have been employed to help understand the structural properties of HNO complexes and HNO reactivities in some interesting biologically relevant systems. The following computational aspects were reviewed in this work: 1) structural and energetic properties of HNO isomers; 2) interactions between HNO and non-metal molecules; 3) structural and spectroscopic properties of HNO metal complexes; 4) HNO reactions with biologically important non-metal systems; 5) involvement of HNO in reactions of metal complexes and metalloproteins. Results indicate that computational investigations are very helpful to elucidate interesting experimental phenomena and provide new insights into unique structural, spectroscopic, and mechanistic properties of HNO involvement in biology.

E versus Z diazeniumdiolation of acetoacetate-derived carbanions

Nitric oxide adds to methyl acetoacetate in the presence of KOH in methanol at room temperature to form potassium acetylsydnonate N-oxide (K1) with an (E)-diazeniumdiolation and potassium acetate diazenium diolate (K(2)2) from a (Z)-diazeniumdiolation. A study of the reaction with LiOH, NaOH, and NMe(4)OH and with ethyl acetate substrate reveals that the temperature of the reaction greatly influences the nitric oxide reactivity. At 23 °C, nitric oxide adds to give both E and Z products, whereas at -5 °C the gas reacts almost exclusively to give Z addition. The (Z)-diazeniumdiolation products, namely, the alkali metal and NMe(4)(+) salts of methyl and ethylbutenoate-2-diazeniumdiolate-3-hydroxylate (3(2-) and 4(2-)), are isolated in good yields. The alkali metal salts are not amenable for recrystallization because of their ready decomposition in aqueous solutions. However, [NMe(4)](2)[MeC(O)C(N(2)O(2))CO(2)Me] is readily recrystallized from a methanol/acetonitrile solvent mixture. The crystals are unambiguously characterized by X-ray crystallography. NMR spectra for all of the 3(2-) and 4(2-) salts reveal the presence of two isomers in aq solutions. But the structure of the NMe(4)(+) salt contains only one of the isomers. Our attempts to cyclize the isolated and purified butenoatediazeniumdiolates from the (Z)-diazeniumdiolation to the E-containing sydnonate products were unsuccessful. TGA/DSC data for all of the products demonstrate the thermal instability of the salts at high temperatures. The salts decompose exothermally possibly with the release of N(2)O among other gases.

Stabilization and detection of nitroxyl by iron and cobalt porphyrins in solution and on surfaces

Nitroxyl (HNO/NO-) is a small short-lived molecule that has been suggested to be produced by nitric oxide (NO) synthases under certain conditions. As for NO , biologically relevant targets of HNO are mainly heme-proteins and therefore, it has been difficult to discriminate the physio-pathological role of each molecule conclusively. Therefore, accurate discrimination between them is still an unresolved matter. On the other hand, there is only scarce information about nitroxyl-metalloporphyrin complexes. Hence, there is growing interest in obtaining and characterizing stable heme model nitroxyl complexes. In this review we show how HNO and NO can be discriminated electrochemically by a Co porphyrin attached to a gold surface, and how nitroxyl can be stabilized by coordination to an electron-poor Fe porphyrin. The Co porphyrin with four anchors, cobalt(II)-5,10,15,20-tetrakis[3-(p-acetylthio-propoxy)phenyl]porphyrin [Co(P)] was covalently attached to gold electrodes, and its reactions with NO and HNO donors were studied electrochemically. By fixing the potential to values that oxidize CoIII(P)NO-, HNO can be selectively detected by amperometric techniques. On the other hand, the one-electron chemical reduction of FeII(TFPPBr8)NO (TFPPBr8= 2 ,3,7,8,12,13,17,18-octa-β-bromo-5,10,15,20-[tetrakis-(pentafluorophenyl)]porphyrin) with cobaltocene yields the significantly stable {FeNO}8nitroxyl anion complex, [Co(C5H5)2]+[Fe(TFPPBr8)NO]-, which was isolated and characterized by several spectroscopies and DFT calculations. This species is intermediate between FeIINO-and FeINO , which is contrasted with the predominant FeIINO-character of known non-heme {FeNO}8complexes.

Layer-by-layer assembly and characterization of multilayers of a manganese porphyrin linked poly(4-vinylpyridinium) derivative and poly(styrenesulfonic acid-o-maleic) acid

Multilayers of manganese(III) porphyrin-linked poly(vinylpyridinium) (MnTMPyP-PVPMe) polyelectrolyte and poly(styrenesulfonic acid-o-maleic) acid (PSS) have been assembled on gold, quartz, and indium tin oxide surfaces by a layer-by-layer (LBL) technique. The assembly process was monitored by measuring their absorption spectra and frequency change after each assembly, both of which confirmed the formation of three-dimensional MnTMPyP-PVPMe/PSS multilayers. The Soret absorption band of porphyrin red shifted about 8 nm in the multilayer compared with that in the dilute aqueous solution. The average mass changes for each assembly of MnTMPyP-PVPMe and PSS were estimated to be about 2.9 and 0.25 μg/cm(2), respectively. X-ray photoelectron spectra revealed that the as-prepared multilayers were composed of S 2p, C 1s, N 1s, O 1s, and Mn 2p, corresponding to polymers of MnTMPyP-PVPMe and PSS. A rough surface was observed after the assembly of MnTMPyP-PVPMe on the gold surface, but it became smoother when the PSS layer was adsorbed. The significant difference in the mass change and film morphology after the assembly of MnTMPyP-PVPMe compared to those after the assembly of PSS was ascribed to the reason that the MnTMPyP-PVPMe polyelectrolyte contained large metalloporphyrin macrocycles, which were axially coordinated to the pyridyl substituents of the PVP polymeric backbones. The cyclic voltammograms revealed two couples of redox waves in the phosphate electrolyte solution at pH 11, which corresponded to the electron-transfer processes of Mn(II)/Mn(III) and Mn(III)/Mn(IV) of polymeric manganese porphyrin MnTMPyP-PVPMe. The charge-transfer process was also investigated. Finally, the present MnTMPyP-PVPMe/PSS multilayers were used as a heterogeneous catalyst for the decoloration of an azo dye.

Rapid and selective nitroxyl (HNO) trapping by phosphines: kinetics and new aqueous ligations for HNO detection and quantitation

Recent studies distinguish the biological and pharmacological effects of nitroxyl (HNO) from its oxidized/deprotonated product nitric oxide (·NO), but the lack of HNO detection methods limits the understanding its in vivo mechanisms and the identification of endogenous sources. We previously demonstrated that reaction of HNO with triarylphosphines provides aza-ylides and HNO-derived amides, which may serve as stable HNO biomarkers. We now report a kinetic analysis for the trapping of HNO by phosphines, ligations of enzyme-generated HNO, and compatibility studies illustrating the selectivity of phosphines for HNO over other physiologically relevant nitrogen oxides. Quantification of HNO using phosphines is demonstrated using an HPLC-based assay and ligations of phosphine carbamates generate HNO-derived ureas. These results further demonstrate the potential of phosphine probes for reliable biological detection and quantification of HNO.

The specificity of nitroxyl chemistry is unique among nitrogen oxides in biological systems

The importance of nitric oxide in mammalian physiology has been known for nearly 30 years. Similar attention for other nitrogen oxides such as nitroxyl (HNO) has been more recent. While there has been speculation as to the biosynthesis of HNO, its pharmacological benefits have been demonstrated in several pathophysiological settings such as cardiovascular disorders, cancer, and alcoholism. The chemical biology of HNO has been identified as related to, but unique from, that of its redox congener nitric oxide. A summary of these findings as well as a discussion of possible endogenous sources of HNO is presented in this review.

The chemistry of nitroxyl-releasing compounds

Nitroxyl (HNO) demonstrates a diverse and unique biological profile compared to nitric oxide, a redox-related compound. Although numerous studies support the use of HNO as a therapeutic agent, the inherent chemical reactivity of HNO requires the use of donor molecules. Two general chemical strategies currently exist for HNO generation from nitrogen-containing molecules: (i) the disproportionation of hydroxylamine derivatives containing good leaving groups attached to the nitrogen atom and (ii) the decomposition of nitroso compounds (X-N=O, where X represents a good leaving group). This review summarizes the synthesis and structure, the HNO-releasing mechanisms, kinetics and by-product formation, and alternative reactions of six major groups of HNO donors: Angeli's salt, Piloty's acid and its derivatives, cyanamide, diazenium diolate-derived compounds, acyl nitroso compounds, and acyloxy nitroso compounds. A large body of work exists defining these six groups of HNO donors and the overall chemistry of each donor requires consideration in light of its ability to produce HNO. The increasing interest in HNO biology and the potential of HNO-based therapeutics presents exciting opportunities to further develop HNO donors as both research tools and potential treatments.

Acyloxy nitroso compounds as nitroxyl (HNO) donors: kinetics, reactions with thiols, and vasodilation properties

Acyloxy nitroso compounds hydrolyze to nitroxyl (HNO), a nitrogen monoxide with distinct chemistry and biology. Ultraviolet-visible spectroscopy and mass spectrometry show hydrolysis rate depends on pH and ester group structure with the observed rate being trifluoroacetate (3) > acetate (1) > pivalate (2). Under all conditions, 3 rapidly hydrolyzes to HNO. A combination of spectroscopic, kinetic, and product studies show that addition of thiols increases the decomposition rate of 1 and 2, leading to hydrolysis and HNO. Under conditions that favor thiolates, the thiolate directly reacts with the nitroso group, yielding oximes without HNO formation. Biologically, 3 behaves like Angeli's salt, demonstrating thiol-sensitive nitric oxide-mediated soluble guanylate cyclase-dependent vasorelaxation, suggesting HNO-mediated vasorelaxation. The slow HNO-donor 1 demonstrates weak thiol-insensitive vasorelaxation, indicating HNO release kinetics determine HNO bioavailability and activity. These results show that acyloxy nitroso compounds represent new HNO donors capable of vasorelaxation depending on HNO release kinetics.

Superoxide dismutase mimics: chemistry, pharmacology, and therapeutic potential

Oxidative stress has become widely viewed as an underlying condition in a number of diseases, such as ischemia-reperfusion disorders, central nervous system disorders, cardiovascular conditions, cancer, and diabetes. Thus, natural and synthetic antioxidants have been actively sought. Superoxide dismutase is a first line of defense against oxidative stress under physiological and pathological conditions. Therefore, the development of therapeutics aimed at mimicking superoxide dismutase was a natural maneuver. Metalloporphyrins, as well as Mn cyclic polyamines, Mn salen derivatives and nitroxides were all originally developed as SOD mimics. The same thermodynamic and electrostatic properties that make them potent SOD mimics may allow them to reduce other reactive species such as peroxynitrite, peroxynitrite-derived CO(3)(*-), peroxyl radical, and less efficiently H(2)O(2). By doing so SOD mimics can decrease both primary and secondary oxidative events, the latter arising from the inhibition of cellular transcriptional activity. To better judge the therapeutic potential and the advantage of one over the other type of compound, comparative studies of different classes of drugs in the same cellular and/or animal models are needed. We here provide a comprehensive overview of the chemical properties and some in vivo effects observed with various classes of compounds with a special emphasis on porphyrin-based compounds.

Direct detection of nitroxyl in aqueous solution using a tripodal copper(II) BODIPY complex

The synthesis, photophysical properties, and a biological application of BODIPY-triazole 1 (BOT1) are described. BOT1 juxtaposes a BODIPY fluorophore with a tripodal ligand platform via a triazole bridge. The triazole linker is afforded by azide-alkyne "click" chemistry and comprises the third arm of the tripodal architecture. BOT1 binds Cu(2+) in aqueous solution to form a Cu(II)[BOT1] complex in which emission from the BODIPY moiety is efficiently quenched. This complex exhibits a prompt turn-on response when exposed to nitroxyl, and this emission response is specific to HNO over other reactive nitrogen and oxygen species. Notably, Cu(II)[BOT1] does not fluoresce in the presence of nitric oxide, making this system the first discrete molecular probe capable of detecting HNO over NO under physiologically relevant conditions. Fluorescence microscopy experiments establish that Cu(II)[BOT1] is membrane-permeable and can successfully signal the presence of HNO in live cells.

Alternative photoinduced release of HNO or NO from an acyl nitroso compound, depending on environmental polarity

A hydrophilic hetero-Diels-Alder cycloadduct was synthesized as a novel photocontrollable donor of reactive nitrogen species. Production of either nitric oxide (NO) or nitroxyl (HNO) was photoinduced from this compound, depending on the environmental polarity.

Kinetic feasibility of nitroxyl reduction by physiological reductants and biological implications

Nitroxyl (HNO), the one-electron reduced and protonated congener of nitric oxide (NO), is a chemically unique species with potentially important biological activity. Although HNO-based pharmaceuticals are currently being considered for the treatment of chronic heart failure or stroke/transplant-derived ischemia, the chemical events leading to therapeutic responses are not established. The interaction of HNO with oxidants results in the well-documented conversion to NO, but HNO is expected to be readily reduced as well. Recent thermodynamic calculations predict that reduction of HNO is biologically accessible. Herein, kinetic analysis suggests that the reactions of HNO with several mechanistically distinct reductants are also biologically feasible. Product analysis verified that the reductants had in fact been oxidized and that in several instances HNO had been converted to hydroxylamine. Moreover, a theoretical analysis suggests that in the reaction of HNO with thiol reductants, the pathway producing sulfinamide is significantly more favorable than that leading to disulfide. Additionally, simultaneous production of HNO and NO yielded a biphasic oxidative capacity.

Nitroxyl (HNO): the Cinderella of the nitric oxide story

Until recently, most of the biological effects of nitric oxide (NO) have been attributed to its uncharged state (NO*), yet NO can also exist in the reduced state as nitroxyl (HNO or NO(-)). Putatively generated from both NO synthase (NOS)-dependent and -independent sources, HNO is rapidly emerging as a novel entity with distinct pharmacology and therapeutic advantages over its redox sibling, NO*. Thus, unlike NO*, HNO can target cardiac sarcoplasmic ryanodine receptors to increase myocardial contractility, can interact directly with thiols and is resistant to both scavenging by superoxide (*O2-) and tolerance development. HNO donors are protective in the setting of heart failure in which NO donors have minimal impact. Here, we discuss the unique pharmacology of HNO versus NO* and highlight the therapeutic potential of HNO donors in the treatment of cardiovascular disease.

N-nitrosomelatonin outcompetes S-nitrosocysteine in inhibiting glyceraldehyde 3-phosphate dehydrogenase: first evidence that N-nitrosomelatonin can modify protein function

Low-molecular-weight S-nitrosothiols (RSNOs) are well known for their capability to transnitrosate cysteine residues of enzymes thereby altering their catalytic activity. It is unknown, however, whether N-nitrosomelatonin (NOMela) which is highly effective in transnitrosating low-molecular-weight thiols (RSHs) can also alter protein function. In the present study, we report on such a capability with glyceraldehyde 3-phosphate dehydrogenase (GAPDH) as a target enzyme. Reaction of NOMela with GAPDH resulted in an increase of RSNOs at the expense of RSHs. Somewhat surprisingly, NOMela was about 10-fold more effective than S-nitrosocysteine in inhibiting GAPDH. Vitamin C and glutathione increased the NOMela-dependent inhibition of the enzyme by accelerating the intermediacy of nitroxyl which is also highly effective in nitrosating RSHs. The occurrence of this intermediate during the NOMela-vitamin C reaction was verified by using Mn(III)-tetrakis(1-methyl-4-pyridyl)porphorin pentachloride as nitroxyl scavenger. The NOMela-dependent inactivation of GAPDH was so effective that this reaction can be used to quantify NOMela with high sensitivity.

Xerogel optical sensor films for quantitative detection of nitroxyl

Xerogel sensing films were synthesized via sol-gel chemistry were used to fabricate optical nitroxyl (HNO) sensors [corrected] Selective detection of HNO in solution was achieved by monitoring the rates of manganese(III) meso-tetrakis(4-sulfonatophenyl) porphyrinate (MnIIITPPS) reductive nitrosylation in the anaerobic interior of aminoalkoxysilane-derived xerogel films. Nitroxyl permeability in sensor films deposited in round-bottom 96-well plates was enhanced via incorporation of trimethoxysilyl-terminated poly(amidoamine-organosilicon) dendrimers in the xerogel network. The selectivity of MnIIITPPS for HNO, the overall sensitivity, and the working dynamic range of the resulting sensors were characterized. The HNO-sensing microtiter plates were used to quantify pH-dependent HNO generation by the recently described HNO-donor sodium 1-(isopropylamino)diazene-1-ium-1,2-diolate (IPA/NO), and compare HNO production efficiency between IPA/NO and Angeli's salt, a traditional HNO-donor.

The pharmacology of nitroxyl (HNO) and its therapeutic potential: not just the Janus face of NO

Nitroxyl (HNO), the 1-electron reduced and protonated congener of nitric oxide (NO), has received recent attention as a potential pharmacological agent for the treatment of heart failure and as a preconditioning agent for the mitigation of ischemia-reperfusion injury. Interest in the pharmacology and biology of HNO has prompted examination, or in some instances reexamination, of many of its chemical properties. Such studies have provided insight into the chemical basis for the biological effects of HNO, although the biochemical mechanisms for many of these effects remain to be established. In this review, a brief description of the biologically relevant chemistry of HNO is given, followed by a more detailed discussion of the pharmacology and potential toxicology of HNO.

Ab initio molecular dynamics study of manganese porphine hydration and interaction with nitric oxide

The authors use ab initio molecular dynamics and the density functional theory+U (DFT+U) method to compute the hydration environment of the manganese ion in manganese (II) and manganese (III) porphines (MnP) dispersed in liquid water. These are intended as simple models for more complex water soluble porphyrins, which have important physiological and electrochemical applications. The manganese ion in Mn(II)P exhibits significant out-of-porphine plane displacement and binds strongly to a single H2O molecule in liquid water. The Mn in Mn(III)P is on average coplanar with the porphine plane and forms a stable complex with two H2O molecules. The residence times of these water molecules exceed 15 ps. The DFT+U method correctly predicts that water displaces NO from Mn(III)P-NO, but yields an ambiguous spin state for the MnP(II)-NO complex.

Structure and dynamics of dioxygen bound to cobalt and iron heme

In this study we use ab initio molecular dynamics simulations to analyze the structure and dynamics of the oxygen ligand in models of the oxymyoglobin active site and its cobalt-substituted analog. Our calculations are performed for iron-porphyrin and cobalt-porphyrin complexes with imidazole and oxygen as axial ligands, and we investigate the effect of the distal histidine in the structure and dynamics of the metal-oxygen unit (MeO(2), Me = Fe, Co). We find that the interaction between the distal histidine and the oxygen ligand is stronger for the cobalt complex than for the iron one, consistent with the superoxide ion character of the bound O(2). The dynamics of the O(2) ligand can be described as oscillations of the O-O axis projection on the porphyrin plane within a porphyrin quadrant combined with frequent jumps from one quadrant to another. However, the ligand motion is significantly faster for CoO(2) compared to FeO(2). As a result, the iron complex shows localized ligand sites, whereas for cobalt several configurations are possible. This gives support to the highly dynamic motion of the oxygen ligand found in several experiments on cobalt oxymyoglobin and model complexes and underlines the higher mobility of the CoO(2) fragment compared to FeO(2).