Nitrosobenzene: electrochemical, UV-visible and EPR spectroscopic studies on the nitrosobenzene free radical generation and its interaction with glutathione

Stepwise Reduction of Redox Noninnocent Nitrosobenzene to Aniline via a Rare Phenylhydroxylamino Intermediate on a Thiolate-Bridged Dicobalt Scaffold

Nitrosobenzene (PhNO) and phenylhydroxylamine (PhNHOH) are of paramount importance because of their involvement as crucial intermediates in the biological metabolism and catalytic transformation of nitrobenzene (PhNO2) to aniline (PhNH2). However, a complete reductive transformation cycle of PhNO to PhNH2 via the PhNHOH intermediate has not been reported yet. In this context, we design and construct a new thiolate-bridged dicobalt scaffold that can accomplish coordination activation and reductive transformation of PhNO. Notably, an unprecedented reversible ligand-based redox sequence PhNO0 ↔ PhNO•- ↔ PhNO2- can be achieved on this well-defined {CoIII(μ-SPh)2CoIII} functional platform. Further detailed reactivity investigations demonstrate that the PhNO0 and PhNO•- complexes cannot react with the usual hydrogen and hydride donors to afford the corresponding phenylhydroxylamino (PhNHO-) species. However, the double reduced PhNO2- complex can readily undergo N-protonation with an uncommon weak proton donor PhSH to selectively yield a stable dicobalt PhNHO- bridged complex with a high pKa value of 13-16. Cyclic voltammetry shows that there are two successive reduction events at E1/2 = -0.075 V and Ep = -1.08 V for the PhNO0 complex, which allows us to determine both bond dissociation energy (BDEN-H) of 59-63 kcal·mol-1 and thermodynamic hydricity (ΔGH-) of 71-75 kcal·mol-1 of the PhNHO- complex. Both values indicate that the PhNO•- complex is not a potent hydrogen abstractor and the PhNO0 complex is not an efficient hydride acceptor. In the presence of BH3 as a combination of protons and electrons, facile N-O bond cleavage of the coordinated PhNHO- group can be realized to generate PhNH2 and a dicobalt hydroxide-bridged complex. Overall, we present the first stepwise reductive sequence, PhNO0 ↔ PhNO•- ↔ PhNO2- ↔ PhNHO- → PhNH2.

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.

Genome-wide gene-deletion screening identifies mutations that significantly enhance explosives vapor detection by a microbial sensor

Genetically engineered microbial biosensors, capable of detecting traces of explosives residues above buried military ordnance and emitting an optical signal in response, may potentially serve for the standoff detection of buried landmines. A promising candidate for such an application is a previously reported Escherichia coli-based reporter strain that employs the yqjF gene promoter as its sensing element; however, for this sensor to be able to detect actual landmines reliably, it was necessary for its detection sensitivity and signal intensity to be enhanced. In this study, a high-throughput approach was employed to screen the effects of individual gene deletions on yqjF activation by 2,4-dinitrotoluene (DNT). Several genes were identified, the deletion of which elicited a significant enhancement of yqjF induction by DNT. The most promising of these mutations were introduced into the sensor strain, individually or in pairs, yielding a considerable increase in signal intensity and a lowering of the detection threshold. A strain harboring two of the identified mutations, ygdD and eutE, appears to be the most sensitive microbial biosensor currently described for the detection of traces of landmine explosives.

Radical chemistry of nitrosoarenes: concepts, synthetic applications and directions

The radical chemistry of nitrosoarenes has recently emerged as an attractive and important topic in organic chemistry.

Structure and reactivity of a mononuclear gold(II) complex

Mononuclear gold(II) complexes are very rare labile species. Transient gold(II) species have been suggested in homogeneous catalysis and in medical applications, but their geometric and electronic structures have remained essentially unexplored: even fundamental data, such as the ionic radius of gold(II), are unknown. Now, an unprecedentedly stable neutral gold(II) complex of a porphyrin derivative has been isolated, and its structural and spectroscopic features determined. The gold atom adopts a 2+2 coordination mode in between those of gold(III) (four-coordinate square planar) and gold(I) (two-coordinate linear), owing to a second-order Jahn-Teller distortion enabled by the relativistically lowered 6s orbital of gold. The reactivity of this gold(II) complex towards dioxygen, nitrosobenzene and acids is discussed. This study provides insight on the ionic radius of gold(II), and allows it to be placed within the homologous series of nd⁹ Cu/Ag/Au divalent ions and the 5d^8/9/10 Pt/Au/Hg 'relativistic' triad in the periodic table.

Quantifying the electron donating capacities of sulfide and dissolved organic matter in sediment pore waters of wetlands

Electron donating capacity (EDC) values were determined for a set of pore water samples collected from the sediments of four separate wetlands in the Cottonwood Lakes Study Area in Jamestown, ND by mediated electrochemical analysis, reaction with substituted nitro(so)benzenes, and calculation based on measured organic carbon and sulfide concentrations. The samples were taken from four hydrologically connected and increasingly sulfidic wetlands within the study site. Parallel trends in EDC values related to hydrologic conditions and to in situ reduced sulfur content were observed by all three methodologies. In particular, it was found that sulfide and dissolved organic matter (DOM) are the primary and secondary reductants, respectively, in these systems. The efficacy of these reductants in transforming organic contaminants, however, is largely driven by native pore water reduced sulfur content. Manipulation of the systems demonstrate that while DOM is capable of reducing highly oxidized contaminants or reactive intermediates, this likely only occurs once the reducing capacity of the sulfide is exhausted. Sulfide therefore was the dominant electron donor in the pore water samples.

Optical writing and reading with a photoactivatable carbazole

The fluorescence of a carbazole chromophore can be activated irreversibly under optical control with the photoinduced opening of an oxazine ring. In proximity to silver nanoparticles, the quantum efficiency of this photochemical transformation and that of the emissive process increase significantly. The plasmonic effects responsible for such enhancements, together with the photochemical and photophysical properties engineered into this particular photoactivatable fluorophore, permit the optical writing and reading of microscaled patterns at low illumination intensities.

Biological and chemical study of fused tri- and tetracyclic indazoles and analogues with important antiparasitic activity

A series of fused tri- and tetracyclic indazoles and analogues compounds (NID) with potential antiparasitic effects were studied using voltamperometric and spectroscopic techniques. Nitroanion radicals generated by cyclic voltammetry were characterized by electron spin resonance spectroscopy (ESR) and their spectral lines were explained and analyzed using simulated spectra. In addition, we examined the interaction between radical species generated from nitroindazole derivatives and glutathione (GSH). Biological assays such as activity against Trypanosoma cruzi and cytotoxicity against macrophages were carried out. Finally, spin trapping and molecular modeling studies were also done in order to elucidate the potentials action mechanisms involved in the trypanocidal activity.

Potent 5-nitrofuran derivatives inhibitors of Trypanosoma cruzi growth: electrochemical, spectroscopic and biological studies

Cyclic voltammetry and electron spin resonance techniques were used in the investigation of several potential antiprotozoal containing thiosemicarbazone and carbamate nitrofurans. In the electrochemical behaviour, a self-protonation process involving the nitro group was observed. The reactivity of the nitro anion radical for these derivatives with glutathione, a biological relevant thiol, was also studied in means of cyclic voltammetry. These studies demonstrated that glutathione could react with radical species from 5-nitrofuryl system. Furthermore, from the voltammetric results, some parameters of biological significance as E(7)(1) (indicative of the biological nitro anion radical formation), and [Formula: see text] (thermodynamic indicator the of oxygen redox cycling) have been calculated. We also evaluated the stability of the nitro anion radical in terms of the dimerization constant (k(d)). The nitrofuran-free radicals from cyclic voltammetry were characterized by electron spin resonance. A clear dependence between both the thiosemicarbazone or carbamate substructure and the length of the linker, furyl- or furylpropenyl-spacer, and the delocalization of the unpaired electron was observed. Through of biological assays we obtained important parameters that account for the selective anti-trypanosomal activity of these derivatives. The trypomastigote viability study showed that all derivatives are as active as in the epimastigote form of the parasite in a doses dependent manner.

Integrated Study of the Dinitrobenzene Electroreduction Mechanism by Electroanalytical and Computational Methods

Electroreduction of 1,2-, 1,3-, and 1,4-dinitrobenzenes in DMF has been investigated by a set of experimental (cyclic voltammetry, chronoamperometry, and controlled potential electrolysis) and theoretical methods (digital simulation and quantum chemical calculations). The transformation of only one nitro group is observed in the presence of proton donors. The process selectivity is provided by reactions of radical anions' intermediate products. The key reactions here are protonation of radical anions of nitrosonitrobenzenes and N-O bond cleavage in radical anions of N-(nitrophenyl)-hydroxylamines.

Comparative spectroscopic and electrochemical study of nitroindazoles: 3-alcoxy, 3-hydroxy and 3-oxo derivatives

Cyclic voltammetry and electron spin resonance techniques were used in the investigation of novel 3-alkoxy- and 3-hydroxy-1-[omega-(dialkylamino)alkyl]-5-nitroindazole derivatives. A self-protonation process involving the protonation of the nitro group was observed. The reactivity of the nitro-anion radical for these derivatives with glutathione, a biological relevant thiol, was also studied by cyclic voltammetry. These studies demonstrated that glutathione could react with radical species from 5-nitroindazole system. Also we demonstrated that nitro-anion radicals show three different patterns of delocalization where the indazole 1-lateral chain does not have major influence.

Optical and redox properties of ruthenium phthalocyanine complexes tuned with axial ligand substituents

The optical and electrochemical properties of the ruthenium phthalocyanine complexes [[(t-Bu)4Pc]Ru(4-Rpy)2], where R = NO2, Me, NH2, and NMe2, are reported. The electron density at the macrocycle may be adjusted using the axial ligand substituents, which have varying electron-donating/withdrawing strengths. Electrochemical data show that the axial pyridine ligand substituents exert significant influence over the phthalocyanine ring-based redox processes. The axial ligands also influence the electronic absorption properties of the complexes with influence also being observed in the electrogenerated oxidized and reduced species.

Indazole N-oxide derivatives as antiprotozoal agents: Synthesis, biological evaluation and mechanism of action studies

A series of indazole N-oxide derivatives have been synthesized and their antichagasic and leishmanocidal properties studied. 3-Cyano-2-(4-iodophenyl)-2H-indazole N1-oxide exhibited interesting antichagasic activity on the two parasitic strains and the two parasitic stages evaluated. Furthermore, besides its trypanocidal activity, 3-cyano-2-(4-nitrophenyl)-2H-indazole N1-oxide showed leishmanocidal activity in the three parasitic strains evaluated. To gain insight into the mechanism of action, electrochemical behaviour, ESR experiment, inhibition of parasitic respiration and QSAR were performed.

ESR and electrochemical study of 5-nitroindazole derivatives with antiprotozoal activity

The electrochemistry of 3-alkoxy- and 3-hydroxy-1-[omega-(dialkylamino)alkyl]-5-nitroindazole derivatives were characterized using cyclic voltammetry in DMSO. The nitro reduction process was studied and this was affected by the acid moieties present in these compounds. A nitro anion self-protonation process was observed. This phenomenon was studied by cyclic voltammetry in presence of increasing amount of NaOH. The reactivity of the nitro anion radical of these derivatives with glutathione was also studied by cyclic voltammetry. The oxidizing effect of glutathione is supported by the parallel decrease of the anodic peak current and increase of the cathodic peak in the cyclic voltammograms, corresponding to the wave of the nitro anion radical from uncharged species with the addition of glutathione. Nitro anion radicals obtained by electrolytic reduction of these derivatives were measured and analyzed in DMSO using electron spin resonance spectroscopy.

Substrate radical intermediates in soluble methane monooxygenase

EPR spin-trapping experiments were carried out using the three-component soluble methane monooxygenase (MMO). Spin-traps 5,5-dimethyl-1-pyrroline N-oxide (DMPO), alpha-4-pyridyl-1-oxide N-tert-butylnitrone (POBN), and nitrosobenzene (NOB) were used to investigate the possible formation of substrate radical intermediates during catalysis. In contrast to a previous report, the NADH-coupled oxidations of various substrates did not produce any trapped radical species when DMPO or POBN was present. However, radicals were detected by these traps when only the MMO reductase component and NADH were present. DMPO and POBN were found to be weak inhibitors of the MMO reaction. In contrast, NOB is a strong inhibitor for the MMO-catalyzed nitrobenzene oxidation reaction. When NOB was used as a spin-trap in the complete MMO system with or without substrate, EPR signals from an NOB radical were detected. We propose that a molecule of NOB acts simultaneously as a substrate and a spin-trap for MMO, yielding the long-lived radical and supporting a stepwise mechanism for MMO.

Revisiting the interaction of the radical anion metabolite of nitrofurantoin with glutathione

There have been several conflicting reports as to the scavenging nature of glutathione toward the nitro radical anion of the drug nitrofurantoin. We produced the radical anion enzymatically using the xanthine oxidase/hypoxanthine system at pH 7.4 and pH 9.0 in the presence of various levels of glutathione from 10 to 100 mM and monitored any changes in the radical concentration via electron spin resonance spectroscopy. Independent of glutathione concentration, there was no decrease in the steady-state concentration of the radical. In fact, there was an average 30% increase in the concentration of the radical anion, which suggests enhanced enzyme activity in the presence of glutathione (GSH). These results, together with observations of the effects of glutathione on the stability of the radical anion generated by radiolysis or dithionite, rule out any detectable reaction between the nitrofurantoin radical anion and GSH under physiologically relevant conditions.