Mononitrosyl Iron Complexes Research Articles

Dynamics of nitrergic system activation in the rat brain provoked by experimentally induced seizures

Epilepsy is a pathophysiological condition displaying a highly diverse phenotype. Consequently, comprehending the mechanisms underlying seizures necessitates moving beyond a simplistic model focused on the imbalance between the classical excitatory and inhibitory neurotransmitter systems. Nitric oxide (NO), a nonclassical and multifunctional gaseous neurotransmitter, has the potential to exert a profound influence on epileptic reactivity. Unfortunately, numerous studies have not provided clear answers about its involvement in the pathophysiology of epilepsy. The objective of our study was to delineate the temporal dynamics of alterations in nitrergic system activation after experimentally induced seizures. Seizures were induced in 2-month-old male Wistar rats by an administration of pilocarpine. Over a 6-hour observation period, seizure behaviour intensity was continuously evaluated using a modified Racine scale. At intervals of 6, 12, 24, 48, or 96 h post-chemoconvulsant administration, NO spin trapping was conducted with ferrous-diethyldithiocarbamate complexes (Fe(DETC)2). Electron paramagnetic resonance (EPR) spectroscopy was employed to quantify mononitrosyl iron complexes (NO-Fe(DETC)2) in the brain. The temporal kinetic of NO release after seizures revealed a rise in NO synthesis during the initial 12 h. Subsequently, a sharp decline occurred, returning to baseline 96 h after pilocarpine injection. Notably, our research suggests that the level of NO synthesis does not interfere with the severity of the epileptic seizures that occur. In light of this, we propose that the nitrergic system is quickly activated in the epileptic brain as a compensatory mechanism of the central nervous system. However, under usual conditions, this activation is insufficient to effectively attenuate seizures.

Sequential Deoxygenation of CO2 and NO2- via Redox-Control of a Pyridinediimine Ligand with a Hemilabile Phosphine.

The deoxygenation of environmental pollutants CO2 and NO2- to form value-added products is reported. CO2 reduction with subsequent CO release and NO2- conversion to NO are achieved via the starting complex Fe(PPhPDI)Cl2 (1). 1 contains the redox-active pyridinediimine (PDI) ligand with a hemilabile phosphine located in the secondary coordination sphere. 1 was reduced with SmI2 under a CO2 atmosphere to form the direduced monocarbonyl Fe(PPhPDI)(CO) (2). Subsequent CO release was achieved via oxidation of 2 using the NOx- source, NO2-. The resulting [Fe(PPhPDI)(NO)]+ (3) mononitrosyl iron complex (MNIC) is formed as the exclusive reduction product due to the hemilabile phosphine. 3 was investigated computationally to be characterized as {FeNO}7, an unusual intermediate-spin Fe(III) coupled to triplet NO- and a singly reduced PDI ligand.

Mononuclear mononitrosyl iron complex with 8-mercaptoquinoline. Synthesis, structure and properties

A mononuclear iron mononitrosyl complex with two 8-mercaptoquinolate ligands was obtained by two different methods. It donates NO when dissolved in DMSO and then diluted with water. The complex was studied by X-ray diffraction, IR, Mössbauer, and EPR spectroscopy. The strong covalent bonding is observed in the Fe(II)-NO• fragment which, together with strong antiferromagnetic coupling, gives the resulting S = 1/2 for the {FeNO}7-unit.The study of the magnetic properties has shown that there is a tendency for the molecules of the complex to dimerize in the crystal structure due to intermolecular ferromagnetic exchange coupling between them through π-stacking at low temperatures.

Positive (Regulatory) and Negative (Cytotoxic) Effects of Dinitrosyl Iron Complexes on Living Organisms.

The proposed in our studies mechanism of dinitrosyl iron complex (DNIC) formation through the main step of disproportionation of two NO molecules in complex with Fe2+ ion leads to emergence of the resonance structure of dinitrosyl-iron fragment of DNIC, [Fe2+(NO)(NO+)]. The latter allowed suggesting capacity of these complexes to function as donor of both neutral NO molecules as well as nitrosonium cations (NO+), which has been demonstrated in experiments. Analysis of biological activity of DNICs with thiol-containing ligands presented in this review demonstrates that NO molecules and nitrosonium cations released from the complexes exert respectively positive (regulatory) and negative (cytotoxic) effects on living organisms. It has been suggested to use dithiocarbamate derivatives to enhance selective release of nitrosonium cations from DNIC in living organisms followed by simultaneous incorporation of the released NO molecules into the biologically non-active mononitrosyl iron complexes with dithiocarbamate derivatives.

Thionitrite (SNO- ) and Perthionitrite (SSNO- ) are Simple Synthons for Nitrosylated Iron Sulfur Clusters.

S/N crosstalk species derived from the interconnected reactivity of H2 S and NO facilitate the transport of reactive sulfur and nitrogen species in signaling, transport, and regulatory processes. We report here that simple Fe2+ ions, such as those that are bioavailable in the labile iron pool (LIP), react with thionitrite (SNO- ) and perthionitrite (SSNO- ) to yield the dinitrosyl iron complex [Fe(NO)2 (S5 )]- . In the reaction of FeCl2 with SNO- we were able to isolate the unstable intermediate hydrosulfido mononitrosyl iron complex [FeCl2 (NO)(SH)]- , which was characterized by X-ray crystallography. We also show that [Fe(NO)2 (S5 )]- is a simple synthon for nitrosylated Fe-S clusters via its reduction with PPh3 to yield Roussin's Red Salt ([Fe2 S2 (NO)4 ]2- ), which highlights the role of S/N crosstalk species in the assembly of fundamental Fe-S motifs.

Thionitrite (SNO−) and Perthionitrite (SSNO−) are Simple Synthons for Nitrosylated Iron Sulfur Clusters

AbstractS/N crosstalk species derived from the interconnected reactivity of H2S and NO facilitate the transport of reactive sulfur and nitrogen species in signaling, transport, and regulatory processes. We report here that simple Fe2+ ions, such as those that are bioavailable in the labile iron pool (LIP), react with thionitrite (SNO−) and perthionitrite (SSNO−) to yield the dinitrosyl iron complex [Fe(NO)2(S5)]−. In the reaction of FeCl2 with SNO− we were able to isolate the unstable intermediate hydrosulfido mononitrosyl iron complex [FeCl2(NO)(SH)]−, which was characterized by X‐ray crystallography. We also show that [Fe(NO)2(S5)]− is a simple synthon for nitrosylated Fe−S clusters via its reduction with PPh3 to yield Roussin's Red Salt ([Fe2S2(NO)4]2−), which highlights the role of S/N crosstalk species in the assembly of fundamental Fe−S motifs.

Physico-Chemistry of Dinitrosyl Iron Complexes as a Determinant of Their Biological Activity.

In this article we minutely discuss the so-called “oxidative” mechanism of mononuclear form of dinitrosyl iron complexes (M-DNICs) formations proposed by the author. M-DNICs are proposed to be formed from their building material—neutral NO molecules, Fe2+ ions and anionic non-thiol (L−) and thiol (RS−) ligands based on the disproportionation reaction of NO molecules binding with divalent ion irons in pairs. Then a protonated form of nitroxyl anion (NO−) appearing in the reaction is released from this group and a neutral NO molecule is included instead. As a result, M-DNICs are produced. Their resonance structure is described as [(L−)2Fe2+(NO)(NO+)], in which nitrosyl ligands are represented by NO molecules and nitrosonium cations in equal proportions. Binding of hydroxyl ions with the latter causes conversion of these cations into nitrite anions at neutral pH values and therefore transformation of DNICs into the corresponding high-spin mononitrosyl iron complexes (MNICs) with the resonance structure described as [(L−)2Fe2+(NO)]. In case of replacing L− by thiol-containing ligands, which are characterized by high π-donor activity, electron density transferred from sulfur atoms to iron-dinitrosyl groups neutralizes the positive charge on nitrosonium cations, which prevents their hydrolysis, ensuring relatively a high stability of the corresponding M-DNICs with the resonance structure [(RS−)2Fe2+ (NO, NO+)]. Therefore, M-DNICs with thiol-containing ligands, as well as their binuclear analogs (B-DNICs, respective resonance structure [(RS−)2Fe2+2 (NO, NO+)2]), can serve donors of both NO and NO+. Experiments with solutions of B-DNICs with glutathione or N-acetyl-L-cysteine (B-DNIC-GSH or B-DNIC-NAC) showed that these complexes release both NO and NO+ in case of decomposition in the presence of acid or after oxidation of thiol-containing ligands in them. The level of released NO was measured via optical absorption intensity of NO in the gaseous phase, while the number of released nitrosonium cations was determined based on their inclusion in S-nitrosothiols or their conversion into nitrite anions. Biomedical research showed the ability of DNICs with thiol-containing ligands to be donors of NO and NO+ and produce various biological effects on living organisms. At the same time, NO molecules released from DNICs usually have a positive and regulatory effect on organisms, while nitrosonium cations have a negative and cytotoxic effect.

Dinitrosyl Iron Complexes with Thiol-Containing Ligands Exist in Living Organisms Mainly in the Binuclear Form

The levels of the mononitrosyl iron complex with diethyldithiocarbamate that form in the liver of mice in vivo and in vitro after intraperitoneal injection of binuclear dinitrosyl iron complexes with N-acetyl-L-cysteine or glutathione, S-nitrosoglutathione, sodium nitrite, or the vasodilating drug isosorbide dinitrate (Isoket®) have been assessed by electron paramagnetic resonance (EPR). The levels of the complex in mice that received binuclear dinitrosyl iron complexes with thiol-containing ligands or S-nitrosoglutathione do not change after the treatment of liver preparations with the strong reducing agent dithionite, in contrast to those formed after nitrite or isosorbide dinitrate administration, whose levels sharply increase after the same treatment. It is inferred that in the latter case an EPR-active mononitrosyl iron complex with diethyldithiocarbamate is produced with the absence or presence of dithionite in the reaction of NO formed from nitrite with Fe2+-diethyldithiocarbamate and Fe3+-diethyldithiocarbamate complexes, respectively. In the former case, the mononitrosyl iron complex with diethyldithiocarbamate is produced by transition of iron-mononitrosyl fragments from already present iron-dinitrosyl groups of binuclear dinitrosyl complexes, whose content is three to four times higher than the content of the mononuclear form of these complexes in the tissue. The results we obtained indicate that when dinitrosyl iron complexes with thiol-containing ligands, either introduced into the body or produced with the participation of endogenous NO, appear in animal tissues in vivo, these complexes are presented in these tissues mainly in their diamagnetic, EPR-silent binuclear form.

ДИНИТРОЗИЛЬНЫЕ КОМПЛЕКСЫ ЖЕЛЕЗА C ТИОЛСОДЕРЖАЩИМИ ЛИГАНДАМИ ПРЕДСТАВЛЕНЫ В ЖИВЫХ ОРГАНИЗМАХ В ОСНОВНОМ ИХ БИЯДЕРНОЙ ФОРМОЙ

The number of mononitrosyl iron complexes with diethyldithiocarbamate, formed in the liver of mice in vivo and in vitro after intraperitoneal injection of binuclear dinitrosyl iron complexes with N-acetyl-L-cysteine or glutathione, S-nitrosoglutathione, sodium nitrite or the vasodilating drug Isoket® was assessed by electron paramagnetic resonance (EPR). The number of the said complexes, in contrast to the complexes, formed after nitrite or Isoket administration, the level of which sharply increased after treatment of liver preparations with a strong reducing agent - dithionite, did not change in the presence of dithionite. It was concluded that, in the first case, EPR-detectable mononitrosyl iron complexes with diethyldithiocarbamate in the absence and presence of dithionite appeared as a result of the reaction of NO formed from nitrite with Fe2+-dieth- yldithiocarbamate and Fe3+-diethyldithiocarbamate complexes, respectively. In the second case, mononitrosyl iron complexes with diethyldithiocarbamate appeared as a result of the transition of iron-mononitosyl fragments from ready-made iron-dinitrosyl groups of binuclear dinitrosyl complexes, which is three to four times higher than the content of the mononuclear form of these complexes in the tissue...

Hemilabile Proton Relays and Redox Activity Lead to {FeNO} x and Significant Rate Enhancements in NO2- Reduction.

Incorporation of the triad of redox activity, hemilability, and proton responsivity into a single ligand scaffold is reported. Due to this triad, the complexes Fe(PyrrPDI)(CO)2 (3) and Fe(MorPDI)(CO)2 (4) display 40-fold enhancements in the initial rate of NO2- reduction, with respect to Fe(MeOPDI)(CO)2 (7). Utilizing the proper sterics and p Ka of the pendant base(s) to introduce hemilability into our ligand scaffolds, we report unusual {FeNO} x mononitrosyl iron complexes (MNICs) as intermediates in the NO2- reduction reaction. The {FeNO} x species behave spectroscopically and computationally similar to {FeNO}7, an unusual intermediate-spin Fe(III) coupled to triplet NO- and a singly reduced PDI ligand. These {FeNO} x MNICs facilitate enhancements in the initial rate.

Dinitrosyl iron complexes with thiol-containing ligands in plant tissues

The formation of dinitrosyl iron complexes with thiol-containing ligands in plant tissues (parsley and apple leaves) in the presence of nitric monoxide was demonstrated using electron paramagnetic resonance. In two types of tissues dinitrosyl iron complexes are predominantly represented by the binuclear diamagnetic form. This diamagnetic form can be transformed in EPR-detectable mononitrosyl iron complexes with diethyldithiocarbamate due to the ability of diethyldithiocarbamate to accept the iron-mononitrosyl groups from iron-dinitrosyl fragments of binuclear complexes. A similar transformation was observed under the effect of diethyldithiocarbamate on a mononuclear paramagnetic form of dinitrosyl iron complexes. The significant amount of binuclear dinitrosyl iron complexes found in plant tissues suggests that these complexes can be considered as a “working form” of nitric monoxide, which is recognized now as a universal regulator of metabolic processes in plants as well as in other organisms.

The binuclear form of dinitrosyl iron complexes with thiol-containing ligands in animal tissues

It has been established that treatment of mice with sodium nitrite, S-nitrosoglutathione and the water-soluble nitroglycerine derivative isosorbide dinitrate (ISDN) as NO donors initiates in vivo synthesis of significant amounts of EPR-silent binuclear dinitrosyl iron complexes (B-DNIC) with thiol-containing ligands in the liver and other tissues of experimental mice. This effect is especially apparent if NO donors are administered to mice simultaneously with the Fe2+-citrate complex. Similar results were obtained in experiments on isolated liver and other mouse tissues treated with gaseous NО in vitro and during stimulation of endogenous NO synthesis in the presence of inducible NO synthase. B-DNIC appeared in mouse tissues after in vitro treatment of tissue samples with an aqueous solution of diethyldithiocarbamate (DETC), which resulted in the transfer of iron-mononitrosyl fragments from B-DNIC to the thiocarbonyl group of DETC and the formation of EPR-detectable mononitrosyl iron complexes (MNIC) with DETC. EPR-Active MNIC with N-methyl-d-glucamine dithiocarbamate (MGD) were synthesized in a similar way. MNIC-MGD were also formed in the reaction of water-soluble MGD-Fe2+ complexes with sodium nitrite, S-nitrosoglutathione and ISDN.

Synthetic modeling chemistry of iron-sulfur clusters in nitric oxide signaling.

Nitric oxide (NO) is an important signaling molecule that is involved in many physiological and pathological functions. Iron-sulfur proteins are one of the main reaction targets for NO, and the [Fe-S] clusters within these proteins are converted to various iron nitrosyl species upon reaction with NO, of which dinitrosyl iron complexes (DNICs) are the most prevalent. Much progress has been made in identifying the origin of cellular DNIC generation. However, it is not well-understood which other products besides DNICs may form during [Fe-S] cluster degradation nor what effects DNICs and other degradation products can have once they are generated in cells. Even more elusive is an understanding of the manner by which cells cope with unwanted [Fe-S] modifications by NO. This Account describes our synthetic modeling efforts to identify cluster degradation products derived from the [2Fe-2S]/NO reaction in order to establish their chemical reactivity and repair chemistry. Our intent is to use the chemical knowledge that we generate to provide insight into the unknown biological consequences of cluster modification. Our recent advances in three different areas are described. First, new reaction conditions that lead to the formation of previously unrecognized products during the reaction of [Fe-S] clusters with NO are identified. Hydrogen sulfide (H2S), a gaseous signaling molecule, can be generated from the reaction between [2Fe-2S] clusters and NO in the presence of acid or formal H• (e(-)/H(+)) donors. In the presence of acid, a mononitrosyl iron complex (MNIC) can be produced as the major iron-containing product. Second, cysteine analogues can efficiently convert MNICs back to [2Fe-2S] clusters without the need for any other reagents. This reaction is possible for cysteine analogues because of their ability to labilize NO from MNICs and their capacity to undergo C-S bond cleavage, providing the necessary sulfide for [2Fe-2S] cluster formation. Lastly, unique dioxygen reactivity of various types of DNICs has been established. N-bound neutral {Fe(NO)2}(10) DNICs react with O2 to generate low-temperature stable peroxynitrite (ONOO(-)) species, which then carry out nitration chemistry in the presence of phenolic substrates, relevant to tyrosine nitration chemistry. The reaction between S-bound anionic {Fe(NO)2}(9) DNICs and O2 results in the formation of Roussin's red esters (RREs) and thiol oxidation products, chemistry that may be important in biological cysteine oxidation. The N-bound cationic {Fe(NO)2}(9) DNICs can spontaneously release NO, and this property can be utilized in developing a new class of NO-donating agents with anti-inflammatory activity.

Mono- and binuclear dinitrosyl iron complexes with thiol-containing ligands in various biosystems

It has been shown that dinitrosyl iron complexes with thiol-containing ligands, bound with modified bovine serum albumin with high amount of thiol groups, appeared in baker yeast or in animal tissues in the presence of exogenous or endogenous nitric oxide, respectively, are represented predominantly by EPR-silent binuclear form. This form can be transformed into EPR-active mononuclear form of dinitrosyl iron complexes with an increase in pH to basic values, into EPR-active form of mononuclear iron nitrosyl complexes in case of bielectronic recovery of the binuclear form of dinitrosyl iron complexes or under the action of dithiocarbamate derivatives. The latter induced the transformation of dinitrosyl iron complexes into EPR-active mononitrosyl iron complexes with dithiocarbamates. A significant amount of binuclear dinitrosyl iron complexes with thiol-containing ligands in living systems and identical biological activity of these complexes and endogenous nitric oxide systems allow of considering endogenous binuclear dinitrosyl iron complexes as a "working form" of endogenous nitric oxide recognized now as a universal regulator of biological processes.

New Synthetic Routes to Iron-Sulfur Clusters: Deciphering the Repair Chemistry of [2Fe-2S] Clusters from Mononitrosyl Iron Complexes.

The nitrosylation of inorganic protein cofactors, specifically that of [Fe-S] clusters to form iron nitrosyls, plays a number of important roles in biological systems. In some of these cases, it is expected that a repair process reverts the nitrosylated iron species to intact [Fe-S] clusters. The repair of nitrosylated [2Fe-2S] cluster, primarily in the form of protein-bound dinitrosyl iron complexes (DNICs), has been observed in vitro and in vivo, but the mechanism of this process remains uncertain. The present work expands upon a previous observation (Fitzpatrick et al. J. Am. Chem. Soc. 2014, 136, 7229) of the ability of mononitrosyl iron complexes (MNICs) to be converted into [2Fe-2S] clusters by the addition of nothing other than a cysteine analogue. Herein, each of the critical elementary steps in the cluster repair has been dissected to elucidate the roles of the cysteine analogue. Systematic variations of a cysteine analogue employed in the repair reaction suggest that (i) the bidentate coordination of a cysteine analogue to MNIC promotes NO release from iron, and (ii) deprotonation of the α carbon of the ferric-bound cysteine analogue leads to the C-S cleavage en route to the formation of [2Fe-2S] cluster. The [2Fe-2S] cluster bearing a cysteine analogue has also been synthesized from thiolate-bridged iron dimers of the form [Fe2(μ-SR)2(SR)4](0/2-), which implies that such species may be present as intermediates in the cluster repair. In addition to MNICs, mononuclear tetrathiolate ferric or ferrous species have been established as another form of iron from which [2Fe-2S] clusters can be generated without need for any other reagent but a cysteine analogue. The results of these experiments bring to light new chemistry of classic coordination complexes and provides further insight into the repair of NO-modified [2Fe-2S] clusters.

Iron(III) bound by hydrosulfide anion ligands: NO-promoted stabilization of the [Fe(III)-SH] motif.

Spontaneous transformation of the thermally stable [HS](-)-bound {Fe(NO)2}(9) dinitrosyl iron complex (DNIC) [(HS)2Fe(NO)2](-) (1) into [(NO)2Fe(μ-S)]2(2-) (Roussin's red salt (RRS)) along with release of H2S, probed by NBD-SCN (NBD = nitrobenzofurazan), was observed when DNIC 1 was dissolved in water at ambient temperature. The reversible transformation of RRS into DNIC 1 (RRS → DNIC 1) in the presence of H2S was demonstrated. In contrast, the thermally unstable hydrosulfide-containing mononitrosyl iron complex (MNIC) [(HS)3Fe(III)(NO)](-) (3) and [Fe(III)(SH)4](-) (5) in THF/DMF spontaneously dimerized into the first structurally characterized Fe(III)-hydrosulfide complexes [(NO)(SH)Fe(μ-S)]2(2-) (4) with two {Fe(NO)}(7) motifs antiferromagnetically coupled and [(SH)2Fe(μ-S)]2(2-) (6) resulting from two Fe(III) (S = 5/2) centers antiferromagnetically coupled to yield an S = 0 ground state with thermal occupancy of higher spin states, respectively. That is, the greater the number of NO ligands bound to [2Fe2S], the larger the antiferromagnetic coupling constant. On the basis of DFT computation and the experimental (and calculated) reduction potential (E1/2) of complexes 1, 3, and 5, the NO-coordinate ligand(s) of complexes 1 and 3 serves as the stronger electron-donating ligand, compared to thiolate, to reduce the effective nuclear charge (Zeff) of the iron center and prevent DNIC 1 from dimerization in an organic solvent (MeCN).

Transformation of a mononitrosyl iron complex to a [2Fe-2S] cluster by a cysteine analogue.

Reversible modification of iron-sulfur clusters by nitric oxide acts as a genetic switch in a group of regulatory proteins. While the conversion of [Fe-S] clusters to iron-nitrosyls has been widely studied in the past, little is known about the reverse process, the repair of [Fe-S] clusters. Reported here is a system in which a mononitrosyl iron complex (MNIC), (PPN)[Fe(S(t)Bu)3(NO)] (1), is converted to a [2Fe-2S] cluster, (PPN)2[Fe2S2(SCH2CH2C(O)OMe)4] (2). This conversion requires only the addition of a cysteine analogue, 3-mercaptomethylpropionate (MMP), at room temperature without the need for any other reagents. The identity of 2 was confirmed spectroscopically, chemically, crystallographically, and analytically. Mass spectrometry and (34)S labeling studies support that the bridging sulfides in 2 derive from the added MMP, the cysteine analogue. The NO lost during the conversion of 1 to 2 is trapped in a dinitrosyl iron side product, (PPN)[Fe(SCH2CH2C(O)OMe)2(NO)2] (4). The present system implies that MNICs are likely intermediates in the repair of NO-damaged [2Fe-2S] clusters and that cysteine is a viable molecule responsible for the destabilization of MINCs and the formation of [2Fe-2S] clusters.

Theoretical Analysis of Fe K‐edge XANES on Mononitrosyl Iron Complex [(NO)Fe(S2C6H4)2][PPN

AbstractX‐ray absorption spectroscopy (XAS) becomes one of the important techniques to characterize the active metal site of crystalline or non‐crystalline materials. The x‐ray absorption near structure (XANES) of Fe K‐edge is often used to monitor the variation of the oxidation states and the local geometric structures in a series of Fe complexes or in situ reactions such as synthetic catalysts and metalloproteins. We provide a detailed interpretation of Fe K‐edge XANES features on the mononitrosyl iron complex, [(NO)Fe(S2C6H4)2][PPN], based on the theoretical calculation results of FDM (finite difference method) and TDDFT (time‐dependent density functional theory). The pre‐edge peak is mainly the transition of 1s(Fe) → 3dz2(Fe)(mixing with 3pz(S)). The following rising edge to white line region is related to Fe‐S and Fe‐N bonding. To apply the results, we have studied the variation of XANES features with systematically changing of Fe‐N bond lengths. The increasing of Fe‐N bond lengths leads to the XANES feature shifting to lower energy owing to reducing the orbitals overlapped between Fe and N.

Nitrate-to-Nitrite-to-Nitric Oxide Conversion Modulated by Nitrate-Containing {Fe(NO)2}9 Dinitrosyl Iron Complex (DNIC)

Nitrosylation of high-spin [Fe(κ(2)-O(2)NO)(4)](2-) (1) yields {Fe(NO)}(7) mononitrosyl iron complex (MNIC) [(κ(2)-O(2)NO)(κ(1)-ONO(2))(3)Fe(NO)](2-) (2) displaying an S = 3/2 axial electron paramagnetic resonance (EPR) spectrum (g(⊥) = 3.988 and g(∥) = 2.000). The thermally unstable nitrate-containing {Fe(NO)(2)}(9) dinitrosyl iron complex (DNIC) [(κ(1)-ONO(2))(2)Fe(NO)(2)](-) (3) was exclusively obtained from reaction of HNO(3) and [(OAc)(2)Fe(NO)(2)](-) and was characterized by IR, UV-vis, EPR, superconducting quantum interference device (SQUID), X-ray absorption spectroscopy (XAS), and single-crystal X-ray diffraction (XRD). In contrast to {Fe(NO)(2)}(9) DNIC [(ONO)(2)Fe(NO)(2)](-) constructed by two monodentate O-bound nitrito ligands, the weak interaction between Fe(1) and the distal oxygens O(5)/O(7) of nitrato-coordinated ligands (Fe(1)···O(5) and Fe(1)···O(7) distances of 2.582(2) and 2.583(2) Å, respectively) may play important roles in stabilizing DNIC 3. Transformation of nitrate-containing DNIC 3 into N-bound nitro {Fe(NO)}(6) [(NO)(κ(1)-NO(2))Fe(S(2)CNEt(2))(2)] (7) triggered by bis(diethylthiocarbamoyl) disulfide ((S(2)CNEt(2))(2)) implicates that nitrate-to-nitrite conversion may occur via the intramolecular association of the coordinated nitrate and the adjacent polarized NO-coordinate ligand (nitrosonium) of the proposed {Fe(NO)(2)}(7) intermediate [(NO)(2)(κ(1)-ONO(2))Fe(S(2)CNEt(2))(2)] (A) yielding {Fe(NO)}(7) [(NO)Fe(S(2)CNEt(2))(2)] (6) along with the release of N(2)O(4) (·NO(2)) and the subsequent binding of ·NO(2) to complex 6. The N-bound nitro {Fe(NO)}(6) complex 7 undergoes Me(2)S-promoted O-atom transfer facilitated by imidazole to give {Fe(NO)}(7) complex 6 accompanied by release of nitric oxide. This result demonstrates that nitrate-containing DNIC 3 acts as an active center to modulate nitrate-to-nitrite-to-nitric oxide conversion.

Dinitrosyl iron complexes with glutathione as NO and NO+ donors

It has been found that heating of solutions of the binuclear form of dinitrosyl iron complexes (B-DNIC) with glutathione in a degassed Thunberg apparatus (рН 1.0, 70°С, 6h) results in their decomposition with a concomitant release of four gaseous NO molecules per one B-DNIC. Further injection of air into the Thunberg apparatus initiates fast oxidation of NO to NO2 and formation of two GS-NO molecules per one B-DNIC. Under similar conditions, the decomposition of B-DNIC solutions in the Thunberg apparatus in the presence of air is complete within 30–40min and is accompanied by formation of four GS-NO molecules per one B-DNIC. It is suggested that the latter events are determined by oxidation of B-DNIC iron and concominant release of four nitrosonium ions (NO+) from each complex. Binding of NO+ to thiol groups of glutathione provokes GS-NO synthesis. At neutral рН, decomposition of B-DNIC is initiated by strong iron chelators, viz., о-phenanthroline and N-methyl-d-glucamine dithiocarbamate (MGD). In the former case, the reaction occurs under anaerobic conditions (degassed Thunberg apparatus) and is accompanied by a release of four NO molecules from B-DNIC. Under identical conditions, MGD-induced decomposition of B-DNIC gives two EPR-active mononuclear mononitrosyl iron complexes with MGD (MNIC–MGD) able to incorporate two iron molecules and two NO molecules from each B-DNIC. The other two NO molecules released from B-DNIC (most probably, in the form of nitrosonium ions) bind to thiol groups of MGD to give corresponding S-nitrosothiols. Acidification of test solutions to рН 1.0 initiates hydrolysis of MGD and, as a consequence, decomposition of MNIC–MGD and the S-nitrosated form of MGD; the gaseous phase contains four NO molecules (as calculated per each B-DNIC). The data obtained testify to the ability of B-DNIC with glutathione (and, probably, of B-DNIC with other thiol-containing ligands) to release both NO molecules and nitrosonium ions upon their decomposition. As far as nitrosyl iron complexes with non-thiol-containing ligands predominantly represented by the mononuclear mononitrosyl iron form (MNIC) are concerned, their decomposition yields exclusively NO molecules.