Protein-bound Dinitrosyl Iron Complexes Research Articles

Conjugation of bone grafts with NO-delivery dinitrosyl iron complexes promotes synergistic osteogenesis and angiogenesis in rat calvaria bone defects.

Craniofacial/jawbone deformities remain a significant clinical challenge in restoring facial/dental functions and esthetics. Despite the reported therapeutics for clinical bone tissue regeneration, the bioavailability issue of autografts and limited regeneration efficacy of xenografts/synthetic bone substitutes, however, inspire continued efforts towards functional conjugation and improvement of bioactive bone graft materials. Regarding the potential of nitric oxide (NO) in tissue engineering, herein, functional conjugation of NO-delivery dinitrosyl iron complex (DNIC) and osteoconductive bone graft materials was performed to optimize the spatiotemporal control over the delivery of NO and to activate synergistic osteogenesis and angiogenesis in rat calvaria bone defects. Among three types of biomimetic DNICs, [Fe2(μ-SCH2CH2COOH)2(NO)4] (DNIC-COOH) features a steady kinetics for cellular uptake by MC3T3-E1 osteoblast cells followed by intracellular assembly of protein-bound DNICs and release of NO. This steady kinetics for intracellular delivery of NO by DNIC-COOH rationalizes its biocompatibility and wide-spectrum cell proliferation effects on MC3T3-E1 osteoblast cells and human umbilical vein endothelial cells (HUVECs). Moreover, the bridging [SCH2CH2COOH]- thiolate ligands in DNIC-COOH facilitate its chemisorption to deproteinized bovine bone mineral (DBBM) and physisorption onto TCP (β-tricalcium phosphate), respectively, which provides a mechanism to control the kinetics for the local release of loaded DNIC-COOH. Using rats with calvaria bone defects as an in vivo model, DNIC-DBBM/DNIC-TCP promotes the osteogenic and angiogenic activity ascribed to functional conjugation of osteoconductive bone graft materials and NO-delivery DNIC-COOH. Of importance, the therapeutic efficacy of DNIC-DBBM/DNIC-TCP on enhanced compact bone formation after treatment for 4 and 12 weeks supports the potential for clinical application to regenerative medicine.

Cell-Penetrating Delivery of Nitric Oxide by Biocompatible Dinitrosyl Iron Complex and Its Dermato-Physiological Implications.

After the discovery of endogenous dinitrosyl iron complexes (DNICs) as a potential biological equivalent of nitric oxide (NO), bioinorganic engineering of [Fe(NO)2] unit has emerged to develop biomimetic DNICs [(NO)2Fe(L)2] as a chemical biology tool for controlled delivery of NO. For example, water-soluble DNIC [Fe2(μ-SCH2CH2OH)2(NO)4] (DNIC-1) was explored for oral delivery of NO to the brain and for the activation of hippocampal neurogenesis. However, the kinetics and mechanism for cellular uptake and intracellular release of NO, as well as the biocompatibility of synthetic DNICs, remain elusive. Prompted by the potential application of NO to dermato-physiological regulations, in this study, cellular uptake and intracellular delivery of DNIC [Fe2(μ-SCH2CH2COOH)2(NO)4] (DNIC-2) and its regulatory effect/biocompatibility toward epidermal cells were investigated. Upon the treatment of DNIC-2 to human fibroblast cells, cellular uptake of DNIC-2 followed by transformation into protein-bound DNICs occur to trigger the intracellular release of NO with a half-life of 1.8 ± 0.2 h. As opposed to the burst release of extracellular NO from diethylamine NONOate (DEANO), the cell-penetrating nature of DNIC-2 rationalizes its overwhelming efficacy for intracellular delivery of NO. Moreover, NO-delivery DNIC-2 can regulate cell proliferation, accelerate wound healing, and enhance the deposition of collagen in human fibroblast cells. Based on the in vitro and in vivo biocompatibility evaluation, biocompatible DNIC-2 holds the potential to be a novel active ingredient for skincare products.

Effect of Oxidation and Protonation States on [2Fe-2S] Cluster Nitrosylation Giving {Fe(NO)2}9 Dinitrosyl Iron Complexes (DNICs).

The nitrosylation of biological Fe/S clusters to give protein-bound dinitrosyl iron complexes (DNICs) is physiologically important. Biomimetic studies on the reaction of synthetic [2Fe-2S] clusters with NO have so far been limited to diferric model complexes. This work now compares the nitrosylation of [2Fe-2S] clusters with SN- or NN-chelating benzimidazolate/thiophenolate or bis(benzimidazolate) capping ligands in their diferric (12- and 22-) and mixed-valent (FeIIFeIII, 13-, and 23-) forms. Furthermore, the effect of protonation of the imidazole part of the SN ligand has been probed on both the nitrosylation reaction and properties of the resulting DNIC. The reaction of 12- and 22- with 4 equiv NO yields the new anionic {Fe(NO)2}9 DNICs 3- and 4-, respectively, which have been comprehensively characterized, including X-ray crystallography of their PPN+ salts. Nitrosylation of mixed-valent [2Fe-2S] clusters 13- and 23- first leads to slow oxidation to the corresponding diferric congeners, followed by core degradation and DNIC formation. In the case of 23-, a second diferric intermediate very similar to 22- is detected by UV-vis spectroscopy, but could not be further identified. Nitrosylation of 1H2 gives the neutral, N-protonated DNIC 3H, and acid/base titrations show that interconversion between 3- and 3H is reversible. Peripheral ligand protonation leads to a blue shift of the NO stretching vibrations by about 23 cm-1 and a significant shift of the reduction potential to less negative values (Δ E1/2 = 0.26 V), but no effect on 57Fe Mössbauer parameters is observed. Density functional theory calculations based on the structure of 3- indicate that the electronic ground-state properties of 3- and 3H are similar, although the NO(π*) → Fe 3d π-donation is slightly increased and π-backbonding is slightly decreased upon protonation. As a result, protonation has a significant effect on the NO stretching frequencies, but only minor effects on the Fe-(NO)2 modes. This is confirmed by nuclear inelastic scattering of 3- and 3H, which shows no clear influence of protonation on the energy of the Fe-(NO)2 bending and stretching modes occurring in the range 400-600 cm-1, but characteristic changes below 350 cm-1 that reflect perturbation of free rotary motion of the thiophenolate and benzimidazole ring systems of the capping ligand after N-protonation. These findings add to the understanding of [2Fe-2S] cluster nitrosylation and will help to identify DNICs resulting from the reaction of NO with Fe/S cofactors featuring alternative, proton-responsive histidine ligands such as the Rieske and mitoNEET [2Fe-2S] clusters.

Dinitrosyl iron complexes with thiol-containing ligands as a base for developing drugs with diverse therapeutic activities: Physicochemical and biological substantiation

Dinitrosyl iron complexes (DNICs) with thiol-containing ligands occur in animal tissues as paramagnetic (EPR-active) mononuclear and diamagnetic (EPR-silent) binuclear species with the presence of nitric oxide. They provide stabilization and storage (within protein-bound DNICs) of nitric oxide, as well as its transport (within low-molecular-weight DNICs) to biological targets to serve as donors of not only nitric oxide itself but also of the nitrosonium ion (NO+). The latter function determines the ability of DNICs to S-nitrosylate various thiol-containing proteins. In this way, the complexes participate in a wide range of physiological and biochemical processes. With respect to the high and diverse level of biological activity and dose dependence of DNICs, they mimic the endogenous nitric oxide system. Taken together with the broad occurrence of DNICs (predominantly in the diamagnetic form) in animal tissues, this fact points to their role as an “operating form” of nitric oxide. It is thought that drugs designed on the basis of DNICs can substantially improve the efficiency of modern medical practice.

Binding of Nitric Oxide in CDGSH-type [2Fe-2S] Clusters of the Human Mitochondrial Protein Miner2

Iron-sulfur proteins are among the primary targets of nitric oxide in cells. Previous studies have shown that iron-sulfur clusters hosted by cysteine residues in proteins are readily disrupted by nitric oxide forming a protein-bound dinitrosyl iron complex, thiolate-bridged di-iron tetranitrosyl complex, or octanitrosyl cluster. Here we report that human mitochondrial protein Miner2 [2Fe-2S] clusters can bind nitric oxide without disruption of the clusters. Miner2 is a member of a new CDGSH iron-sulfur protein family that also includes two mitochondrial proteins: the type II diabetes-related mitoNEET and the Wolfram syndrome 2-linked Miner1. Miner2 contains two CDGSH motifs, and each CDGSH motif hosts a [2Fe-2S] cluster via three cysteine and one histidine residues. Binding of nitric oxide in the reduced Miner2 [2Fe-2S] clusters produces a major absorption peak at 422 nm without releasing iron or sulfide from the clusters. The EPR measurements and mass spectrometry analyses further reveal that nitric oxide binds to the reduced [2Fe-2S] clusters in Miner2, with each cluster binding one nitric oxide. Although the [2Fe-2S] cluster in purified human mitoNEET and Miner1 fails to bind nitric oxide, a single mutation of Asp-96 to Val in mitoNEET or Asp-123 to Val in Miner1 facilitates nitric oxide binding in the [2Fe-2S] cluster, indicating that a subtle change of protein structure may switch mitoNEET and Miner1 to bind nitric oxide. The results suggest that binding of nitric oxide in the CDGSH-type [2Fe-2S] clusters in mitochondrial protein Miner2 may represent a new nitric oxide signaling mode in cells.

Hypotensive Effect and Accumulation of Dinitrosyl Iron Complexes in Blood and Tissues after Intravenous and Subcutaneous Injection.

Subcutaneous injection of Oxacom with glutathione-bound dinitrosyl iron complex as the active principle produced a slower drop of mean BP and longer accumulation of protein-bound dinitrosyl iron complexes in whole blood and tissues than intravenous injection of this drug, while durations of hypotensive effect in both cases were practically identical. In contrast to intravenous injection of the drug, its subcutaneous administration was not characterized by a high concentration of protein-bound dinitrosyl iron complexes in the blood at the onset of experiment; in addition, accumulation of these NO forms in the lungs was more pronounced after subcutaneous injection than after intravenous one.

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.

Redox activities of mono- and binuclear forms of low-molecular and protein-bound dinitrosyl iron complexes with thiol-containing ligands

EPR, optical, electrochemical and stopped-flow methods were used to demonstrate that Fe(NO)2 fragments in paramagnetic mononuclear and diamagnetic binuclear forms of dinitrosyl iron complexes with glutathione are reversibly reduced by a two-electron mechanism to be further transformed from the initial state with d7 configuration into states with the d8 and d9 electronic configurations of the iron atom. Under these conditions, both forms of DNIC display identical optical and EPR characteristics in state d9 suggesting that reduction of the binuclear form of DNIC initiates their reversible decomposition into two mononuclear dinitrosyl iron fragments, one of which is EPR-silent (d8) and the other one is EPR-active (d9). Both forms of DNIC produce EPR signals with the following values of the g-factor: g⊥=2.01, g||=1.97, gaver.=2.0. M-DNIC with glutathione manifest an ability to pass into state d9, however, only in solutions with a low content of free glutathione. Similar transitions were established for protein-bound М- and B-DNIC with thiol-containing ligands.

Transformations of dinitrosyl iron complexes in an isolated rat heart after introduction of this substance into perfusion medium

The objective of the present research was to study transformations of various physiological NO forms in an isolated rat heart, perfused with the medium, containing dinitrosyl iron complexes with glutathione ligand (DNIC-GH). We showed that such aerobic perfusion resulted in accumulation of mostly diamagnetic NO physiological forms (S-nitrosothiols) in myocardial tissue. They were transformed into protein-bound mononuclear dinitrosyl iron complexes during subsequent total ischemia. Meantime, DNIC-GH injection on the onset of ischemia resulted in the changes in the state of mitochondrial respiratory state, characterized by the increase in myocardial concentration of flavosemiquinones.

Introduction of dinitrosyl iron complexes with thiol-containing ligands into animal organism by inhalation method

The possibility of water-soluble dinitrosyl iron complexes (DNIC) with thiol-containing ligands introduction into lungs and other tissues of mice by free inhalation of little drops (2-3 microns diameter) of the solutions of these complexes was investigated. Little drops of 2-20 mM solutions of the complexes were obtained by using an inhalation apparatus (compressor nebulizer). A cloud of these little drops was then inhaled by animals in a closed chamber. A maximal amount of protein-bound DNICs formed in mouse lungs was 0.6 micromoles per kilogram of tissue weight. The amount of DNIC in lungs, liver and blood decreased to the undetected level within 2-3 hours after inhalation. No cytotoxic effect of DNIC formed in lungs on Mycobacterium tuberculosis was found in mice infected with these mycobacteria.

Dinitrosyl iron complexes with glutathione largely relieve rats of experimental endometriosis

A study was made of the effect of binuclear dinitrosyl iron complexes (DNIC) with glutathione in rats with experimental endometriosis. The latter was induced in an autotransplantation model, where two fragments of endometrium with myometrium (2 × 2 mm) from the left uterine horn were grafted to the inner surface of the anterior abdominal wall. After 4 weeks, the test animals received i.p. injections of 0.5 mL DNIC-glutathione at a dose of 12.5 μmol/kg daily for 12 days. This treatment more than halved the total volume of endometrioid tumors. Remarkably, tumor growths from grafts in control rats were often attended by tumors spontaneously arising nearby or in other locations; no such secondary tumors were observed in DNIC-treated animals. The EPR signal with gav = 2.03 characteristic of protein-bound DNIC with thiol ligands was recorded in liver and endometrioid implants of control as well as treated animals. Activation of ribonucleotide reductase, detected by a doublet EPR signal at g = 2.0 with 2.3-mT hyperfine splitting, was found in small tumors. The beneficial effect of DNIC-glutathione was suggested to be due to DNIC breakdown near the tumors, with release of a large amount of molecular nitric oxide and nitrosonium ions that resulted in selective local cytotoxicity.

Distribution and pharmacokinetics of dinitrosyl iron complexes in rat organs

Dinitrosyl iron complexes (DNICs) have been traced in rat blood and organs after intravenous infusion of Oxacom. It is shown that the active principle (DNIC with glutathione) is rapidly distributed through the organism and deposited in blood and organs as protein-bound DNICs. The specific levels of DNIC in the main body organs are comparable, whereas its apparent lifetimes relate as blood < heart = lung < liver < kidney. Spin trapping assays indicate that protein-bound DNICs are a major but not the only form of NO deposition; the next largest depot is most probably formed by S-nitrosothiols. The gradual release of NO from such pools ensures the smooth and prolonged hypotensive effect of Oxacom.

The Metal Core Structures in the Recombinant Escherichia coli Transcriptional Factor SoxR

X-ray absorption, circular dichroism, and EPR spectroscopy were employed to investigate the metal-core structures in the Escherichia coli transcriptional factor SoxR under reduced, oxidized, and nitrosylated conditions. The spectroscopic data revealed that the coordination environments of the metal active centers varied only very slightly between the reduced and oxidized states, similar to most other proteins containing iron-sulfur clusters. Upon nitrosylation of oxidized SoxR, however, we observed a low-temperature EPR spectrum characteristic of a protein dinitrosyl iron complex (DNIC), with an intensity corresponding to about two DNICs per iron sulfur cluster in the protein, according to spin quantification relative to a low-molecular-weight DNIC standard. In addition, there was no evidence for dichroic spectral features in the responsive region of the nitrosyl iron complexes, as well as for Fe-Fe back-scattering in the fitting of the Fe extended X-ray absorption fine structure (EXAFS) spectrum. Instead the Fe EXAFS spectrum of the nitrosylated SoxR core exhibited the same first- and second-shell coordination environments characteristic of modeled small molecular DNICs, indicating that each of the [2 Fe-2 S] cores in the homodimeric SoxR was dissociated into two individual DNICs. Similar nitrosylation of the reduced mixed-valence SoxR for 1 min led to degradation of the iron-sulfur clusters to give several iron species, including one with EPR signals characteristic of a reduced Roussin's red ester (rRRE), a diamagnetic species, presumably Roussin's red ester (RRE), and a small amount of DNIC. We also undertook in vivo time-course studies of E. coli cells containing recombinant SoxR after rapid purging of the cells with exogenous NO gas. Rapid freeze-quenched EPR experiments demonstrated rapid formation of the SoxR rRRE species, followed by fast breakup of this precursor intermediate to form the stable protein-bound DNIC species. Accordingly, under nitrosative stress, we believe that the response of SoxR to NO could depend on the intracellular redox state of E. coli, the central modulator of which could be exploited to deduce the appropriate mechanism to sense the presence of NO for physiological regulation.

Peptide-Bound Dinitrosyliron Complexes (DNICs) and Neutral/Reduced-Form Roussin’s Red Esters (RREs/rRREs): Understanding Nitrosylation of [Fe–S] Clusters Leading to the Formation of DNICs and RREs Using a De Novo Design Strategy

This manuscript describes the interaction of low-molecular-weight DNICs with short peptides designed to explore the stability and structure of DNIC-peptide/RRE-peptide constructs. Although characterization of protein-bound and low-molecular-weight DNICs is possible via EPR, XAS, and NRVS, this study demonstrates that the combination of aqueous IR ν(NO) and UV-vis spectra can serve as an efficient tool to characterize and discriminate peptide-bound DNICs and RREs. The de novo chelate-cysteine-containing peptides KC(A)(n)CK-bound (n = 1-4) dinitrosyliron complexes KC(A)(n)CK-DNIC (CnA-DNIC) and monodentate-cysteine-containing peptides KCAAK-/KCAAHK-bound Roussin's red esters (RREs) KCAAK-RRE/KCAAHK-RRE were synthesized and characterized by aqueous IR, UV-vis, EPR, CD, XAS, and ESI-MS. In contrast to the inertness of chelate-cysteine-containing peptide-bound DNICs toward KCAAK/KCAAHK, transformation of KCAAK-RRE/KCAAHK-RRE into CnA-DNIC triggered by CnA and reversible transformation between CnA-DNIC and CnA-RRE via {Fe(NO)(2)}(9)-{Fe(NO)(2)}(10) reduced-form peptide-bound RREs demonstrate that the {Fe(NO)(2)}(9) motif displays a preference for chelate-cysteine-containing peptides over monodentate-cysteine-containing peptides. Also, this study may signify that nitrosylation of [Fe-S] proteins generating protein-bound RREs, reduced protein-bound RREs, or protein-bound DNICs are modulated by both the oxidation state of iron and the chelating effect of the bound proteins of [Fe-S] clusters.

Penile erectile activity of dinitrosyl iron complexes with thiol-containing ligands

It has been established that intracavernous injections of water-soluble dinitrosyl iron complexes (DNIC) with glutathione or cysteine (0.4–6.0μmoles/kg) to male rats induce short-term (2–3min) penile erection along with a short-term drop of arterial pressure and appearance of protein-bound DNIC in cavernous tissue and circulating blood. The duration of erection and the hypotensive activity of DNIC increase dramatically after simultaneous intracavernous injection of DNIC and the phosphodiesterase-5 inhibitor papaverine. Surgical denervation of cavernous bodies does not influence the erectile activity of DNIC. No penile erection takes place after intravenous (instead of intracavernous) injection of the same dose of DNIC; in this case, protein-bound DNIC are detected only in the blood. These findings suggest that water-soluble DNIC with thiol-containing ligands (cysteine or glutathione) can be used as a basis in the design of a novel class of drugs for treating erectile dysfunctions.

Iron–sulfur proteins are the major source of protein-bound dinitrosyl iron complexes formed in Escherichia coli cells under nitric oxide stress

Protein-bound dinitrosyl iron complexes (DNICs) have been observed in prokaryotic and eukaryotic cells under nitric oxide (NO) stress. The identity of proteins that bind DNICs, however, still remains elusive. Here we demonstrate that iron–sulfur proteins are the major source of protein-bound DNICs formed in Escherichia coli cells under NO stress. Expression of recombinant iron–sulfur proteins, but not proteins without iron–sulfur clusters, almost doubles the amount of protein-bound DNICs formed in E. coli cells after NO exposure. Purification of recombinant proteins from the NO-exposed E. coli cells further confirms that iron–sulfur proteins, but not proteins without iron–sulfur clusters, are modified, forming protein-bound DNICs. Deletion of the iron–sulfur cluster assembly proteins IscA and SufA to block the [4Fe–4S] cluster biogenesis in E. coli cells largely eliminates the NO-mediated formation of protein-bound DNICs, suggesting that iron–sulfur clusters are mainly responsible for the NO-mediated formation of protein-bound DNICs in cells. Furthermore, depletion of the “chelatable iron pool” in wild-type E. coli cells effectively removes iron–sulfur clusters from proteins and concomitantly diminishes the NO-mediated formation of protein-bound DNICs, indicating that iron–sulfur clusters in proteins constitute at least part of the chelatable iron pool in cells.

Identification of Protein-Bound Dinitrosyl Iron Complexes by Nuclear Resonance Vibrational Spectroscopy

We have applied (57)Fe nuclear resonance vibrational spectroscopy (NRVS) to identify protein-bound dinitrosyl iron complexes. Intense NRVS peaks due to vibrations of the N-Fe-N unit can be observed between 500 and 700 cm(-1) and are diagnostic indicators of the type of iron dinitrosyl species present. NRVS spectra for four iron dinitrosyl model compounds are presented and used as benchmarks for the identification of species formed in the reaction of Pyrococcus furiosus ferredoxin D14C with nitric oxide.

Oxygen is required for the l-cysteine-mediated decomposition of protein-bound dinitrosyl–iron complexes

Increasing evidence suggests that iron–sulfur proteins are the primary targets of nitric oxide (NO). Exposure of Escherichia coli cells to NO readily converts iron–sulfur proteins to protein-bound dinitrosyl–iron complexes (DNICs). Although the protein-bound DNICs are stable in vitro under aerobic or anaerobic conditions, they are efficiently repaired in aerobically growing E. coli cells even without new protein synthesis. The cellular repair mechanism for the NO-modified iron–sulfur proteins remains largely elusive. Here we report that, unlike aerobically growing E. coli cells, starved E. coli cells fail to reactivate the NO-modified iron–sulfur proteins. Significantly, the addition of l-cysteine, but not other related biological thiols, results in decomposition of the protein-bound DNICs in starved E. coli cells and in cell extracts under aerobic conditions. However, under anaerobic conditions, l-cysteine has little or no effect on the protein-bound DNICs in starved E. coli cells or in vitro, suggesting that oxygen is required for the l-cysteine-mediated decomposition of the protein-bound DNICs. Additional studies reveal that l-cysteine is able to release the DNIC from the protein and bind to it, and the l-cysteine-bound DNICs are rapidly disrupted by oxygen, resulting in the eventual decomposition of the protein-bound DNICs under aerobic conditions.

Ironsulfur Proteins Comprise the Major Source of Protein-bound Dinitrosyl Iron Complexes Formed in Escherichia coli Cells under Nitric Oxide Stress

Ironsulfur Proteins Comprise the Major Source of Protein-bound Dinitrosyl Iron Complexes Formed in Escherichia coli Cells under Nitric Oxide Stress

Relative Binding Affinity of Thiolate, Imidazolate, Phenoxide, and Nitrite Toward the {Fe(NO)2} Motif of Dinitrosyl Iron Complexes (DNICs): The Characteristic Pre-Edge Energy of {Fe(NO)2}9 DNICs

The synthesis, characterization, and transformation of the anionic {Fe(NO)(2)}(9) dinitrosyl iron complexes (DNICs) [(NO)(2)Fe(ONO)(2)](-) (1), [(NO)(2)Fe(OPh)(2)](-) (2), [(NO)(2)Fe(OPh)(C(3)H(3)N(2))](-) (3) (C(3)H(3)N(2) = imidazolate), [(NO)(2)Fe(OPh)(-SC(4)H(3)S)](-) (4), [(NO)(2)Fe(p-OPhF)(2)](-) (5), and [(NO)(2)Fe(SPh)(ONO)](-) (6) were investigated. The binding affinity of ligands ([SPh](-), [-SC(4)H(3)S](-), [C(3)H(3)N(2)](-), [OPh](-), and [NO(2)](-)) toward the {Fe(NO)(2)}(9) motif follows the ligand-displacement series [SPh](-) approximately [-SC(4)H(3)S](-) > [C(3)H(3)N(2)](-) > [OPh](-) > [NO(2)](-). The findings, the pre-edge energy derived from the 1s --> 3d transition in a distorted T(d) environment of the Fe center falling within the range of 7113.4-7113.8 eV for the anionic {Fe(NO)(2)}(9) DNICs, implicate that the iron metal center of DNICs is tailored to minimize the electronic changes accompanying changes in coordinated ligands. Our results bridging the ligand-substitution reaction study and X-ray absorption spectroscopy study of the electronic richness of the {Fe(NO)(2)}(9) core may point the way to understanding the reasons for nature's choice of combinations of cysteine, histidine, and tyrosine in protein-bound DNICs and rationalize that most DNICs characterized/proposed nowadays are bound to the proteins almost through the thiolate groups of cysteinate/glutathione side chains in biological systems.