Transformation of the {Fe(NO)2}9 Dinitrosyl Iron Complexes (DNICs) into S-Nitrosothiols (RSNOs) Triggered by Acid-Base Pairs

Reactivity of Thiolate and Hydrosulfide with a Mononuclear {FeNO}7 Complex Featuring a Very High N-O Stretching Frequency

Synthesis, characterization, electronic structure, and redox reactions of a mononuclear {FeNO}7 complex with a very high N-O stretching frequency in solution are presented. Nitrosylation of [(LKP)Fe(DMF)]2+ (1) (LKP = tris((1-methyl-4,5-diphenyl-1H-imidazol-2-yl)methyl)amine) produced a five-coordinate {FeNO}7 complex, [(LKP)Fe(NO)]2+ (2). While complex 2 could accommodate an additional water molecule to generate a six-coordinate {FeNO}7 complex, [(LKP)Fe(NO)(H2O)]2+ (3), the coordinated H2O in 3 dissociates to generate 2 in solution. The molecular structure of 2 features a nearly linear Fe-N-O unit with an Fe-N distance of 1.744(4) Å, N-O distance of 1.162(5) Å, and Collapse

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Affiliation(s)

Soumik Karmakar School of Chemical Sciences, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India
Suman Patra School of Chemical Sciences, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India
Koushik Pramanik School of Chemical Sciences, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India
Amit Adhikary Department of Chemistry, Technology Campus, University of Calcutta, JD Block, Sector III, Salt Lake, Kolkata 700098, India
Abhishek Dey School of Chemical Sciences, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India
Amit Majumdar School of Chemical Sciences, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032, India

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Promotion of S-nitrosation of cysteine by a {Co(NO)2}10 complex

S-Nitrosothiols (SNOs) serve as endogenous carriers and donors of NO within living cells, releasing nitrosonium ions (NO+), NO, or other nitroso derivatives. In this study, we present a bioinspired {Co(NO)2}10 complex 1 that achieved S-nitrosation towards Cys residues. The incorporation of a ferrocenyl group in 1 allowed for fine-tuning of the nitrosation reaction, taking advantage of the redox ability of Cys residues. Complex 1 was synthesized and characterized, demonstrating its NO translation reactivity. Furthermore, complex 1 successfully converted Cys into S-nitrosocysteine (Cys-SNO), as confirmed by UV-Vis, IR, and XAS spectroscopy. This study presents a promising approach for S-nitrosation of Cys residues for further exploration in the modification of Cys-containing peptides.

The Biologically Relevant Coordination Chemistry of Iron and Nitric Oxide: Electronic Structure and Reactivity

Nitric oxide (NO) is an important signaling molecule that is involved in a wide range of physiological and pathological events in biology. Metal coordination chemistry, especially with iron, is at the heart of many biological transformations involving NO. A series of heme proteins, nitric oxide synthases (NOS), soluble guanylate cyclase (sGC), and nitrophorins, are responsible for the biosynthesis, sensing, and transport of NO. Alternatively, NO can be generated from nitrite by heme- and copper-containing nitrite reductases (NIRs). The NO-bearing small molecules such as nitrosothiols and dinitrosyl iron complexes (DNICs) can serve as an alternative vehicle for NO storage and transport. Once NO is formed, the rich reaction chemistry of NO leads to a wide variety of biological activities including reduction of NO by heme or non-heme iron-containing NO reductases and protein post-translational modifications by DNICs. Much of our understanding of the reactivity of metal sites in biology with NO and the mechanisms of these transformations has come from the elucidation of the geometric and electronic structures and chemical reactivity of synthetic model systems, in synergy with biochemical and biophysical studies on the relevant proteins themselves. This review focuses on recent advancements from studies on proteins and model complexes that not only have improved our understanding of the biological roles of NO but also have provided foundations for biomedical research and for bio-inspired catalyst design in energy science.

Protective Effect of Dinitrosyl Iron Complexes Bound with Hemoglobin on Oxidative Modification by Peroxynitrite

Dinitrosyl iron complexes (DNICs) are a physiological form of nitric oxide (•NO) in an organism. They are able not only to deposit and transport •NO, but are also to act as antioxidant and antiradical agents. However, the mechanics of hemoglobin-bound DNICs (Hb-DNICs) protecting Hb against peroxynitrite-caused, mediated oxidative modification have not yet been scrutinized. Through EPR spectroscopy we show that Hb-DNICs are destroyed under the peroxynitrite action in a dose-dependent manner. At the same time, DNICs inhibit the oxidation of tryptophan and tyrosine residues and formation of carbonyl derivatives. They also prevent the formation of covalent crosslinks between Hb subunits and degradation of a heme group. These effects can arise from the oxoferryl heme form being reduced, and they can be connected with the ability of DNICs to directly intercept peroxynitrite and free radicals, which emerge due to its homolysis. These data show that DNICs may ensure protection from myocardial ischemia.

Dinitrosyl Iron Complexes (DNICs). From Spontaneous Assembly to Biological Roles

Dinitrosyl iron complexes (DNICs) are spontaneously and rapidly generated in cells. Their assembly requires nitric oxide (NO), biothiols, and nonheme iron, either labile iron or iron-sulfur clusters. Despite ubiquitous detection by electron paramagnetic resonance in NO-producing cells, the DNIC's chemical biology remains only partially understood. In this Forum Article, we address the reaction mechanisms for endogenous DNIC formation, with a focus on a labile iron pool as the iron source. The capability of DNICs to promote S-nitrosation is discussed in terms of S-nitrosothiol generation associated with the formation and chemical reactivity of DNICs. We also highlight how elucidation of the chemical reactivity and the dynamics of DNICs combined with the development of detection/quantification methods can provide further information regarding their participation in physiological and pathological processes.

Post-Translational S-Nitrosylation of Proteins in Regulating Cardiac Oxidative Stress

Like other post-translational modifications (PTMs) of proteins, S-nitrosylation has been considered a key regulatory mechanism of multiple cellular functions in many physiological and disease conditions. Emerging evidence has demonstrated that S-nitrosylation plays a crucial role in regulating redox homeostasis in the stressed heart, leading to discoveries in the mechanisms underlying the pathogenesis of heart diseases and cardiac protection. In this review, we summarize recent studies in understanding the molecular and biological basis of S-nitrosylation, including the formation, spatiotemporal specificity, homeostatic regulation, and association with cellular redox status. We also outline the currently available methods that have been applied to detect S-nitrosylation. Additionally, we synopsize the up-to-date studies of S-nitrosylation in various cardiac diseases in humans and animal models, and we discuss its therapeutic potential in cardiac protection. These pieces of information would bring new insights into understanding the role of S-nitrosylation in cardiac pathogenesis and provide novel avenues for developing novel therapeutic strategies for heart diseases.

How is Nitric Oxide (NO) Converted into Nitrosonium Cations (NO+) in Living Organisms? (Based on the Results of Optical and EPR Analyses of Dinitrosyl Iron Complexes with Thiol-Containing Ligands)

The present work provides theoretical and experimental foundations for the ability of dinitrosyl iron complexes (DNICs) with thiol-containing ligands to be not only the donors of neutral NO molecules, but also the donors of nitrosonium cations (NO+) in living organisms ensuring S-nitrosation of various proteins and low-molecular-weight compounds. It is proposed that the emergence of those cations in DNICs is related to disproportionation reaction of NO molecules, initiated by their binding with Fe2+ ions (two NO molecules per one ion). At the same time, possible hydrolysis of iron-bound nitrosonium cations is prevented by the electron density transition to nitrosonium cations from sulfur atoms of thiol-containing ligands, which are included in the coordination sphere of iron. It allows supposing that iron in iron-nitrosyl complexes of DNICs has a d 7 electronic configuration. This supposition is underpinned by experimental data revealing that a half of nitrosyl ligands are converted into S-nitrosothiols (RSNOs) when those complexes decompose, with the other half of those ligands released in the form of neutral NO molecules.

Fe in biosynthesis, translocation, and signal transduction of NO: toward bioinorganic engineering of dinitrosyl iron complexes into NO-delivery scaffolds for tissue engineering

The ubiquitous physiology of nitric oxide enables the bioinorganic engineering of [Fe(NO)₂]-containing and NO-delivery scaffolds for tissue engineering.

Thiolate-based dinitrosyl iron complexes: Decomposition and detection and differentiation from S-nitrosothiols

Dinitrosyl iron complexes (DNIC) spontaneously form in aqueous solutions of Fe(II), nitric oxide (NO), and various anions. They exist as an equilibrium between diamagnetic, dimeric (bi-DNIC) and paramagnetic, monomeric (mono-DNIC) forms. Thiolate groups (e.g., on glutathione or protein cysteine residues) are the most biologically relevant anions to coordinate to Fe(II). Low molecular weight DNIC have previously been suggested to be important mediators of NO biology in cells, and emerging literature supports their role in the control of iron-dependent cellular processes. Recently, it was shown that DNIC may be one of the most abundant NO-derived products in cells and may serve as intermediates in the cellular formation of S-nitrosothiols. In this work, we examined the stability of low molecular weight DNIC and investigated issues with their detection in the presence of other NO-dependent metabolites such as S-nitrosothiols. By using spectrophotometric, Electron Paramagnetic Resonance, ozone-based chemiluminesence, and HPLC techniques we established that at neutral pH, bi-DNIC remain stable for hours, whereas excess thiol results in decomposition to form nitrite. NO was also detected during the decomposition, but no S-nitrosothiol formation was observed. Importantly, mercury chloride accelerated the degradation of DNIC; thus, the implications of this finding for the diagnostic use of mercury chloride in the detection of S-nitrosothiols were determined in simple and complex biological systems. We conclude S-nitrosothiol levels may have been substantially overestimated in all methods where mercury chloride has been used.

Toward biocompatible dinitrosyl iron complexes: sugar-appended thiolates

Herein, we report the synthesis, characterization, and NO transfer/release studies of the first ‘sugar appended’ DNICs, which harness the hydrophilicity and cell targeting potential of the thiosugar, in addition to showing increased stability in air.

How are nitrosothiols formed de novo in vivo?

The biological mechanisms of de novo formation of cellular nitrosothiols (as opposed to transnitrosation) are reviewed. The approach is to introduce chemical foundations for each mechanism, followed by evidence in biological systems. The general categories include mechanisms involving nitrous acid, NO autoxidation and oxidant stress, redox active and inactive metal ions, and sulfide/persulfide. Important conclusions/speculations are that de novo cellular thiol nitrosation (1) is an oxidative process, and so should be considered within the family of other thiol oxidative modifications, (2) may not involve a single dominant process but depends on the specific conditions, (3) does not involve O₂ under at least some conditions, and (4) may serve to provide a "substrate pool" of protein cysteine nitrosothiol which could, through subsequent enzymatic transnitrosation/denitrosation, be "rearranged" to accomplish the specificity and regulatory control required for effective post-translational signaling.

Dinitrosyl iron complexes (DNICs): from biomimetic synthesis and spectroscopic characterization toward unveiling the biological and catalytic roles of DNICs

Dinitrosyl iron complexes (DNICs) have been recognized as storage and transport agents of nitric oxide capable of selectively modifying crucial biological targets via its distinct redox forms (NO(+), NO(•) and NO(-)) to initiate the signaling transduction pathways associated with versatile physiological and pathological responses. For decades, the molecular geometry and spectroscopic identification of {Fe(NO)2}(9) DNICs ({Fe(NO)x}(n) where n is the sum of electrons in the Fe 3d orbitals and NO π* orbitals based on Enemark-Feltham notation) in biology were limited to tetrahedral (CN = 4) and EPR g-value ∼2.03, respectively, due to the inadequacy of structurally well-defined biomimetic DNICs as well as the corresponding spectroscopic library accessible in biological environments. The developed synthetic methodologies expand the scope of DNICs into nonclassical square pyramidal and trigonal bipyramidal (CN = 5) and octahedral (CN = 6) {Fe(NO)2}(9) DNICs, as well as two/three accessible redox couples for mononuclear {Fe(NO)2}(9/10) and dinuclear [{Fe(NO)2}(9/10)-{Fe(NO)2}(9/10)] DNICs with biologically relevant S/O/N ligation modes. The unprecedented molecular geometries and electronic states of structurally well-defined DNIC models provide the foundation to construct a spectroscopic library for uncovering the identity of DNICs in biological environments as well as to determine the electronic structures of the {Fe(NO)2} core in qualitative and quantitative fashions by a wide range of spectroscopic methods. On the basis of (15)N NMR, electron paramagnetic resonance (EPR), IR, cyclic voltammetry (CV), superconducting quantum interference device (SQUID) magnetometry, UV-vis, single-crystal X-ray crystallography, and Fe/S K-edge X-ray absorption and Fe Kβ X-ray emission spectroscopies, the molecular geometry, ligation modes, nuclearity, and electronic states of the mononuclear {Fe(NO)2}(9/10) and dinuclear [{Fe(NO)2}(9/10)-{Fe(NO)2}(9/10)] DNICs could be characterized and differentiated. In addition, Fe/S K-edge X-ray absorption spectroscopy of tetrahedral DNICs deduced the qualitative assignment of Fe/NO oxidation states of {Fe(NO)2}(9) DNICs as a resonance hybrid of {Fe(II)((•)NO)(NO(-))}(9) and {Fe(III)(NO(-))2}(9) electronic states; the quantitative NO oxidation states of [(PhS)3Fe(NO)](-), [(PhS)2Fe(NO)2](-), and [(PhO)2Fe(NO)2](-) were further achieved by newly developed valence to core Fe Kβ X-ray emission spectroscopy as -0.58 ± 0.18, -0.77 ± 0.18, and -0.95 ± 0.18, respectively. The in-depth elaborations of electronic structures provide credible guidance to elucidate (a) the essential roles of DNICs modeling the degradation and repair of [Fe-S] clusters under the presence of NO, (b) transformation of DNIC into S-nitrosothiol (RSNO)/N-nitrosamine (R2NNO) and NO(+)/NO(•)/NO(-), (c) nitrite/nitrate activation producing NO regulated by redox shuttling of {Fe(NO)2}(9) and {Fe(NO)2}(10) DNICs, and (d) DNICs as H2S storage and cellular permeation pathway of DNIC/Roussin's red ester (RRE) for subsequent protein S-nitrosylation. The consolidated efforts on biomimetic synthesis, inorganic spectroscopy, chemical reactivity, and biological functions open avenues to the future designs of DNICs serving as stable inorganic NO(+)/NO(•)/NO(-) donors for pharmaceutical applications.

Redox conversions of dinitrosyl iron complexes with natural thiol-containing ligands

Using the electron paramagnetic resonance (EPR) and optical spectrophotometric methods, it has been established that biologically active, water-soluble dinitrosyl iron complexes (DNIC) with glutathione are predominantly represented by the diamagnetic binuclear form (B-DNIC) even in the presence of a 10-fold excess of glutathione non-incorporated into DNIC at neutral pH. With the increase in рН to 10-11, B-DNIC are fully converted into the paramagnetic mononuclear form (М-DNIC) with a characteristic EPR signal at g⊥=2.04, g‖=2.014 and gaver.=2.03. After treatment with a strong reducing agent sodium dithionite, both М- and B-DNIC are converted into the paramagnetic form with a characteristic EPR signal at g⊥=2.01, g‖=1.97 and gaver.=2.0. Both forms display similar absorption spectra with absorption bands at 960 and 640nm and a bend at 450nm. After oxidation by atmospheric oxygen, this situation is reversed, which manifests itself in the disappearance of the EPR signal at gaver.=2.0 and complete regeneration of initial absorption spectra of М- or B-DNIC with characteristic absorption bands at 390 or 360 and 310nm, respectively. Treatment of bovine serum albumin (BSA) solutions with gaseous NO in the presence of Fe(2+) and cysteine yields BSA-bound М-DNIC (М-DNIC-BSA). After treatment with sodium dithionite, the latter undergo transformations similar to those established for low-molecular М-DNIC with glutathione. Based on the complete coincidence of the optical and the EPR characteristics of sodium dithionite-treated М- and B-DNIC and other findings, it is suggested that sodium dithionite-reduced B-DNIC are subject to reversible decomposition into М-DNIC. The reduction and subsequent oxidation of М- and B-DNIC are interpreted in the paradigm of the current concepts of the initial electronic configurations of М- and B-DNIC (d(7) ({Fe(NO)2}(7)) and d(7)-d(7) ({Fe(NO)2}(7)-{Fe(NO)2}(7)), respectively).

Conversion of {Fe(NO)2}10 dinitrosyl iron to nitrato iron(III) species by molecular oxygen

A new {Fe(NO)(2)}(10) dinitrosyl iron complex possessing a 2,9-dimethyl-1,10-phenanthroline ligand has been prepared. This complex exhibits dioxygenase activity, converting NO to nitrate (NO(3)(-)) anions. During the oxygenation reaction, formation of reactive nitrating species is implicated, as shown in the effective o-nitration with a phenolic substrate.

A Conspectus of Cellular Mechanisms of Nitrosothiol Formation from Nitric Oxide

Although chemical mechanisms for the formation of nitrosothiol from •NO have been studied extensively "in the test tube", surprisingly little is known regarding the mechanism(s) of how nitrosothiols are formed in vivo. This lack of understanding has hampered more general acceptance of the concept of cysteine nitrosothiol formation as a generally applicable, regulated, and functionally significant protein posttranslational modification (as opposed to multiple other •NO-induced thiol modifications). Here we provide a brief overview/summary of the cellular formation of nitrosothiols from •NO via two possible mechanisms involving oxygen or transition metals.