Developing iron nitrosyl complexes as NO donor prodrugs

Structure, properties, and decomposition in biological systems of a new nitrosyl iron complex with 2-methoxythiophenolyls, promising for the treatment of cardiovascular diseases

A new promising binuclear tetranitrosyl iron complex with 2-methoxythiophenolyl of the composition [Fe2(C7H7OS)2(NO)4] (complex 1), which acts on the therapeutic targets of cardiovascular diseases, guanylate and adenylate cyclase, has been synthesized. X-ray diffraction data show the presence of two isoforms of complex 1; according to quantum chemical calculations, the structure of only the trans isomer is stable in solutions. The processes of transformation of complex 1 in DMSO, in aqueous solutions, as well as in the presence of bovine serum albumin, reduced glutathione, and mucin were studied. DMSO promotes the decomposition of the original complex 1 into mononuclear products. In biological systems, the mechanisms of decomposition of the complex 1 differ from aqueous solutions. In albumin solution, a gradual formation of a high-molecular-weight dinitrosyl complex is observed, obtained by coordinating the [Fe(NO)2]+ fragment with the amino acid groups of the protein. In the presence of mucin, an EPR signal from stable mononitrosyl products is observed. The introduction of glutathione into the system of the complex 1 leads to the replacement of one initial thioligand with glutathione. In the model systems under study, a more efficient and prolonged generation of NO groups is observed compared to a buffer solution. The obtained data on the influence of the environment on the properties of the complex 1 in combination with a study of their effect on enzymes allow us to recommend it for further study as a potential drug with vasodilator, antianginal, and hypotensive properties.

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.

One- and two-dimensional PbII compounds resulting from reaction of PbBr2 and Pb(SCN)2 with pyrimidine-2-thione

Pyrimidine-2-thione (HSpym) reacts with lead(II) thiocyanate and lead(II) bromide in N,N-dimethylformamide (DMF) to form poly[(μ-isothiocyanato-κ²N:S)(μ₄-pyrimidine-2-thiolato-κ⁶N¹,S:S:S:S,N³)lead(II)], [Pb(C₄H₃N₂S)(NCS)]_n or [Pb(Spym)(NCS)]_n, (I), and the polymeric one-dimensional (1D) compound catena-poly[[μ₄-bromido-di-μ-bromido-(μ-pyrimidine-2-thiolato-κ³N¹,S:S)(μ-pyrimidine-2-thione-κ³N¹,S:S)dilead(II)] N,N-dimethylformamide monosolvate], {[Pb₂Br₃(C₄H₃N₂S)(C₄H₄N₂S)]·C₃H₇NO}_n or {[Pb₂Br₃(Spym)(HSpym)]·DMF}_n, (IIa), respectively. Poly[μ₄-bromido-di-μ₃-bromido-(μ-pyrimidine-2-thiolato-κ³N¹,S:S)(μ-pyrimidine-2-thione-κ³N¹,S:S)dilead(II)], [Pb₂Br₃(C₄H₃N₂S)(C₄H₄N₂S)]_n or [Pb₂Br₃(Spym)(HSpym)]_n, (IIb), could be obtained as a mixture with (IIa) when using a lesser amount of solvent. In the crystal structures of the pseudohalide/halide Pb^II stable compounds, coordination of anionic and neutral HSpym has been observed. Both Spym^- (in the thiolate tautomeric form) and NCS^- ligands were responsible for the two-dimensional (2D) arrangement in (I). The Br^- ligands establish the 1D polymeric arrangement in (IIa). Eight-coordinated metal centres have been observed in both compounds, when considering the Pb...S and Pb...Br interactions. Both compounds were characterized by FT-IR and diffuse reflectance spectroscopies, as well as by powder X-ray diffraction. Compound (IIa) and its desolvated version (IIb) represent the first structurally characterized Pb^II compounds containing neutral HSpym and anionic Spym^- ligands. After a prolonged time in solution, (IIa) is converted to another compound due to complete deprotonation of HSpym. The structural characterization of (I) and (II) suggests HSpym as a good candidate for the removal of Pb^II ions from solutions containing thiocyanate or bromide ions.

A divergent mode of activation of a nitrosyl iron complex with unusual antiangiogenic activity

Nitric oxide (NO) and nitroxyl (HNO) have gained broad attention due to their roles in several physiological and pathophysiological processes. Remarkably, these sibling species can exhibit opposing effects including the promotion of angiogenic activity by NO compared to HNO, which blocks neovascularization. While many NO donors have been developed over the years, interest in HNO has led to the recent emergence of new donors. However, in both cases there is an expressive lack of iron-based compounds. Herein, we explored the novel chemical reactivity and stability of the trans-[Fe(cyclam)(NO)Cl]Cl₂ (cyclam = 1,4,8,11-tetraazacyclotetradecane) complex. Interestingly, the half-life (t_1/2) for NO release was 1.8 min upon light irradiation, vs 5.4 h upon thermal activation at 37 °C. Importantly, spectroscopic evidence supported the generation of HNO rather than NO induced by glutathione. Moreover, we observed significant inhibition of NO donor- or hypoxia-induced HIF-1α (hypoxia-inducible factor 1α) accumulation in breast cancer cells, as well as reduced vascular tube formation by endothelial cells pretreated with the trans-[Fe(cyclam)(NO)Cl]Cl₂ complex. Together, these studies provide the first example of an iron-nitrosyl complex with anti-angiogenic activity as well as the potential dual activity of this compound as a NO/HNO releasing agent, which warrants further pharmacological investigation.

Synthetic advances inspired by the bioactive dinitrosyl iron unit

Resulting from biochemical iron-NO interactions, dinitrosyl iron complexes (DNICs) are small organometallic-like molecules, considered to serve as vehicles for NO transport and storage in vivo. Formed by the interaction of NO with cellular iron sulfur clusters or with the cellular labile iron pool, DNICs have been documented to be the largest NO-derived adduct in cells, even surpassing the well-known nitrosothiols (RSNOs). Continuing efforts in biological chemistry are aimed at understanding the movement of DNICs in and out of cells, and their important role in NO-induced iron efflux leading to apoptosis in cells. Intrigued by the integrity of the unique dinitrosyl iron unit (DNIU) and the possibility of roles for it in human physiology or medicinal applications, the understanding of fundamental properties such as ligand effects on its ability to switch between two redox levels has been pursued through biomimetic complexes. Using metallodithiolates and N-heterocyclic carbenes (NHCs) as ligands to Fe(NO)2, the synthesis of a library of novel DNICs, in both the oxidized, {Fe(NO)2}(9), and reduced, {Fe(NO)2}(10), forms (Enemark-Feltham notation), offers opportunity to examine structural, reactivity, and spectroscopic features. The raison d'etre for the MN2S2·Fe(NO)2 synthesis development is for the potential to exploit the ease of accessing two redox levels on two different metal sites, a property presumably required for achieving two electron redox processes in base metals. Hence such molecules may be viewed as synthetic analogues of [NiFe]- or [FeFe]-hydrogenase active sites in nature, both of which use bridging thiolates for connection of the two centers. A particular success was the development of an Fe(NO)N2S2·Fe(NO)2(+/0) redox pair for proton reduction electrocatalysis. Monomeric, reduced NHC-DNICs of the L2Fe(NO)2 type are synthesized via the Fe(CO)2(NO)2 precursor, and oxidized thiolate-containing forms are derived from the dimeric (μ-RS)2[Fe(NO)2]2. Monomeric NHC-DNICs are four coordinate, pseudotetrahedral compounds with planar Fe(NO)2 units in which the slightly bent Fe-NO groups are directed symmetrically inward at both redox levels. They serve as stable analogues of biological histidine binding sites. In agreement with IR data, Mössbauer spectroscopic parameters, and DFT computations, the prototypic NHC-DNICs indicate extensive delocalization of the electron density of iron via π-backbonding. Such π-delocalization presents an unusual reaction path for the one electron process of RS(-)/RSSR interconversion. Comparisons with imidazole-DNICs find NHCs to be the "better" ligands to Fe(NO)2 and prompted investigations in (a) possible relationships between such imidazole- and NHC-containing DNICs, (b) systems that might mimic the reactivity of DNICs with the endogenous gaseotransmitter CO, and (c) mechanistic details of such processes. In a broader context, these studies aim to further describe the behavior of the {Fe(NO)2} unit as a single molecular entity when subjected to various ligand environments and reaction conditions.

Recent Advances in Multinuclear Metal Nitrosyl Complexes

The coordination chemistry of metal nitrosyls has expanded rapidly in the past decades due to major advances of nitric oxide and its metal compounds in biology. This review article highlights advances made in the area of multinuclear metal nitrosyl complexes, including Roussin's salts and their ester derivatives from 2003 to present. The review article focuses on isolated multinuclear metal nitrosyl complexes and is organized into different sections by the number of metal centers and bridging ligands.

Chemistry of nitrosyliron complexes supported by a β-diketiminate ligand

Several nitrosyl complexes of Fe and Co have been prepared using the sterically hindered Ar-nacnac ligand (Ar-nacnac = anion of [(2,6-diisopropylphenyl)NC(Me)](2)CH). The dinitrosyliron complexes [Fe(NO)(2)(Ar-nacnac)] (1) and (Bu(4)N)[Fe(NO)(2)(Ar-nacnac)] (2) react with [Fe(III)(TPP)Cl] (TPP = tetraphenylporphine dianion) to generate [Fe(II)(NO)(TPP)] and the corresponding mononitrosyliron complexes. The factors governing NO transfer with dinitrosyliron complexes (DNICs) 1 and 2 are evaluated, together with the chemistry of the related mononitrosyliron complex, [Fe(NO)Br(Ar-nacnac)] (4). The synthesis and properties of the related cobalt dinitrosyl [Co(NO)(2)(Ar-nacnac)] (3) is also discussed for comparison to DNICs 1 and 2. The solid-state structures of several of these compounds as determined by X-ray crystallography are reported.

trans-Bis(μ-2-hydroxy-ethanethiol-ato-κS:S)bis-[dinitro-syliron(II)](Fe-Fe)

The title complex, [Fe₂(C₂H₅OS)₂(NO)₄], lies on a crystallographic inversion center. The Fe—Fe distance is characteristic of a metal–metal bond. In the crystal structure, inter­molecular O—H⋯O hydrogen bonds link complex mol­ecules into a two-dimensional network.