NO Releasing and Anticancer Properties of Octahedral Ruthenium–Nitrosyl Complexes with Equatorial 1H-Indazole Ligands

Recent advances on the development of NO-releasing molecules (NORMs) for biomedical applications

Nitric oxide (NO) is an important biological messenger as well as a signaling molecule that participates in a broad range of physiological events and therapeutic applications in biological systems. However, due to its very short half-life in physiological conditions, its therapeutic applications are restricted. Efforts have been made to develop an enormous number of NO-releasing molecules (NORMs) and motifs for NO delivery to the target tissues. These NORMs involve organic nitrate, nitrite, nitro compounds, transition metal nitrosyls, and several nanomaterials. The controlled release of NO from these NORMs to the specific site requires several external stimuli like light, sound, pH, heat, enzyme, etc. Herein, we have provided a comprehensive review of the biochemistry of nitric oxide, recent advancements in NO-releasing materials with the appropriate stimuli of NO release, and their biomedical applications in cancer and other disease control.

Shedding light on cellular dynamics: the progress in developing photoactivated fluorophores

Photoactivated fluorophores (PAFs) are highly effective imaging tools that exhibit a removal of caging groups upon light excitation, resulting in the restoration of their bright fluorescence. This unique property allows for precise control over the spatiotemporal aspects of small molecule substances, making them indispensable for studying protein labeling and small molecule signaling within live cells. In this comprehensive review, we explore the historical background of this field and emphasize recent advancements based on various reaction mechanisms. Additionally, we discuss the structures and applications of the PAFs. We firmly believe that the development of more novel PAFs will provide powerful tools to dynamically investigate cells and expand the applications of these techniques into new domains.

Ru(II) based dual nitric oxide donors: electrochemical and photochemical reactivities and vasorelaxant effect with no cytotoxicity

The synthesized complexes, cis-[Ru(NO)(NO2)(phen)2](PF6)2 (NONO2P) and cis-[Ru(NO)(NO2)(bpy)2](PF6)2 (NONO2B), were characterized by using elemental analysis, voltammetry and electronic and vibrational spectroscopy. Under electrochemical and photochemical stimulation in an aqueous medium, there are indications of the formation of complexes, which suggests that the nitro and nitrosyl groups are converted into nitric oxide. Both compounds do not show cytotoxic activity against human umbilical vein endothelial cells (HUVECs). The cis-[Ru(NO)(NO2)(phen)2](PF6)2 complex presented vasorelaxation activity in superior mesenteric arteries from Wistar rats: the biphasic concentration-response curve indicates two sites of action. In the presence of NO scavengers, we observed an impaired relaxing effect induced by NONO2P, suggesting that the vasorelaxant effect is due to NO production from this compound.

Reactivity of a nitrosyl ruthenium complex and its potential impact on the fate of DNA - An in vitro investigation

The role of metal complexes on facing DNA has been a topic of major interest. However, metallonitrosyl compounds have been poorly investigated regarding their reactivities and interaction with DNA. A nitrosyl compound, cis-[Ru(bpy)₂(SO₃)(NO)](PF₆)(A), showed a variety of promising biological activities catching our attention. Here, we carried out a series of studies involving the interaction and damage of DNA mediated by the metal complex A and its final product after NO release, cis-[Ru(bpy)₂(SO₃)(H₂O](B). The fate of DNA with these metal complexes was investigated upon light or chemical stimuli using electrophoresis, electronic absorption spectroscopy, circular dichroism, size-exclusion resin, mass spectrometry, electron spin resonance (ESR) and viscometry. Since many biological disorders involve the production of oxidizing species, it is important to evaluate the reactivity of these compounds under such conditions as well. Indeed, the metal complex B exhibited important reactivity with H₂O₂ enabling DNA degradation, with detection of an unusual oxygenated intermediate. ESR spectroscopy detected mainly the DMPO-OOH adduct, which only emerges if H₂O₂ and O₂ are present together. This result indicated HOO^• as a key radical likely involved in DNA damage as supported by agarose gel electrophoresis. Notably, the nitrosyl ruthenium complex did not show evidence of direct DNA damage. However, its aqua product should be carefully considered as potentially harmful to DNA deserving further in vivo studies to better address any genotoxicity.

Dissociation of Bipyridine and Coordination with Nitrosyl: Cyclometalated Ruthenium Nitrosyl Complex

A novel family of ruthenium nitrosyl complexes [Ru(bpy)(C^∧N)(MeCN)NO](PF₆)₂ (2a-2e, bpy = 2,2'-bipyridine, HC^∧N = 2-phenylpyridine and its derivatives) has been prepared by reacting cyclometalated ruthenium complexes [Ru(bpy)₂(C^∧N)][PF₆] (1a-1e) with NO⁺, which were comprehensively characterized by mass, IR, NMR, and UV-vis spectra as well as the single-crystal X-ray structure determinations. Herein, the coordination geometry of Ru atoms in 2a-2e is a distorted octahedron and {Ru^II-NO⁺}⁶ is present in these complexes. Theoretical calculations suggest that the reactions involving dissociation of one bipyridine and coordination with NO⁺ proceed spontaneously (ΔG < 0) and the transformation from 1a-1e to the intermediates is dominated by substituents (ΔG_RI varies from -1.19 to -1.53 eV), which influence the binding energy between Ru(II) and NO⁺ in complexes 2a-2e (-89.42 to -101.17 kcal/mol) and thus control the photorelease of NO on a certain scale. The weak absorption bands in the visible region could be attributed to the contribution of dπ(Ru^II) → π*(NO⁺), which were enhanced greatly under light, indicating the possible release of NO. The photoinduced NO, as well as singlet oxygen (¹O₂), was then confirmed by EPR spectra, and the amount of NO released from 2a-2e was estimated via Griess reagent assay. The cytotoxicity of these complexes with or without visible light irradiation was also investigated using an MTT assay.

Syntheses, Rearrangements, and Structural Analyses of Unsaturated Nitrogen Donor Ligands Derived from Diphenyldiazomethane and the Chiral Rhenium Lewis Acid [(η5-C5H5)Re(NO)(PPh3)]+

Diphenyldiazomethane and a labile chlorobenzene complex of [(η5-C5H5)Re(NO)(PPh3)]+ BF4– react to give the η1 adduct [(η5-C5H5)Re(NO)(PPh3)(NNCPh2)]+ BF4– (73%). When this is con-ducted in the presence of copper powder, a 3-phenyl-1H-indazole...

A novel triruthenium nitrosyl bearing a quinolinic ligand: a comparison of its spectroscopic behavior with its pyridine analogues

A comparative analysis of ruthenium nitrosyl spectroscopic data helps unravel the electronic character of the unit {RuNO}6[RuIIIRuIIIO].

Characteristic vibrational frequencies of osmium(ii) nitrosyl complexes probed by Raman spectroscopy and DFT calculations

Raman spectroscopy at variable temperature provides experimental frequencies for osmium(ii) nitrosyl complexes. Vibrational transitions are assigned using DFT calculations.

Ruthenium-nitrosyl complexes as NO-releasing molecules and potential anticancer drugs

The synthesis of new types of mono- and polynuclear ruthenium nitrosyl complexes is driving progress in the field of NO generation for a variety of applications. Light-induced Ru-NO bond dissociation...

The Ruthenium Nitrosyl Moiety in Clusters: Trinuclear Linear μ-Hydroxido Magnesium(II)-Diruthenium(II), μ3-Oxido Trinuclear Diiron(III)-Ruthenium(II), and Tetranuclear μ4-Oxido Trigallium(III)-Ruthenium(II) Complexes

The ruthenium nitrosyl moiety, {RuNO}⁶, is important as a potential releasing agent of nitric oxide and is of inherent interest in coordination chemistry. Typically, {RuNO}⁶ is found in mononuclear complexes. Herein we describe the synthesis and characterization of several multimetal cluster complexes that contain this unit. Specifically, the heterotrinuclear μ₃-oxido clusters [Fe₂RuCl₄(μ₃-O)(μ-OMe)(μ-pz)₂(NO)(Hpz)₂] (6) and [Fe₂RuCl₃(μ₃-O)(μ-OMe)(μ-pz)₃(MeOH)(NO)(Hpz)][Fe₂RuCl₃(μ₃-O)(μ-OMe)(μ-pz)₃(DMF)(NO)(Hpz)] (7·MeOH·2H₂O) and the heterotetranuclear μ₄-oxido complex [Ga₃RuCl₃(μ₄-O)(μ-OMe)₃(μ-pz)₄(NO)] (8) were prepared from trans-[Ru(OH)(NO)(Hpz)₄]Cl₂ (5), which itself was prepared via acidic hydrolysis of the linear heterotrinuclear complex {[Ru(μ-OH)(μ-pz)₂(pz)(NO)(Hpz)]₂Mg} (4). Complex 4 was synthesized from the mononuclear Ru complexes (H₂pz)[trans-RuCl₄(Hpz)₂] (1), trans-[RuCl₂(Hpz)₄]Cl (2), and trans-[RuCl₂(Hpz)₄] (3). The new compounds 4–8 were all characterized by elemental analysis, ESI mass spectrometry, IR, UV–vis, and ¹H NMR spectroscopy, and single-crystal X-ray diffraction, with complexes 6 and 7 being characterized also by temperature-dependent magnetic susceptibility measurements and Mössbauer spectroscopy. Magnetometry indicated a strong antiferromagnetic interaction between paramagnetic centers in 6 and 7. The ability of 4 and 6–8 to form linkage isomers and release NO upon irradiation in the solid state was investigated by IR spectroscopy. A theoretical investigation of the electronic structure of 6 by DFT and ab initio CASSCF/NEVPT2 calculations indicated a redox-noninnocent behavior of the NO ancillary ligand in 6, which was also manifested in TD-DFT calculations of its electronic absorption spectrum. The electronic structure of 6 was also studied by an X-ray charge density analysis.

Mononuclear trans-[Ru(OH)NO(Hpz)₄]²⁺ proved to be a source of μ-hydroxido and μ₃- and/or μ₄-oxido bridging groups, which could be incorporated into the heterotrinuclear complexes {[Ru(μ-OH)(μ-pz)₂(pz)(NO)(Hpz)]₂Mg}, [Fe₂RuCl₄(μ₃-O)(μ-OMe)(μ-pz)₂(NO)(Hpz)₂], and [Fe₂RuCl₃(μ₃-O)(μ-OMe)(μ-pz)₃(MeOH)(NO)(Hpz)][Fe₂RuCl₃(μ₃-O)(μ-OMe)(μ-pz)₃(DMF)(NO)(Hpz)] (7·MeOH·2H₂O) and the heterotetranuclear μ₄-oxido complex [Ga₃RuCl₃(μ₄-O)(μ-OMe)₃(μ-pz)₄(NO)]. The structures obtained were all confirmed by SC-XRD, including an X-ray charge density analysis that revealed the electronic structure of the RuFe2 cluster. Two of these nitrosyl complexes underwent photoinduced isomerization with generation of the nitrosyl linkage isomers MS1 and MS2, as revealed by IR spectroscopy at 10 K.

Nitric oxide release and related light-induced cytotoxicity of ruthenium nitrosyls with coordinated nicotinate derivatives

The synthetic approaches for the preparation of trans(NO,OH)-cis(NO₂,NO₂)-[RuNO(L)₂(NO₂)₂OH], where L = ethyl nicotinate (I) and methyl nicotinate (II), are reported. The structures of the complexes are characterized by X-ray diffraction and analyzed by Hirshfeld surface analysis. Both compounds show a nitric oxide release reaction under 445 or 532 nm irradiation of dimethyl sulfoxide (DMSO) solutions, which is studied by combined ultraviolet-visible- (UV-vis), infrared- (IR), and electron paramagnetic resonance (EPR) spectroscopy and density functional theory (DFT) calculations. The charge transfer from the OH-Ru-NO chain and nitrite ligands to the antibonding orbitals of Ru-NO is responsible for the photo-cleavage of the ruthenium-nitrosyl bond. The elimination of NO leads to a side reaction, namely the protonation of the parent hydroxyl compound. The cytotoxicity and photo-induced cytotoxicity investigations of both compounds on the breast adenocarcinoma cell line MCF-7 reveal that (I) and (II) are cytotoxic with IC₅₀ values of 27.5 ± 2.8 μM and 23.3 ± 0.3 μM, respectively. Moreover, (I) shows an increase of the toxicity after light irradiation by 7 times (IC₅₀ = 4.1 ± 0.1), which makes it a prominent target for deeper biological investigations.

Insights into metalloproteins and metallodrugs from electron paramagnetic resonance spectroscopy

Metal ions play an important role in diverse biological processes, and much of the basic knowledge derived from studying native bioinorganic systems are applied in the synthesis of new molecules with the aim of diagnosing and treating diseases. At first glance, metalloproteins and metallodrugs are very different systems, but metal ion coordination, redox chemistry and substrate binding play essential roles in advancing both of these research fields. In this article, we discuss recent metalloprotein and metallodrug studies where electron paramagnetic resonance spectroscopy served as a major tool to gain a better understanding of metal-based structures and their function.

Nitrosyl linkage photoisomerization in heteroleptic fluoride ruthenium complexes derived from labile nitrate precursors

Four complexes with trans-ON–Ru–F coordinate were synthesized from their nitrate precursors. Upon light irradiation, complexes I–III show reversible formation of highly stable linkage isomers MS2 which leads to a higher photogeneration temperature of MS1.

Near-IR light-induced photorelease of nitric oxide (NO) on ruthenium nitrosyl complexes: formation, reactivity, and biological effects

Polypyridyl backbone nitrosyl complexes of ruthenium with the molecular framework [RuII(antpy)(bpy)NO+/˙]n+ [4](PF6)3 (n = 3), [4](PF6)2 (n = 2), where antpy = 4'-(anthracene-9-yl)-2,2':6',2''-terpyridine and bpy = 2,2'-bipyridine, were synthesized via a stepwise synthetic route from the chloro precursor [RuII(antpy)(bpy)(Cl)](PF6) [1](PF6) and [RuII(antpy)(bpy)(CH3CN)](PF6)2 [2](PF6)2 and [RuII(antpy)(bpy)(NO2)](PF6) [3](PF6). After column chromatographic purification, all the synthesized complexes were fully characterized using different spectroscopic and analytical techniques including mass spectroscopy, 1H NMR, FT-IR and UV-vis spectrophotometry. The Ru-NO stretching frequency of [4](PF6)3 was observed at 1941 cm-1, which suggests moderately strong Ru-NO bonding. A massive shift in the νNO frequency occurred at Δν = 329 cm-1 (solid) upon reducing [4](PF6)3 to [4](PF6)2. To understand the molecular integrity of the complexes, the structure of [3](PF6) was successfully determined by X-ray crystallography. The redox properties of [4](PF6)3 were thoroughly investigated together with the other precursor complexes. The rate constants for the first-order photo-release of NO from [4](PF6)3 and [4](PF6)2 were determined to be 8.01 × 10-3 min-1 (t1/2 ∼ 86 min) and 3.27 × 10-2 min-1 (t1/2 ∼ 21 min), respectively, when exposed to a 200 W Xenon light. Additionally, the photo-cleavage of Ru-NO occurred within ∼2 h when [4](PF6)3 was irradiated with an IR light source (>700 nm) at room temperature. The first-order rate constant of 9.4 × 10-3 min-1 (t1/2 ∼ 73 min) shows the efficacy of the system and its capability to release NO in the photo-therapeutic window. The released NO triggered by light was trapped by reduced myoglobin, a biologically relevant target protein. The one-electron reduction of [4](PF6)3 to [4](PF6)2 was systematically carried out chemically (hydrazine hydrate), electrochemically and biologically. In the biological reduction, it was found that the reduction is much slower with double-stranded DNA compared to a single-stranded oligonucleotide (CAAGGCCAACCGCGAGAAGATGAC). Moreover, [4](PF6)3 exhibited significant photo-toxicity to the VCaP prostate cancer cell line upon irradiation with a visible light source (IC50 ∼ 8.97 μM).

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.

Multistep Photochemical Reactions of Polypyridine-Based Ruthenium Nitrosyl Complexes in Dimethylsulfoxide

The photorelease of nitric oxide (NO·) has been investigated in dimethylsulfoxide (DMSO) on two compounds of formula [Ru(R-tpy)(bpy)(NO)](PF₆)₃, in which bpy stands for 2,2′-bipyridine and R-tpy for the 4′-R-2,2′:6′,2″-terpyridine with R = H and MeOPh. It is observed that both complexes are extremely sensitive to traces of water, leading to an equilibrium between [Ru(NO)] and [Ru(NO₂)]. The photoproducts of formula [Ru(R-tpy)(bpy)(DMSO)](PF₆)₂ are further subjected to a photoreaction leading to a reversible linkage isomerization between the stable Ru-DMSO_(S) (sulfur linked) and the metastable Ru-DMSO_(O) (oxygen linked) species. A set of 4 [Ru(R-tpy)(bpy)(DMSO)]²⁺ complexes (R = H, MeOPh, BrPh, NO₂Ph) is investigated to characterize the ratio and mechanism of the isomerization which is tentatively related to the difference in absorbance between the Ru-DMSO_(S) and Ru-DMSO_(O) forms. In addition, the X-ray crystal structures of [Ru(tpy)(bpy)(NO)](PF₆)₃ and [Ru(MeOPh-tpy)(bpy)(DMSO_(S))](PF₆)₂ are presented.

Real-time redox monitoring of a nitrosyl ruthenium complex acting as NO-donor agent in a single A549 cancer cell with multiplex Fourier-transform infrared microscopy

Multiplex Fourier-transform infrared microscopy (μFT-IR) helped to monitor trans-[Ru(NO) (NH₃)₄ (isn)]³⁺(I), uptake by A549 lung carcinoma cell, as well as the generation of its product, nitric oxide (NO), inside the cell. Chronoamperometry with NO-sensor and μFT-IR showed that exogenous NADH and the A549 cell induced the NO release redox mechanism. Chemical imaging confirmed that (I) was taken up by the cell, and that its localization coincided with its consumption in the cellular environment within 15 min of exposure. The Ru-NO absorption band in the IR spectrum shifted from 1932 cm^-1, when NO was coordinated to Ru as {Ru^II-NO⁺}³⁺, to 1876 cm^-1, due the formation of reduced species {Ru^II-NO⁰}²⁺, a precursor of NO release. Futhermore, the μFT-IR spectral profile demonstrated that, as a result of the NO action on the target, NO interacted with nucleic acids, which provided a biochemical response that is detectable in living cells.

Chemical and photochemical behavior of ruthenium nitrosyl complexes with terpyridine ligands in aqueous media

The synthesis and behavior in water of a set of various cis(Cl,Cl)-[R-tpyRuCl₂(NO)](PF₆) and trans(Cl,Cl)-[R-tpyRuCl₂(NO)](PF₆) (R = fluorenyl, phenyl, thiophenyl; tpy = 2,2':6',2''-terpyridine) complexes are presented. In any case, one chlorido ligand is substituted by a hydroxo ligand and the final species arises as a single trans(NO,OH) isomer, whatever the nature of the starting cis/trans(Cl,Cl) complexes. Six X-ray crystal structures are presented for cis(Cl,Cl)-[thiophenyl-tpyRuCl₂(NO)](PF₆) (cis-3a), trans(Cl,Cl)-[thiophenyl-tpyRuCl₂(NO)](PF₆) (trans-3a), trans(NO,OH)-[phenyl-tpyRu(Cl)(OH)(NO)](PF₆) (4a), trans(NO,OH)-[thiophenyl-tpyRu(Cl)(OH)(NO)](PF₆) (4b), trans(NO,OEt)-[phenyl-tpyRu(Cl)(OEt)(NO)](PF₆) (5a), and trans(NO,OH)-[phenyl-tpyRu(Cl)(OEt)(NO)](PF₆) (5b) compounds. The different cis/trans(Cl,Cl) complexes exhibit an intense low-lying transition in the λ = 330-390 nm range, which appears to be slightly blue-shifted after Cl → OH substitution. In water, both cis/trans(Cl,Cl) isomers are converted to a single trans(NO,OH) isomer in which one chlorido- is replaced by one hydroxo-ligand, which avoids tedious separation workout. The water stable trans(NO,OH)-species all release NO with quantum yields of 0.010 to 0.075 under irradiation at 365 nm. The properties are discussed with computational analysis performed within the framework of Density Functional Theory.

Sousa EH, de França Lopes LG, Teixeira EH, Vasconcelos MA, Martins PHR, Medeiros EJT, Batista AA, Holanda AKM. Biphosphinic ruthenium complexes as the promising antimicrobial agents

There is an urgent need for new antimicrobial compounds to combat the growing threat of widespread antibiotic resistance. Ruthenium compounds have shown promising activities including two biphosphinic compounds as described here.

A novel Pt(iv) mono azido mono triazolato complex evolves azidyl radicals following irradiation with visible light

A novel Pt^IV azido triazolato complex exists as an equilibrium between two species in d₃-MeCN and evolves azide radicals (but not hydroxide radicals) when irradiated with visible light.

The platinum(iv) azido complex trans,trans,trans-[Pt^IV(N₃)₂(OH)₂(py)₂] (1) undergoes cycloaddition with 1,4-diphenyl-2-butyne-1,4-dione (2) under mild, catalyst-free conditions, affording a number of mono and bis click products. The major mono click product (3) exists in MeCN as an equilibrium mixture between two species; 3a and 3b rapidly interconvert through nucleophilic attack of the axial Pt–OH group at the adjacent Ph–CO group. The kinetic and thermodynamic parameters for this interconversion have been measured by selective saturation-transfer NMR spectroscopic experiments and are consistent with cyclisation at the Pt centre. Complex 3b was also characterised by X-ray crystallography. Visible light irradiation (440–480 nm) of 3 in d₃-MeCN produces azidyl radicals (N₃˙), as demonstrated by EPR spin-trapping with DMPO; no generation of hydroxyl radicals was observed. ¹H–¹⁹⁵Pt HMBC NMR confirmed that the photoproducts were Pt^IV rather than Pt^II species, and HPLC was consistent with these being [3–N₃]⁺ species; no facile photoejection of the triazolato ligand was observed, consistent with MS/MS fragmentation of 3. When 3 was irradiated in the presence of 5′-GMP, no 5′-GMP photoproducts were observed, suggesting that complex 3 is likely to exhibit significantly simplified biological activity (release of azidyl radicals but not DNA binding) compared with complex 1.

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.