Research paperFurther insights into ruthenium(II) piano-stool complexes with N-alkyl imidazoles
Introduction
In recent years, ruthenium is very often seen in complexes with a specific activity toward cancer cells. There are a number of reviews in which the synthesis and the mechanisms of its action were reported [1], [2], [3], [4].
The complex of Ru(III), NAMI-A (imidazolium [trans-tetrachloro(1H-imidazole)(S-dimethylsulfoxide)ruthenate(III)]) (Fig. 1) has undergone phase I of a clinical trial [5]. NAMI-A is a ruthenium-based compound with selective anti-metastatic activity as evidenced in experimental models of solid tumors [6]. However, further attention is directed to the RAPTA compounds ([Ru(η6-arene)(PTA)X2], PTA = 1,3,5-triaza-7-phosphaadamantane) as representatives of Ru(II) compounds which show anti-tumor activity in vitro [7]. Further, [Ru(η6-p-cymene)Cl2(PTA)] (denoted as RAPTA-C) [8] has shown moderate effects on solid tumor metastases (Fig. 1) [5]. In recent years, attention is directed toward organometallic ruthenium(II) compounds with imidazole ligands as well. Imidazoles are an important class of heterocyclic compounds and include many substances that are significant both biologically and chemically [9]. As can be seen in Dyson's papers, these complexes have shown a broad spectrum of cytotoxic properties in vitro as well as in vivo [10], [11] (Fig. 1). They have compared the ruthenium(II) complexes coordinated with diverse ligands (investigating in case of certain ligands even their homology sequence) and different arene molecules (π-bonding arene: benzene, p-cymene, toluene). The various number of imidazole rings with different alkyl groups were used as well. A higher cytotoxicity showed a complex with longer alkyl chain ([Ru(η6-p-cymene)Cl(N-BuIm)2][Cl] (compound 9b in [10]) vs. [Ru(η6-p-cymene)Cl(N-MeIm)2][Cl] (compound 9a in [10]) Further, if one compare cytotoxicity of the diverse ruthenium(II) complexes containing the same imidazole ligand (for example, N-Methylimidazole), it can be noticed that the higher number of coordinated imidazoles induce the higher cytotoxicity. However, mechanism of antitumor activity of imidazole compounds was not investigated in detail, except a few papers describing DNA and protein interactions with some organometallic ruthenium(II)-arene compounds [12] and the mechanism of cell death using flow cytometry [13], [14]. In addition to imidazole complexes of ruthenium, many others piano-stool [Ru(η6-p-cymene)L] compounds have shown significant antitumor activity [15]. Thus, [(η6-p-cymene)RuCl2(2‐X‐5‐aminopyridine)] (X = F, Cl, Br) exhibited moderate in vitro activity against A549 and MCF‐7 human cancer cells [15a], as well as water-soluble ruthenium(II)-NHC complexes (NHC = N-heterocyclic carbene) which are demonstrated remarkable cytotoxic activity against Caco-2 and MCF-7 cell lines [15b]. However, Ru(II)-p-cymene with versatile acetazolamide ligand are not cytotoxic to cancer cells and non-tumorigenic cells [16]. This is in contrast to other complexes belonging to the family [(η6-arene)RuCl(N,N-ligand)] but not unexpected for sulfonamide derivatives.
Our research group studied coordination and biological properties of transition metals (gold(I), gold(III), platinum(II) and rhodium(III)) with different imidazole ligands for a number of years [17]. In this paper, we describe the synthesis, characterization and crystal structures of two Ru(II)-p-cymene-L (L = N-Methylimidazole and N-Propylimidazole) complexes: [Ru(η6-p-cymene)(N-MeIm)3]Cl2·2H2O (1) and [Ru(η6-p-cymene)(N-PrIm)Cl2] (2) having a different level of heterocycle saturation. Also, we investigated the interaction of complexes with calf thymus DNA (CT-DNA) for the determination of the binding parameters and the complex ability to displace ethidium bromide (EB) from the CT-DNA-EB performed by fluorescence spectroscopy. In addition, docking simulations toward DNA dodecamer have been done in order to establish free binding energies of the Ligand—DNA system. PM6-D3H4 reaction path evaluation has been done for [Ru(Ar)(H2O] ↔ Gua(N7) in order to demonstrate the binding strength of [Ru(η6-p-cymene)(N-PrIm)Cl2] (2) on DNA. The in vitro antitumor activities of the metal complexes on human cancer cell lines MCF-7, A-549, HT-29, HeLa and MRC-5 have been investigated as well, and first results on the mechanisms of antiproliferative activity of the new Ru(II) complexes against human cervix adenocarcinoma cells were reported. The type of cell death was assessed by flow cytometry and cell morphology assessment. The effects on expression of proteins included in apoptotic signaling pathways (Bcl-2, Bax, Caspase-3, and Poly(ADP-ribose)polymerase, PARP) as well as cell cycle phase distribution of HeLa cells were monitored. A Western Blot analysis was used to evaluate the expression level of apoptosis-associated proteins.
Section snippets
Materials and physical measurements
Reagent grade, commercially available, chemicals were used without further purification. [Ru-(η6-p-cymene)Cl2]2, toluene, CH3OH, NaOH, HCl, NaCl, Tris-HCl, D2O, CDCl3, highly polymerized calf thymus DNA (CT-DNA) and ethidium bromide were purchased from Sigma-Aldrich, and used as received. UV–Vis spectra and fluorescence measurements were carried out in tris(hydroxymethyl)aminomethane (Tris, 10 mM) and NaCl (150 mM) and adjusted to pH 7.4 with hydrochloric acid.
Elemental microanalyses for C, H,
Coordination chemistry
This paper deals with the two Ru(II) complexes coordinated by two different aromatic imidazole ligands. In the reaction between dichloro(p-cymene)ruthenium(II) dimer and the monodentate N-Methylimidazole or N-Propylimidazole ligands we prepared two complexes: yellow [Ru(η6-p-cymene)(N-MeIm)3]Cl2·2H2O (1) and orange [Ru(η6-p-cymene)(N-PrIm)Cl2] (2) (Scheme 1). The ligands, N-Methylimidazole and N-Propylimidazole are coordinated to Ru(II) via nitrogen atom from aromatic imidazole ring. The
Conclusion
Two new Ru(II)-p-cymene-L complexes, [Ru(η6-p-cymene)(N-MeIm)3]Cl2·2H2O (1) and [Ru(η6-p-cymene)(N-PrIm)Cl2] (2), were isolated and characterized using experimental and computational techniques. Crystal structures of the (1) and (2) have been verified by X-ray diffraction analysis. Interaction these two new Ru(II) complexes with DNA has been studied. Specifically, UV spectroscopy studies have revealed their ability to bind to DNA. The binding strength of the complexes to DNA as calculated
Acknowledgement
The authors are grateful to the Ministarstvo Prosvete, Nauke i Tehnološkog Razvoja, RS for the financial support (Project No. III41010).
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Anticancer evaluation of ruthenium(III) complexes with N-donor ligands tethered to coumarin or uracil moieties
2019, Inorganica Chimica ActaCitation Excerpt :Although the metal complexes exhibit electronic spectral changes symbolic of DNA intercalators, the low intrinsic binding constants (Kb) suggest otherwise. In particular, the comparable Kb values of 1 and 2 with other groove-binding ruthenium complexes advocate that the primary mode of DNA interaction of 1 (6.67 × 102 M−1) and 2 (2 × 102 M−1), for instance, the groove-binding ruthenium cymene complexes, [Ru(η6-p-cymene)(N-MeIm)3]l2 (N-MeIm = N-methylimidazole) and [Ru(η6-p-cymene)(NPrIm)Cl2] (NPrIm = N-propylimidazole) afforded Kb values in the order of 104 M−1 [47]. In contrast, metal complex cationic species of [Ru(bipy)2(Lb)]2+ (Lb = bidentate N-donor chelating ligands) have been shown to be partial DNA intercalators, where the planar aromatic Lb chromophores facilitate DNA intercalation and the more sterically demanding [Ru(bipy)2(Lb)]2+ are found in the DNA minor groove [48].
Experimental and theoretical characterization of the strong effects on DNA stability caused by half-sandwich Ru(II) and Ir(III) bearing thiabendazole complexes
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