Interaction between [Ru(η6-p-cym)(H2O)3]2+ and dl-serine or dl-isoserine: The role of the side chain alcoholic OH group in metal ion binding

Role of the (pseudo)halido ligand in ruthenium(II) p-cymene α-amino acid complexes in speciation, protein reactivity and cytotoxicity

The reactions of the dimeric complexes [RuX₂(η⁶-p-cymene)]₂ (X = Br, I, SCN) with L-proline (ProH) and trans-4-hydroxy-L-proline (HypH), in methanol in the presence of NaOH, afforded [RuX(κ²N,O-Pro)(η⁶-p-cymene)] (X = Br, 1b; I, 1c; SCN, 1d) and [RuX(κ²N,O-Hyp)(η⁶-p-cymene)] (X = Br, 2b; I, 2c; SCN, 2d), respectively. Alternatively, the one-pot, sequential addition of the appropriate α-amino carboxylate and X^- salt to [RuCl₂(η⁶-p-cymene)]₂ led to [RuX(κ²N,O-Pro)(η⁶-p-cymene)] (X = N₃, 1e; NO₂, 1f; CN 1g) and [Ru(N₃)(κ²N,O-Hyp)(η⁶-p-cymene)] (2e). Complexes [Ru(κ³N,O,O'-O₂CCH(NH₂)(R)O)(η⁶-p-cymene)] (R = CH₂, 3h; R = CHMe, 4h; R = CH₂CH₂, 5h) were prepared from the reaction of [RuCl₂(η⁶-p-cymene)]₂ with the appropriate α-amino acid and NaOH in refluxing isopropanol. Treatment of the L-serine (SerH₂) derivative [RuCl(κ²N,O-SerH)(η⁶-p-cymene)] (3a) with 1,3,5-triaza-7-phosphaadamantane (PTA) in water at reflux produced [Ru(κ²N,O-Ser)(κP-PTA)(η⁶-p-cymene)]Cl ([3i]Cl). The products were isolated in good to excellent yields, and were characterized by elemental analysis, IR and multinuclear NMR spectroscopy. The structures of 1f and 2b-e were ascertained by X-ray diffraction studies. The behaviour of the complexes in water and cell culture medium was investigated by multinuclear NMR and UV-Vis spectroscopy, revealing a considerable influence of the monodentate ligand on the aqueous chemistry. Complexes 1d-e, 2d-e, 3h, 4h and [3i]Cl, showing substantial inertness in aqueous media, were assessed for their cytotoxicity towards A2780 and A2780cisR cancer cell lines and the noncancerous HEK 293T cell line. A selection of compounds was also investigated for Ru uptake in A2780 cells and interactions with cytochrome c as a model protein. Combined, these studies provide insights into the previously debated role of the 'leaving' ligand on the biological activity of Ru(II) arene α-amino acid complexes.

The interaction of half-sandwich (η5-Cp*)Rh(III) cation with histidine containing peptides and their ternary species with (N,N) bidentate ligands

Our goal was to explore the possible interactions of the potential metallodrug (η⁵-Cp*)Rh(III) complexes with histidine containing biomolecules (peptides/proteins) in order to understand the most important thermodynamic factors influencing the biospeciation and biotransformation of (η⁵-Cp*)Rh(III) complexes. To this end, here we report systematic solution thermodynamic and solution structural study on the interaction of (η⁵-Cp*)Rh(III) cation with histidine containing peptides and their constituents ((N-methyl)imidazole, GGA-OH, GGH-OH, histidine-amide, HGG-OH, GHG-NH₂), based on extensive ¹H NMR, ESI-MS and potentiometric investigations. The comparative evaluation of our data indicated that (η⁵-Cp*)Rh(III) cation is able to induce the deprotonation of amide nitrogen well below pH 7. Consequently, at physiological pH the peptides are coordinated to Rh(III) by tridentate manner, with the participation of amide nitrogen. At pH 7.4 the (η⁵-Cp*)Rh(III) binding affinity of peptides follow the order GGA-OH < < GGH-OH < < histidine-amide < HGG-OH < GHG-NH₂, i.e. the observed binding strength essentially depends on the presence and position of histidine within the peptide sequence. We also performed computational study on the possible solution structures of complexes present at near physiological pH. At pH 7.4 all histidine containing peptides form ternary complexes with strongly coordinating (N,N) bidentate ligands (ethylenediamine or bipyridyl), in which the peptides are monodentately coordinated to Rh(III) through their imidazole N¹‑nitrogens. In addition, the strongest chelators histidine-amide, HGG-OH and GHG-NH₂ are also able to displace these powerful bidentate ligands from the coordination sphere of Rh(III).

Binding mechanisms of half-sandwich Rh(III) and Ru(II) arene complexes on human serum albumin: a comparative study

Abstract

Various half-sandwich ruthenium(II) arene complexes and rhodium(III) arene complexes have been intensively investigated due to their prominent anticancer activity. The interaction of the organometallic complexes of Ru(η⁶-p-cymene) and Rh(η⁵-C₅Me₅) with human serum albumin (HSA) was studied in detail by a combination of various methods such as ultrafiltration, capillary electrophoresis, ¹H NMR spectroscopy, fluorometry and UV–visible spectrophotometry in the presence of 100 mM chloride ions. Binding characteristics of the organometallic ions and their complexes with deferiprone, 2-picolinic acid, maltol, 6-methyl-2-picolinic acid and 2-quinaldic acid were evaluated. Kinetic aspects and reversibility of the albumin binding are also discussed. The effect of low-molecular-mass blood components on the protein binding was studied in addition to the interaction of organorhodium complexes with cell culture medium components. The organometallic ions were found to bind to HSA to a high extent via a coordination bond. Release of the bound metal ions was kinetically hindered and could not be induced by the denaturation of the protein. Binding of the Ru(η⁶-p-cymene) triaqua cation was much slower (ca. 24 h) compared to the rhodium congener (few min), while their complexes interacted with the protein relatively fast (1–2 h). The studied complexes were bound to HSA coordinatively. The highly stable and kinetically inert 2-picolinate Ru(η⁶-p-cymene) complex bound in an associative manner preserving its original entity, while lower stability complexes decomposed partly or completely upon binding to HSA. Fast, non-specific and high-affinity binding of the complexes on HSA highlights their coordinative interaction with various types of proteins possibly decreasing effective drug concentration.

Graphic abstract

Electronic supplementary material

The online version of this article (10.1007/s00775-019-01683-0) contains supplementary material, which is available to authorized users.

Mechanistic study on substitution reaction of a citrato(p-cymene)Ru(ii) complex with sulfur-containing amino acids

Thorough kinetic study revealed characteristics of the reaction mechanism for arene ruthenium(ii) complexes with bio-related ligands.

Comparative solution equilibrium and structural studies of half-sandwich ruthenium(II)(η6-toluene) complexes of picolinate derivatives

Five Ru(II)(η⁶-toluene) complexes formed with 2-picolinic acid and its various derivatives have been synthesized and characterized. X-ray structures of four complexes are also reported. Complex formation processes of [Ru(II)(η⁶-toluene)(H₂O)₃]²⁺ organometallic cation with the metal-free ligands were studied in aqueous solution in the presence of chloride ions by the combined use of ¹H NMR spectroscopy, UV-visible spectrophotometry and pH-potentiometry. Solution stability, chloride ion affinity and lipophilicity of the complexes were characterized together with in vitro cytotoxic and antiproliferative activity in cancer cell lines being sensitive and resistant to classic chemotherapy and in normal cells as well. Formation of mono complexes such as [Ru(η⁶-toluene)(L)(Z)]^+/0 (L: completely deprotonated ligand; Z = H₂O/Cl^-) with high stability and [Ru(η⁶-toluene)(L)(OH)] was found in solution. The pK_a values (8.3-8.7) reflect the formation of low amount of mixed hydroxido species at pH 7.4 at 0.2 M KCl ionic strength. The complexes are fairly hydrophilic and show moderate chloride ion affinity and fast chloride-water exchange processes. The studied complexes exhibit no cytotoxic activity in human cancer cells (IC₅₀ > 100 μM), only complexes formed with 2-picolinic acid (1) and its 3-methyl derivative (2) represented a moderate antiproliferative effect (IC₅₀ = 84.8 (1), 79.2 μM (2)) on a multidrug resistant colon adenocarcinoma cell line revealing considerable multidrug resistant selectivity. Complexes 1 and 2 bind to human serum albumin covalently and relatively slowly with moderate strength at multiple binding sites without ligand cleavage.

[(η(6)-p-cymene)Ru(H2O)3](2+) binding capability of aminohydroxamates - A solution and solid state study

Complex forming capabilities of [(η(6)-p-cymene)Ru(H2O)3](2+) with aminohydroxamates (2-amino-N-hydroxyacetamide (α-alahaH), 3-amino-N-hydroxypropanamide (β-alahaH) and 4-amino-N-hydroxybutanamide (γ-abhaH)) having the primary amino group in different chelatable position to the hydroxamic function were studied by pH-potentiometry, NMR and MS methods. Formation of stable [O,O] and mixed [O,O][N,N] chelated mono- and dinuclear species is detected in partially slow with α-alahaH and β-alahaH or in fast processes with γ-abhaH and the formation constants of the complexes present in aqueous solution are reported. Synthesis, spectral (NMR, IR) and ESI mass spectrometric characterization of novel dinuclear α-alaninehydroximato complexes containing the half-sandwich type Ru(II) core is described. The crystal and molecular structure of [{(η(6)-p-cymene)Ru}2(μ(2)-α-alahaH-1)(H2O)Br]Br∙H2O (1) and [{(η(6)-p-cymene)Ru}2(μ(2)-α-alahaH-1)(H2O)Cl]BF4∙H2O (2) was determined by single crystal X-ray diffraction method. In the complexes one half-sandwich core is coordinated by a hydroxamate [O,O] chelate while the other one by [Namino,Nhydroxamate] fashion of the bridging ligand. In both cases the remaining coordination sites of one of the Ru cores are taken by a halide ion whiles the other one by a water molecule. Reaction of 2 with 9-methylguanine indicates the N7 coordination of this simple DNA model. Complexes 1 and 2 were tested for their in vitro cytotoxicity using human-derived cancer cell lines (A2780, MCF-7, SKOV-3, HCT-116, HeLa) and showed no anti-proliferative activity in the micromolar concentration range.

Half-sandwich complexes of ruthenium, osmium, rhodium and iridium with dl-methionine or S-methyl-l-cysteine: a solid state and solution equilibrium study

Half-sandwich type platinum metal ions form stable 1 : 1 complexes with dl-methionine and S-methyl-l-cysteine resulting in diastereomers that epimerize at the S donor.

A green and sustainable phosphine-free NHC-ruthenium catalyst for selective oxidation of alcohols to carboxylic acids in water

An efficient novel protocol is applied for direct synthesis of carboxylic acids from primary alcohols through catalytic dehydrogenation. This reaction occurs in water as the solvent and as the oxygen-atom source and no toxic oxidizing agent is involved.