A study on the properties and reactivity of naphthoquinone–cobalt(III) prototypes for bioreductive prodrugs

Cobalt(III)-py2en systems as potential carriers of β-ketoester-based ligands

Two cobalt(III) complexes containing different β-ketoesters, namely [CoIII(L1)(py2en)](ClO4)2·H2O (1) and [CoIII(L2)(py2en)](ClO4)2 (2) (py2en = N,N'-bis(pyridin-2-ylmethyl)ethylenediamine; L1- = methylacetoacetate; L2- = ethyl 4-chloroacetoacetate) have been prepared and investigated as prototypes of bioreductive prodrugs. The presence of β-ketoester and py2en ligands in 1 and 2, as well as the perchlorate counterions, was supported by IR spectroscopy and CHN elemental analysis. The composition molecular structure of both complexes was confirmed by NMR spectroscopy and ESI mass spectrometry. Structural information was also obtained for 2via X-ray diffraction analysis. The redox properties indicate that 1 and 2 are suitable for reduction under biological conditions. Investigation of DNA-interacting suggest that 1 and 2 bind DNA via electrostatic forces. Both complexes may be employed as possible platforms for the delivery of biologically active compounds, since their reaction with ascorbic acid in PBS at pH 6.2 and 7.4 at 37°C results in the release of the β-ketoester ligands upon Co(III)/Co(II) reduction.

Bidirectional noninnocence of hinge-like deprotonated bis-lawsone on selective ruthenium platform: a function of varying ancillary ligands

The present work aimed to obtain discrete heavier metal complexes of unperturbed deprotonated bis-lawsone (hinge-like H₂L = 2,2'-bis(3-hydroxy-1,4-napthoquinone). This is primarily due to its limited examples with lighter metal ions (Co, Zn, and Ga) and the fact that our earlier approach with the osmium ion facilitated its functionalisation. Herein, we demonstrated the successful synthesis and structural characterisation of L^2--derived diruthenium [(bpy)₂Ru^II(μ-L^2-)Ru^II(bpy)₂](ClO₄)₂ [1](ClO₄)₂ (S = 0), (acac)₂Ru^III(μ-L^2-)Ru^III(acac)₂2 (S = 1) and monoruthenium (pap)₂Ru(L^2-) 3 (S = 0) derivatives (bpy = 2,2'-bipyridine, acac = acetylacetonate, and pap = 2-phenylazopyridine). The crystal structures established that (i) O,O^-/O,O^- donating five-membered bis-bidentate and O^-,O^- donating seven-membered bidentate chelating modes of deprotonated L^2- in rac (ΔΔ/ΛΛ) diastereomeric [1](ClO₄)₂, 2 and 3, respectively. (ii) The L^2- bridging unit in [1](ClO₄)₂, 2 and 3 underwent twisting its two naphthoquinone rings with respect to the ring connecting C-C bond by 73.01°, 62.15° and 59.12°, respectively. (iii) Intermolecular π-π interactions (∼3.5 Å) between the neighbouring molecules. The paramagnetic complex 2 (S = 1) with two non-interacting Ru(III) (S = 1/2) ions exhibited weak antiferromagnetic coupling only at very low temperatures. In agreement with the magnetic results, 2 displayed typical Ru^III-based anisotropic EPR in CH₃CN (<g>/Δg: 2.314/0.564) but without any forbidden g_1/2 signal at 120 K. The complexes exhibited multiple redox processes in CH₃CN in the experimental potential window of ± 2.0 V versus SCE. The analysis of the redox steps via a combined experimental and theoretical (DFT/TD-DFT) approach revealed the involvement of L^2- to varying extents in both the oxidative and reductive processes as a consequence of its bidirectional redox non-innocent feature. The mixing of the frontier orbitals of the metal ion and L^2- due to their closeness in energy indeed led to the resonating electronic form in certain redox states instead of any precise electronic structural state.

Investigation of cobalt(III)-phenylalanine complexes for hypoxia-activated drug delivery

Four cobalt(iii)-phenylalanine complexes, [Co(Phe)(py2en)](ClO4)2·H2O (1), [Co(Phe)(TPA)](ClO4)2·H2O (2), [Co(Phe)(py2enMe2)](ClO4)2·H2O (3) and [Co(bipy)2(Phe)](ClO4)2·H2O (4), were investigated as prototype models for hypoxia-activated delivery of melphalan - a phenylalanine derivative anticancer drug of the class of nitrogen mustards. Single crystal X-ray diffraction analysis provided the molecular structures of 1-4, as a single isomer/conformer. According with NMR and theoretical calculations, the solid-state structures of 2 and 4 are maintained in solutions. For complexes 1 and 3, though, a mixture of isomers was found in DMSO solutions: Λ-cisα(exo,exo) and Δ-cisβ1(exo,exo) for 1 (3 : 2 ratio), and Λ-cisα(exo,exo) and Δ-cisα(exo,exo) for 3 (5 : 1 ratio). Theoretical calculations point to a re-equilibration reaction of the solid-state Λ-cisβ1 isomer of 1 in solution. Electrochemical analysis revealed a correlation between the electron-donor capacity of the ancillary ligands and the redox potentials of the complexes. The potentials varied from +0.01 for 1 to +0.31 V vs. SHE for 4 in aqueous media and indicate that reduction should be achieved in biological media. The integrity of the complexes in pH 5.5 and 7.4 buffered solutions was confirmed by UV-Vis monitoring up to 24 h at 25 °C. Reduction by ascorbic acid (AA) shows an O2-dependent dissociation of the l-Phe for complexes 1-3, with higher conversion rates at pH 7.4. For complex 4, a fast dissociation of l-Phe was observed, with conversion rates unaffected by the pH and presence of O2.

Lipophilic Ga Complex with Broad-Spectrum Antimicrobial Activity and the Ability to Overcome Gallium Resistance in both Pseudomonas aeruginosa and Staphylococcus aureus

Antibiotic resistance (AR) necessitates the discovery of new antimicrobials with alternative mechanisms of action to those employed by conventional antibiotics. One such strategy utilizes Ga³⁺ to target iron metabolism, a critical process for survival. Still, Ga-based therapies are generally ineffective against Gram-positive bacteria and promote Ga resistance. In response to these drawbacks, we report a lipophilic Ga complex, [Ga₂L₃(bpy)₂] (L = 2,2'-bis(3-hydroxy-1,4-naphthoquinone; bpy = 2,2'-bipyridine)), effective against drug-resistant Pseudomonas aeruginosa (DRPA; minimum inhibitory concentration, MIC = 10 μM = 14.8 μg/mL) and methicillin-resistant Staphylococcus aureus (MRSA, MIC = 100 μM = 148 μg/mL) without iron-limited conditions. Importantly, [Ga₂L₃(bpy)₂] shows noticeably delayed and decreased resistance in both MRSA and DRPA, with only 8× MIC in DRPA and none in MRSA after 30 passages. This is likely due to the dual mode of action afforded by Ga (disruption of iron metabolism) and the ligand (reactive oxygen species production). Overall, [Ga₂L₃(bpy)₂] demonstrates the utility of lipophilic metal complexes with multiple modes of action in combatting AR in Gram-positive and Gram-negative bacteria.

Evaluation of cobalt(III) complexes as potential hypoxia-responsive carriers of esculetin

Differentiation between hypoxic and normoxic tissues have been exploited for the development of selective chemotherapeutic agents. In this context, cobalt(III)-based coordination compounds have been designed and investigated as prospective hypoxia-responsive drug delivery systems. Three cobalt(III) complexes, namely [Co^III(esc)(py₂en)]ClO₄·(CH₃OH)₂ (1) [Co^III(esc)(TPA)]ClO₄·3H₂O (2) and [Co^III(bipy)₂(esc)]ClO₄·2.5H₂O (3) (py₂en = N,N'-bis(pyridin-2-ylmethyl)ethylenediamine, TPA = tris(2-pyridylmethyl)amine, bipy = 2,2'-bipyridine and esc = 6,7-dihydroxycoumarin or esculetin), were prepared and investigated as potential carriers of esculetin. The spectroscopic and electrochemical properties of 1-3 were investigated and compared. Reactions of the complexes with biologically relevant reducing agents, viz. ascorbic acid, cysteine and glutathione, were monitored spectroscopically for 24 h, in pH 6.2 and 7.4 PBS phosphate buffer saline (PBS) solutions at 37 °C, under air, argon and dioxygen atmospheres. Dissociation of esculetin was observed upon Co³⁺/Co²⁺ reduction preferably under hypoxic conditions, with more effective conversion rates for 3 > 2 > 1. These results illustrate the importance to modulate the Co³⁺/Co²⁺ redox potential through the donor-acceptor properties of the ancillary ligands. Complex 3 is cytotoxic against HCT-116 but not against HT-29 and HEK-293 cells. In addition, DNA-binding studies indicate that interactions of 1 and 3 with the biomolecule are electrostatic.

Effect of ligand substituents on nickel and copper [N4] complexes: electronic and redox behavior, and reactivity towards protons

The ligand substituents have a major effect on the redox potentials, catalytic efficiency and robustness of the complexes in HER.

Hypoxia-activated prodrugs and redox-responsive nanocarriers

Hypoxia is one of the marked features of malignant tumors, which is associated with several adaptation changes in the microenvironment of tumor cells. Therefore, targeting tumor hypoxia is a research hotspot for cancer therapy. In this review, we summarize the developing chemotherapeutic drugs for targeting hypoxia, including quinones, nitroaromatic/nitroimidazole, N-oxides, and transition metal complexes. In addition, redox-responsive bonds, such as nitroimidazole groups, azogroups, and disulfide bonds, are frequently used in drug delivery systems for targeting the redox environment of tumors. Both hypoxia-activated prodrugs and redox-responsive drug delivery nanocarriers have significant effects on targeting tumor hypoxia for cancer therapy. Hypoxia-activated prodrugs are commonly used in clinical trials with favorable prospects, while redox-responsive nanocarriers are currently at the experimental stage.

Novel CoIII complexes containing fluorescent coumarin-N-acylhydrazone hybrid ligands: Synthesis, crystal structures, solution studies and DFT calculations

A series of new Co^III complexes of the type [Co(dien)(L1-L3)]ClO₄ (1-3), containing fluorescent coumarin-N-acylhydrazonate hybrid ligands, (E)-N'-(1-(7-oxido-2-oxo-2H-chromen-3-yl)ethylidene)-4-R-benzohydrazonate [where R=H (L1^2-), OCH₃ (L2^2-) or Cl (L3^2-)], were obtained and isolated in the low spin Co^III configuration. Single-crystal X-ray diffraction showed that the coumarin-N-acylhydrazones act as tridentate ligands in their deprotonated form (L^2-). The cation (+1) complexes contain a diethylenetriamine (dien) as auxiliary ligand and their structures were calculated by DFT studies which were also performed for the Co^II (S=1/2 and S=3/2) configurations. The LS Co^II (S=1/2) concentrated the spin density on the O-Co-O axis while the HS Co^II (S=3/2) exhibited a broad spin density distribution around the metallic center. Cyclic voltammetry studies showed that structural modifications made in the L^2- ligands caused a slight influence on the electronic density of the metal center, and the E_1/2 values for the Co^III/Co^II redox couple increased following the electronic effect of the R-substituent, in the order: 2 (R=OCH₃)<1 (R=H)<3 (R=Cl). The theoretical redox potentials (E°) of the process Co^III→Co^II were calculated for both Co^II spin states (S=1/2 and S=3/2) and a better correlation was found for Co^III→Co^II (S=1/2), compared with experimental values vs SHE (E^°_calc=-0.37, -0.36 and -0.32V vs E^°_exp.=-0.371, -0.406 and -0.358V, for 1-3 respectively). Complexes 1-3 exhibited a very intense absorption band around 470nm, assigned by DFT calculations as π-π* transitions from the delocalized coumarin-N-acylhydrazone system. 1-3 were very stable in MeOH for several days. Likewise, 1-3 were stable in phosphate buffer containing sodium ascorbate after 15h, which was attributed to the high chelate effect and σ-donor ability of the L^2- and dien ligands.

Investigation of cobalt(iii)–TPA complexes as potential bioreductively activated carriers for naphthoquinone-based drugs

Differently from CH₃, halogens as substituents foster incorporation of methoxide into NQs upon complexation.

Copper(II), cobalt(II) and nickel(II) complexes of lapachol: synthesis, DNA interaction, and cytotoxicity

Three novel copper(II), cobalt(II), and nickel(II) complexes of lapachol (Lap) containing 110-phenanthroline (phen) ligand, [M(Lap)₂(phen)] (M=Cu(II), 1, Co(II), 2, and Ni(II), 3), have been synthesized and characterized using, elemental analysis and spectroscopic studies. Their interactions with calf thymus DNA (CT DNA) were investigated using viscosity, thermal denaturation, circular dichorism, fluorescence quenching, and electronic absorption spectroscopy. The DNA cleavage abilities of 1-3 have been studied, where cleavage activity of copper complex 1 is more than the complexes 2 and 3. The in vitro cytotoxic potential of the complexes 1-3 against human cervical carcinoma (HeLa), human liver hepatocellular carcinoma (HepG-2), and human colorectal adenocarcinoma (HT-29) cells indicated their promising antitumor activity with quite low IC₅₀ values in the range of .15-2.41 μM, which are lower than those of cisplatin.

Investigation of cobalt(iii)-triazole systems as prototypes for hypoxia-activated drug delivery

A novel cobalt(iii)-triazole system was developed for hypoxia-activated drug delivery with an [O₂]-dependent reduction to cobalt(ii) followed by ligand dissociation.

Lawsone in organic synthesis

Lawsone has been used as the starting material for the synthesis of a variety of biologically active compounds and materials.