From Cyclen to 12-Crown-4 Copper(II) Complexes: Exchange of Donor Atoms Improves DNA Cleavage Activity

Experimental and computational investigation of heteroatom substitution in nucleolytic Cu(II) cyclen complexes for balancing stability and redox activity

Cu(II) complexes of cyclen-based ligands CuL¹-CuL⁶ were synthesized and characterized. The corresponding ligands L¹-L⁶ comprise different donor sets including S and O atoms. Whereas cyclen (L¹) is commercially available, L²-L⁶ were synthesized according to protocols available in the literature. Cleavage activity of the complexes towards plasmid DNA was tested in the presence and absence of ascorbate as a reducing agent (oxidative vs. hydrolytic cleavage). As previously shown, the substitution of N donor atoms with hard donor O atoms leads to efficient oxidative nucleases, but dissociation of the complex upon reduction. We thus opted for S substitution (soft donors) to stabilize the reduced Cu(I) species. Increasing the S content, however, leads to species that are difficult to reoxidize in order to ensure efficient oxidative DNA cleavage. We are showing by experimental (cyclic voltammetry) and computational means (DFT) that the rational combination of O and S atoms next to two nitrogen donors within the macrocycle (oxathiacyclen complex CuL⁶) leads to the stabilization of both redox states. The complex thus exhibits the highest oxidative DNA cleavage activity within this family of cyclen-based Cu(II) complexes - without leaching of the metal ion during reduction.

Ability of Azathiacyclen Ligands To Stop Cu(Aβ)-Induced Production of Reactive Oxygen Species: [3N1S] Is the Right Donor Set

Alzheimer's disease (AD) is an incurable neurodegenerative disease that leads to the progressive and irreversible loss of mental functions. The amyloid beta (Aβ) peptide involved in the disease is responsible for the production of damaging reactive oxygen species (ROS) when bound to Cu ions. A therapeutic approach that consists of removing Cu ions from Aβ to alter this deleterious interaction is currently being developed. In this context, we report the ability of five different 12-membered thiaazacyclen ligands to capture Cu from Aβ and to redox silence it. We propose that the presence of a sole sulfur atom in the ligand increases the rate of Cu capture and removal from Aβ, while the kinetic aspect of the chelation was an issue encountered with the 4N parent ligand. The best ligand for removing Cu from Aβ and inhibiting the associated ROS production is the 1-thia-4,7,10-triazacyclododecane [3N1S]. Indeed the replacement of more N by S atoms makes the corresponding Cu complexes easier to reduce and thus able to produce ROS on their own. In addition, the ligand with three sulfur atoms has a weaker affinity for Cu^II than Aβ, and is thus unable to remove Cu from CuAβ.

Nuclease-like metalloscissors: Biomimetic candidates for cancer and bacterial and viral infections therapy

Despite the extensive and rapid discovery of modern drugs for treatment of cancer, microbial infections, and viral illnesses; these diseases are still among major global health concerns. To take inspiration from natural nucleases and also the therapeutic potential of metallopeptide antibiotics such as the bleomycin family, artificial metallonucleases with the ability of promoting DNA/RNA cleavage and eventually affecting cellular biological processes can be introduced as a new class of therapeutic candidates. Metal complexes can be considered as one of the main categories of artificial metalloscissors, which can prompt nucleic acid strand scission. Accordingly, biologists, inorganic chemists, and medicinal inorganic chemists worldwide have been designing, synthesizing and evaluating the biological properties of metal complexes as artificial metalloscissors. In this review, we try to highlight the recent studies conducted on the nuclease-like metalloscissors and their potential therapeutic applications. Under the light of the concurrent Covid-19 pandemic, the human need for new therapeutics was highlighted much more than ever before. The nuclease-like metalloscissors with the potential of RNA cleavage of invading viral pathogens hence deserve prime attention.

Toward Catalytic Antibiotics: Redesign of Fluoroquinolones to Catalytically Fragment Chromosomal DNA

A library of ciprofloxacin-nuclease conjugates was designed and synthesized to investigate their potential as catalytic antibiotics. The Cu(II) complexes of the new designer compounds (i) showed excellent in vitro hydrolytic and oxidative DNase activity, (ii) showed good antibacterial activity against both Gram-negative and Gram-positive bacteria, and (iii) proved to be highly potent bacterial DNA gyrase inhibitors via a mechanism that involves stabilization of the fluoroquinolone-topoisomerase-DNA ternary complex. Furthermore, the Cu(II) complexes of two of the new designer compounds were shown to fragment supercoiled plasmid DNA into linear DNA in the presence of DNA gyrase, demonstrating a "proof of concept" in vitro. These ciprofloxacin-nuclease conjugates can therefore serve as models with which to develop next-generation, in vivo functioning catalytic antimicrobials.

Expanding the Ligand Classes Used for Mn(II) Complexation: Oxa-aza Macrocycles Make the Difference

We report two macrocyclic ligands based on a 1,7-diaza-12-crown-4 platform functionalized with acetate (tO2DO2A²⁻) or piperidineacetamide (tO2DO2AM^Pip) pendant arms and a detailed characterization of the corresponding Mn(II) complexes. The X−ray structure of [Mn(tO2DO2A)(H₂O)]·2H₂O shows that the metal ion is coordinated by six donor atoms of the macrocyclic ligand and one water molecule, to result in seven-coordination. The Cu(II) analogue presents a distorted octahedral coordination environment. The protonation constants of the ligands and the stability constants of the complexes formed with Mn(II) and other biologically relevant metal ions (Mg(II), Ca(II), Cu(II) and Zn(II)) were determined using potentiometric titrations (I = 0.15 M NaCl, T = 25 °C). The conditional stabilities of Mn(II) complexes at pH 7.4 are comparable to those reported for the cyclen-based tDO2A²⁻ ligand. The dissociation of the Mn(II) chelates were investigated by evaluating the rate constants of metal exchange reactions with Cu(II) under acidic conditions (I = 0.15 M NaCl, T = 25 °C). Dissociation of the [Mn(tO2DO2A)(H₂O)] complex occurs through both proton− and metal−assisted pathways, while the [Mn(tO2DO2AM^Pip)(H₂O)] analogue dissociates through spontaneous and proton-assisted mechanisms. The Mn(II) complex of tO2DO2A²⁻ is remarkably inert with respect to its dissociation, while the amide analogue is significantly more labile. The presence of a water molecule coordinated to Mn(II) imparts relatively high relaxivities to the complexes. The parameters determining this key property were investigated using ¹⁷O NMR (Nuclear Magnetic Resonance) transverse relaxation rates and ¹H nuclear magnetic relaxation dispersion (NMRD) profiles.

Copper(II) Complexes with Tetradentate Piperazine-Based Ligands: DNA Cleavage and Cytotoxicity

Five-coordinate Cu(II) complexes, [Cu(Ln)X]ClO4/PF6, where Ln = piperazine ligands bearing two pyridyl arms and X = ClO4− for Ln = L1 (1-ClO4), L2 (2-ClO4), L3 (3-ClO4), and L6 (6-ClO4) as well as [Cu(Ln)Cl]PF6 for Ln = L1 (1-Cl), L4 (4-Cl), and L5 (5-Cl) have been synthesized and characterized by spectroscopic techniques. The molecular structures of the last two complexes were determined by X-ray crystallography. In aqueous acetonitrile solutions, molar conductivity measurements and UV-VIS spectrophotometric titrations of the complexes revealed the hydrolysis of the complexes to [Cu(Ln)(H2O)]2+ species. The biological activity of the Cu(II) complexes with respect to DNA cleavage and cytotoxicity was investigated. At micromolar concentration within 2 h and pH 7.4, DNA cleavage rate decreased in the order: 1-Cl ≈ 1-ClO4 > 3-ClO4 ≥ 2-ClO4 with cleavage enhancements of up to 23 million. Complexes 4-Cl, 5-Cl, and 6-ClO4 were inactive. In order to elucidate the cleavage mechanism, the cleavage of bis(4-nitrophenyl)phosphate (BNPP) and reactive oxygen species (ROS) quenching studies were conducted. The mechanistic pathway of DNA cleavage depends on the ligand’s skeleton: while an oxidative pathway was preferable for 1-Cl/1-ClO4, DNA cleavage by 2-ClO4 and 3-ClO4 predominantly proceeds via a hydrolytic mechanism. Complexes 1-ClO4, 3-ClO4, and 5-Cl were found to be cytotoxic against A2780 cells (IC50 30–40 µM). In fibroblasts, the IC50 value was much higher for 3-ClO4 with no toxic effect.

Peptide Hydrolysis by Metal (Oxa)cyclen Complexes: Revisiting the Mechanism and Assessing Ligand Effects

The mechanism responsible for peptide bond hydrolysis by Co(III) and Cu(II) complexes with (oxa)cyclen ligands has been revisited by means of computational tools. We propose that the mechanism starts by substrate coordination and an outer-sphere attack on the amide C atom of a solvent water molecule assisted by the metal hydroxo moiety as a general base, which occurs through six-membered ring transition states. This new mechanism represents a more likely scenario than the previously proposed mechanisms that involved an inner-sphere nucleophilic attack through more strained four-membered rings transition states. The corresponding computed overall free-energy barrier of 25.2 kcal mol^-1 for hydrolysis of the peptide bond in Phe-Ala by a cobalt(III) oxacyclen catalyst (1) is consistent with the experimental values obtained from rate constants. Also, we assessed the influence of the nature of the ligand throughout a systematic replacement of N by O atoms in the (oxa)cyclen ligand. Increasing the number of coordinating O atoms accelerates the reaction by increasing the Lewis acidity of the metal ion. On the other hand, the higher reactivity observed for the copper(II) oxacyclen catalyst with respect to the analogous Co(III) complex can be attributed to the larger Brönsted basicity of the copper(II) hydroxo ligand. Ultimately, the detailed understanding of the ligand and metal nature effects allowed us to identify the double role of the metal hydroxo complexes as Lewis acids and Brönsted bases and to rationalize the observed reactivity trends.

Cu(II)-Based Paramagnetic Probe to Study RNA-Protein Interactions by NMR

Paramagnetic NMR techniques allow for studying three-dimensional structures of RNA-protein complexes. In particular, paramagnetic relaxation enhancement (PRE) data can provide valuable information about long-range distances between different structural components. For PRE NMR experiments, oligonucleotides are typically spin-labeled using nitroxide reagents. The current work describes an alternative approach involving a Cu(II) cyclen-based probe that can be covalently attached to an RNA strand in the vicinity of the protein's binding site using "click" chemistry. The approach has been applied to study binding of HIV-1 nucleocapsid protein 7 (NCp7) to a model RNA pentanucleotide, 5'-ACGCU-3'. Coordination of the paramagnetic metal to glutamic acid residue of NCp7 reduced flexibility of the probe, thus simplifying interpretation of the PRE data. NMR experiments showed attenuation of signal intensities from protein residues localized in proximity to the paramagnetic probe as the result of RNA-protein interactions. The extent of the attenuation was related to the probe's proximity allowing us to construct the protein's contact surface map.

Crystal structure of (perchlorato-κO)(1,4,7,10-tetra-aza-cyclo-dodecane-κ4N)copper(II) perchlorate

The crystal structure of the title salt, [Cu(ClO4)(C8H20N4)]ClO4, is reported. The CuII ion exhibits a square-pyramidal geometry and is coordinated by the four N atoms of the neutral 1,4,7,10-tetra-aza-cyclo-dodecane (cyclen) ligand and an O atom from one perchlorate anion, with the second perchlorate ion hydrogen-bonded to one of the amine N atoms of the cyclen ligand. Additional N-H⋯O hydrogen bonds between the amine H atoms and the coordinating and non-coordinating perchlorate groups create a three-dimensional network structure. Crystals were grown from a concentrated methanol solution at ambient temperature, resulting in no co-crystallization of solvent.

Variational first hyperpolarizabilities of 2,3-naphtho-15-crown-5 ether derivatives with cation-complexing: a potential and selective cation detector

When the crown ether and its cation derivatives display obvious contrasts in NLO properties, the NLO-based detection of cations is achieved.