Reactivity of N-Heterocyclic Carbene Half-Sandwich Ru-, Os-, Rh-, and Ir-Based Complexes with Cysteine and Selenocysteine: A Computational Study

Detailed mechanism of a DNA/RNA nucleobase substituting bridging ligand in diruthenium(II,III) and dirhodium(II,II) tetraacetato paddlewheel complexes: protonation of the leaving acetate is crucial

Paddlewheel complexes of bimetallic scaffolds are emerging metallic agents in the bioinorganic chemistry landscape. In the most commonly employed construct, these complexes are decorated by the carboxylate moiety, prompting their possible deployment to target either protein or nucleic acid targets. In this study, density functional investigation was performed to assess viable mechanistic routes for the substitution of one acetate ligand with one chelating purine, i.e. adenine or guanine, in diruthenium and dirhodium tetraacetate paddlewheel complexes. This study evidenced the relevant stages of the process at an atomistic scale of resolution and provided for the encompassed rate-determining chemical events. Therefore, calculations indicated that acetate decomplexation as well as the concomitant nucleobase bridging proceeded gradually via a multistep process that included protonation of the leaving acetate. The present picture of the mechanism is envisioned to be relevant to the design and interpretation of experiments focused on the reaction of diruthenium and/or dirhodium tetracarboxylate complexes with nucleobases and eventuating in the formation of either nucleobase bridged-complexes or in the dismantling of the bimetallic construct.

Dimolybdenum (II,II) paddlewheel complexes bearing non-steroidal anti-inflammatory drug ligands: Insights into the chemico-physical profile and first biological assessment

Multinuclear complexes are metal compounds featured by adjacent bound metal centers that can lead to unconventional reactivity. Some M2L4-type paddlewheel dinuclear complexes with monoanionic bridging ligands feature promising properties, including therapeutic ones. Molybdenum has been studied for the formation of multiple-bonded M2+ compounds due to their unique scaffold, redox, and spectroscopic properties as well as for applications in several fields including catalysis and biology. These latter are much less explored and only sporadic studies have been carried out. Here, a series of four dimolybdenum (II,II) carboxylate paddlewheel complexes were synthesized using different Non-Steroidal Anti-Inflammatory Drugs (NSAIDs) as ligands. The reaction of (NH4)5[Mo2Cl9]·H2O with the selected NSAIDs in methanol produced the complexes Mo2(μ-O2CR)4 where RCO2 is ibuprofen (1), naproxen (2), aspirin (3) and indomethacin (4). The products were obtained in good yields and extensively characterized with integrated techniques. Stability and solution behaviour were studied using a mixed experimental and computational approach. Finally, the biological activity of 1 and 3 (i.e. the most reactive and the most stable compounds of the series, respectively) was preliminarily assessed confirming the disassembling of the molecules in the biological milieu. Overall, some very interesting results emerged for these unconventional compounds from a mechanistic point of view.

The binding of diruthenium (II,III) and dirhodium (II,II) paddlewheel complexes at DNA/RNA nucleobases: Computational evidences of an appreciable selectivity toward the AU base pairs

Multiple medicinal strategies involve modifications of the structure of DNA or RNA, which disrupt their correct functioning. Metal complexes with medicinal effects, also known as metallodrugs, are among the agents intended specifically for the attack onto nucleosides. The diruthenium (II,III) and dirhodium (II,II) paddlewheel complexes constitute promising dual acting drugs due to their ability to release the therapeutically active bridging ligands upon their substitution by endogenous ligands. In this paper, we study the structure and the stability of the complexes formed by the diruthenium (II,III) and dirhodium (II,II) paddlewheel complexes coordinated in axial positions with the DNA/RNA nucleobases or base pairs, assuming the attainable metalation at all the accessible pyridyl nitrogens. Dirhodium complexes coordinate at the pyridyl nitrogens more strongly than the diruthenium complexes. On the other hand, we found that the diruthenium scaffold binds more selectively to nucleobase targets. Furthermore, we reveal a tighter coordination of diruthenium complex at the adenine-uracil base pair, compared to adenine-thymine, hence constituting a scarce instance of RNA-selectivity. We envision that the here reported computational outcomes may pace future experiments addressing the binding of diruthenium and dirhodium paddlewheel complexes at either single nucleobases or DNA/RNA fragments.

Unveiling the mechanism of activation of the Te(IV) prodrug AS101. New chemical insights towards a better understanding of its medicinal properties

AS101 (Ammonium trichloro (dioxoethylene-O,O') tellurate) is an important hypervalent Te-based prodrug. Recently, we started a systematic investigation on AS101 with the aim to correlate its promising biological effects as a potent immunomodulator drug with multiple medicinal applications and its specific chemical properties. To date, a substantial agreement on the rapid conversion of the initial AS101 species into the corresponding TeOCl3- anion does exist, and this latter species is reputed as the pharmacologically active one. However, we realized that TeOCl3- could quickly undergo further steps of conversion in an aqueous medium, eventually producing the TeO2 species. Using a mixed experimental and theoretical investigation approach, we characterized the conversion process leading to TeO2 occurring both in pure water and in reference buffers at physiological-like pH. Our findings may offer a valuable "chemical tool" for a better description, interpretation -and optimization- of the mechanism of action of AS101 and Te-based compounds. This might be a starting point for improved AS101-based medicinal application.

Diruthenium Paddlewheel Complexes Attacking Proteins: Axial versus Equatorial Coordination

Metallodrugs are an important group of medicinal agents used for the treatment of various diseases ranging from cancers to viral, bacterial, and parasitic diseases. Their distinctive features include the availability of a metal centre, redox activity, as well as the ability to multitarget. Diruthenium paddlewheel complexes are an intensely developing group of metal scaffolds, which can securely coordinate bidentate xenobiotics and transport them to target tissues, releasing them by means of substitution reactions with biomolecular nucleophiles. It is of the utmost importance to gain a complete comprehension of which chemical reactions happen with them in physiological milieu to design novel drugs based on these bimetallic scaffolds. This review presents the data obtained in experiments and calculations, which clarify the chemistry these complexes undergo once administered in the proteic environment. This study demonstrates how diruthenium paddlewheel complexes may indeed embody a new paradigm in the design of metal-based drugs of dual-action by presenting and discussing the protein metalation by these complexes.

Noble Metal Nanoparticles with Nanogel Coatings: Coinage Metal Thiolate-Stabilized Glutathione Hydrogel Shells

Developing biocompatible nanocoatings is crucial for biomedical applications. Noble metal colloidal nanoparticles with biomolecular shells are thought to combine diverse chemical and optothermal functionalities with biocompatibility. Herein, we present nanoparticles with peptide hydrogel shells that feature an unusual combination of properties: the metal core possesses localized plasmon resonance, whereas a few-nanometer-thick shells open opportunities to employ their soft framework for loading and scaffolding. We demonstrate this concept with gold and silver nanoparticles capped by glutathione peptides stacked into parallel β-sheets as they aggregate on the surface. A key role in the formation of the ordered structure is played by coinage metal(I) thiolates, i.e., Ag(I), Cu(I), and Au(I). The shell thickness can be controlled via the concentration of either metal ions or peptides. Theoretical modeling of the shell's molecular structure suggests that the thiolates have a similar conformation for all the metals and that the parallel β-sheet-like structure is a kinetic product of the peptide aggregation. Using third-order nonlinear two-dimensional infrared spectroscopy, we revealed that the ordered secondary structure is similar to the bulk hydrogels of the coinage metal thiolates of glutathione, which also consist of aggregated stacked parallel β-sheets. We expect that nanoparticles with hydrogel shells will be useful additions to the nanomaterial toolbox. The present method of nanogel coating can be applied to arbitrary surfaces where the initial deposition of the seed glutathione monolayer is possible.

Mechanism of Action of Antitumor Au(I) N-Heterocyclic Carbene Complexes: A Computational Insight on the Targeting of TrxR Selenocysteine

The targeting of human thioredoxin reductase is widely recognized to be crucially involved in the anticancer properties of several metallodrugs, including Au(I) complexes. In this study, the mechanism of reaction between a set of five N-heterocyclic carbene Au(I) complexes and models of the active Sec residue in human thioredoxin reductase was investigated by means of density functional theory approaches. The study was specifically addressed to the kinetics and thermodynamics of the tiled process by aiming at elucidating and explaining the differential inhibitory potency in this set of analogous Au(I) bis-carbene complexes. While the calculated free energy profile showed a substantially similar reactivity, we found that the binding of these Au(I) bis-carbene at the active CysSec dyad in the TrxR enzyme could be subjected to steric and orientational restraints, underlining both the approach of the bis-carbene scaffold and the attack of the selenol group at the metal center. A new and detailed mechanistic insight to the anticancer activity of these Au(I) organometallic complexes was thus provided by consolidating the TrxR targeting paradigm.

Mechanistic Evaluations of the Effects of Auranofin Triethylphosphine Replacement with a Trimethylphosphite Moiety

Auranofin, a gold(I)-based complex, is under clinical trials for application as an anticancer agent for the treatment of nonsmall-cell lung cancer and ovarian cancer. In the past years, different derivatives have been developed, modifying gold linear ligands in the search for new gold complexes endowed with a better pharmacological profile. Recently, a panel of four gold(I) complexes, inspired by the clinically established compound auranofin, was reported by our research group. As described, all compounds possess an [Au{P(OMe)3}]+ cationic moiety, in which the triethylphosphine of the parent compound auranofin was replaced with an oxygen-rich trimethylphosphite ligand. The gold(I) linear coordination geometry was complemented by Cl-, Br-, I-, and the auranofin-like thioglucose tetraacetate ligand. As previously reported, despite their close similarity to auranofin, the panel compounds exhibited some peculiar and distinctive features, such as lower log P values which can induce relevant differences in the overall pharmacokinetic profiles. To get better insight into the P-Au strength and stability, an extensive study was carried out for relevant biological models, including three different vasopressin peptide analogues and cysteine, using 31P NMR and LC-ESI-MS. A DFT computational study was also carried out for a better understanding of the theoretical fundamentals of the disclosed differences with regard to triethylphosphine parent compounds.

Auranofin Targeting the NDM-1 Beta-Lactamase: Computational Insights into the Electronic Configuration and Quasi-Tetrahedral Coordination of Gold Ions

Recently, the well-characterized metallodrug auranofin has been demonstrated to restore the penicillin and cephalosporin sensitivity in resistant bacterial strains via the inhibition of the NDM-1 beta-lactamase, which is operated via the Zn/Au substitution in its bimetallic core. The resulting unusual tetrahedral coordination of the two ions was investigated via the density functional theory calculations. By assessing several charge and multiplicity schemes, coupled with on/off constraining the positions of the coordinating residues, it was demonstrated that the experimental X-ray structure of the gold-bound NDM-1 is consistent with either Au(I)-Au(I) or Au(II)-Au(II) bimetallic moieties. The presented results suggest that the most probable mechanism for the auranofin-based Zn/Au exchange in NDM-1 includes the early formation of the Au(I)-Au(I) system, superseded by oxidation yielding the Au(II)-Au(II) species bearing the highest resemblance to the X-ray structure.

Inorganic Drugs as a Tool for Protein Structure Solving and Studies on Conformational Changes

Inorganic drugs are capable of tight interactions with proteins through coordination towards aminoacidic residues, and this feature is recognized as a key aspect for their pharmacological action. However, the "protein metalation process" is exploitable for solving the phase problem and structural resolution. In fact, the use of inorganic drugs bearing specific metal centers and ligands capable to drive the binding towards the desired portions of the protein target could represent a very intriguing and fruitful strategy. In this context, a theoretical approach may further contribute to solve protein structures and their refinement. Here, we delineate the main features of a reliable experimental-theoretical integrated approach, based on the use of metallodrugs, for protein structure solving.

Biological Activities of Ruthenium NHC Complexes: An Update

Ruthenium N-heterocyclic carbene (NHC) complexes have unique physico-chemical properties as catalysts and a huge potential in medicinal chemistry and pharmacology, exhibiting a variety of notable biological activities. In this review, the most recent studies on ruthenium NHC complexes are summarized, focusing specifically on antimicrobial and antiproliferative activities. Ruthenium NHC complexes are generally active against Gram-positive bacteria, such as Bacillus subtilis, Staphylococcus aureus, Micrococcus luteus, Listeria monocytogenes and are seldom active against Gram-negative bacteria, including Salmonella typhimurium, Pseudomonas aeruginosa and Escherichia coli and fungal strains of Candida albicans. The antiproliferative activity was tested against cancer cell lines of human colon, breast, cervix, epidermis, liver and rat glioblastoma cell lines. Ruthenium NHC complexes generally demonstrated cytotoxicity higher than standard anticancer drugs. Further studies are needed to explore the mechanism of action of these interesting compounds.

Kinetics of Reactions of Dirhodium and Diruthenium Paddlewheel Tetraacetate Complexes with Nucleophilic Protein Sites: Computational Insights

Recently, dirhodium and diruthenium paddlewheel complexes have drawn attention as perspective anticancer drugs. In this study, the kinetics of reaction of typical paddlewheel scaffolds Rh₂(μ-O₂CCH₃)₄(H₂O)₂, Ru₂(μ-O₂CCH₃)₄(H₂O)Cl, and [Ru₂(μ-O₂CCH₃)₄(HO)Cl]^- with protein nucleophiles were investigated by means of the density functional theory. The substitution of axial ligands─water and chloride─by the models of protein residue side chains was analyzed, revealing the binding selectivity displayed by these paddlewheel metal scaffolds. The substitution of water is under a thermodynamic control, in which, although the Arg, Cys^-, and Sec^- residues are the most favorable, their binding is expected to be scarcely selective in a biological context. On the other hand, the replacement of the axial water with a more stable hydroxo ligand induces the chloride substitution in diRu complexes, which also targets Arg, Cys^-, and Sec^-, although with a moderately higher activation barrier for any examined protein residue. Additionally, the carried out characterization of the geometrical parameters of the transition states permitted determination of the impact of an increased steric hindrance of diRh and diRu complexes on their protein site selectivity. This study corroborates the idea of the substitution of the acetate ligands with biologically active, but more hindering, carboxylate ligands, in order to yield dual acting metallodrugs. This study allows us to assume that the delivery of diRu paddlewheel complexes in their monoanionic form [Ru₂(μ-O₂CR)₄(OH)Cl]^- decorated by the bulky substituents R may constitute an approach to augment the selectivity toward anticancer targets, such as TrxR in tumor cells, although under the condition that such a selectivity is operative only in high pH conditions.