Silver in biology and medicine: opportunities for metallomics researchers

Bacteriogenic synthesis of morphologically diverse silver nanoparticles and their assessment for methyl orange dye removal and antimicrobial activity

Nanotechnology and nanoparticles have gained massive attention in the scientific community in recent years due to their valuable properties. Among various AgNPs synthesis methods, microbial approaches offer distinct advantages in terms of cost-effectiveness, biocompatibility, and eco-friendliness. In the present research work, investigators have synthesized three different types of silver nanoparticles (AgNPs), namely AgNPs-K, AgNPs-M, and AgNPs-E, by using Klebsiella pneumoniae (MBC34), Micrococcus luteus (MBC23), and Enterobacter aerogenes (MBX6), respectively. The morphological, chemical, and elemental features of the synthesized AgNPs were analyzed by using UV-Vis spectroscopy (UV-Vis), Fourier transform-infrared spectroscopy (FTIR), X-ray diffraction (XRD), field emission scanning electron microscope (FESEM) and energy-dispersive spectroscopy (EDX). UV-Vis absorbance peaks were obtained at 475, 428, and 503 nm for AgNPs-K, AgNPs-M, and AgNPs-E, respectively. The XRD analysis confirmed the crystalline nature of the synthesized AgNPs, having peaks at 26.2°, 32.1°, and 47.2°. At the same time, the FTIR showed bands at 599, 963, 1,693, 2,299, 2,891, and 3,780 cm-1 for all the types of AgNPs indicating the presence of bacterial biomolecules with the developed AgNPs. The size and morphology of the AgNPs varied from 10 nm to several microns and exhibited spherical to porous sheets-like structures. The percentage of Ag varied from 37.8% (wt.%) to 61.6%, i.e., highest in AgNPs-K and lowest in AgNPs-M. Furthermore, the synthesized AgNPs exhibited potential for environmental remediation, with AgNPs-M exhibiting the highest removal efficiency (19.24% at 120 min) for methyl orange dye in simulated wastewater. Further, all three types of AgNPs were evaluated for the removal of methyl orange dye from the simulated wastewater, where the highest dye removal percentage was 19.24% at 120 min by AgNPs-M. Antibacterial potential of the synthesized AgNPs assessment against both Gram-positive (GPB) Bacillus subtilis (MBC23), B. cereus (MBC24), and Gram-negative bacteria Enterococcus faecalis (MBP13) revealed promising results, with AgNPs-M, exhibiting the largest zone of inhibition (12 mm) against GPB B. megaterium. Such investigation exhibits the potential of the bacteria for the synthesis of AgNPs with diverse morphology and potential applications in environmental remediation and antibacterial therapy-based synthesis of AgNPs.

Silver Organometallics that are Highly Potent Thioredoxin and Glutathione Reductase Inhibitors: Exploring the Correlations of Solution Chemistry with the Strong Antibacterial Effects

The antibacterial activity of silver species is well-established; however, their mechanism of action has not been adequately explored. Furthermore, issues of low-molecular silver compounds with cytotoxicity, stability, and solubility hamper their progress to drug leads. We have investigated silver N-heterocyclic carbene (NHC) halido complexes [(NHC)AgX, X = Cl, Br, and I] as a promising new type of antibacterial silver organometallics. Spectroscopic studies and conductometry established a higher stability for the complexes with iodide ligands, and nephelometry indicated that the complexes could be administered in solutions with physiological chloride levels. The complexes showed a broad spectrum of strong activity against pathogenic Gram-negative bacteria. However, there was no significant activity against Gram-positive strains. Further studies clarified that tryptone and yeast extract, as components of the culture media, were responsible for this lack of activity. The reduction of biofilm formation and a strong inhibition of both glutathione and thioredoxin reductases with IC50 values in the nanomolar range were confirmed for selected compounds. In addition to their improved physicochemical properties, the compounds with iodide ligands did not display cytotoxic effects, unlike the other silver complexes. In summary, silver NHC complexes with iodide secondary ligands represent a useful scaffold for nontoxic silver organometallics with improved physicochemical properties and a distinct mechanism of action that is based on inhibition of thioredoxin and glutathione reductases.

Cytosine-Rich Oligonucleotide and Electrochemically Reduced Graphene Oxide Nanocomposite for Ultrasensitive Electrochemical Ag+ Sensing

Silver ions (Ag+) are crucial in various fields, but pose environmental and health risks at high concentrations. This study presents a straightforward approach for the ultra-trace detection of Ag+, utilizing a composite of a cytosine-rich oligonucleotide (CRO) and an electrochemically reduced graphene oxide (ERGO). Initially, ERGO was synthesized on a glassy carbon electrode (GCE) through the reduction of graphene oxide (GO) via cyclic voltammetry. A methylene blue-tagged CRO (MB-CRO) was then anchored to the ERGO surface through π-π interactions, resulting in the formation of an MB-CRO-modified ERGO electrode (MB-CRO/ERGO-GCE). The interaction with Ag+ ions induced the formation of silver-mediated C-Ag+-C coordination, prompting the MB-CRO to adopt a hairpin structure. This conformational change led to the desorption of the MB-CRO from the ERGO-GCE, causing a variation in the redox current of the methylene blue associated with the MB-CRO. Electrochemical assays revealed that the sensor exhibits extraordinary sensitivity to Ag+ ions, with a linear detection range from 1 femtomolar (fM) to 100 nanomolars (nM) and a detection limit of 0.83 fM. Moreover, the sensor demonstrated high selectivity for Ag+ ions and several other benefits, including stability, reproducibility, and straightforward fabrication and operational procedures. Additionally, real sample analyses were performed using the modified electrode to detect Ag+ in tap and pond water samples, yielding satisfactory recovery rates.

Electrochemical Devices in Cutaneous Wound Healing

In healthy skin, vectorial ion transport gives rise to a transepithelial potential which directly impacts many physiological aspects of skin function. A wound is a physical defect that breaches the epithelial barrier and changes the electrochemical environment of skin. Electroceutical dressings are devices that manipulate the electrochemical environment, host as well as microbial, of a wound. In this review, electroceuticals are organized into three mechanistic classes: ionic, wireless, and battery powered. All three classes of electroceutical dressing show encouraging effects on infection management and wound healing with evidence of favorable impact on keratinocyte migration and disruption of wound biofilm infection. This foundation sets the stage for further mechanistic as well as interventional studies. Successful conduct of such studies will determine the best dosage, timing, and class of stimulus necessary to maximize therapeutic efficacy.

Cu+/Ag+ Competition in Type I Copper Proteins (T1Cu)

Due to the similarity in the basic coordination behavior of their mono-charged cations, silver biochemistry is known to be linked to that of copper in biological systems. Still, Cu⁺/²⁺ is an essential micronutrient in many organisms, while no known biological process requires silver. In human cells, copper regulation and trafficking is strictly controlled by complex systems including many cytosolic copper chaperones, whereas some bacteria exploit the so-called "blue copper" proteins. Therefore, evaluating the controlling factors of the competition between these two metal cations is of enormous interest. By employing the tools of computational chemistry, we aim to delineate the extent to which Ag⁺ might be able to compete with the endogenous copper in its Type I (T1Cu) proteins, and where and if, alternatively, it is handled uniquely. The effect of the surrounding media (dielectric constant) and the type, number, and composition of amino acid residues are taken into account when modelling the reactions in the present study. The obtained results clearly indicate the susceptibility of the T1Cu proteins to a silver attack due to the favorable composition and geometry of the metal-binding centers, along with the similarity between the Ag⁺/Cu⁺-containing structures. Furthermore, by exploring intriguing questions of both metals' coordination chemistry, an important background for understanding the metabolism and biotransformation of silver in organisms is provided.

Zn(II) to Ag(I) Swap in Rad50 Zinc Hook Domain Leads to Interprotein Complex Disruption through the Formation of Highly Stable Agx(Cys)y Cores

The widespread application of silver nanoparticles in medicinal and daily life products increases the exposure to Ag(I) of thiol-rich biological environments, which help control the cellular metallome. A displacement of native metal cofactors from their cognate protein sites is a known phenomenon for carcinogenic and otherwise toxic metal ions. Here, we examined the interaction of Ag(I) with the peptide model of the interprotein zinc hook (Hk) domain of Rad50 protein from Pyrococcus furiosus, a key player in DNA double-strand break (DSB) repair. The binding of Ag(I) to 14 and 45 amino acid long peptide models of apo- and Zn(Hk)₂ was experimentally investigated by UV-vis spectroscopy, circular dichroism, isothermal titration calorimetry, and mass spectrometry. The Ag(I) binding to the Hk domain was found to disrupt its structure via the replacement of the structural Zn(II) ion by multinuclear Ag_x(Cys)_y complexes. The ITC analysis indicated that the formed Ag(I)-Hk species are at least 5 orders of magnitude stronger than the otherwise extremely stable native Zn(Hk)₂ domain. These results show that Ag(I) ions may easily disrupt the interprotein zinc binding sites as an element of silver toxicity at the cellular level.

Silver and copper-benznidazole derivatives as potential antiparasitic metallodrugs: Synthesis, characterization, and biological evaluation

Currently the only drug available to treat Chagas disease in Brazil is benznidazole (BZN). Therefore, there is an urgent need to discover and develop new anti- Trypanosoma cruzi candidates. In our continuous effort to enhance clinical antiparasitic drugs using synergistic strategy, BZN was coordinated to silver and copper ions to enhance its effectiveness to treat that illness. In this work, the syntheses of four novel metal-BZN complexes, [Ag(BZN)₂]NO₃·H₂O (1), [CuCl₂(BZN)(H₂O)]·1/2CH₃CN (2), [Ag(PPh₃)₂(BZN)₂]NO₃·H₂O (3), and [Cu(PPh₃)₂(BNZ)₂]NO₃·2H₂O (4), and their characterization using multiple analytical and spectroscopic techniques such as Infrared (FTIR), Nuclear Magnetic Resonance (¹H, ¹³C, ³¹P), UV-Visible (UV-Vis), Electron Paramagnetic Resonance (EPR), conductivity and elemental analysis are described. IC₅₀ (Half-maximal inhibitory concentration) values of Ag-BZN compounds are about five to ten times lower than benznidazole itself in both proliferation stages of the parasite (epimastigotes and amastigotes). The cytotoxicity of both compounds in human cells (fibroblasts and hepatocytes) are comparable to BZN, indicating that Ag-BZN complexes can be more selective than BZN.

Non-invasive radionuclide imaging of trace metal trafficking in health and disease: "PET metallomics"

Several specific metallic elements must be present in the human body to maintain health and function. Maintaining the correct quantity (from trace to bulk) and location at the cell and tissue level is essential. The study of the biological role of metals has become known as metallomics. While quantities of metals in cells and tissues can be readily measured in biopsy and autopsy samples by destructive analytical techniques, their trafficking and its role in health and disease are poorly understood. Molecular imaging with radionuclides - positron emission tomography (PET) and single photon emission computed tomography (SPECT) - is emerging as a means to non-invasively study the acute trafficking of essential metals between organs, non-invasively and in real time, in health and disease. PET scanners are increasingly widely available in hospitals, and methods for producing radionuclides of some of the key essential metals are developing fast. This review summarises recent developments in radionuclide imaging technology that permit such investigations, describes the radiological and physicochemical properties of key radioisotopes of essential trace metals and useful analogues, and introduces current and potential future applications in preclinical and clinical investigations to study the biology of essential trace metals in health and disease.

Antimicrobial Properties of Amino-Acid-Derived N-Heterocyclic Carbene Silver Complexes

Complexes {Ag[NHC^Mes,R]}_n (R = H, 2a; Me, 2b and 2b’; ⁱPr, 2c; ⁱBu, 2d), were prepared by treatment of imidazolium precursor compounds [Im^Mes,R] (2-(3-mesityl-1H-imidazol-3-ium-1-yl)acetate, 1a, (S)-2-alkyl(3-mesityl-1H-imidazol-3-ium-1-yl)acetate, 1b–d, and (R)-2-methyl(3-mesityl-1H-imidazol-3-ium-1-yl)acetate, 1b’, with Ag₂O under appropriate conditions. They were characterised by analytical, spectroscopic (IR, ¹H, and ¹³C NMR and polarimetry), and X-ray methods (2a). In the solid state, 2a is a one-dimensional coordination polymer, in which the silver(I) cation is bonded to the carbene ligand and to the carboxylate group of a symmetry-related Ag[NHC^Mes,H] moiety. The coordination environment of the silver centre is well described by the DFT study of the dimeric model {Ag[NHC^Mes,H]}₂. The antimicrobial properties of these complexes were evaluated versus Gram-negative bacteria E. coli and P. aeruginosa. From the observed MIC and MBC values (minimal inhibitory concentration and minimal bactericidal concentration, respectively), complex 2b’ showed the best antimicrobial properties (eutomer), which were significantly better than those of its enantiomeric derivative 2b (distomer). Additionally, analysis of MIC and MBC values of 2a–d reveal a clear structure–antimicrobial effect relationship. Antimicrobial activity decreases when the steric properties of the R alkyl group in {Ag[NHC^Mes,R]}_n increase.

Bi2O3 nanoparticles exhibit potent broad-spectrum antimicrobial activity and the ability to overcome Ag-, ciprofloxacin- and meropenem-resistance in P. aeruginosa: the next silver bullet of metal antimicrobials?

Antimicrobial resistance is a persistent threat to global public health. In order to combat the spread of pathogenic bacteria, numerous antimicrobial materials have been incorporated into wound dressings and medical devices such as implants and catheters. The most frequently utilized of these materials are Ag-salts and Ag-nanoparticles (AgNPs) due to their low minimum inhibitory concentrations (MICs) against common Gram-negative pathogenic bacteria such as P. aeruginosa. However, such Ag-based materials are limited to treating Gram-negative bacteria and prone to generating Ag-resistant phenotypes after only 7 consecutive exposures to these materials at a sub-inhibitory concentration. Here, we demonstrate α-Bi2O3 NPs as potential replacements for such materials, i.e., α-Bi2O3 NPs that exhibit potent broad-spectrum antibacterial activity (MIC = 0.75 μg mL-1 against P. aeruginosa; MIC = 2.5 μg mL-1 against S. aureus). Furthermore, these NPs are effective against Ag-resistant and carbapenem-resistant bacteria (MICs = 1.0 μg mL-1 and 1.25 μg mL-1, respectively) and also show a synergistic effect with meropenem (mero) in P. aeruginosa bacteria, allowing for the use of meropenem with smaller therapeutic doses (fractional inhibitory concentration = 0.45). Finally, unlike other materials that have been explored as effective antimicrobials, α-Bi2O3 NPs do not contribute to the development of Bi-resistant phenotypes after 30 passages of consecutive exposure to a sub-lethal dose of such NPs. Our results demonstrate that Bi-based materials represent a critical tool against multidrug resistant bacteria and require greater attention within the community. We anticipate this study to inspire broader investigation into the use of other metal oxides as antimicrobial materials, particularly those that limit the development of resistant phenotypes.

Metallodrug Profiling against SARS-CoV-2 Target Proteins Identifies Highly Potent Inhibitors of the S/ACE2 interaction and the Papain-like Protease PLpro

The global spread of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has called for an urgent need for dedicated antiviral therapeutics. Metal complexes are commonly underrepresented in compound libraries that are used for screening in drug discovery campaigns, however, there is growing evidence for their role in medicinal chemistry. Based on previous results, we have selected more than 100 structurally diverse metal complexes for profiling as inhibitors of two relevant SARS-CoV-2 replication mechanisms, namely the interaction of the spike (S) protein with the ACE2 receptor and the papain-like protease PLpro . In addition to many well-established types of mononuclear experimental metallodrugs, the pool of compounds tested was extended to approved metal-based therapeutics such as silver sulfadiazine and thiomersal, as well as polyoxometalates (POMs). Among the mononuclear metal complexes, only a small number of active inhibitors of the S/ACE2 interaction was identified, with titanocene dichloride as the only strong inhibitor. However, among the gold and silver containing complexes many turned out to be very potent inhibitors of PLpro activity. Highly promising activity against both targets was noted for many POMs. Selected complexes were evaluated in antiviral SARS-CoV-2 assays confirming activity for gold complexes with N-heterocyclic carbene (NHC) or dithiocarbamato ligands, a silver NHC complex, titanocene dichloride as well as a POM compound. These studies might provide starting points for the design of metal-based SARS-CoV-2 antiviral agents.

The biochemical fate of Ag+ ions in Staphylococcus aureus, Escherichia coli, and biological media

Silver is commonly included in a range of household and medical items to provide bactericidal action. Despite this, the chemical fate of the metal in both mammalian and bacterial systems remains poorly understood. Here, we applied a metallomics approach using X-ray absorption spectroscopy (XAS) and size-exclusion chromatography hyphenated with inductively coupled plasma mass spectrometry (SEC-ICP-MS) to advance our understanding of the biochemical fate of silver ions in bacterial culture and cells, and the chemistry associated with these interactions. When silver ions were added to lysogeny broth, silver was exclusively associated with moderately-sized species (~30 kDa) and bound by thiolate ligands. In two representative bacterial pathogens cultured in lysogeny broth including sub-lethal concentrations of ionic silver, silver was found in cells to be predominantly coordinated by thiolate species. The silver biomacromolecule-binding profile in Staphylococcus aureus and Escherichia coli was complex, with silver bound by a range of species spanning from 20 kDa to >1220 kDa. In bacterial cells, silver was nonuniformly colocalised with copper-bound proteins, suggesting that cellular copper processing may, in part, confuse silver for nutrient copper. Notably, in the treated cells, silver was not detected bound to low molecular weight compounds such as glutathione or bacillithiol.