Coordination Chemistry of Microbial Iron Transport

Azo dye-based on-site assay paper kit for rapid and selective naked-eye and smartphone-assisted detection of deferoxamine drug

Ferric ion {Fe(III)} is an integral part of many structural and functional components in the body and plays critical roles in physiological and pathological processes. High Fe(III) concentrations damage many systems and organs in the body. The most common method used to prevent excessive accumulation of Fe(III) ions is chelation therapy and the most commonly used drug is deferoxamine (DFO). In this study, an azo dye-based test-paper probe was developed for the colorimetric determination of DFO. The bi-dental azo-dye, named DAZS (diazosalicylaldehyde), was synthesized with a 58 % yield as due to the coupling reaction of 4,4'-diaminobibenzyl and salicylaldehyde. The surface of cellulose-based chromatographic papers immersed in DAZS solution was coated with DAZS and peraped DAZS-coated paper (DAZS-paper) was used as a sensor kit. The structure of DAZS, DAZS-paper, and its interaction mechanism with Fe(III) and DFO were elucidated by FTIR, 1H NMR, 13C NMR, STEM, EDX, and AFM techniques. One arm of DAZS, a symmetrical molecule, interacts with the cellulose-based chromatographic paper, while the other arm remains exposed. This exposed arm interacts with the Fe(III), interrupting the intramolecular charge transfer (ICT) mechanism present in the DAZS molecule, and the color of the DAZS-paper becomes lighter. This interaction, which forms the Fe(III) sensing strategy, is disrupted in the presence of DFO. As a result of its strong chelating feature, DFO forms a chelate complex with Fe(III), enabling the separation of Fe(III) from DAZS. With the separation of the Fe(III), the ICT mechanism is activated again in the free part of DAZS, providing the coloration of DAZS-paper. Although the observed coloration is directly proportional to the concentration of DFO, color analysis was performed with a smartphone via the "colorimeter" application. By analyzing the test paper and solution medium with a smartphone, the detection limits (LOD) of DFO were found to be 95.0 nmol L-1 and 82.0 nmol L-1, respectively. In the selectivity study conducted with various Fe(III) chelating agents, DFO was determined with high selectivity. It was found that common metal cations and anions did not significantly affect the developed probe. The developed method, which was verified by a chromatographic reference method, can also be easily applied to the commercial drug Desferal and fetal bovine serum containing DFO.

The design of nanofiber-based sensors for visual identification of pathogenic bacterial contamination

Pathogenic bacteria pose a serious threat to human health, and an excessively long detection time may cause patients to miss the optimal treatment opportunity. Therefore, rapid and accurate bacterial detection methods are highly significant for the diagnosis of diseases. This study aimed to develop a simple and portable detection platform for the highly sensitive colorimetric detection of P. aeruginosa, S. aureus, and E. coli. The functional NFM with a double-layer structure was used as the research material. One layer consisted of a nanofiber film loaded with Fe(III), while the other layer was a hydrophilic polymer.The functional NFM demonstrated a detection limit of 10 CFU/mL for pathogenic bacteria, with a detection time of only 10 min. This platform showed great potential in environmental and food safety detection.

Natural sideromycins and siderophore-conjugated natural products as inspiration for novel antimicrobial agents

The widespread emergence of multidrug-resistant (MDR) Gram-negative pathogens has posed a major challenge to clinical anti-infective therapy, and new effective treatments are urgently needed. A promising "Trojan horse" strategy involves conjugating antibiotics to siderophore molecules; the resulting siderophore-antibiotic conjugates (SACs) deliver antibiotics directly into cells by hijacking the sophisticated iron transport systems of Gram-negative bacteria, bypassing the outer membrane permeability barrier to enhance uptake and antibacterial efficacy. The clinical release of the first siderophore-antibiotic conjugate, cefiderocol, has aroused tremendous interest in the field among researchers and pharmaceutical companies. To date, most of the reported SACs have focused on the conjugation of siderophores to traditional antibacterial drugs. However, these antibacterial agents designed on the basis of the traditional antibiotic skeleton theoretically bear the risk of cross-resistance caused by shared molecular scaffolds. In this case, exploring novel natural product antibacterial conjugate scaffolds to circumvent the risk of early cross-resistance represents a presumably more sustainable approach for the development of SACs. In this review, we systematically summarize the research progress on siderophore-natural product conjugates as novel antimicrobial agents reported since 2010. Additionally, we propose challenges to be overcome and prospects for future development in this field.

The Coordination Chemistry of Two Peptidic Models of NFeoB and Core CFeoB Regions of FeoB Protein: Complexes of Fe(II), Mn(II), and Zn(II)

Often necessary for efficient Fe(II) trafficking into bacterial cell, the Feo system is a vital transporter for many pathogenic bacteria and indispensable for proper development and survival in the host organism during infection. In this work, we present the metal-binding characteristics of the peptidic models of two putative Fe(II)-binding sites of E. coliFeoB: L1 (Ac-477IMRGEATPFVMELPVYHVPH496-CONH2) being a fragment of the Core CFeoB region located between the transmembrane helices and L2 (Ac-38VERKEG43-CONH2), which represents the ExxE motif found within the NFeoB domain. With a variety of physicochemical methods, such as potentiometry, mass spectrometry, NMR, and EPR spectroscopy, we have determined the stability constants and metal-binding residues for the complexes of Fe(II), Mn(II), and Zn(II) with two ligands, L1 and L2, acting as models for the Core CFeoB and ExxE motif. We compare their affinities toward the studied metal ions with the previously studied C-terminal part of the protein and discuss a possible role in metal trafficking by the whole protein.

Ion Mobility-Coupled Mass Spectrometry for Metallophore Detection

Metal chelating small molecules (metallophores) play significant roles in microbial interactions and bacterial survival; however, current methods to identify metallophores are limited by low sensitivity, a lack of metal selectivity, and/or complicated data analysis. To overcome these limitations, we developed a novel approach for detecting metallophores in natural product extracts using ion mobility-coupled mass spectrometry (IM-MS). As a proof of concept, marine bacterial extracts containing known metallophores were analyzed by IM-MS with and without added metals, and the data were compared between conditions to identify metal-binding metabolites. Ions with changes in both mass and mobility were specific to metallophores, enabling their identification within these complex extracts. Additionally, we compared the use of direct infusion (DI) and liquid chromatography (LC) separation with IM-MS. For most samples, DI outperformed LC by minimizing the time required for data collection and simplifying analysis. However, for some samples, LC improved the detection of metallophores likely by reducing ion suppression. IM-MS was then used to identify 10 metallophores in an extract from a marine Micromonospora sp. Overall, incorporating IM-MS facilitated the rapid detection of metal-binding natural products in complex bacterial extracts through the comparison of mass and mobility data in the presence and absence of metals.

Three local plants adapt to ecological restoration of abandoned lead-zinc mines through assembly of rhizosphere bacterial communities

Introduction

The plant restoration and ecological restoration of lead-zinc mines are very important.

Methods

In this study, we used three local plants to carry out ecological restoration of abandoned lead-zinc mining areas and detected the adaptive mechanisms of soil bacterial diversity and function during the ecological restoration of lead-zinc mines through 16S rRNA sequencing.

Results

The results revealed that lead-zinc mining significantly reduced the soil bacterial diversity, including the Shannon, Simpson, and observed species indices, whereas the planting of the three ecological restoration plants restored the soil microbial diversity to a certain extent, leading to increases in the Shannon index and Observed species indices. Mining activities significantly reduced the abundances of RB41 and Bryobacter in the bulk soil compared with those in the nonmining areas, whereas the three ecological restoration plants increased the abundances of RB41 and Bryobacter in the rhizosphere soil compared with those in the bulk soil in the mining areas. Following the planting of the three types of ecologically restored plants, the soil bacterial community structure partially recovered. In addition, different plants have been found to have different functions in the lead-zinc ecological restoration process, including iron complex transport system-permitting proteins and ATP binding cassettes.

Discussion

This study confirms for the first time that plants adapt to the remediation process of abandoned lead-zinc mines by non-randomly assembling rhizosphere bacterial communities and functions, providing a reference for screening microbial remediation bacterial resources and plant microbe joint bioremediation strategies for lead-zinc mines.

Current developments and prospects of the antibiotic delivery systems

Antibiotics have remained the cornerstone for the treatment of bacterial infections ever since their discovery in the twentieth century. The uproar over antibiotic resistance among bacteria arising from genome plasticity and biofilm development has rendered current antibiotic therapies ineffective, urging the development of innovative therapeutic approaches. The development of antibiotic resistance among bacteria has further heightened the clinical failure of antibiotic therapy, which is often linked to its low bioavailability, side effects, and poor penetration and accumulation at the site of infection. In this review, we highlight the potential use of siderophores, antibodies, cell-penetrating peptides, antimicrobial peptides, bacteriophages, and nanoparticles to smuggle antibiotics across impermeable biological membranes to achieve therapeutically relevant concentrations of antibiotics and combat antimicrobial resistance (AMR). We will discuss the general mechanisms via which each delivery system functions and how it can be tailored to deliver antibiotics against the paradigm of mechanisms underlying antibiotic resistance.

Bioinspired Metal-Ligand Networks with Enhanced Stability and Performance: Facile Preparation of Hydroxypyridinone (HOPO)-Functionalized Materials

Bioinspired hydroxypyridinone (HOPO)functionalized materials are shown to display a remarkable capacity for stability and for chelating a wide array of metal ions. This allows for the synthesis of multifunctional networks with diverse physical properties when compared to traditional catechol systems. In the present study, we report a facile, one-pot synthesis of an amino HOPO ligand and simple, scalable incorporation into PEG-acrylate based networks via active ester chemistry. This modular network approach allows for fabrication of patterned HOPO containing networks which can chelate a range of metal ions, such as transition metals (Fe3+) and lanthanides (Ho3+, Tb3+), leading to modulation of mechanical, magnetic, and fluorescent properties. Moreover, networks with tailored, heterogeneous properties can be prepared through localization of metal ion incorporation in 3-dimensions via masking techniques, creating distinctly soft, hard, magnetic, and fluorescent domains.

Siderophores as tools and treatments

In the search for iron, an essential element in many biochemical processes, microorganisms biosynthesise dedicated chelators, known as siderophores, to sequester iron from their environment and actively transport the siderophore complex into the cell. This process has been implicated in bacterial pathogenesis and exploited through siderophore-antibiotic conjugates as a method for selective antibiotic delivery. Here we review this Trojan-horse approach including design considerations and potential in diagnostics and infection imaging.

Efficient density functional theory directed identification of siderophores with increased selectivity towards indium and germanium

Siderophores are promising ligands for application in novel recycling and bioremediation technologies, as they can selectively complex a variety of metals. However, with over 250 known siderophores, the selection of suiting complexants in the wet lab is impractical. Thus, this study established a density functional theory (DFT) based approach to efficiently identify siderophores with increased selectivity towards target metals on the example of germanium and indium. Considering 239 structures, chemically similar siderophores were clustered, and their complexation reactions modeled utilizing DFT. The calculations revealed siderophores with, compared to the reference siderophore desferrioxamine B (DFOB), up to 128 % or 48 % higher selectivity for indium or germanium, respectively. Experimental validation of the method was conducted with fimsbactin A and agrobactin, demonstrating up to 40 % more selective indium binding and at least sevenfold better germanium binding than DFOB, respectively. The results generated in this study open the door for the utilization of siderophores in eco-friendly technologies for the recovery of many different critical metals from various industry waters and leachates or bioremediation approaches. This endeavor is greatly facilitated by applying the herein-created database of geometry-optimized siderophore structures as de novo modeling of the molecules can be omitted.

Preparation of 6-Amino-N-hydroxyhexanamide-Modified Porous Chelating Resin for Adsorption of Heavy Metal Ions

The pollution of water bodies by heavy metal ions has recently become a global concern. In this experiment, a novel chelating resin, D851-6-AHHA, was synthesized by grafting 6-amino-N-hydroxyhexanamide (6-AHHA) onto the (-CH2N-(CH2COOH)2) group of the D851 resin, which contained a hydroxamic acid group, amide group, and some carboxyl groups. This resin was developed for the purpose of removing heavy metal ions, such as Cr(III) and Pb(II), from water. The findings from static adsorption experiments demonstrated the remarkable adsorption effectiveness of D851-6-AHHA resin towards Cr(III) and Pb(II). Specifically, the maximum adsorption capacities for Cr(III) and Pb(II) were determined to be 91.50 mg/g and 611.92 mg/g, respectively. Furthermore, the adsorption kinetics of heavy metal ions by D851-6-AHHA resin followed the quasi-second-order kinetic model, while the adsorption isotherms followed the Langmuir model. These findings suggest that the adsorption process was characterized by monolayer chemisorption. The adsorption mechanism of D851-6-AHHA resin was comprehensively investigated through SEM, XRD, FT-IR, and XPS analyses, revealing a high efficiency of D851-6-AHHA resin in adsorbing Cr(III) and Pb(II). Specifically, the (-C(=O)NHOH) group exhibited a notable affinity for Cr(III) and Pb(II), forming stable multi-elemental ring structures with them. Additionally, dynamic adsorption experiments conducted using fixed-bed setups further validated the effectiveness of D851-6-AHHA resin in removing heavy metal ions from aqueous solutions. In conclusion, the experimental findings underscored the efficacy of D851-6-AHHA resin as a highly efficient adsorbent for remediating water bodies contaminated by heavy metal ions.

Polycatechols inhibit ferroptosis and modulate tau liquid-liquid phase separation to mitigate Alzheimer's disease

Alzheimer's disease (AD) is a complex neurodegenerative disorder that affects learning, memory, and cognition. Current treatments targeting amyloid-β (Aβ) and tau have shown limited effectiveness, necessitating further research on the aggregation and toxicity mechanisms. One of these mechanisms involves the liquid-liquid phase separation (LLPS) of tau, contributing to the formation of pathogenic tau aggregates, although their conformational details remain elusive. Another mechanism is ferroptosis, a type of iron-dependent lipid peroxidation-mediated cell death, which has been implicated in AD. There is a lack of therapeutic strategies that simultaneously target amyloid toxicity and ferroptosis. This study aims to explore the potential of polycatechols, PDP and PLDP, consisting of dopamine and L-Dopa, respectively, as multifunctional agents to modulate the pathological nexus between ferroptosis and AD. Polycatechols were found to sequester the labile iron pool (LIP), inhibit Aβ and tau aggregation, scavenge free radicals, protect mitochondria, and prevent ferroptosis, thereby rescuing neuronal cell death. Interestingly, PLDP promotes tau LLPS, and modulates their intermolecular interactions to inhibit the formation of toxic tau aggregates, offering a conceptually innovative approach to tackle tauopathies. This is a first-of-its-kind polymer-based integrative approach that inhibits ferroptosis, counteracts amyloid toxicity, and modulates tau LLPS to mitigate the multifaceted toxicity of AD.

Novel Penicillin-Based Sulfone-Siderophore Conjugates for Restoring β-Lactam Antibiotic Efficacy

Membrane permeability is a natural defense barrier that contributes to increased bacterial drug resistance, particularly for Gram-negative pathogens. As such, accurate delivery of the antibacterial agent to the target has become a growing research area in the infectious diseases field as a means of improving drug efficacy. Although the efficient transport of siderophore-antibiotic conjugates into the cytosol still remains challenging, great success has been achieved in the delivery of β-lactam antibiotics into the periplasmic space via bacterial iron uptake pathways. Cefiderocol, the first siderophore-cephalosporin conjugate approved by the US Food and Drug Administration, is a good example. These conjugation strategies have also been applied to the precise delivery of β-lactamase inhibitors, such as penicillin-based sulfone 1, to restore β-lactam antibiotic efficacy in multidrug-resistant bacteria. Herein, we have explored the impact on the bacterial activity of 1 by modifying its iron chelator moiety. A set of derivatives functionalized with diverse iron chelator groups and linkages to the scaffold (compounds 2-8) were synthesized and assayed in vitro. The results on the ability of derivatives 2-8 to recover β-lactam antibiotic efficacy in difficult-to-treat pathogens that produce various β-lactamase enzymes, along with kinetic studies with the isolated enzymes, allowed us to identify compound 2, a novel β-lactamase inhibitor with an expanded spectrum of activity. Molecular dynamics simulation studies provided us with further information regarding the molecular basis of the relative inhibitory properties of the most relevant compound described herein.

Iron mobilization from intact ferritin: effect of differential redox activity of quinone derivatives with NADH/O2 and in situ-generated ROS

Ferritins are multimeric nanocage proteins that sequester/concentrate excess of free iron and catalytically synthesize a hydrated ferric oxyhydroxide bio-mineral. Besides functioning as the primary intracellular iron storehouses, these supramolecular assemblies also oversee the controlled release of iron to meet physiologic demands. By virtue of the reducing nature of the cytosol, reductive dissolution of ferritin-iron bio-mineral by physiologic reducing agents might be a probable pathway operating in vivo. Herein, to explore this reductive iron-release pathway, a series of quinone analogs differing in size, position/nature of substituents and redox potentials were employed to relay electrons from physiologic reducing agent, NADH, to the ferritin core. Quinones are well known natural electron/proton mediators capable of facilitating both 1/2 electron transfer processes and have been implicated in iron/nutrient acquisition in plants and energy transduction. Our findings on the structure-reactivity of quinone mediators highlight that iron release from ferritin is dictated by electron-relay capability (dependent on E1/2 values) of quinones, their molecular structure (i.e., the presence of iron-chelation sites and the propensity for H-bonding) and the type/amount of reactive oxygen species (ROS) they generate in situ. Juglone/Plumbagin released maximum iron due to their intermediate E1/2 values, presence of iron chelation sites, the ability to inhibit in situ generation of H2O2 and form intramolecular H-bonding (possibly promotes semiquinone formation). This study may strengthen our understanding of the ferritin-iron-release process and their significance in bioenergetics/O2-based cellular metabolism/toxicity while providing insights on microbial/plant iron acquisition and the dynamic host-pathogen interactions.

Smart responsive in situ hydrogel systems applied in bone tissue engineering

The repair of irregular bone tissue suffers severe clinical problems due to the scarcity of an appropriate therapeutic carrier that can match dynamic and complex bone damage. Fortunately, stimuli-responsive in situ hydrogel systems that are triggered by a special microenvironment could be an ideal method of regenerating bone tissue because of the injectability, in situ gelatin, and spatiotemporally tunable drug release. Herein, we introduce the two main stimulus-response approaches, exogenous and endogenous, to forming in situ hydrogels in bone tissue engineering. First, we summarize specific and distinct responses to an extensive range of external stimuli (e.g., ultraviolet, near-infrared, ultrasound, etc.) to form in situ hydrogels created from biocompatible materials modified by various functional groups or hybrid functional nanoparticles. Furthermore, "smart" hydrogels, which respond to endogenous physiological or environmental stimuli (e.g., temperature, pH, enzyme, etc.), can achieve in situ gelation by one injection in vivo without additional intervention. Moreover, the mild chemistry response-mediated in situ hydrogel systems also offer fascinating prospects in bone tissue engineering, such as a Diels-Alder, Michael addition, thiol-Michael addition, and Schiff reactions, etc. The recent developments and challenges of various smart in situ hydrogels and their application to drug administration and bone tissue engineering are discussed in this review. It is anticipated that advanced strategies and innovative ideas of in situ hydrogels will be exploited in the clinical field and increase the quality of life for patients with bone damage.

Facile and Bioinspired Approach from Gallic Acid for the Synthesis of Biobased Flame Retardant Coatings of Basalt Fibers

A sustainable, bioinspired approach to functionalize basalt fibers with an innovative gallic acid (GA)-iron phenyl phosphonate complex (BF-GA-FeP), for the purpose of improving the flame retardancy in composite materials, is developed. BFs were at first pretreated with O3, obtaining surface free hydroxyl groups that allowed the subsequent covalent immobilization of biosourced GA units on the fiber through ester linkages. Phenolic -OH groups of the GA units were then exploited for the complexation of iron phenyl phosphonate, resulting in the target-complex-coated BF fiber (BF-GA-FeP). Microwave plasma atomic emission spectroscopy and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy analyses of BF-GA-FeP highlighted an increase in iron content, modification of fiber morphology, and occurrence of phosphorus, respectively. BFs, modified with a low amount of the developed complex, were used to reinforce a poly(lactic acid) (PLA) matrix in the production of a biocomposite (PLA/BF-FeP). PLA/BF-FeP showed a higher thermal stability than neat PLA and PLA reinforced with untreated BFs (PLA/BF), as confirmed by thermogravimetric analysis. The cone calorimeter test highlighted several advantages for PLA/BF-FeP, including a prolonged time to ignition, a reduced time to flame out, an 8% decrease in the peak heat release rate, and a 15% reduced fire propagating index compared to PLA/BF.

Discovery of YFJ-36: Design, Synthesis, and Antibacterial Activities of Catechol-Conjugated β-Lactams against Gram-Negative Bacteria

Cefiderocol is the first approved catechol-conjugated cephalosporin against multidrug-resistant Gram-negative bacteria, while its application was limited by poor chemical stability associated with the pyrrolidinium linker, moderate potency against Klebsiella pneumoniae and Acinetobacter baumannii, intricate procedures for salt preparation, and potential hypersensitivity. To address these issues, a series of novel catechol-conjugated derivatives were designed, synthesized, and evaluated. Extensive structure-activity relationships and structure-metabolism relationships (SMR) were conducted, leading to the discovery of a promising compound 86b (Code no. YFJ-36) with a new thioether linker. 86b exhibited superior and broad-spectrum in vitro antibacterial activity, especially against A. baumannii and K. pneumoniae, compared with cefiderocol. Potent in vivo efficacy was observed in a murine systemic infection model. Furthermore, the physicochemical stability of 86b in fluid medium at pH 6-8 was enhanced. 86b also reduced potential the risk of allergy owing to the quaternary ammonium linker. The improved properties of 86b supported its further research and development.

Preclinical characterisation of gallium-68 labeled ferrichrome siderophore stereoisomers for PET imaging applications

BACKGROUND

Siderophores are small iron-binding molecules produced by microorganisms to facilitate iron acquisition from the environment. Radiolabelled siderophores offer a promising solution for infection imaging, as they can specifically target the pathophysiological mechanisms of pathogens. Gallium-68 can replace the iron in siderophores, enabling molecular imaging with positron emission tomography (PET). Stereospecific interactions play a crucial role in the recognition of receptors, transporters, and iron utilisation. Furthermore, these interactions have an impact on the host environment, affecting pharmacokinetics and biodistribution. This study examines the influence of siderophore stereoisomerism on imaging properties, with a focus on ferrirubin (FR) and ferrirhodin (FRH), two cis-trans isomeric siderophores of the ferrichrome type.

RESULTS

Tested siderophores were labelled with gallium-68 with high radiochemical purity. The resulting complexes differed in their in vitro characteristics. [68Ga]Ga-FRH showed less hydrophilic properties and higher protein binding values than [68Ga]Ga-FR. The stability studies confirmed the high radiochemical stability of both [68Ga]Ga-siderophores in all examined media. Both siderophores were found to be taken up by S. aureus, K. pneumoniae and P. aeruginosa with similar efficacy. The biodistribution tested in normal mice showed rapid renal clearance with low blood pool retention and fast clearance from examined organs for [68Ga]Ga-FR, whereas [68Ga]Ga-FRH showed moderate retention in blood, resulting in slower pharmacokinetics. PET/CT imaging of mice injected with [68Ga]Ga-FR and [68Ga]Ga-FRH confirmed findings from ex vivo biodistribution studies. In a mouse model of S. aureus myositis, both radiolabeled siderophores showed radiotracer accumulation at the site of infection.

CONCLUSIONS

The 68Ga-complexes of stereoisomers ferrirubin and ferrirhodin revealed different pharmacokinetic profiles. In vitro uptake was not affected by isomerism. Both compounds had uptake with the same bacterial culture with similar efficacy. PET/CT imaging showed that the [68Ga]Ga-complexes accumulate at the site of S. aureus infection, highlighting the potential of [68Ga]Ga-FR as a promising tool for infection imaging. In contrast, retention of the radioactivity in the blood was observed for [68Ga]Ga-FRH. In conclusion, the stereoisomerism of potential radiotracers should be considered, as even minor structural differences can influence their pharmacokinetics and, consequently, the results of PET imaging.

Deferiprone: new environmental perspectives. Insights into its sequestering ability vs. different metal cations

Deferiprone, generally, is considered an important chelating agent for Fe3+ overload. From a literature data analysis, a lack of information on the interaction of this molecule toward a series of metal cations emerged, inducing to fill out the topic. The complexing ability of deferiprone toward Ca2+, Mg2+, Cd2+ and Pb2+ was studied by potentiometry and 1H NMR spectroscopy, in KCl aqueous solutions at different ionic strength values (0.1 ≤ I/mol dm-3 ≤ 1.0) and T = 298.15 K. The same speciation model featured by the ML, ML2, ML3 and ML(OH) (M = metal and L = deferiprone or DFP) species was obtained for Cd2+ and Pb2+; the formation constants calculated at infinite dilution are: logTβ = 7.23±0.02, 12.47±0.03, 16.70±0.04, and -2.53±0.04, respectively for Cd2+ and 9.91±0.01, 15.99±0.02, 19.93±0.05 and 0.99±0.02 for Pb2+. Only two species, namely ML and ML2, were determined for Ca2+ and Mg2+, whose formation constants at infinite dilution are respectively: 3.72±0.01 and 6.50±0.02, for the first one, 5.31±0.01 and 9.58±0.01, for the second. The ligand sequestering ability and affinity toward M2+ were evaluated by determining the pL0.5 and pM parameters at different pHs and ionic strengths. The results suggest that deferiprone has the best complexing and sequestering ability toward Pb2+, followed by Cd2+, Mg2+ and Ca2+, respectively. 1H NMR studies confirmed the DFP affinity for Cd2+ and Pb2+, and in combination with DFT calculations showed that metal cations are bound to the hydroxo-oxo moiety of the pyridinone ring. The data reported in this study provide information on the possible employment of a small molecule like deferiprone, as a chelating and sequestering agent for Pb2+ accumulation or overload from environmental and biological matrices.

Recognition of Actinides by Siderocalin

Plain simulations and enhanced sampling unveil a novel siderocalin (Scn) recognition mode for An-Ent (where An = actinides and Ent = enterobactin) complexes and identify a "seesaw" relationship between actinide affinity to Ent and Scn recognition to an An-Ent complex. Electrostatic interactions predominantly govern competitive binding in both processes. Additionally, hydrolysis-induced negative charge, water expulsion-driven entropy, and Ent's conformational adaptability collectively enhance high-affinity recognition.

Rhodotorulic Acid and its Derivatives: Synthesis, Properties, and Applications

Siderophores are low molecular weight compounds produced by microorganisms to scavenge iron in iron-deficient environments. Rhodotorulic acid, a natural hydroxamate siderophore, plays a vital role in iron acquisition for fungi and bacteria. As the simplest natural hydroxamate siderophore, it exhibits a high affinity for ferric ions, enabling it to form stable complexes that facilitate iron uptake and transport within microorganisms. This article provides a comprehensive analysis of this hydroxamate siderophore, rhodotorulic acid, its synthesis, physicochemical properties, and biological significance. It also explores its applications in antifungal and plant protection strategies. Insights into RA derivatives reveal distinct biological effects and applications with potential in various fields, from antioxidants to antifungals. Rhodotorulic acid and its derivatives show promise for novel therapies, plant protection strategies, and iron supplementation in agriculture. Understanding their properties could advance science and medicine with sustainable practices.

Fe(II), Mn(II), and Zn(II) Binding to the C-Terminal Region of FeoB Protein: An Insight into the Coordination Chemistry and Specificity of the Escherichia coli Fe(II) Transporter

The interactions between two peptide ligands [Ac763CCAASTTGDCH773 (P1) and Ac743RRARSRVDIELLATRKSVSSCCAASTTGDCH773 (P2)] derived from the cytoplasmic C-terminal region of Eschericha coli FeoB protein and Fe(II), Mn(II), and Zn(II) ions were investigated. The Feo system is regarded as the most important bacterial Fe(II) acquisition system, being one of the key virulence factors, especially in anaerobic conditions. Located in the inner membrane of Gram-negative bacteria, FeoB protein transports Fe(II) from the periplasm to the cytoplasm. Despite its crucial role in bacterial pathogenicity, the mechanism in which the metal ion is trafficked through the membrane is not yet elucidated. In the gammaproteobacteria class, the cytoplasmic C-terminal part of FeoB contains conserved cysteine, histidine, and glutamic and aspartic acid residues, which could play a vital role in Fe(II) binding in the cytoplasm, receiving the metal ion from the transmembrane helices. In this work, we characterized the complexes formed between the whole cytosolic C-terminal sequence of E. coli FeoB (P2) and its key polycysteine region (P1) with Fe(II), Mn(II), and Zn(II) ions, exploring the specificity of the C-terminal region of FeoB. With the help of a variety of potentiometric, spectroscopic (electron paramagnetic resonance and NMR), and spectrometric (electrospray ionization mass spectrometry) techniques and molecular dynamics, we propose the metal-binding modes of the ligands, compare their affinities toward the metal ions, and discuss the possible physiological role of the C-terminal region of E. coli FeoB.

The catechol moiety of obafluorin is essential for antibacterial activity

Obafluorin is a Pseudomonas fluorescens antibacterial natural product that inhibits threonyl-tRNA synthetase (ThrRS). It acts as a broad-spectrum antibiotic against a range of clinically relevant pathogens and comprises a strained β-lactone ring decorated with catechol and 4-nitro-benzyl moieties. The catechol moiety is widespread in nature and its role in the coordination of ferric iron has been well-characterised in siderophores and Trojan horse antibiotics. Here we use a combination of mutasynthesis, bioassays, enzyme assays and metal binding studies to delineate the role of the catechol moiety in the bioactivity of obafluorin. We use P. fluorescens biosynthetic mutants to generate obafluorin analogues with modified catechol moieties. We demonstrate that an intact catechol is required for both antibacterial activity and inhibition of the ThrRS molecular target. Although recent work showed that the obafluorin catechol coordinates Zn2+ in the ThrRS active site, we find that obafluorin is a weak Zn2+ binder in vitro, contrasting with a strong, specific 1 : 1 interaction with Fe3+. We use bioassays with siderophore transporter mutants to probe the role of the obafluorin catechol in Fe3+-mediated uptake. Surprisingly, obafluorin does not behave as a Trojan horse antibiotic but instead exhibits increased antibacterial activity in the presence of Fe3+. We further demonstrate that Fe3+ binding prevents the hydrolytic breakdown of the β-lactone ring, revealing a hitherto unreported function for the catechol moiety in natural product bioactivity.

Intermediates and shunt products of massiliachelin biosynthesis in Massilia sp. NR 4-1

Siderophores are small molecules secreted by microorganisms in order to scavenge iron from the environment. An example is the thiazoline-containing natural product massiliachelin, which is produced by Massilia sp. NR 4-1 under iron-deficient conditions. Based on experimental evidence and genome analysis, it was suspected that this bacterium synthesizes further iron-chelating molecules. After a thorough inspection of its metabolic profile, six previously overlooked compounds were isolated that were active in the chrome azurol S (CAS) assay. Mass spectrometric measurements and nuclear magnetic resonance spectroscopic analyses identified these compounds as possible biosynthetic intermediates or shunt products of massiliachelin. Their bioactivity was tested against one Gram-positive and three Gram-negative bacteria.

[M8L4]8+-Type Squares Self-Assembled by Dipalladium Corners and Bridging Aromatic Dipyrazole Ligands for Iodine Capture

Square-like metallamacrocyclic palladium(II) complexes [M₈L₄]⁸⁺ (1-7) were synthesized by reacting aromatic dipyrazole ligands (H₂L¹-H₂L³ with pyromellitic arylimide-, 1,4,5,8-naphthalenetetracarboxylic arylimide-, and anthracene-based aromatic groups, respectively) with dipalladium corners ([(bpy)₂Pd₂(NO₃)₂](NO₃)₂, [(dmbpy)₂Pd₂(NO₃)₂](NO₃)₂, or [(phen)₂Pd₂(NO₃)₂](NO₃)₂, where bpy = 2,2'-bipyridine, dmbpy = 4,4'-dimethyl-2,2'-bipyridine, and phen = 1,10-phenanthroline) in aqueous solutions via metal-directed self-assembly. Metallamacrocycles 1-7 were fully characterized by ¹H and ¹³C nuclear magnetic resonance spectroscopy and electrospray ionization mass spectrometry, and the square structure of 7·8NO₃^- was further confirmed via single crystal X-ray diffraction. These square-like metallamacrocycles exhibit effective performance for iodine adsorption.

Screening microbial inoculants and their interventions for cross-kingdom management of wilt disease of solanaceous crops- a step toward sustainable agriculture

Microbial inoculants may be called magical bullets because they are small in size but have a huge impact on plant life and humans. The screening of these beneficial microbes will give us an evergreen technology to manage harmful diseases of cross-kingdom crops. The production of these crops is reducing as a result of multiple biotic factors and among them the bacterial wilt disease triggered by Ralstonia solanacearum is the most important in solanaceous crops. The examination of the diversity of bioinoculants has shown that more microbial species have biocontrol activity against soil-borne pathogens. Reduced crop output, lower yields, and greater cost of cultivation are among the major issues caused by diseases in agriculture around the world. It is universally true that soil-borne disease epidemics pose a greater threat to crops. These necessitate the use of eco-friendly microbial bioinoculants. This review article provides an overview of plant growth-promoting microorganisms bioinoculants, their various characteristics, biochemical and molecular screening insights, and modes of action and interaction. The discussion is concluded with a brief overview of potential future possibilities for the sustainable development of agriculture. This review will be useful for students and researchers to obtain existing knowledge of microbial inoculants, their activities, and their mechanisms, which will facilitate the development of environmentally friendly management strategies for cross-kingdom plant diseases.

Siderophores: Chemical Tools for Precise Antibiotic Delivery

The success of precision medicine coupled with the disappointing impact of broad-spectrum antibiotic use on microbiome stability and bacterial resistance, has triggered a shift in antibiotic design strategies toward precision antibiotics. This also includes the implementation of novel vectorization approaches directed to improve the internalization of antibacterial agents into deadly gram-negative pathogens through precise and well-defined mechanisms. The conjugation of antibiotics to siderophores (iron scavengers), which are compounds that are able to afford stable iron-complexes that facilitate the internalization into the cell by using bacterial iron uptake pathways as gateways, is a strategy that has begun to show excellent results with the commercialization of the first antibiotic based on this principle, cefiderocol. This digests review provides an overview of the molecular basis for this antibiotic-siderophore conjugation approach, along with recent successful examples and highlights future challenges facing this booming research area.

High spin Fe(III)-doped nanostructures as T1 MR imaging probes

Magnetic Resonance Imaging (MRI) T₁ contrast agents based on Fe(III) as an alternative to Gd-based compounds have been under intense scrutiny in the last 6-8 years and a number of nanostructures have been designed and proposed for in vivo diagnostic and theranostic applications. Excluding the large family of superparamagnetic iron oxides widely used as T₂ -MR imaging agents that will not be covered by this review, a considerable number and type of nanoparticles (NPs) have been employed, ranging from amphiphilic polymer-based NPs, NPs containing polyphenolic binding units such as melanin-like or polycatechols, mixed metals such as Fe/Gd or Fe/Au NPs and perfluorocarbon nanoemulsions. Iron(III) exhibits several favorable magnetic properties, high biocompatibility and improved toxicity profile that place it as the paramagnetic ion of choice for the next generation of nanosized MRI and theranostic contrast agents. An analysis of the examples reported in the last decade will show the opportunities for relaxivity and MR-contrast enhancement optimization that could bring Fe(III)-doped NPs to really compete with Gd(III)-based nanosystems. This article is categorized under: Diagnostic Tools > In Vivo Nanodiagnostics and Imaging Diagnostic Tools > Diagnostic Nanodevices Therapeutic Approaches and Drug Discovery > Nanomedicine for Oncologic Disease.

Synthesis of temporin L hydroxamate-based peptides and evaluation of their coordination properties with iron(III )

Ferric iron is an essential nutrient for bacterial growth. Pathogenic bacteria synthesize iron-chelating entities known as siderophores to sequestrate ferric iron from host organisms in order to colonize and replicate. The development of antimicrobial peptides (AMPs) conjugated to iron chelators represents a promising strategy for reducing the iron availability, inducing bacterial death, and enhancing simultaneously the efficacy of AMPs. Here we designed, synthesized, and characterized three hydroxamate-based peptides Pep-cyc1, Pep-cyc2, and Pep-cyc3, derived from a cyclic temporin L peptide (Pep-cyc) developed previously by some of us. The Fe³⁺ complex formation of each ligand was characterized by UV-visible spectroscopy, mass spectrometry, and IR and NMR spectroscopies. In addition, the effect of Fe³⁺ on the stabilization of the α-helix conformation of hydroxamate-based peptides and the cotton effect were examined by CD spectroscopy. Moreover, the antimicrobial results obtained in vitro on some Gram-negative strains (K. pneumoniae and E. coli) showed the ability of each peptide to chelate efficaciously Fe³⁺ obtaining a reduction of MIC values in comparison to their parent peptide Pep-cyc. Our results demonstrated that siderophore conjugation could increase the efficacy and selectivity of AMPs used for the treatment of infectious diseases caused by Gram-negative pathogens.

Ligand-Promoted Surface Solubilization of TiO2 Nanoparticles by the Enterobactin Siderophore in Biological Medium

Titanium dioxide nanoparticles (TiO2-NPs) are increasingly used in consumer products for their particular properties. Even though TiO2 is considered chemically stable and insoluble, studying their behavior in biological environments is of great importance to figure their potential dissolution and transformation. The interaction between TiO2-NPs with different sizes and crystallographic forms (anatase and rutile) and the strong chelating enterobactin (ent) siderophore was investigated to look at a possible dissolution. For the first time, direct evidence of anatase TiO2-NP surface dissolution or solubilization (i.e., the removal of Ti atoms located at the surface) in a biological medium by this siderophore was shown and the progressive formation of a hexacoordinated titanium-enterobactin (Ti-ent) complex observed. This complex was characterized by UV-visible and Fourier transform infrared (FTIR) spectroscopy (both supported by Density Functional Theory calculations) as well as electrospray ionization mass spectrometry (ESI-MS) and X-ray photoelectron spectroscopy (XPS). A maximum of ca. 6.3% of Ti surface atoms were found to be solubilized after 24 h of incubation, releasing Ti-ent complexes in the micromolar range that could then be taken up by bacteria in an iron-depleted medium. From a health and environmental point of view, the effects associated to the solubilization of the E171 TiO2 food additive in the presence of enterobactin and the entrance of the Ti-enterobactin complex in bacteria were questioned.

Investigation of the immune escape mechanism of Treponema pallidum

BACKGROUND

Syphilis is a chronic sexually transmitted disease caused by Treponema pallidum subspecies pallidum (T. pallidum), which is a public health problem that seriously affects human health worldwide. T. pallidum is characterized by early transmission and immune escape and is therefore termed an "invisible pathogen".

METHODS

This review systematically summarizes the host's innate and adaptive immune responses to T. pallidum infection as well as the escape mechanisms of T. pallidum.

PURPOSE

To lay the foundation for assessing the pathogenic mechanism and the systematic prevention and treatment of syphilis.

CONCLUSION

The immune escape mechanism of T. pallidum plays an important role in its survival. Exploring the occurrence and development of these mechanisms has laid the foundation for the development of syphilis vaccine.

Achieving Selectivity for Phosphate over Pyrophosphate in Ethanol with Iron(III)-Based Fluorescent Probes

Two iron(III)-based molecular receptors employing 1,2-hydroxypyridinone ligands were developed for phosphate recognition and fluorescence sensing via indicator displacement assay (IDA). The tetra- and pentadentate ligands enable anion recognition by the iron(III) center via its remaining one or two open coordination sites. Weak protective coordination of fluorescein at those sites prevents the formation of μ-oxo dimers in aerated solutions. Its rapid and selective displacement by inorganic phosphate results in a 20-fold increase in the fluorescence of the indicator. Both receptors exhibit high affinity for inorganic phosphate and high selectivity over common competing anions, including halides, acetate, carbonate, and, remarkably, pyrophosphate as well as arsenate. Coordination of phosphate to the iron(III) center was confirmed by ATR-IR and ³¹P NMR spectroscopy.

Degradation of mirubactin to multiple siderophores with varying Fe(III) chelation properties

Three siderophores mirubactins B-D (4-6) were identified as the degradation products of previously isolated mirubactin (1). Their structures were revealed by HR-ESI-MS/MS, NMR analyses, and density functional calculations, among which 4 contains an unusual cyclic amidine functionality. Cyclic voltammetry showed that 5 and 6 have reduced iron complexing capacity. Mirubactin (1) and Fe(III) could also form a stable complex, which may be an ingenious approach to compete for iron acquisition by the producing organisms.

Harnessing rhizobacteria to fulfil inter-linked nutrient dependency on soil and alleviate stresses in plants

Plant rhizo-microbiome comprises of complex microbial communities that colonizes at the interphase of plant roots and soil. Plant-growth-promoting rhizobacteria (PGPR) in the rhizosphere provides important ecosystem services ranging from release of essential nutrients for enhancing soil quality and improving plant health to imparting protection to plants against rising biotic and abiotic stresses. Hence, PGPR serve as restoring agents to rejuvenate soil health and mediate plant fitness in the facet of changing climate. Though, it is evident that nutrients availability in soil are managed through inter-linked mechanisms, how PGPR expediate these processes remain less recognized. Promising results of PGPR inoculation on plant growth are continually reported in controlled environmental conditions, however, their field application often fails due to competition with native microbiota and low colonization efficiency in roots. The development of highly efficient and smart bacterial synthetic communities by integrating bacterial ecological and genetic features provides better opportunities for successful inoculant formulations. This review provides an overview of the inter-play between nutrient availability and disease suppression governed by rhizobacteria in soil followed by the role of synthetic bacterial communities in developing efficient microbial inoculants. Moreover, an outlook on the beneficial activities of rhizobacteria in modifying soil characteristics to sustainably boost agroecosystem functioning is also provided.

Amamistatins isolated from Nocardia altamirensis

Four new phenolic siderophores were isolated from the actinomycete Nocardia altamirensis along with the known natural product amamistatin B and a putative biosynthetic shunt product. The structures of all compounds were elucidated through 1D and 2D NMR analyses as well as mass spectrometry. The iron-chelating properties of the retrieved metabolites were evaluated in a chrome azurol S assay.

Advances in the Synthesis of Enterobactin, Artificial Analogues, and Enterobactin-Derived Antimicrobial Drug Conjugates and Imaging Tools for Infection Diagnosis

Iron is an essential growth factor for bacteria, but although highly abundant in nature, its bioavailability during infection in the human host or the environment is limited. Therefore, bacteria produce and secrete siderophores to ensure their supply of iron. The triscatecholate siderophore enterobactin and its glycosylated derivatives, the salmochelins, play a crucial role for iron acquisition in several bacteria. As these compounds can serve as carrier molecules for the design of antimicrobial siderophore drug conjugates as well as siderophore-derived tool compounds for the detection of infections with bacteria, their synthesis and the design of artificial analogues is of interest. In this review, we give an overview on the synthesis of enterobactin, biomimetic as well as totally artificial analogues, and related drug-conjugates covering up to 12/2021.

1 Introduction

2 Antibiotic Crisis and Sideromycins as Natural Templates for New Antimicrobial Drugs

3 Biosynthesis of Enterobactin, Salmochelins, and Microcins

4 Total Synthesis of Enterobactin and Salmochelins

5 Chemoenzymatic Semi-synthesis of Salmochelins and Microcin E492m Derivatives

6 Synthesis of Biomimetic Enterobactin Derivatives with Natural Tris-lactone Backbone

7 Synthesis of Artificial Enterobactin Derivatives without Tris-lactone Backbone

8 Conclusions

Acetyl-CoA-Mediated Post-Biosynthetic Modification of Desferrioxamine B Generates N- and N-O-Acetylated Isomers Controlled by a pH Switch

Biosynthesis of the hydroxamic acid siderophore desferrioxamine D₁ (DFOD₁, 6), which is the N-acetylated analogue of desferrioxamine B (DFOB, 5), has been delineated. Enzyme-independent Ac-CoA-mediated N-acetylation of 5 produced 6, in addition to three constitutional isomers containing an N-O-acetyl group installed at either one of the three hydroxamic acid groups of 5. The formation of N-Ac-DFOB (DFOD₁, 6) and the composite of N-O-acetylated isomers N-O-Ac-DFOB[001] (6a), N-O-Ac-DFOB[010] (6b), and N-O-Ac-DFOB[100] (6c) (defined as the N-O-Ac motif positioned within the terminal amine, internal, or N-acetylated region of 5, respectively), was pH-dependent, with 6a-6c dominant at pH < 8.5 and 6 dominant at pH > 8.5. The trend in the pH dependence was consistent with the pK_a values of the NH₃⁺ (pK_a ∼ 10) and N-OH (pK_a ∼ 8.5-9) groups in 5. The N- and N-O-acetyl motifs can be conceived as a post-biosynthetic modification (PBM) of a nonproteinaceous secondary metabolite, akin to a post-translational modification (PTM) of a protein. The pH-labile N-O-acetyl group could act as a reversible switch to modulate the properties and functions of secondary metabolites, including hydroxamic acid siderophores. An alternative (most likely minor) biosynthetic pathway for 6 showed that the nonribosomal peptide synthetase-independent siderophore synthetase DesD was competent in condensing N'-acetyl-N-succinyl-N-hydroxy-1,5-diaminopentane (N'-Ac-SHDP, 7) with the dimeric hydroxamic acid precursor (AHDP-SHDP, 4) native to 5 biosynthesis to generate 6. The strategy of diversifying protein structure and function using PTMs could be paralleled in secondary metabolites with the use of PBMs.

Transcription regulation of iron carrier transport genes by ECF sigma factors through signaling from the cell surface into the cytoplasm

Bacteria are usually iron-deficient because the Fe³⁺ in their environment is insoluble or is incorporated into proteins. To overcome their natural iron limitation, bacteria have developed sophisticated iron transport and regulation systems. In gram-negative bacteria, these include iron carriers, such as citrate, siderophores, and heme, which when loaded with Fe³⁺ adsorb with high specificity and affinity to outer membrane proteins. Binding of the iron carriers to the cell surface elicits a signal that initiates transcription of iron carrier transport and synthesis genes, referred to as “cell surface signaling”. Transcriptional regulation is not coupled to transport. Outer membrane proteins with signaling functions contain an additional N-terminal domain that in the periplasm makes contact with an anti-sigma factor regulatory protein that extends from the outer membrane into the cytoplasm. Binding of the iron carriers to the outer membrane receptors elicits proteolysis of the anti-sigma factor by two different proteases, Prc in the periplasm, and RseP in the cytoplasmic membrane, inactivates the anti-sigma function or results in the generation of an N-terminal peptide of ∼50 residues with pro-sigma activity yielding an active extracytoplasmic function (ECF) sigma factor. Signal recognition and signal transmission into the cytoplasm is discussed herein.

Native mass spectrometry-based metabolomics identifies metal-binding compounds

Although metals are essential for the molecular machineries of life, systematic methods for discovering metal-small molecule complexes from biological samples are limited. Here, we describe a two-step native electrospray ionization-mass spectrometry method, in which post-column pH adjustment and metal infusion are combined with ion identity molecular networking, a rule-based data analysis workflow. This method enabled the identification of metal-binding compounds in complex samples based on defined mass (m/z) offsets of ion species with the same chromatographic profiles. As this native electrospray metabolomics approach is suited to the use of any liquid chromatography-mass spectrometry system to explore the binding of any metal, this method has the potential to become an essential strategy for elucidating metal-binding molecules in biology.

Toxicity of the iron siderophore mycobactin J in mouse macrophages: Evidence for a hypoxia response

Mycobacterium tuberculosis, the causative agent of tuberculosis, is an obligate intracellular pathogen that lives within the phagosome of macrophages. Here we demonstrate that the siderophore mycobactin J, produced by the closely related intracellular pathogen Mycobacterium paratuberculosis, is toxic to murine macrophage cells. Its median lethal dose, 10 μM, is lower than that of the iron chelators desferrioxamine B and TrenCAM, an enterobactin analog. To determine the source of this toxicity, we conducted microarray, ELISA, and metabolite profiling experiments. The primary response is hypoxia-like, which implies iron starvation as the underlying cause of the toxicity. This observation is consistent with our recent finding that mycobactin J is a stronger iron chelator than had been inferred from previous studies. Mycobactin J is known to partition into cell membranes and hydrophobic organelles indicating that enhanced membrane penetration is also a likely factor. Thus, mycobactin J is shown to be toxic, eliciting a hypoxia-like response under physiological conditions.

Hydroxypyridinones as a Very Promising Platform for Targeted Diagnostic and Therapeutic Radiopharmaceuticals

Hydroxypyridinones (HOPOs) have been used in the chelation therapy of iron and actinide metals. Their application in metal-based radiopharmaceuticals has also been increasing in recent years. This review article focuses on how multidentate HOPOs can be used in targeted radiometal-based diagnostic and therapeutic radiopharmaceuticals. The general structure of radiometal-based targeted radiopharmaceuticals, a brief description of siderophores, the basic structure and properties of bidentate HOPO, some representative HOPO multidentate chelating agents, radiopharmaceuticals based on HOPO multidentate bifunctional chelators for gallium-68, thorium-227 and zirconium-89, as well as the future prospects of HOPO multidentate bifunctional chelators in other metal-based radiopharmaceuticals are described and discussed in turn. The HOPO metal-based radiopharmaceuticals that have shown good prospects in clinical and preclinical studies are gallium-68, thorium-227 and zirconium-89 radiopharmaceuticals. We expect HOPO multidentate bifunctional chelators to be a very promising platform for building novel targeted radiometal-based diagnostic and therapeutic radiopharmaceuticals.

Hydroxypyridinones as a Very Promising Platform for Targeted Diagnostic and Therapeutic Radiopharmaceuticals

Hydroxypyridinones (HOPOs) have been used in the chelation therapy of iron and actinide metals. Their application in metal-based radiopharmaceuticals has also been increasing in recent years. This review article focuses on how multidentate HOPOs can be used in targeted radiometal-based diagnostic and therapeutic radiopharmaceuticals. The general structure of radiometal-based targeted radiopharmaceuticals, a brief description of siderophores, the basic structure and properties of bidentate HOPO, some representative HOPO multidentate chelating agents, radiopharmaceuticals based on HOPO multidentate bifunctional chelators for gallium-68, thorium-227 and zirconium-89, as well as the future prospects of HOPO multidentate bifunctional chelators in other metal-based radiopharmaceuticals are described and discussed in turn. The HOPO metal-based radiopharmaceuticals that have shown good prospects in clinical and preclinical studies are gallium-68, thorium-227 and zirconium-89 radiopharmaceuticals. We expect HOPO multidentate bifunctional chelators to be a very promising platform for building novel targeted radiometal-based diagnostic and therapeutic radiopharmaceuticals.

Hydroxypyridinones as a Very Promising Platform for Targeted Diagnostic and Therapeutic Radiopharmaceuticals

Hydroxypyridinones (HOPOs) have been used in the chelation therapy of iron and actinide metals. Their application in metal-based radiopharmaceuticals has also been increasing in recent years. This review article focuses on how multidentate HOPOs can be used in targeted radiometal-based diagnostic and therapeutic radiopharmaceuticals. The general structure of radiometal-based targeted radiopharmaceuticals, a brief description of siderophores, the basic structure and properties of bidentate HOPO, some representative HOPO multidentate chelating agents, radiopharmaceuticals based on HOPO multidentate bifunctional chelators for gallium-68, thorium-227 and zirconium-89, as well as the future prospects of HOPO multidentate bifunctional chelators in other metal-based radiopharmaceuticals are described and discussed in turn. The HOPO metal-based radiopharmaceuticals that have shown good prospects in clinical and preclinical studies are gallium-68, thorium-227 and zirconium-89 radiopharmaceuticals. We expect HOPO multidentate bifunctional chelators to be a very promising platform for building novel targeted radiometal-based diagnostic and therapeutic radiopharmaceuticals.

Antimicrobial Activity of Metals and Metalloids

Competition shapes evolution. Toxic metals and metalloids have exerted selective pressure on life since the rise of the first organisms on the Earth, which has led to the evolution and acquisition of resistance mechanisms against them, as well as mechanisms to weaponize them. Microorganisms exploit antimicrobial metals and metalloids to gain competitive advantage over other members of microbial communities. This exerts a strong selective pressure that drives evolution of resistance. This review describes, with a focus on arsenic and copper, how microorganisms exploit metals and metalloids for predation and how metal- and metalloid-dependent predation may have been a driving force for evolution of microbial resistance against metals and metalloids. Expected final online publication date for the Annual Review of Microbiology, Volume 75 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Coordination Complexes to Combat Bacterial Infections: Recent Developments, Current Directions and Future Opportunities

Drug discovery aimed at the efficient eradication of life-threatening bacterial infections, especially in light of the emergence of multi-drug resistance of pathogenic bacteria, has remained a challenge for medicinal chemists over the past several decades. As nutrient acquisition and metabolism at the host-pathogen interface become better elucidated, new drug targets continue to emerge. Metal homeostasis is among these processes, and thus provides opportunities for medicinal inorganic chemists to alter or disrupt these processes selectively to impart bacteriostatic or bacteriotoxic effects. In this minireview, we showcase some of the recent work from the field of metal-based antibacterial agents and highlight divergent strategies and mechanisms of action.

Features of iron accumulation at high concentration in pulcherrimin-producing Metschnikowia yeast biomass

In previous studies it was found that the antimicrobial properties of pulcherrimin-producing Metschnikowia species are related to the formation of a red pigment-pulcherrimin and sequestration of free iron from their growth medium. For strains of Metschnikowia pulcherrima, M. sinensis, M. shaxiensis, and M. fructicola, at a high, ≈80 mg/kg, elemental Fe concentration in agar growth media we observed the essentially different (metal luster, non-glossy rust like, and colored) yeast biomass coatings. For the studied strains the optical and scanning electron microscopies showed the increased formation of chlamydospores that accumulate a red pigment-insoluble pulcherrimin rich in iron. The chlamydospore formation and decay depended on the iron concentration. In this study pulcherrimin in biomass of the selected Metschnikowia strains was detected by Mössbauer spectroscopy. At ≈80 mg/kg elemental Fe concentration the Mössbauer spectra of biomass of the studied strains were almost identical to these of purified pulcherrimin. Iron in pulcherrimin reached ≈1% of biomass by weight which is very high in comparison with elemental Fe percentage in growth medium and is not necessary for yeast growth. The pulcherrimin in biomass was also observed by Mössbauer spectroscopy at lower, ≈5 mg/kg, elemental Fe concentration. Through chemical binding of iron pulcherrimin sequestrates the soluble Fe in the growth media. However, at high Fe concentrations, the chemical and biochemical processes lead to the pulcherrimin accumulation in biomass chlamydospores. When soluble iron is sequestrated or removed from the growth media in this way, it becomes inaccessible for other microorganisms.

A perspective essay on the use of Ga3+ as a proxy for Fe3+ in bioinorganic model studies and its successful use for therapeutic purposes

The use of Ga³⁺ as a structural mimic for Fe³⁺ in model bioinorganic investigations is usually based on a common assumption that Ga³⁺ and Fe³⁺ should form bioligand complexes of similar stabilities due to their similar charge/radius ratio (z/r). However, the literature survey presented here is contrary to this notion, showing that under laboratory conditions often Ga³⁺ forms weaker bioligand complexes than Fe³⁺in aqueous medium. We hypothesize that this is because Ga³⁺ is more aquaphilic than Fe³⁺ as suggested by their relative heats of hydration (ΔH_hyd). The successful use of Ga³⁺ as a therapeutic agent is also briefly reviewed, showing this success often stems from the redox inertness as well as different pharmacokinetics of Ga³⁺ than Fe³⁺, but similar metabolic pathways as Fe³⁺ in human serum.