Genomics‐Driven Discovery of NO‐Donating Diazeniumdiolate Siderophores in Diverse Plant‐Associated Bacteria

Siderophores are key players in bacteria–host interactions, with the main function to provide soluble iron for their producers. Gramibactin from rhizosphere bacteria expands siderophore function and diversity as it delivers iron to the host plant and features an unusual diazeniumdiolate moiety for iron chelation. By mutational analysis of the grb gene cluster, we identified genes (grbD and grbE) necessary for diazeniumdiolate formation. Genome mining using a GrbD‐based network revealed a broad range of orthologous gene clusters in mainly plant‐associated Burkholderia/Paraburkholderia species. Two new types of diazeniumdiolate siderophores, megapolibactins and plantaribactin were fully characterized. In vitro assays and in vivo monitoring experiments revealed that the iron chelators also liberate nitric oxide (NO) in plant roots. This finding is important since NO donors are considered as biofertilizers that maintain iron homeostasis and increase overall plant fitness.     

Iron‐Catalysed Radical Polymerisation by Living Bacteria

The ability to harness cellular redox processes for abiotic synthesis might allow the preparation of engineered hybrid living systems. Towards this goal we describe a new bacteria‐mediated iron‐catalysed reversible deactivation radical polymerisation (RDRP), with a range of metal‐chelating agents and monomers that can be used under ambient conditions with a bacterial redox initiation step to generate polymers. Cupriavidus metallidurans, Escherichia coli, and Clostridium sporogenes species were chosen for their redox enzyme systems and evaluated for their ability to induce polymer formation. Parameters including cell and catalyst concentration, initiator species, and monomer type were investigated. Water‐soluble synthetic polymers were produced in the presence of the bacteria with full preservation of cell viability. This method provides a means by which bacterial redox systems can be exploited to generate “unnatural” polymers in the presence of “host” cells, thus setting up the possibility of making natural–synthetic hybrid structures and conjugates.     

Zinc Homeostasis at the Bacteria/Host Interface—From Coordination Chemistry to Nutritional Immunity

Zinc is one of the most important metal nutrients for species from all kingdoms, being a key structural or catalytic component of hundreds of enzymes, crucial for the survival of both pathogenic microorganisms and their hosts. This work is an overview of the homeostasis of zinc in bacteria and humans. It explains the importance of this metal nutrient for pathogens, describes the roles of zinc sensors, regulators, and transporters, and summarizes various uptake systems and different proteins involved in zinc homeostasis—both those used for storage, buffering, and signaling inside the cell and those excreted in order to obtain Zn^II from the host. The human zinc‐dependent immune system response is explained, with a special focus given to ‘zinc nutritional immunity′, a process that describes the competition between the bacteria or fungus and the host for this metal, during which both the pathogen and host make huge efforts to control zinc availability. This sophisticated tug of war over Zn^II might be considered as a possible target for novel antibacterial therapies.     

Two‐Dimensional Ketone‐Driven Metal–Organic Coordination on Cu(111)

Two‐dimensional metal–organic nanostructures based on the binding of ketone groups and metal atoms were fabricated by depositing pyrene‐4,5,9,10‐tetraone (PTO) molecules on a Cu(111) surface. The strongly electronegative ketone moieties bind to either copper adatoms from the substrate or codeposited iron atoms. In the former case, scanning tunnelling microscopy images reveal the development of an extended metal–organic supramolecular structure. Each copper adatom coordinates to two ketone ligands of two neighbouring PTO molecules, forming chains that are linked together into large islands through secondary van der Waals interactions. Deposition of iron atoms leads to a transformation of this assembly resulting from the substitution of the metal centres. Density functional theory calculations reveal that the driving force for the metal substitution is primarily determined by the strength of the ketone–metal bond, which is higher for Fe than for Cu. This second class of nanostructures displays a structural dependence on the rate of iron deposition.     

Bacteria-mediated precursor-dependent biosynthesis of superparamagnetic iron oxide and iron sulfide nanoparticles

The bacterium Actinobacter sp. has been shown to be capable of extracellularly synthesizing iron based magnetic nanoparticles, namely maghemite (gamma-Fe2O3) and greigite (Fe3S4) under ambient conditions depending on the nature of precursors used. More precisely, the bacterium synthesized maghemite when reacted with ferric chloride and iron sulfide when exposed to the aqueous solution of ferric chloride-ferrous sulfate. Challenging the bacterium with different metal ions resulted in induction of different proteins, which bring about the specific biochemical transformations in each case leading to the observed products. Maghemite and iron sulfide nanoparticles show superparamagnetic characteristics as expected. Compared to the earlier reports of magnetite and greigite synthesis by magnetotactic bacteria and iron reducing bacteria, which take place strictly under anaerobic conditions, the present procedure offers significant advancement since the reaction occurs under aerobic condition. Moreover, reaction end products can be tuned by the choice of precursors used.     

Covalency‐Driven Preservation of Local Charge Densities in a Metal‐to‐Ligand Charge‐Transfer Excited Iron Photosensitizer

Covalency is found to even out charge separation after photo‐oxidation of the metal center in the metal‐to‐ligand charge‐transfer state of an iron photosensitizer. The σ‐donation ability of the ligands compensates for the loss of iron 3d electronic charge, thereby upholding the initial metal charge density and preserving the local noble‐gas configuration. These findings are enabled through element‐specific and orbital‐selective time‐resolved X‐ray absorption spectroscopy at the iron L‐edge. Thus, valence orbital populations around the central metal are directly accessible. In conjunction with density functional theory we conclude that the picture of a localized charge‐separation is inadequate. However, the unpaired spin density provides a suitable representation of the electron–hole pair associated with the electron‐transfer process.     

Antiviral properties of lactoferrin--a natural immunity molecule

Lactoferrin, a multifunctional iron binding glycoprotein, plays an important role in immune regulation and defence mechanisms against bacteria, fungi and viruses. Lactoferrin's iron withholding ability is related to inhibition of microbial growth as well as to modulation of motility, aggregation and biofilm formation of pathogenic bacteria. Independently of iron binding capability, lactoferrin interacts with microbial, viral and cell surfaces thus inhibiting microbial and viral adhesion and entry into host cells. Lactoferrin can be considered not only a primary defense factor against mucosal infections, but also a polyvalent regulator which interacts in viral infectious processes. Its antiviral activity, demonstrated against both enveloped and naked viruses, lies in the early phase of infection, thus preventing entry of virus in the host cell. This activity is exerted by binding to heparan sulphate glycosaminoglycan cell receptors, or viral particles or both. Despite the antiviral effect of lactoferrin, widely demonstrated in vitro studies, few clinical trials have been carried out and the related mechanism of action is still under debate. The nuclear localization of lactoferrin in different epithelial human cells suggests that lactoferrin exerts its antiviral effect not only in the early phase of surface interaction virus-cell, but also intracellularly. The capability of lactoferrin to exert a potent antiviral activity, through its binding to host cells and/or viral particles, and its nuclear localization strengthens the idea that lactoferrin is an important brick in the mucosal wall, effective against viral attacks and it could be usefully applied as novel strategy for treatment of viral infections.     

Iron/Brønsted Acid Catalyzed Asymmetric Hydrogenation: Mechanism and Selectivity‐Determining Interactions

Hydrogenation catalysts involving abundant base metals such as cobalt or iron are promising alternatives to precious metal systems. Despite rapid progress in this field, base metal catalysts do not yet achieve the activity and selectivity levels of their precious metal counterparts. Rational improvement of base metal complexes is facilitated by detailed knowledge about their mechanisms and selectivity‐determining factors. The mechanism for asymmetric imine hydrogenation with Knölker’s iron complex in the presence of chiral phosphoric acids is here investigated computationally at the DFT‐D level of theory, with models of up to 160 atoms. The resting state of the system is found to be an adduct between the iron complex and the deprotonated acid. Rate‐limiting H₂ splitting is followed by a stepwise hydrogenation mechanism, in which the phosphoric acid acts as the proton donor. C?H ??? O interactions between the phosphoric acid and the substrate are involved in the stereocontrol at the final hydride transfer step. Computed enantiomeric ratios show excellent agreement with experimental values, indicating that DFT‐D is able to correctly capture the selectivity‐determining interactions of this system.     

Redox‐Active M8L6 Cubic Hosts with Tetraphenylethylene Faces Encapsulate Organic Dyes for Light‐Driven H2 Production

The design of artificial systems that mimic highly evolved and finely tuned natural enzymes is a promising subject of intensive research. The assembly of O‐symmetric cubic structures with an Fe₈L₆ formula was reported through the direct combination of a C₄‐symmetric tetraphenylethylene‐based ligand with a C₃‐symmetric tris(bipyridine)iron node. The robust metal–organic cubes are rich in π‐electron density and provide favorable interactions with planar polycyclic aromatic hydrocarbons. Within the confined space of the host, the aromatic hydrocarbons molecules are forced closer to the redox active host, and the photoinduced electron transfer (PET) is modified into a pseudo‐intramolecular pathway. These iron vertices within the cubes exhibit suitable redox potential for electrochemical reduction of protons and the well‐modified PET is further tailored to create artificial systems for light‐driven hydrogen evolution from water through the encapsulation of fluorescein dyes. Control experiments based on a mononuclear compound resembling the iron corner of the octahedron suggest an enzymatic dynamic behavior. The new, well‐elucidated reaction pathways and the increased molarity of the reaction within the confined space render these supramolecular systems superior to other relevant systems.     

Iron‐Catalyzed Cross‐Couplings in the Synthesis of Pharmaceuticals: In Pursuit of Sustainability

The scarcity of precious metals has led to the development of sustainable strategies for metal‐catalyzed cross‐coupling reactions. The establishment of new catalytic methods using iron is attractive owing to the low cost, abundance, ready availability, and very low toxicity of iron. In the last few years, sustainable methods for iron‐catalyzed cross‐couplings have entered the critical area of pharmaceutical research. Most notably, iron is one of the very few metals that have been successfully field‐tested as highly effective base‐metal catalysts in practical, kilogram‐scale industrial cross‐couplings. In this Minireview, we critically discuss the strategic benefits of using iron catalysts as green and sustainable alternatives to precious metals in cross‐coupling applications for the synthesis of pharmaceuticals. The Minireview provides an essential introduction to the fundamental aspects of practical iron catalysis, highlights areas for improvement, and identifies new fields to be explored.     

Visualization and In Situ Ablation of Intracellular Bacterial Pathogens through Metabolic Labeling

Protected by the host cells, the hidden intracellular bacteria are typically difficult to kill by common antibiotics and cannot be visualized without complex cellular pretreatments. Herein, we successfully developed a bacteria‐metabolizable dual‐functional probe TPEPy‐d ‐Ala, which is based on d ‐alanine and a photosensitizer with aggregation‐induced emission for fluorescence turn‐on imaging of intracellular bacteria in living host cells and photodynamic ablation in situ. Once metabolically incorporated into bacterial peptidoglycan, the intramolecular motions of TPEPy‐d ‐Ala are inhibited, leading to an enhanced fluorescent signal, which allows the clear visualization of the intracellular bacteria. Moreover, TPEPy‐d ‐Ala can effectively ablate the labeled intracellular bacteria in situ owing to covalent ligation to peptidoglycan, yielding a low intracellular minimum inhibitory concentration (MIC) of 20±0.5 μg mL^?1, much more efficient than that of a commonly used antibiotic, vancomycin.     

Heterogeneous iron-containing catalysts for the reaction of CCl<Subscript>4</Subscript> addition to a multiple bond

The catalytic properties of iron complexes with monoethanolamine chemically immobilized on the surface of silica were studied in the reaction of CCl₄ addition to 1-octene. Unlike the previously studied complexes of other transition metals, the activity of the iron complexes increased with increasing metal content. The analysis of the catalyst composition and reaction products demonstrated that the reaction occurred by a coordination mechanism at low metal concentrations. At high concentrations, the metal mainly occurred in the form of an oxychloride and the catalytic process occurred nonselectively, probably, by an ionic mechanism. The addition occurred in an analogous manner in the presence of iron chloride supported onto unmodified silica gel.     

The Coordination Chemistry and Organometallic Chemistry of Tridentate Oxygen Ligands with π-Donor Properties

Metal complexes are capable of accomplishing almost anything, provided they contain the proper metal/ligand combinations. A host of essential biochemical transformations—but also a great many industrially significant reactions—occur within the coordination spheres of metal ions. For instance, the particular arrangement of ligands in the zinc-containing enzyme carboanhydrase is responsible for an acceleration in the hydration of CO₂ by a factor of 10⁹. It is the ligands that determine whether an iron atom will transfer molecular oxygen, as in the case of hemoglobin, or electrons, as with the cytochromes. By varying the ligands it is possible to establish in advance whether a metal ion in the presence of synthesis gas will cause an olefin to be hydrogenated or hydroformylated. Stated more generally, it is the ligands that stabilize the particular oxidation states of a metal and determine how substrate molecules will be coordinated and undergo reaction. The synthesis of new ligands that confer specific reactivity on metal ions is thus an important challenge for the coordination chemist. The following article describes organometallic compounds of the type [CpCo{P(O)R′R″}₃]^?, which have developed from an extremely unreactive laboratory curiosity into versatile oxygen-containing ligands whose steric and electronic properties promise a series of interesting applications.     

Near‐Infrared‐Light‐Triggered Antimicrobial Indium Phosphide Quantum Dots

The emergence of multidrug‐resistant (MDR) pathogens represents one of the most urgent global public health crises. Light‐activated quantum dots (QDs) are alternative antimicrobials, with efficient transport, low cost, and therapeutic efficacy, and they can act as antibiotic potentiators, with a mechanism of action orthogonal to small‐molecule drugs. Furthermore, light‐activation enhances control over the spatiotemporal release and dose of the therapeutic superoxide radicals from QDs. However, the limited deep‐tissue penetration of visible light needed for QD activation, and concern over trace heavy metals, have prevented further translation. Herein, we report two indium phosphide (InP) QDs that operate in the near‐infrared and deep‐red light window, enabling deeper tissue penetration. These heavy‐metal‐free QDs eliminate MDR pathogenic bacteria, while remaining non‐toxic to host human cells. This work provides a pathway for advancing QD nanotherapeutics to combat MDR superbugs.     

Elucidation of Gram-Positive Bacterial Iron(III) Reduction for Kaolinite Clay Refinement

Recently, microbial-based iron reduction has been considered as a viable alternative to typical chemical-based treatments. The iron reduction is an important process in kaolin refining, where iron-bearing impurities in kaolin clay affects the whiteness, refractory properties, and its commercial value. In recent years, Gram-negative bacteria has been in the center stage of iron reduction research, whereas little is known about the potential use of Gram-positive bacteria to refine kaolin clay. In this study, we investigated the ferric reducing capabilities of five microbes by manipulating the microbial growth conditions. Out of the five, we discovered that Bacillus cereus and Staphylococcus aureus outperformed the other microbes under nitrogen-rich media. Through the biochemical changes and the microbial behavior, we mapped the hypothetical pathway leading to the iron reduction cellular properties, and found that the iron reduction properties of these Gram-positive bacteria rely heavily on the media composition. The media composition results in increased basification of the media that is a prerequisite for the cellular reduction of ferric ions. Further, these changes impact the formation of biofilm, suggesting that the cellular interaction for the iron(III)oxide reduction is not solely reliant on the formation of biofilms. This article reveals the potential development of Gram-positive microbes in facilitating the microbial-based removal of metal contaminants from clays or ores. Further studies to elucidate the corresponding pathways would be crucial for the further development of the field.     

Reversible Unfolding–Refolding of Rubredoxin: A Single‐Molecule Force Spectroscopy Study

In metalloproteins, metal centers serve as active sites for a range of functional purposes and as important structural elements to facilitate protein folding and assembly. It is challenging to observe the reversible unfolding and refolding of metalloproteins because of a loss or decomposition of the metal center. Here, the reversible unfolding–refolding of the iron–sulfur protein rubredoxin was observed directly using single‐molecule force spectroscopy. The results demonstrate that the iron can remain attached to the CXXC motif when rubredoxin is unfolded. Upon relaxation, the unfolded rubredoxin can refold into its native holo state with the reconstituted FeS₄ center. The possible loss of iron from the unfolded protein prevents rubredoxin from refolding into its native holo state. These results demonstrated that unfolding of rubredoxin is reversible, as long as the iron remains attached, and provide experimental evidence for the iron‐priming mechanism for the folding of rubredoxin.     

Iron chelator induces MIP-alpha/CCL20 in human intestinal epithelial cells: implication for triggering mucosal adaptive immunity

A previous report by this laboratory demonstrated that bacterial iron chelator (siderophore) triggers inflammatory signals, including the production of CXC chemokine IL-8, in human intestinal epithelial cells (IECs). Microarray-based gene expression profiling revealed that iron chelator also induces macrophage inflammatory protein 3 alpha (MIP-3alpha)/CC chemokine-ligand 20 (CCL20). As CCL20 is chemotactic for the cells involved in host adaptive immunity, this suggests that iron chelator may stimulate IECs to have the capacity to link mucosal innate and adaptive immunity. The basal medium from iron chelator deferoxamine (DFO)-treated HT-29 monolayers was as chemotactic as recombinant human CCL20 at equivalent concentrations to attract CCR6(+) cells. The increase of CCL20 protein secretion appeared to correspond to that of CCL20 mRNA levels, as determined by real-time quantitative RT-PCR. The efficacy of DFO at inducing CCL20 mRNA was also observed in human PBMCs and in THP-1 cells, but not in human umbilical vein endothelial cells. Interestingly, unlike other proinflammatory cytokines, such as TNF-alpha and IL-1beta, a time-dependent experiment revealed that DFO slowly induces CCL20, suggesting a novel mechanism of action. A pharmacologic study also revealed that multiple signaling pathways are differentially involved in CCL20 production by DFO, while some of those pathways are not involved in TNF-alpha-induced CCL20 production. Collectively, these results demonstrate that, in addition to some bacterial products known to induce host adaptive immune responses, direct chelation of host iron by infected bacteria may also contribute to the initiation of host adaptive immunity in the intestinal mucosa.     

Visible‐Light‐Driven Water Oxidation with a Polyoxometalate‐Complexed Hematite Core of 275 Iron Atoms

Although metal oxide nanocrystals are often highly active, rapid aggregation (particularly in water) generally precludes detailed solution‐state investigations of their catalytic reactions. This is equally true for visible‐light‐driven water oxidation with hematite α‐Fe₂O₃ nanocrystals, which bridge a conceptual divide between molecular complexes of iron and solid‐state hematite photoanodes. We herein report that the aqueous solubility and remarkable stability of polyoxometalate (POM)‐complexed hematite cores with 275 iron atoms enable investigations of visible‐light‐driven water oxidation at this frontier using the versatile toolbox of solution‐state methods typically reserved for molecular catalysis. The use of these methods revealed a unique mechanism, understood as a general consequence of fundamental differences between reactions of solid‐state metal oxides and freely diffusing “fragments” of the same material.     

Development of an <Emphasis Type="Italic">Agrobacterium</Emphasis>-Mediated Transformation Method and Evaluation of Two Exogenous Constitutive Promoters in Oleaginous Yeast <Emphasis Type="Italic">Lipomyces starkeyi</Emphasis>

The biogenic iron oxide/hydroxide materials possess useful combination of physicochemical properties and are considered for application in various areas. Their production does not require special investments because these compounds are formed during cultivation of neurophilic iron bacteria. Bacteria from genus Leptothrix develop intensively in the Sphaerotilus-Leptothrix group of bacteria isolation medium and feeding media of Fedorov and Lieske. These media are different in their composition which determined the present study as an attempt to clear up the reasons that define the differences in the composition of the laboratory-obtained biomasses and the natural biomass finds. FTIRS, Mössbauer spectroscopy, and XRD were used in the research. Comparative analysis showed that the biomass and control samples contain iron compounds (α-FeOOH, γ-FeOOH, β-FeOOH, γ-Fe₂O₃) in different ratios. The biomass samples were enriched in oxyhydroxides of higher dispersion. Organic residuals of bacterial origin, SO₄, CO₃, and PO₄ groups were registered in the biogenic materials.     

Single‐Atom Iron Catalysts on Overhang‐Eave Carbon Cages for High‐Performance Oxygen Reduction Reaction

Single‐atom catalysts have drawn great attention, especially in electrocatalysis. However, most of previous works focus on the enhanced catalytic properties via improving metal loading. Engineering morphologies of catalysts to facilitate mass transport through catalyst layers, thus increasing the utilization of each active site, is regarded as an appealing way for enhanced performance. Herein, we design an overhang‐eave structure decorated with isolated single‐atom iron sites via a silica‐mediated MOF‐templated approach for oxygen reduction reaction (ORR) catalysis. This catalyst demonstrates superior ORR performance in both alkaline and acidic electrolytes, comparable to the state‐of‐the‐art Pt/C catalyst and superior to most precious‐metal‐free catalysts reported to date. This activity originates from its edge‐rich structure, having more three‐phase boundaries with enhanced mass transport of reactants to accessible single‐atom iron sites (increasing the utilization of active sites), which verifies the practicability of such a synthetic approach.