Divalent Transition Metal Ions Research Articles

Self-Healing and Moldable Poly(2-isopropenyl-2-oxazoline) Supramolecular Hydrogels Based on a Transient Metal Coordination Network

Materials that are capable of autonomous healing upon damage is the focus of many research groups because of the numerous potential applications of these materials, such as coatings, actuators for robotic applications, and wound healing. Although significant progress in the area has been achieved, the synthesis and large-scale applications of self-healing and moldable materials continue to be challenging. The present study highlights the use of poly(2-isopropenyl-2-oxazoline) partially modified with 2,2':6',2 ''-terpyridine-4'-carboxylic acid (TPy) for the synthesis of self-healing and moldable metallo-supramolecular hydrogels by metal coordination. The hydrogelation was triggered by the addition of divalent transition metal ions (e.g., Fe2+, Ni2+, Co2+, and Zn2+). Using this approach, metallo-supramolecular hydrogels with tunable rheological properties could be obtained, while evidencing the correlation between the hydrogel mechanical properties and the binding strength and kinetic lability of the supramolecular cross-linking motifs. The resulting hydrogels exhibited rapid self-healing properties at room temperature as confirmed by rheological measurements. Moreover, hydrogels cross-linked with Co2+ and Zn2+ showed a gel-sol transition, indicative of the higher kinetic lability of their complexes. The hydrogels exhibited fast and excellent repeatable autonomic healing capacity even at higher cross-linking density, making them useful as autonomously repairing materials and coatings. Compared to the conventional metallo-hydrogels, the herein developed hydrogels can be molded into a variety of complex geometrical shapes, which could be potentially applied in various areas from soft robotics to dynamic self-healing coatings.

Zinc promotes liquid–liquid phase separation of tau protein

Tau is a microtubule-associated protein that plays a major role in Alzheimer's disease (AD) and other tauopathies. Recent reports indicate that, in the presence of crowding agents, tau can undergo liquid-liquid phase separation (LLPS), forming highly dynamic liquid droplets. Here, using recombinantly expressed proteins, turbidimetry, fluorescence microscopy imaging, and fluorescence recovery after photobleaching (FRAP) assays, we show that the divalent transition metal zinc strongly promotes this process, shifting the equilibrium phase boundary to lower protein or crowding agent concentrations. We observed no tau LLPS-promoting effect for any other divalent transition metal ions tested, including Mn2+, Fe2+, Co2+, Ni2+, and Cu2+ We also demonstrate that multiple zinc-binding sites on tau are involved in the LLPS-promoting effect and provide insights into the mechanism of this process. Zinc concentration is highly elevated in AD brains, and this metal ion is believed to be an important player in the pathogenesis of this disease. Thus, the present findings bring a new dimension to understanding the relationship between zinc homeostasis and the pathogenic process in AD and related neurodegenerative disorders.

Mechanism of Ion Permeation in the Eukaryotic Copper Transporter CTR

Copper uptake in eukaryotic cells is facilitated by the CTR copper transport proteins. Crystal structure of a CTR homolog from xx S. salar shows that CTR is a homotrimer and that each protomer has 3 transmembrane helices. A central permeation pathway is formed by the three subunits and two Cu binding sites were identified, each formed by three highly conserved methionines. It remains unknown how each of the Cu binding site contribute to selective permeation of Cu, and whether Cu transport is coupled to transport of another ion. We expressed and purified CTRs from human and C. elegans, and examined their functions in cell-free transport and binding assays. Because Cu+ is not stable in solution, we measured Ag+ transport in CTR. CTR transports Ag+ at a rate at least 10 fold higher than divalent transition metal ions such as nickel (Ni2+) or cobalt (Co2+). Mutations to the conserved methionines significantly reduce Ag+ transport but have little effect on Ni2+ transport. Ni2+ transport was instead affected by mutation to a conserved glutamate residue. These results indicate that CTR favors monovalent transition metal ions and that mono- and divalent cations may interact with CTR at different sites. This conclusion is also supported by affinity measurement of different ions. We also discovered that the ion transport process is electroneutral, indicating that permeation of Ag+ or other transition metal ions is accompanied by transport of another ion. After testing all possible candidates, we arrived at the conclusion that hydroxyl ions (OH−) are co-transported with the metal ions. It is not clear if CTR transports the two ions individually or together as a neutral metal hydroxide complex ion. Our study revealed for the first time the mechanism of ion selectivity and transport in CTR.

Introducing Seven Transition Metal Ions into Terpyridine-Based Supramolecules: Self-Assembly and Dynamic Ligand Exchange Study.

In coordination-driven self-assembly, 2,2':6',2″-terpyridine (tpy) has gained extensive attention in constructing supramolecular architectures on the basis of ⟨tpy-M-tpy⟩ connectivity. In direct self-assembly of large discrete structures, however, the metal ions were mainly limited to Cd(II), Zn(II), and Fe(II) ions. Herein, we significantly broaden the spectrum of metal ions with seven divalent transition metal ions M(II) (M = Mn, Fe, Co, Ni, Cu, Zn, Cd) to assemble a series of supramolecular fractals. In particular, Mn(II), Co(II), Ni(II), and Cu(II) were reported for the first time to form such large and discrete structures with ⟨tpy-M-tpy⟩ connectivity. In addition, the structural stabilities of those supramolecules in the gas phase and the kinetics of the ligand exchange process in solution were investigated using mass spectrometry. Such a fundamental study gave the relative order of structural stability in the gas phase and revealed the inertness of coordination in solution depending on the metal ions. Those results would guide the future study in tpy-based supramolecular chemistry in terms of self-assembly, characterization, property, and application.

Effect of Transition-Metal Ions on the Conformation of Encephalin Investigated by Hydrogen/Deuterium Exchange and Theoretical Calculations.

We have studied the effects of different 3d orbitals in divalent transition-metal ions [G2+ = Mn2+ (d5), Fe2+ (d6), Co2+ (d7), Ni2+ (d8), Cu2+ (d9), or Zn2+ (d10)] on the conformations of leucine encephalin (LE) and methionine encephalin (ME) in the gas phase using hydrogen/deuterium exchange mass spectrometry (HDX-MS) and theoretical calculations at the molecular level. The HDX-MS reveals a 1:1 stoichiometric monovalent complex of [LE/ME + G - H]+ and observed that the different HDX reactivities follow the trend Fe2+ < Co2+ < Ni2+ < Mn2+ < Cu2+ ≈ Zn2+ and that [ME + Mn/Cu/Zn - H]+ > [LE + Mn/Cu/Zn - H]+, while [LE + Fe/Co/Ni - H]+ > [ME + Fe/Co/Ni - H]+. We cross-correlated the collision-induced dissociation energies of the complexes with the HDX results and found that the more stable the complex, the harder it is for it to undergo HDX. Furthermore, we used theoretical calculations to optimize the favorable conformations of the complexes and found the same interaction structure of G2+ coordination with the five carbonyl oxygens of LE/ME that have different bond lengths. Finally, we calculated the proton affinity (PA) values of the optimized complexes in order to interpret the HDX observations that the higher the PA values, the more difficult it is for the complex to undergo HDX. Overall, both the experiments and the theoretical calculations show that the six metal ions have different effects on the LE/ME conformation, with the low-energy stability of the G2+ 3d orbitals corresponding to more dramatic effects on the LE/ME conformation. In addition, the hardness of the ionic acid corresponding to the fully filled Mn2+ and half-filled Zn2+ orbitals also contributes strongly to the coordination effect; the conformation effect of Fe2+/Co2+/Ni2+ on LE is greater than that on ME, whereas the conformation effect of Mn2+/Cu2+/Zn2+ on ME is greater than that on LE.

Potentiometric and Electrochemical studies for the interaction of Zinc (II), Lead (II), Cadmium (II) with HEPES and HEPPSO as Zwitterionic Buffer Ligands at Different Temperature

The complexation of the divalent transition metal ions Zn (II), Pb (II) or Cd (II) with two important biologically zwitterionic buffers (N-[2-hydroxyethyl] piperazine-N'-[2-ethanesulfonic acid]) (HEPES) and (N-[2-hydroxyethyl] piperazine-N'-[2-hydroxy-propanesulfonic acid]) (HEPPSO) in an aqueous medium, were studied using potentiometric pH titration in 1:1 ratio at four different temperatures 288 K, 298 K, 308 K and 318 K at ionic strength 0.1 molL-1 KNO3. The formation constants and thermodynamic parameters of different protonated, normal, monohydroxy and dihydroxy complexes for the binary systems M(II) + HEPES and M(II) + HEPPSO have been evaluated. The electrochemical behavior of the free Pb(II) and its binary complexes with HEPES and HEPPSO was studied by cyclic voltammetry on glassy carbon electrode. Upon temperature elevation the rate of the reduction process of the Pb2+-HEPPSO is higher than that of Pb2+-HEPES. The diffusion coefficient, the activation energies and the standard rate constant (Ks) for the diffusion process of Pb(II) complexes were calculated.

The Solution Structure and Dynamics of Cd-Metallothionein from Helix pomatia Reveal Optimization for Binding Cd over Zn.

Metallothioneins (MTs) are cysteine-rich polypeptides that are naturally found coordinated to monovalent and/or divalent transition metal ions. Three metallothionein isoforms from the Roman snail Helix pomatia are known. They differ in their physiological metal load and in their specificity for transition metal ions such as Cd2+ (HpCdMT isoform) and Cu+ (HpCuMT isoform) or in the absence of a defined metal specificity (HpCd/CuMT isoform). We have determined the solution structure of the Cd-specific isoform (HpCdMT) by nuclear magnetic resonance spectroscopy using recombinant isotopically labeled protein loaded with Zn2+ or Cd2+. Both structures display two-domain architectures, where each domain comprises a characteristic three-metal cluster similar to that observed in the β-domains of vertebrate MTs. The polypeptide backbone is well-structured over the entire sequence, including the interdomain linker. Interestingly, the two domains display mutual contacts, as observed before for the metallothionein of the snail Littorina littorea, to which both N- and C-terminal domains are highly similar. Increasing the length of the linker motionally decouples both domains and removes mutual contacts between them without having a strong effect on the stability of the individual domains. The structures of Cd6- and Zn6-HpCdMT are nearly identical. However, 15N relaxation, in particular 15N R2 rates, is accelerated for many residues of Zn6-HpCdMT but not for Cd6-HpCdMT, revealing the presence of conformational exchange effects. We suggest that this snail MT isoform is evolutionarily optimized for binding Cd rather than Zn.

Structural, Electronic, and Spectral Properties of Metal Dimethylglyoximato [M(DMG)2; M = Ni2+, Cu2+] Complexes: A Comparative Theoretical Study

Dimethylglyoxime (DMG) usually forms thermodynamically stable chelating complexes with selective divalent transition-metal ions. Electronic and spectral properties of metal-DMG complexes are highly dependent on the nature of metal ions. Using range-separated hybrid functional augmented with dispersion corrections within density functional theory (DFT) and time-dependent DFT, we present a detailed and comprehensive study on structural, electronic, and spectral (both IR and UV-vis) properties of M(DMG)2 [M = Ni2+, Cu2+] complexes. Ni(DMG)2 results are thoroughly compared with Cu(DMG)2 and also against available experimental data. Stronger H-bonding leads to greater stability of Ni(DMG)2 with respect to isolated ions (M2+ and DMG-) compared to Cu(DMG)2. In contrast, a relatively larger reaction enthalpy for Cu(DMG)2 formation from chemically relevant species is found than that of Ni(DMG)2 because of the greater binding enthalpy of [Ni(H2O)6]2+ than that of [Cu(H2O)6]2+. In dimers, Ni(DMG)2 is found to be 6 kcal mol-1 more stable than Cu(DMG)2 due to a greater extent of dispersive interactions. Interestingly, a modest ferromagnetic coupling (588 cm-1) is predicted between two spin-1/2 Cu2+ ions present in the Cu(DMG)2 dimer. Additionally, the potential energy curves calculated along the O-H bond coordinate for both complexes suggest asymmetry and symmetry in the H-bonding interactions between the H-bond donor and acceptor O centers in the solid-state and in solution, respectively, well corroborating with early experimental findings. Interestingly, a lower proton transfer barrier is obtained for the Ni(DMG)2 compared to its Cu-analogue due to stronger H-bonding in the former complex. In fact, relatively weaker H-bonding in Cu(DMG)2 results in blue-shifted O-H stretching modes compared to that in Ni(DMG)2. On the other hand, qualitatively similar optical absorption spectra are obtained for both complexes with red-shifted peaks found for the Cu(DMG)2. Finally, computational models for axial mono- and diligand (aqua and ammonia) coordinated M(DMG)2 complexes are predicted to be energetically feasible and stable with relatively greater binding stability obtained for the ammonia-coordination.

Efficient antimicrobial properties of layered double hydroxide assembled with transition metals via a facile preparation method

Mg2+ in MgAl-layered double hydroxides nanoparticles was substituted with different divalent transition metal ions (MAl-LDHs, M: Mg2+, Cu2+, Ni2+, Co2+, and Mn2+) via a facile method to be used as antibacterial agents. The phase structural and morphological characterizations of MAl-LDHs were investigated by XRD, FTIR spectroscopy and TEM. The results have shown that all of MAl-LDHs had typical layered structures except MnAl-LDH which contained Mn3O4 phases. Particular morphology of MnAl-LDH with ellipsoids, spherical and rod-like structure and CuAl-LDH with rod-like shape existed. IC50 (the concentrations providing 50% antibacterial activity) values of CuAl-LDH, NiAl-LDH, CoAl-LDH, and MnAl-LDH in broth dilution tests were ∼800-1500 μg/mL. Dosages of CuAl-LDH, CoAl-LDH, and MnAl-LDH with >10 mm inhibition zone in disk diffusion tests were ∼150–300 μg/disk. Antibacterial mechanism of MAl-LDHs may be attributed to the synergistic factors including effected surroundings, surface interactions, morphology of particles, ROS and metal ions. The results indicate a facile method to synthesis LDHs based effective antibacterial agents with the potential application in the area of water treatment and antibacterial coating.

Covalent Poly(2‐Isopropenyl‐2‐Oxazoline) Hydrogels with Ultrahigh Mechanical Strength and Toughness through Secondary Terpyridine Metal‐Coordination Crosslinks

AbstractThe present study reports the synthesis of poly(2‐isopropenyl‐2‐oxazoline) (PiPOx) dual‐crosslinked hydrogels by both covalent and physical (i.e., metal–ligand coordination) interactions. First, chemical crosslinking of a modified PiPOx polymer containing terpyridine (TPy) unit is achieved by reacting with azelaic acid (non‐anedioic acid). Transient crosslinks are subsequently introduced by complexation of the TPy units with different divalent transition metal ions. This strategy provides access to hydrogels with superior mechanical properties compared to the pure covalently crosslinked PiPOx hydrogels. The mechanical properties and water uptake of the hydrogels could be easily controlled by swelling in different aqueous metal ion solutions. PiPOx hydrogels swollen in Zn2+ solution are found to possess ultrahigh compression strength (9 MPa), remarkable toughness (99 MJ m−3) and outstanding self‐recoverability (98% toughness recovery after swelling for 60 min without external stimuli), which are among the highest reported in literature to date. These remarkable properties are assigned to the thermodynamically stable, but kinetically labile Zn2+‐TPy complexes that produce a dynamic network with fewer imperfections and better adaptive properties under mechanical stress compared to those with other metal ions.

Metal-dependent assembly of a protein nano-cage.

Short, alpha-helical coiled coils provide a simple, modular method to direct the assembly of proteins into higher order structures. We previously demonstrated that by genetically fusing de novo-designed coiled coils of the appropriate oligomerization state to a natural trimeric protein, we could direct the assembly of this protein into various geometrical cages. Here, we have extended this approach by appending a coiled coil designed to trimerize in response to binding divalent transition metal ions and thereby achieve metal ion-dependent assembly of a tetrahedral protein cage. Ni2+ , Co2+ , Cu2+ , and Zn2+ ions were evaluated, with Ni2+ proving the most effective at mediating protein assembly. Characterization of the assembled protein indicated that the metal ion-protein complex formed discrete globular structures of the diameter expected for a complex containing 12 copies of the protein monomer. Protein assembly could be reversed by removing metal ions with ethylenediaminetetraacetic acid or under mildly acidic conditions.

Removal of toxic tellurium (IV) compounds via bioreduction using flucloxacillin in aqueous acidic medium: A kinetic and mechanistic approach

This paper describes a novel method for the removal of potassium tellurite (TeIV), a toxic tellurium (IV) compound, via its bioreduction using the drug flucloxacillin (Flx) in an aqueous sulfuric acid solution. The kinetics of the bioreduction process were monitored using UV–Vis absorption spectra at an ionic strength of 2.0 mol dm−3 and 298 K. The reaction between TeIV and Flx was set at a 1:1 stoichiometry. The reduction reaction followed first-order kinetics for [Flx] and fractional-first-order kinetics for [TeIV] and [H+]. The effects of ionic strength and relative permittivity of the reaction medium were also explored. Supplementation with divalent transition metal ions enhanced the reduction rate. The reaction products were identified, in order of their stoichiometric results, spot tests and FT-IR spectra as 3-(2-chloro-6-fluorophenyl)-5-methylisoxazol-4-carbocylic acid, 5,5-dimethyl-thiazolidine-2,4-dicarboxlic acid, ammonium ion, carbon dioxide and elemental tellurium (Te0). The reaction rate dependence on temperature was studied, and the activation and thermodynamic parameters were assessed and discussed. The derived rate-law expression was found to be in excellent accordance with the acquired investigational outcomes. A conceivable reaction mechanism has been provided, which includes a reaction between the protonated flucloxacillin (Flx+) and tellurous acid (H2TeO3) as the essential reactive species, resulting in the construction of an intermediate complex. Such complex decays in the rate-determining step to yield the final reaction products.

Structural diversity in polymeric and discrete complexes constructed by divalent transition metals and unsymmetrical quasi semirigid pyridinecarboxylate isomers

Nine polymeric and discrete complexes including a couple of genuine supramolecular isomers have been successfully prepared from divalent transition metal ions (Mn(II), Co(II), Ni(II), Cu(II), and Zn(II)) and unsymmetrical quasi semirigid pyridinecarboxylate isomers, InMe-n-py (n = 2, 3, 4). Structural analyses reveal that the unsymmetrical quasi semirigid ligands manage the metal(II) ions to form discrete (0D) armed metallocycle and dinuclear paddlewheel molecule, 1D ribbon, simple 2D rhombus (4,4) grid, 2D chiral wavy square (4,4) grid, and 2D double-edged layer with decorated (4,4) topology. Noncovalent hydrogen bonds and aromatic stacking interactions are found to be accessorial secondary interactions that are helpful for the extension and stabilization of the final inorganic−organic hybrid supramolecular networks of metal(II) complexes. Positional isomerism and conformational diversity of the unsymmetrical quasi semirigid ligands and the coordination preference of divalent metal ions play key roles in dominating structure variation and network complexity. Thermogravimetric analysis of mass loss showed that the frameworks of these complexes are thermally stable up to ca. 220–400 °C. Variable-temperature, solid-state magnetic susceptibility studies suggest that Mn(II) complexes 1 and 2 exhibit antiferromagnetic exchange coupling.

Toward efficient and air-stable carbon-based all-inorganic perovskite solar cells through substituting CsPbBr3 films with transition metal ions

The perovskite film with large grain size and low defect states density is technically important to improve the power conversion efficiency and environmental stability of perovskite solar cells (PSCs). In the present work, we raise a compositional engineering strategy of inorganic CsPbBr3 perovskite films by substituting partial Pb2+ with divalent transition metal ions having smaller ionic radius (TM2+ = Mn2+, Ni2+, Cu2+ and Zn2+). The photovoltaic performances of hole transporting layer-free all-inorganic CsPbBr3 PSCs are markedly improved through maximizing grain size with few grain boundaries as well as low trap states density and minimizing energy loss in charge carrier transfer. A champion power conversion efficiency as high as 9.18% is achieved for the all-inorganic CsPb0.995Zn0.005Br3 PSC free of encapsulation, which shows a remarkable long-term stability over 760 h in 80% humidity air atmosphere at 25 °C. The high efficiency and improved stability demonstrate compositional engineering is a promising strategy to forward high-quality perovskite films and high-performance of all-inorganic CsPbBr3 PSCs.

Synthesis, Spectral and Antimicrobial Investigation of 2-(Naphthalenel-ylamino)-2- Phenylacetonitrile and 1, 10-Phenanthroline with Five Divalent Transition Metal Ions

Five new mixed ligand metal complexes have been synthesized by the reaction of divalent transition metal ions (Hg, Ni, Zn, Cu and Cd) with 2-(naphthalen-l-ylamino)-2-phenylacetonitrile (L1 ) and 1,10-phenanthroline (L2). The coordination likelihood of the two ligands toward metal ions has been suggested in the light of elemental analysis, UV-Vis spectra, FTIR, 1H-NMR, flam atomic absorption, molar conductance and magnetic studies. Results data suggest that the octahedral geometry for all the prepared complexes. Antibacterial examination of synthesized complexes in vitro was performed against four bacterias. Firstly, Gram-negative bacteria namely, Pseudomonas aerugin and Escherichia. Secondly, Gram-positive bacteria namely, Bacillus subtilis, Staphylococcuaurouss. Results data exhibit that the synthesized complexes exhibited more biological activity than tetracycline pharmaceutical.

Characterization of the interactions between banana condensed tannins and biologically important metal ions (Cu2+, Zn2+ and Fe2+)

To characterize the interactions of banana condensed tannins (BCT) with copper (Cu2+), zinc (Zn2+) and iron ions (Fe2+), various methods and techniques were utilized in the current work. Our findings indicated that BCT isolated from green bananas (cv. Brazil) were a mixture of flavan-3-ol oligomers with degree of polymerization in the range of 3-5 and with the trimers as the main component. The (-)-epigallocatechin and (-)-epicatechin were the dominant units of oligomer structures. BCT could interact with divalent transition metal ions mainly via catechol group-ion exchange and pyrogallol group-ion exchange, while static quenching mechanism was the main action mode of BCT fluorescence quenching by metal ions. The effectiveness of precipitation of metal ions by BCT decreased with the order of Fe2+ > Cu2+ > Zn2+. Alternatively, the characteristics of BCT-metal ion coprecipitates confirmed the interactions between BCT and divalent metal ions. Therefore, characterization of the interaction mechanisms between BCT and biologically important metal ions should help in the design of strategies to prevent the metal-induced toxicity in individuals with a high exposure of transition metal ions.

Speciation and siting of divalent transition metal ions in silicon-rich zeolites. An FTIR study

Abstract Speciation and location of Co2+, Mn2+ and Ni2+ in the extraframework positions of the dehydrated zeolite matrix of ferrierite structure were studied in detail using FTIR spectroscopy of antisymmetric T–O–T vibrations of the zeolite framework. Me2+–ferrierites were prepared by the ion exchange of the NH4– and Na–zeolite forms and by impregnation of the NH4 form. Bare Me2+ occupies all three known cationic sites in dehydrated cationic zeolite. The wavenumbers of bands of individual cations in individual sites were identified. At low Me2+ loadings (Me2+/Al &lt; 0.15), Me2+ replaces two protonic sites and exclusively bare Me2+ is present in dehydrated samples. Sets of such samples were employed for the estimation of extinction coefficients of Co2+, Mn2+ and Ni2+ in cationic sites. These coefficients differ for individual cations but are the same for a cation at different sites. Ion exchange to the NH4 form allows preparation of samples with maximum possible loading of bare Me2+ only for Co2+. In the case of Mn2+, exchange to the Na-parent zeolite or impregnation is required for this purpose while samples with maximum loading by bare Ni2+ can be prepared only by impregnation.

Facile synthesis of sulfide-based chalcogenide as hole-transporting materials for cost-effective efficient perovskite solar cells

Sulfide-based chalcogenide quaternary semiconductors Cu2MSnS4 where M = Zn, Mn, Ni and, Co have emerged as a promising material for photovoltaic application due to their opto-electrical properties, friendly environment and earth-abundant composition. Herein, we report on the surfactant-assisted, solvent-free, hydrothermal synthesis of Cu2MSnS4 (M = Zn, Mn, Ni, Co) nanoparticles and their application in photovoltaics as well. The kesterite and stannite crystal structure of the investigated compounds were confirmed by powder X-ray diffraction. Raman, Photoluminescence and UV–Vis reflectance spectroscopies were employed to study the lattice vibrational and the optical properties. High-resolution transmittance electron microscopy was performed to study the morphology of nanoparticles size, as well as the crystallinity of chalcogenide compounds. In the presence of Cu2MSnS4 with different divalent transition metal ions (M) = Mn2+, Co2+, Ni2+ and Zn2+ as a hole transport material, the perovskite-sensitized TiO2 films showed power conversion efficiencies of 8.35, 6.24, 7.55 and 4.16%, respectively.

Transition Metal Sequestration by the Host-Defense Protein Calprotectin.

In response to microbial infection, the human host deploys metal-sequestering host-defense proteins, which reduce nutrient availability and thereby inhibit microbial growth and virulence. Calprotectin (CP) is an abundant antimicrobial protein released from neutrophils and epithelial cells at sites of infection. CP sequesters divalent first-row transition metal ions to limit the availability of essential metal nutrients in the extracellular space. While functional and clinical studies of CP have been pursued for decades, advances in our understanding of its biological coordination chemistry, which is central to its role in the host-microbe interaction, have been made in more recent years. In this review, we focus on the coordination chemistry of CP and highlight studies of its metal-binding properties and contributions to the metal-withholding innate immune response. Taken together, these recent studies inform our current model of how CP participates in metal homeostasis and immunity, and they provide a foundation for further investigations of a remarkable metal-chelating protein at the host-microbe interface and beyond.

Ratio-Controlled Precursors of Anderson-Evans Polyoxometalates: Synthesis, Structural Transformation, and Magnetic and Catalytic Properties of a Series of Triol Ligand-Decorated {M2Mo6} Clusters (M = Cu2+, Co2+, Ni2+, Zn2+).

A series of triol ligand [CH3C(CH2OH)3] covalently decorated polyoxometalates (POMs), which could be ascribed to the primary complexes with structural formulas {M2[Mo2O4(CH3C(CH2O)3)2]3}2- (M = Cu2+, Co2+, Ni2+, Zn2+), have been synthesized in organic solvents. Single-crystal X-ray structural analysis reveals that the synthesized polyanionic clusters are comprised of three {Mo2} units and two divalent transition-metal ions connecting to each other in an alternating style, where all {Mo2} blocks were covalently decorated by two triol ligands in the trans conformation. The 1/3 molar ratio of M/Mo in the prepared complexes was higher than those ratios in typical Anderson-Evans, Wells-Dawson, and Keggin POMs. With a decrease in the M/Mo molar ratio of a Mo-contained reactant to 1/6 and/or the addition of acetic acid to the reaction solution, the primary complexes acting as precursors transformed continuously into the corresponding triol-ligand-decorated Anderson-Evans POMs. Detailed investigations were conducted by using different isopolymolybdates in various solvent environments, and several Anderson-Evans POMs in different triol-ligand-decorated fashions were obtained from the primary complexes. In addition, we also realized the transformation between the Anderson-Evans clusters in different decoration fashions by simply controlling the acidity in solution. Magnetic measurement showed a general property, but the catalytic experiments demonstrated that CoII- and Zn II-containing POMs displayed a higher efficiency for the selective oxidation of thioanisole to sulfoxide.