Specific Coordination Site Research Articles

Precise Synthesis of Dual-Single-Atom Electrocatalysts through Pre-Coordination-Directed in Situ Confinement for CO2 Reduction.

Dual-single-atom catalysts (DSACs) are the next paradigm shift in single-atom catalysts because of the enhanced performance brought about by the synergistic effects between adjacent bimetallic pairs. However, there are few methods for synthesizing DSACs with precise bimetallic structures. Herein, a pre-coordination strategy is proposed to precisely synthesize a library of DSACs. This strategy ensures the selective and effective coordination of two metals via phthalocyanines with specific coordination sites, such as -F- and -OH-. Subsequently, in situ confinement inhibits the migration of metal pairs during high-temperature pyrolysis, and obtains the DSACs with precisely constructed metal pairs. Despite changing synthetic parameters, including transition metal centers, metal pairs, and spatial geometry, the products exhibit similar atomic metal pairs dispersion properties, demonstrating the universality of the strategy. The pre-coordination strategy synthesized DSACs shows significant CO2 reduction reaction performance in both flow-cell and practical rechargeable Zn-CO2 batteries. This work not only provides new insights into the precise synthesis of DSACs, but also offers guidelines for the accelerated discovery of efficient catalysts.

Precise Synthesis of Dual‐Single‐Atom Electrocatalysts through Pre‐Coordination‐Directed in Situ Confinement for CO2 Reduction

AbstractDual‐single‐atom catalysts (DSACs) are the next paradigm shift in single‐atom catalysts because of the enhanced performance brought about by the synergistic effects between adjacent bimetallic pairs. However, there are few methods for synthesizing DSACs with precise bimetallic structures. Herein, a pre‐coordination strategy is proposed to precisely synthesize a library of DSACs. This strategy ensures the selective and effective coordination of two metals via phthalocyanines with specific coordination sites, such as −F− and −OH−. Subsequently, in situ confinement inhibits the migration of metal pairs during high‐temperature pyrolysis, and obtains the DSACs with precisely constructed metal pairs. Despite changing synthetic parameters, including transition metal centers, metal pairs, and spatial geometry, the products exhibit similar atomic metal pairs dispersion properties, demonstrating the universality of the strategy. The pre‐coordination strategy synthesized DSACs shows significant CO2 reduction reaction performance in both flow‐cell and practical rechargeable Zn‐CO2 batteries. This work not only provides new insights into the precise synthesis of DSACs, but also offers guidelines for the accelerated discovery of efficient catalysts.

Computational Analyses of the AtTPC1 (Arabidopsis Two-Pore Channel 1) Permeation Pathway.

Two Pore Channels (TPCs) are cation-selective voltage- and ligand-gated ion channels in membranes of intracellular organelles of eukaryotic cells. In plants, the TPC1 subtype forms the slowly activating vacuolar (SV) channel, the most dominant ion channel in the vacuolar membrane. Controversial reports about the permeability properties of plant SV channels fueled speculations about the physiological roles of this channel type. TPC1 is thought to have high Ca2+ permeability, a conclusion derived from relative permeability analyses using the Goldman–Hodgkin–Katz (GHK) equation. Here, we investigated in computational analyses the properties of the permeation pathway of TPC1 from Arabidopsis thaliana. Using the crystal structure of AtTPC1, protein modeling, molecular dynamics (MD) simulations, and free energy calculations, we identified a free energy minimum for Ca2+, but not for K+, at the luminal side next to the selectivity filter. Residues D269 and E637 coordinate in particular Ca2+ as demonstrated in in silico mutagenesis experiments. Such a Ca2+-specific coordination site in the pore explains contradicting data for the relative Ca2+/K+ permeability and strongly suggests that the Ca2+ permeability of SV channels is largely overestimated from relative permeability analyses. This conclusion was further supported by in silico electrophysiological studies showing a remarkable permeation of K+ but not Ca2+ through the open channel.

A Redox-Switchable, Allosteric Coordination Complex.

A redox-regulated molecular tweezer complex was synthesized via the weak-link approach. The PtII complex features a redox-switchable hemilabile ligand (RHL) functionalized with a ferrocenyl moiety, whose oxidation state modulates the opening of a specific coordination site. Allosteric regulation by redox agents gives reversible access to two distinct structural states-a fully closed state and a semi-open state-whose interconversion was studied via multinuclear NMR spectroscopy, cyclic voltammetry, and UV-vis-NIR spectroscopy. Two structures in this four-state system were further characterized via SCXRD, while the others were modeled through DFT calculations. This fully reversible, RHL-based system defines an unusual level of electrochemical control over the occupancy of a specific coordination site, thereby providing access to four distinct coordination states within a single system, each defined and differentiated by structure and oxidation state.

Coupling Photoluminescence and Ionic Conduction Properties Using the Different Coordination Sites of Ureasil-Polyether Hybrid Materials.

In this article, we demonstrate that each functional group of ureasil organic-inorganic hybrid (OIH) materials can act as a specific coordination site for a given active guest species, hence allowing the possibility of combining different functional properties. To illustrate this concept, the sol-gel process was used to produce diurea cross-linked siloxane-polyethylene oxide (U-PEO) and siloxane-polypropylene oxide (U-PPO) hybrid host frameworks with similar molecular weights (1900 and 2000 g mol-1 for PEO and PPO, respectively), with Li+ and Eu3+ as active guest ions providing ionic conduction and photoluminescence (PL) properties, respectively. Comparison of Fourier transform infrared spectra and small-angle X-ray scattering results for single-doped (using Li+ or Eu3+) and co-doped (using Li+ and Eu3+) U-PEO and U-PPO hosts showed that in every case, there was specific coordination of Eu3+ by the carbonyl group of the urea bridge and of Li+ by ether-type oxygen of the PEO and PPO chains. Optical analyses demonstrated that loading with Li+ did not affect the luminescence properties of the Eu3+-loaded OIH. Although loading with Eu3+ had a small effect on ionic transport, co-doping with Li+ ions ensured macroscopic ion-conduction of the transparent and luminescent hybrid material. The results suggested that the combination of both properties in a transparent elastomeric material could be useful for the development of multifunctional devices. The results suggested that the combination of both properties in a transparent elastomeric material could be useful for the development of multifunctional polyelectrolytes applied in the field of dual luminescent devices such as photoelectrochromic smart windows.

Sequential Recognition of Zinc and Pyrophosphate Ions in a Terpyridine-Functionalized Single Nanochannel.

A highly selective recognition system for zinc(II) and pyrophosphate ions is constructed using a single conical nanochannel covalently functionalized with N'-{4-[(2,2':6',2''-terpyridine)-4-yl]benzyl}ethane-1,2-diamine (TPYD). The immobilized TPYD acts as a specific coordination site for Zn2+ to form TPYD-Zn2+ complexes in preference over other metal ions, and subsequently, the resulting Zn2+ -coordinated nanochannel can be used as a selective recognition element for the pyrophosphate ion based on the coordination reaction between hydroxyl oxygen atoms of pyrophosphate and Zn2+ . Ion recognition is monitored by measuring the current-voltage curves of the solutions. The ionic current of the TPYD-functionalized system at -2.0 V undergoes a clear decrease after exposure to Zn2+ ions and is followed with an obvious increase after subsequent treatment with a pyrophosphate solution. The change of ionic current can be primarily attributed to the variation of charge density of the nanochannel. This functionalized single nanochannel might provide a simple and universal means to recognize other targets by modifying different functional molecules onto the inner surfaces of nanochannels.

Transferrin-inspired vehicles based on pH-responsive coordination bond to combat multidrug-resistant breast cancer.

A novel biomimetic drug delivery system (BDDS) inspired by the pH-dependent ferric ion-transport and release manner of transferrin (Tf) was developed for combating multidrug-resistant breast cancer. Tf-inspired carrier was synthesized by modifying bovine serum albumin (BSA) with histamine (HA) through amide reaction to provide superior specific coordination sites for ferric ion-drug complexes, and self-assembled into nanoparticles (NPs) induced by coordination bond. Tf-inspired NPs were prepared via environment-friendly method, and well redispersed in saline after lyophilization. When internalized into tumor cells by SPARC (secreted protein acidic and rich in cysteine) mediated endocytosis, Tf-inspired NPs bypassed and decreased the P-glycoprotein-mediated drug efflux and led to more effective treatment of multidrug-resistant breast cancer compared with free drugs both invitro and invivo due to the enhanced cellular uptake and rapid pH-responsive drug release. Moreover, Tf-inspired NPs exhibited good biocompatibility and low systemic toxicity. Thus, our results demonstrate that Tf-inspired NPs based on coordination bond represent as a smart drug delivery strategy to combat multidrug-resistant cancer and have great potential for clinical applications in cancer therapy.

Analytical Gel Filtration for Probing Heavy Metal Transfer between Proteins

Heavy metals can cause damage to biomolecules such as proteins and DNA in multiple ways. Cells therefore strive for keeping intracellular (heavy) metal ions bound to specific proteins that are capable of handling detoxification, export or integration as cofactors. Metal binding proteins usually provide specific coordination sites that bind certain ions with ultrahigh affinity, with the thermodynamic driving force being the stability of organometallic complexes. However, the metal binding properties of these proteins can be highly variable. Therefore the transfer of specific ions between separate proteins or even between distinct binding sites located on one and the same protein does not always follow affinity gradients, but depends on particular protein interactions that are difficult to predict. We established a method suitable to probe metal transfer between two proteins, provided the proteins are amenable to purification and in vitro handling. It consists of the loading with metals, the co-incubation and the separation of metal-exchanging proteins with subsequent determination of bound metal content. The method is exemplified by experimental data of ours probing the transfer of copper(I) between the membrane-extrinsic metal binding domain MBD2 and the transmembrane domain of CopA, a copper export ATPase from Escherichia coli (Drees et al., 2015).

ChemInform Abstract: Pentacoordination for Pincer and Related Terdentate Coordination Compounds: Revisiting Structural Properties and Trends for D8 Transition Metal Systems

AbstractReview: 116 refs.

Synthetic procedures and structural features of new di- and tri-nuclear chelate compounds of Cd(II) with sulfur and nitrogen donor ligands

Cd(dmpymt) 2 reacts with CdCl 2, CdBr 2·4H 2O, CdI 2, 2,2′-bipyridine and 1,10-phenanthroline to give the dimeric chelates [Cd(dmpymt)(bpy)Cl] 2 and [Cd(dmpymt)(phen)Cl] 2, as well as the tri-nuclear complexes [Cd 3(dmpymt) 4(bpy) 2Br 2] and [Cd 3(dmpymt) 4(bpy) 2I 2] (dmpymtH = 4,6-dimethylpyrimidine-2-thione; bpy = 2,2′-bipyridine; phen = 1,10-phenantroline). In all complexes the Cd(II) centers present the coordination number six. The new compounds are examples of the managing of the final aggregation state of thiolate metal complexes by introducing co-ligands to block specific coordination sites of the metal center.

Current Status of Imaging Infections with Radiolabeled Anti-Infective Agents

Infection specific radiopharmaceuticals can be used for diagnosis, decision-making in therapy and treatment follow-up. Research has been ongoing to develop infection specific markers since clinically used tracers cannot discriminate between infection and inflammation. A specific radiopharmaceutical for infection imaging should satisfy the following criteria: high and specific uptake at the infection site, rapid infection detection and background clearance, minimal accumulation in non-target tissues, low toxicity, zero immune response and especially the ability to differentiate infection from sterile inflammation. Radiolabeled anti-infective agents can fulfill the majority of these requirements. Technetium- 99m (99mTc) labeled antibiotics have potential to differentiate sterile inflammation from infection. There are numerous studies reporting the use of radiolabeled antibacterial and antifungal agents for infection detection. Other promising agents are antimicrobial peptides (AMPs) since they preferentially bind to bacteria membranes instead of those of mammalian cells thus distinguishing between infection and sterile inflammation. Synthetic AMPs derived from human natural peptides offers possibility for studying the effects of polymerization and substituting the amino acid sequence to design a specific micro-organism seeking tracer. 99mTc-labeled anti-infectives are ideal as infection-seeking agents because of its direct and fast accumulation. Clinical studies already undertaken and further evaluation with different pathogen types such as viruses, fungi, parasites and intracellular pathogens in humans will improve the potential of these compounds. Radiochemical techniques for labeling anti-infectives have been developed to optimize biodistribution and targeting properties of tracers. An important issue is the technetium-99m specific coordination site in the anti-infective molecule and its chemical and biochemical characterization.

Lys and Arg in UBI: A specific site for a stable Tc-99m complex?

The aim of this study was to help establish if ubiquicidin peptide 29-41 fragment (UBI) contains a specific site for 99mTc labeling by a new direct method under alkaline conditions. Since this peptide does not have cysteine residues, it is possible that neighboring arginine and lysine in the peptide amino acid sequence (Thr-Gly-Arg-Ala-Lys-Arg-Arg-Met-Gln-Tyr-Asn-Arg-Arg) could be a specific coordination site to form a stable 99mTc-UBI complex. Following direct labeling, the in vitro stability of 99mTc-UBI was compared to UBI radiolabeled by one indirect method using HYNIC/tricine and HYNIC/tricine/EDDA. Radiochemical purity of 99mTc-UBI averaged 97% compared to 88% for 99mTc-HYNIC-UBI/tricine and 98% for 99mTc-HYNIC-UBI/tricine/EDDA. Both 99mTc-HYNIC-UBI (tricine or EDDA) and 99mTc-UBI showed stability in human serum and solutions of cysteine. 99mTc-UBI radiochemical purity 24 h after dilution in 0.9% NaCl was greater than 90% at pH 9 and greater than 95% at pH 6.5. Under one set of experimental conditions, in vitro binding to bacteria of 99mTc-UBI was 35% and identical to that of 99mTc-HYNIC-UBI/tricine and 99mTc-HYNIC-UBI/tricine/EDDA at 32% and 31% respectively. The biodistribution of 99mTc-UBI in mice showed a rapid renal clearance. To help identify the site(s) of 99mTc binding following direct labeling, molecular mechanics and quantum-mechanical calculations were performed which showed that the amine groups of Arg 7 and Lys are the most probable site. The calculations show that these groups can form a square pyramid with two water molecules for the Tc cation ( dxysp 3 ). It will be necessary to isolate and characterize the 99Tc(V)( O)-UBI .(H 2O) n complex to confirm these results.

The electronic structure of Cu+, Ag+, and Au+ zeolites.

A variety of procedures have been used to prepare d10-zeolite materials. The electronic structure of these materials can be regarded to a first approximation as a superposition of the framework, of the charge compensating ions, of solvent molecules and of guest species. Zeolite oxygen to d10-ion charge transfer transitions dominate the electronic spectra if the ions coordinate to the zeolite oxygens. Specific coordination sites can influence the energy and the intensity of these transitions remarkably. Intra guest transitions dominate in quantum dot materials, as discussed in detail for luminescent Ag2S zeolite A. The zeolite is not needed for the photocatalytic water oxidation on Ag+/AgCl photo anodes with visible light. It can, however, be used to increase the active surface area substantially.

Surface studies of a chalcogenide glass ferric ion‐selective electrode Part 2: The effects of inorganic ions, organic ligands and seawater on sensor response

AbstractX‐ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS) and electrochemical impedance spectroscopy (EIS) have been used to investigate the effect of inorganic ions, organic ligands and seawater on the response of the FeIII ion‐selective electrode (ISE). X‐ray photoelectron spectroscopy has shown that the formation of antimony(V) oxide/hydroxide on the chalcogenide glass membrane soaked in seawater is dictated by electrode kinetics. It has been found that selenide atoms in the chalcogenide glass are replaced by oxygen in the presence of seawater. Furthermore, EIS aging studies demonstrated that the chalcogenide glass membrane experiences a sluggish charge transfer reaction in seawater. By contrast, SIMS imaging and depth profiling on various regions of a seawater‐exposed membrane identified a phase comprising iron oxide–hydroxide–carbonate–carboxylate. Furthermore, SIMS revealed a uniform chemisorption of oxygen on the membrane surface, and adsorption does not appear to be related to specific coordination sites. X‐ray photoelectron spectroscopy failed to detect any chloride chemisorption on the membrane surface, whereas in situ EIS showed that chloride along with hydroxide ions facilitate the charge transfer process of the FeIII ISE. Moreover, the effect of humic acid on the FeIII ISE was studied using EIS. The results of this XPS, SIMS and EIS study have enabled the derivation of a response mechanism for the FeIII ISE in seawater. Copyright © 2002 John Wiley & Sons, Ltd.

Differential effects of transition metal cations on the conformation and biological activities of nerve growth factor

Direct effects of Zn2+ on the conformation and biological activity of nerve growth factor (NGF) have previously been described. Zn2+ binds to specific coordination sites within NGF and induces conformational changes within domains that participate in receptor recognition processes. Recent theoretical considerations indicate that other transition metal cations (particularly, Cu2+and Pd2+) are capable of forming similar complexes with NGF. Inactivation of NGF by transition metal cations is inhibitory to neuronal regeneration and sprouting, and can lead to cell death under conditions where NGF is required for survival in PC12 cells. In this study we investigated the influence of various metal ions on NGF conformation, geometry of NGF amino terminal peptide and NGF-mediated biological effects in FC12 cells. A number of metal ions (Zn2+, Cu2+ and Pd2+) alter NGF conformation in cell-free assays and inhibit NGF-mediated cell survival. Other metals have been shown to be either toxic to PC12 cells by mechanisms independent of NGF activity (e.g. Ag+, Hg2+) or non-toxic to the cells under conditions tested (e.g. Al3+, Cr3+). In conclusion, several metal cations are capable of inhibiting NGF activity, thereby blocking NGF-mediated cell survival and plasticity.

Pseudotetrahedral Organocobalt(III) Compounds Containing Specific Coordination Sites for Brønsted Acids

Transmetallation of the lithiated derivative 2,6-bis[dimethyl(amino)methyl]phenyllithium with the cobalt(III) centre of the dimer bis[(η5-cyclopentadienyl)cobalt(iodo)(μ-iodo)] has been found to afford the organocobalt compound [CoI{2-(Me2NCH2)-6-(Me2NH+CH2)C6H3}(η5-C5H5)]I, 3, in which the potentially terdentate (pincer) ligand is bound to Co through the formerly lithiated aryl carbon atom and one NMe2 unit only, whilst the second NMe2 group is protonated by HI. In the presence of aqueous KPF6, 3 has been found to undergo both substitution of the iodide bound to Co by a water molecule and deprotonation of the Me2NH group to afford [CoI{2,6-(Me2NCH2)2C6H3}(η5-C5H5)(H2O)]PF6, 4a. It was found that the water ligand in 4a could be substituted by neutral ligands such as PMe2Ph as well as by Bronsted acids such as HCl, HI, HF, or HOAc to afford the compounds [CoX{2-(Me2NCH2)-6-(Me2NH+CH2)C6H3}(η5-C5H5)]PF6, 5a−d, where X = Cl, I, F, or OAc, respectively. Crystal structure analyses of some representative examples of these compounds, including 4a, have shown unambiguously that an intramolecular hydrogen bond is present between X and the proton of the Me2NH+ unit. 1H-NMR evidence that this interaction persists in solution has been found for compounds 4a, 5a, 5c, and 5d.

Pseudotetrahedral Organocobalt(III) Compounds Containing Specific Coordination Sites for Brønsted Acids

Pseudotetrahedral Organocobalt(III) Compounds Containing Specific Coordination Sites for Brønsted Acids

Co 2+ as a structural probe in acetate liquids and glasses

Compositions of lithium acetate and lead acetate form homogeneous liquids when heated above 100°C. When such liquids are doped with Co 2+, the liquid/glass is colored blue at relatively high temperatures (100°C–150°C) and varying shades of red under ambient conditions. The color change is gradual, continuous, and reversible with temperature, even through the liquid-glass transition. This thermochromic property has been attributed to Co 2+ changing coordination from tetrahedral to octahedral with decreasing temperature, due to the rotation of two acetate ligands that become bidentate. Additives to the glass composition can control the acetate glass structure, which preferentially stabilizes a specific coordination site of Co 2+ and the resulting color of the liquid/glass. The color changes are not directly attributable to changes in the basicity or polymerization of the acetate liquid/glass network. Acetate liquids/glasses lack a covalently bonded network, and thus are not appropriate models for silicate structures.

Interaction of the transition metal ions with natural peptides

NMR, CD, EPR and absorption spectra studies as well as polarographic and potentiometric studies on metal ion interaction with two natural peptide i.e. glutathione (GSH) and thyrotropin releasing factor (TRF, L-pyroglutamyl-L-histydyl-L-prolinamide) have revealed quite unusual features of both tripeptides as the chelating agents [1-4]. In the case of Cu(II), Co(II) and Ni(II)-TRF systems, the tripeptide acts as the tridentate ligand with formation of the metal ion bonds with N3 imidazol, N − of the peptide linkage between Pyr and His and the amide nitrogen of pyroglutamic acid. CD spectra studies have shown that the conformation of the chelate rings is very sensitive on the deprotonation process of N1 imidazole nitrogen (see also [4]). GSH and its oxidized form GSSG with Cu(II) and Co(II) forms very interesting redox system [2]. Both forms of the glutathione are extremely sensitive on the presence of cupric ions in the solution, especially at higher pH region. All studies have shown that the cysteine residue is the most specific coordination site for all studied metal ions i.e. Cu(II), Co(II) and Ni(II).