Thiophilic Metal Research Articles

Three-Component Synthesis of 1-Substituted 5-Aminotetrazoles Promoted by Bismuth Nitrate.

A nontoxic bismuth-promoted multicomponent synthesis of 5-aminotetrazoles and bistetrazoles is reported. The reaction of phenyl isothiocyanate, NaN3, and amine (primary aliphatic, aromatic, and aliphatic diamine) promoted by Bi(NO3)3·5H2O under microwave heating affords good yields, short reaction times, simple workup, and purification without column chromatography. A set of diagnostic 1H NMR signals was identified as a guide for quickly elucidating the exclusive (or main) regioisomer formed, with the stronger electron donor group located at heterocyclic nitrogen 1. This regioselectivity is strongly dependent on the electronic density of the amine. It is opposite to that obtained by several thiourea desulfurization methods promoted by thiophilic metals and metal-free protocols.

Mining flotation reagents: Quantitative and robust analysis of metal-xanthate complexes in water

Xanthates, common mining flotation reagents, strongly bind thiophilic metals such as copper (Cu), lead (Pb), cadmium (Cd), and zinc (Zn) and consequentially change their bioavailability and mobility upon their discharge into the environment. However, accurate quantification of the metal-xanthate complexes has remained elusive. This study develops a novel and robust method that realizes the accurate quantification of the metal-xanthate complexes resulted from single and multiple reactions of three typical xanthates (ethyl, isopropyl, and butyl xanthates) and four thiophilic metals (Cu, Pb, Cd, and Zn) in water samples. This method uses sulfur (S2-) dissociation, followed by tandem solid phase extraction of C18 + PWAX and subsequent LC-MS/MS analysis. It has a wide linearity range (1–1000 μg/L, R2 ≥ 0.995), low method detection limits (0.002–0.036 μg/L), and good recoveries (70.6–107.0 %) at 0.01–10 mg/L of xanthates. Applications of this method showed ubiquitous occurrence of the metal-xanthate complexes as the primary species in flotation wastewaters, which the concentrations were 4.6–28.9-fold higher than those previously determined. It is the first quantitative method established for the analysis of metal-xanthate complexes in water samples, which is of great importance to comprehensively understand the fate and risks of xanthates in the environment.

Catalytic Metal Ion-Substrate Coordination during Nonenzymatic RNA Primer Extension.

Nonenzymatic template-directed RNA copying requires catalysis by divalent metal ions. The primer extension reaction involves the attack of the primer 3'-hydroxyl on the adjacent phosphate of a 5'-5'-imidazolium-bridged dinucleotide substrate. However, the nature of the interaction of the catalytic metal ion with the reaction center remains unclear. To explore the coordination of the catalytic metal ion with the imidazolium-bridged dinucleotide substrate, we examined catalysis by oxophilic and thiophilic metal ions with both diastereomers of phosphorothioate-modified substrates. We show that Mg2+ and Cd2+ exhibit opposite preferences for the two phosphorothioate substrate diastereomers, indicating a stereospecific interaction of the divalent cation with one of the nonbridging phosphorus substituents. High-resolution X-ray crystal structures of the products of primer extension with phosphorothioate substrates reveal the absolute stereochemistry of this interaction and indicate that catalysis by Mg2+ involves inner-sphere coordination with the nonbridging phosphate oxygen in the pro-SP position, while thiophilic cadmium ions interact with sulfur in the same position, as in one of the two phosphorothioate substrates. These results collectively suggest that during nonenzymatic RNA primer extension with a 5'-5'-imidazolium-bridged dinucleotide substrate the interaction of the catalytic Mg2+ ion with the pro-SP oxygen of the reactive phosphate plays a crucial role in the metal-catalyzed SN2(P) reaction.

Dithiocarbamate ligands as heavy metal expectorants from aqueous solutions

The uncontrolled concentrations of heavy metals introduced into the water sources need an effective method for their removal. The dithiocarbamate (dtc) ligands are the most popular precipitating agents used for the removal of metals from wastewater. Presently, two novel dtc ligands, one without methylene linker (K2L1) and the other with methylene linker (K2L2) were used for the removal of eight heavy metals ions including Fe2+, Co2+, Ni2+, Cu2+, Cd2+, Ag+, Zn2+ and Pb2+ from the aqueous solution. These dtc ligands contain acyl (NH-C = O) and S, S (thiolate) functional groups. The results indicated that the metal removal efficiency of both ligands was almost 100% at pH 4 and 7 and free of the linker effect and acyl functional groups but depends on the S, S binding sites. It exhibited a distinct selectivity for the removal of metal ions (Pb2+ ∼ Cu2+ ∼ Ag1+ > Cd2+ > Co2+ > Ni2+ > Zn2+ > Fe2+). The developed methods have the potential for the removal of heavy metals and can be used for other thiophilic metals that are water-soluble. HIGHLIGHTS Water treatment, contaminated by heavy metal ions. Synthesis and characterization of two novel dithiocarbamate ligands. Effect of methylene (-CH2-) linker of dithiocarbamtes on heavy metals extraction. pH and temperature effect on the extraction efficiency of ligands.

Enhanced Separation of Sulfur and Metals from Polymetallic Sulfur Slag through Recrystallizing Regulation of Sulfur Crystals

Elemental sulfur is an important non-metallic material that is widely used in various fields including chemical, metallurgical and sulfuric acid industries. Polymetallic sulfur slag (PSS) is an important secondary resource for the recovery of sulfur. However, separation of sulfur from PSS is difficult due to the tight binding of some thiophilic metals to sulfur. Herein, we proposed a recrystallization method for the effective separation of sulfur and metals by using organic solvents to control the particle size of sulfur crystals. It was suggested that the particle size of sulfur recovered in the close to saturation state of toluene is about 2000 μm, while the purity is as high as 99.6%. Moreover, the recovery rate is over 95%, which is more than the current commonly used flotation method. The growth mechanism of sulfur crystals under different saturation conditions lays a foundation for the deepening of the growth theory of large-size sulfur crystals. This method realized the effective separation of sulfur from metal sulfides such as FeS2 and ZnS, which provided an important guidance for the recovery of sulfur resources through a novel method of recrystallizing regulation.

Kinetic Acceleration of Lithium Polysulfide Conversion via a Copper-Iridium Alloying Catalytic Strategy in Li-S Batteries.

To solve the shuttle effect of soluble lithium polysulfides (LiPSs), a porous N-doped carbon-supported copper-iridium alloy catalyst composite (CuIr/NC) has been synthesized and served as a modified cathode sulfur host for lithium-sulfur batteries (LSBs). The metal-organic framework-derived calcined carbon frameworks build efficient conductive channels for fast ion/electron transport. Furthermore, alloying noble metals Ir with thiophilic metal Cu provides abundant active sites to effectively capture LiPSs and accelerate the catalytic conversion process, originating from modulating the surface electronic structure of the metal Cu by introducing Ir atoms to affect the 3d-orbital distribution. All of the above are strongly supported by a range of characterization studies and density functional theory calculations. Benefiting from the above advantages, the LSBs generally show satisfactory cycling performance. Apart from exhibiting a terrific initial specific capacity of 1288 mA h g-1 at 0.2 C, they can also keep long-term cycling stability under a high current density up to 5 C together with a slow specific capacity decay ratio (0.033%) per cycle after 1000 cycles. In addition, it is worth mentioning that a high areal capacity (4.7 mA h cm-2) with a low E/S ratio (6.2 μL mg-1) could still be accomplished at higher sulfur loading (4.3 mg cm-2).

Prebiotic Environments with Mg2+ and Thiophilic Metal Ions Increase the Thermal Stability of Cysteine and Non-cysteine Peptides.

Wet-dry cycles driven by heating to high temperatures are frequently invoked for the prebiotic synthesis of peptides. Similarly, iron-sulfur clusters are often cited as an example of an ancient catalyst that helped prune early chemical systems into metabolic-like pathways. Because extant iron-sulfur clusters are metallocofactors of protein enzymes and nearly ubiquitous across biology, a reasonable hypothesis is that prebiotic iron-sulfur peptides formed on the early Earth. However, iron-sulfur clusters are coordinated by multiple cysteine residues, and the stability of cysteines to the heat steps of wet-dry cycles has not been determined. It, therefore, has remained unclear if the peptides needed to stabilize the formation of iron-sulfur clusters could have formed. If not, then iron-sulfur-dependent activity may have emerged later, when milder, more biological-like peptide synthesis machinery took hold. Here, we report the thermal stability of cysteine-containing peptides. We show that temperatures of 150 °C lead to the rapid degradation of cysteinyl peptides. However, the presence of Mg2+ at environmentally reasonable concentrations leads to significant protection. Thiophilic metal ions also protect against degradation at 150 °C but require concentrations not frequently observed in the environment. Nevertheless, cysteine-containing peptides are stable at lower, prebiotically plausible temperatures in seawater, carbonate lake, and ferrous lake conditions. The data are consistent with the persistence of cysteine-containing peptides on the early Earth in environments rich in metal ions. High concentrations of Mg2+ are common intra- and extra-cellularly, suggesting that the protection afforded by Mg2+ may reflect conditions that were present on the prebiotic Earth.

Anatase Nanocrystals Covalently Functionalized with EDTA-diol: Interaction with Aromatic Sulfur.

The current study addresses the generation of surface-modified nanograined anatase (5 nm) by a single-step in situ process. The EDTA-diol (ester) functionality existing on the surface of anatase nanoparticles was proved with the help of a battery of physicochemical techniques, including FTIR, XPS, and NMR spectral measurements. The sample showed excellent dispersity in ethanol. The anatase lattice showed disorder and had oxygen vacancies. From the XPS analysis, the existence of 14% of Ti3+ was established. The functionalized sample remained thermally stable up to 250 °C, beyond which it transformed into a graphite-titania nanocomposite. The interfacial ligand (ester) to metal (Ti) charge transfer (LMCT) transitions were present in the UV-visible spectrum of the sample and indicated the functionalization to be covalent. Unsaturated OH-groups on the surface yielded a ζ-potential value of -39.8 mV. The covalently functionalized nanotitania was exploited in the adsorptive desulfurization of thiophene, an aromatic sulfur model compound. The consequences of surface adsorption of thiophene were analyzed with the help of UV-visible, FTIR, and 1H and 13C NMR spectra and EDS and XPS measurements. The adsorption data, derived from the batch process, were fitted successfully to pseudo-second-order kinetics and the Temkin model. The adsorption capacity of covalently functionalized titania was compared with several other high surface area adsorbents containing thiophilic metal ions.

Probing Metal-Dependent Phosphate Binding for the Catalysis of the 17E DNAzyme.

The RNA-cleaving 17E DNAzyme exhibits different levels of cleavage activity in the presence of various divalent metal ions, with Pb2+ giving the fastest cleavage. In this study, the metal-phosphate interaction is probed to understand the trend of activity with different metal ions. For the first-row transition metals, the lowest activity shown by Ni2+ correlates with the inhibition by the inorganic phosphate and its water ligand exchange rate, suggesting inner-sphere metal coordination. Cleavage activity with the two stereoisomers of the phosphorothioate-modified substrates, Rp and Sp, indicated that Mg2+, Mn2+, Fe2+, and Co2+ had the highest Sp:Rp activity ratio of >900. Comparatively, the activity was much less affected using the thiophilic metals, including Pb2+, suggesting inner-sphere coordination. The pH-rate profiles showed that Pb2+ was different than the rest of the metal ions in having a smaller slope and a similar fitted apparent pKa and the pKa of metal-bound water. Combining previous reports and our current results, we propose that Pb2+ most likely plays the role of a general acid while the other metal ions are Lewis acid catalysts interacting with the scissile phosphate.

Zn/Ni and Zn/Pd Heterobimetallic Coordination Polymers with [SSC-N(CH2COO)2]3− Ligands

In the construction of heterobimetallic coordination polymers based on dithiocarbamato–carboxylate (DTCC ligands), platinum as a thiophilic metal center can be replaced by the cheaper nickel or palladium. The compounds Zn[Pd(HL)2] and Zn2[M(L)2] (M = Ni, Pd; L = {SSC-N(CH2COO)2}3−) were prepared in a sequential approach starting from K3(L). The products were characterized by IR and NMR spectroscopy, thermal analyses, and single-crystal X-ray diffraction. The products decompose under nitrogen between 300 and 400 °C. Zn[Pd(HL)2] · 6H2O forms polymeric chains in the solid state, and the Zn2[M(L)2] · 14H2O (M = Ni, Pd) exhibit two-dimensional polymeric structures, each being isotypic with the respective Zn/Pt analogs. While the carboxylate groups in all these products are coordinated to zinc in a κO-monodentate mode, a structural variant of Zn2[Ni(L)2] having κO:κO′-briding carboxylate groups was also obtained. Exchange of the metal sites in the two Ni/Zn compounds was not observed, and these compounds are therefore diamagnetic.

Highly sensitive and selective strategy for imaging Hg2+ using near-infrared squaraine dye in live cells and zebrafish

This article describes the rational design and synthesis of a monothiosquaraine (MTSQ) fluorescent Hg2+ probe that exhibits near-infrared fluorescence, ultrahigh brightness, fast response, high sensitivity and excellent selectivity. Utilizing the strategy of the incorporation of the heavy atom effect and self-assembly caused quenching, the background fluorescence of MTSQ was greatly lowered and the sensitivity in PBS buffer solution was highly enhanced with a very low limit of detection (LOD = 1.2 nM). Systematic studies revealed excellent selectivity and specificity for Hg2+ over other thiophilic metal ions such as Ag+, Pb2+, and Cd2+. The mechanistic investigation indicates that self-assembly and desulfurization of MTSQ are responsible for Hg2+-triggered significantly higher fluorescence enhancement. With high sensitivity, optimal pH range of 3–7, weak background, good water-solubility and low cytotoxicity, MTSQ therefore can be applied for imaging Hg2+ in live cells and zebrafish. The very simple but effective strategy reported here can provide a more convenient approach for designing NIR probes.

Further insights into the metal ion binding abilities and the metalation pathway of a plant metallothionein from Musa acuminata.

The superfamily of metallothioneins (MTs) combines a diverse group of metalloproteins, sharing the characteristics of rather low molecular weight and high cysteine content. The latter provides MTs with the capability to coordinate thiophilic metal ions, in particular those with a d 10 electron configuration. The sub-family of plant MT3 proteins is only poorly characterized and there is a complete lack of three-dimensional structure information. Building upon our previous results on the Musa acuminata MT3 (musMT3) protein, the focus of the present work is to understand the metal cluster formation process, the role of the single histidine residue present in musMT3, and the metal ion binding affinity. We concentrate our efforts on the coordination of ZnII and CdII ions, using CoII as a spectroscopic probe for ZnII binding. The overall protein-fold is analysed with a combination of limited proteolytic digestion, mass spectrometry, and dynamic light scattering. Histidine coordination of metal ions is probed with extended X-ray absorption fine structure spectroscopy and CoII titration experiments. Initial experiments with isothermal titration calorimetry provide insights into the thermodynamics of metal ion binding.

A Silver-Specific DNAzyme with a New Silver Aptamer and Salt-Promoted Activity.

Most RNA-cleaving DNAzymes require a metal ion to interact with the scissile phosphate for activity. Therefore, few unmodified DNAzymes work with thiophilic metals because of their low affinity for phosphate. Recently, an Ag+-specific Ag10c DNAzyme was reported via in vitro selection. Herein, Ag10c is characterized to rationalize the role of the strongly thiophilic Ag+. Systematic mutation studies indicate that Ag10c is a highly conserved DNAzyme and its Ag+ binding is unrelated to C-Ag+-C interaction. Its activity is enhanced by increasing Na+ concentrations in buffer. At the same metal concentration, activity decreases in the following order: Li+ > Na+ > K+. Ag10c binds one Na+ ion and two Ag+ ions for catalysis. The pH-rate profile has a slope of ∼1, indicating a single deprotonation step. Phosphorothioate substitution at the scissile phosphate suggests that Na+ interacts with the pro-Rp oxygen of the phosphate, and dimethyl sulfate footprinting indicates that the DNAzyme loop is a silver aptamer binding two Ag+ ions. Therefore, Ag+ exerts its function allosterically, while the scissile phosphate interacts with Na+, Li+, Na+, or Mg2+. This work suggests the possibility of isolating thiophilic metal aptamers based on DNAzyme selection, and it also demonstrates a new Ag+ aptamer.

Sunlight and ultrasound-assisted synthesis of photoluminescent silver nanoclusters: A unique ‘Knock out’ sensor for thiophilic metal ions

We demonstrate the fine tuning of sunlight and ultrasound in green synthesis of photoluminescent silver nanoclusters (AgNCs) having quantum dot (QD) character for the first time. The use of hazardous additives (e.g. reducing agents, organic solvents, etc.) is minimized. The role of sunlight (which ruptures the S–S linkage) and ultrasound (which produces free radicals) in controlling various synthetic parameter as well as properties are carefully monitored and scientifically explained. The synthesized materials have the capabilities in sensing thiophilic metals ions (Hg2+, Pb2+, Cd2+, Zn2+) up to nanomolar (nM) and milimolar (mM) ranges by photoluminescence (PL) spectroscopy and PL microscopy respectively. The sensing of metal ions is based on fluorescence quenching. Hard soft acid base principle (HSAB) plays a major role to recognise the synthesized material as a unique ‘knock out’ sensor. The HR-MS and HR-TEM show that the silver nanocluster is made of 2–6 silver atoms having diameter in the range of 2–3nm respectively. The synthesized materials may be a good candidate for bio-labelling, bio-sensing and cell imaging.

An Efficient Lanthanide-Dependent DNAzyme Cleaving 2'-5'-Linked RNA.

RNA can form two types of linkage. In addition to the predominant 3'-5' linkage, 2'-5'-linked RNA is also important in biology, medicine, and prebiotic studies. Here, in vitro selection was used to isolate a DNAzyme that specifically cleaves 2'-5' RNA by using Ce(3+) as the metal cofactor, but leaves the 3'-5' counterpart intact. This Ce5 DNAzyme requires trivalent light lanthanide ions and shows a rate of 0.16 min(-1) in the presence of 10 μm Ce(3+) ; the activity decreases with heavier lanthanide ions. This is the fastest DNAzyme reported for this reaction, and it might enable applications in chemical biology. As a proof-of-concept, using this DNAzyme, the reactions between phosphorothioate-modified RNA and strongly thiophilic metals (Hg(2+) and Tl(3+) ) were studied as a function of pH.

Supported carbon dots decorated with metallothionein for selective cadmium adsorption and removal

Abstract Carbon dots are prepared and immobilized onto spherical SiO2 through a one-step thermal oxidation and then decorated with metallothionein (MT), a protein with high affinity towards thiophilic metals. The MT-carbon dots composites are characterized by means of FT-IR, SEM and TGA, giving rise to a MT loading amount of 823 μg g−1. The adsorption of cadmium by the composites is a fast process and follows Langmuir model. In comparison with native SiO2, a 2- and 2.4-folds improvement on the static and dynamic adsorption capacity of the composites for cadmium are obtained, respectively. Moreover, the adsorption efficiency is not affected by the presence of other metals. Finally, the composites are successfully applied for the removal of cadmium in a series of environmental water samples.

Biochemical Characterization of a Lanthanide-Dependent DNAzyme with Normal and Phosphorothioate-Modified Substrates.

A trivalent lanthanide (Ln(3+))-dependent RNA-cleaving DNAzyme, Ce13d, was recently isolated via in vitro selection. Ce13d is active in the presence of all Ln(3+) ions. Via introduction of a single phosphorothioate (PS) modification at the cleavage site, its activity with Ln(3+) decreases while all thiophilic metals can activate this DNAzyme. This property is unique to Ce13d and is not found in many other tested DNAzymes. This suggests the presence of a well-defined but general metal binding site. Herein, a systematic study of Ce13d with the PO substrate (using Ce(3+)) and the PS substrate (using Cd(2+)) is performed. In both the PO and PS systems, the highest activity was with ∼10 μM metal ions. Higher concentrations of Ce(3+) completely inhibit the activity, while Cd(2+) only slows the activity. A comparison of different metal ions suggests that the role of metal is to neutralize the phosphate negative charge. Both systems follow a similar pH-rate profile with a single deprotonation step, indicating similar reaction mechanisms. The activity difference between the Rp and Sp form of the PS substrate is <10-fold, which is much smaller than most known RNA-cleaving enzymes. Mutation studies identified eight highly conserved purines, among which the two adenines play mainly structural roles, while the guanines are likely to be involved in metal binding. Ce13d can serve as a model system for further understanding of DNAzyme biochemistry and bioinorganic chemistry.

Rational evolution of Cd2+-specific DNAzymes with phosphorothioate modified cleavage junction and Cd2+sensing

In vitro selection of RNA-cleaving DNAzymes is a powerful method for isolating metal-specific DNA. A few successful examples are known, but it is still difficult to target some thiophilic metals such as Cd2+ due to limited functional groups in DNA. While using modified bases expands the chemical functionality of DNA, a single phosphorothioate modification might boost its affinity for thiophilic metals without complicating the selection process or using bases that are not commercially available. In this work, the first such in vitro selection for Cd2+ is reported. After using a blocking DNA and negative selections to rationally direct the library outcome, a highly specific DNAzyme with only 12 nucleotides in the catalytic loop is isolated. This DNAzyme has a cleavage rate of 0.12 min−1 with 10 μM Cd2+ at pH 6.0. The Rp form of the substrate is cleaved ∼100-fold faster than the Sp form. The DNAzyme is most active with Cd2+ and its selectivity against Zn2+ is over 100 000-fold. Its application in detecting Cd2+ is also demonstrated. The idea of introducing single modifications in the fixed region expands the scope of DNA/metal interactions with minimal perturbation of DNA structure and property.

Unique fluorogenic ratiometric fluorescent chemodosimeter for rapid sensing of CN(-) in water.

A new benzimidazole-spiropyran conjugate chemosensor molecule (BISP) has been synthesized and characterized by (1)H NMR spectroscopy, mass spectrometry (ESI-MS), and elemental analysis. The two isomeric forms (BISP↔BIMC) were shown to be highly selective and sensitive to CN(-) among the ten anions studied in aqueous HEPES buffer, as shown by fluorescence and absorption spectroscopy and even by visual color changes, with a detection limit of 1.7 μM for BIMC. The reaction of CN(-) with BIMC was monitored by (1)H NMR spectroscopy, high-resolution mass spectrometry (HRMS), UV/Vis measurements, and fluorescence spectroscopy in HEPES buffer of pH 7.4. TDDFT calculations were performed in order to correlate the electronic properties of the chemosensor with its cyanide complex. Further, titration against thiophilic metal ions like Au(3+), Cu(2+), Ag(+), and Hg(2+) with [BIMC-CN] in situ showed that it acts as a secondary recognition ensemble toward Au(3+) and Cu(2+) by switch-on fluorescence. In addition, a reversible logic-gate property of BIMC has been demonstrated through a feedback loop in the presence of CN(-) and Au(3+) ions, respectively. Furthermore, the use of BIMC to detect CN(-) in live cells by fluorescence imaging has also been demonstrated. Notably, test strips based on BIMC were fabricated, which could serve as convenient and efficient CN(-) test kits.

Sensing parts-per-trillion Cd(2+), Hg(2+), and Pb(2+) collectively and individually using phosphorothioate DNAzymes.

Cadmium, mercury, and lead are collectively banned by many countries and regions in electronic devices due to their extremely high toxicity. To date, no sensing method can detect them as a group and also individually with sufficient sensitivity and selectivity. An RNA-cleaving DNAzyme (Ce13d) was recently reported to be active with trivalent lanthanides, which are hard Lewis acids. In this work, phosphorothioate (PS) modifications were systematically made on Ce13d. A single PS modification at the substrate cleavage site shifts the activity from being dependent on lanthanides to soft thiophilic metals. By incorporating the PS modification to another DNAzyme, a sensor array was prepared to detect each metal. Individual sensors have excellent sensitivity (limit of detection = 4.8 nM Cd(2+), 2.0 nM Hg(2+), and 0.1 nM Pb(2+)). This study provides a new route to obtain metal-specific DNAzymes by atomic replacement and also offers important mechanistic insights into metal binding and DNAzyme catalysis.