Mercury(II) Complexes of Imidazole and Histidine

Highly efficient Hg2+ removal via a competitive strategy using a Co-based metal organic framework ZIF-67

The stronger coordination ability of mercury ions with organic ligands than the metal ions in metal organic framework (MOFs) provides an accessible way to separate mercury ions from solution using specific MOFs. In this study, a Co-based MOF (ZIF-67, Co(mIM)₂) was synthesized. It did not introduce specific functional groups, such as -SH and -NH₂, into its structure through complicated steps. It separate Hg²⁺ from wastewater with a new strategy, which utilized the stronger coordination ability of Hg²⁺ with the nitrogen atom on the imidazole ring of the organic ligand than the Co²⁺ ions. Hg²⁺ replaced Co²⁺ nodes from ZIF-67 and formed a more stable precipitate with mIM. The experimental results showed that this new strategy was efficient. ZIF-67 exhibited Hg²⁺ adsorption capacity of 1740 mg/g, much higher than the known MOFs sorbents. mIMs is the reaction center and ZIF-67 can improve its utilization. The sample color faded from purple to white due to the loss of cobalt ion. It is a great feature of ZIF-67 that allows users to judge whether the sorbent is deactivated intuitively. ZIF-67 can be sustainable recycled by adding organic ligands to the solution after treatment due to its simple synthesis method at room temperature. It's a high-efficient and sustainable sorbent for Hg²⁺ separation from wastewater.

Hg(II) Binding to Thymine Bases in DNA

The compounds of mercury can be highly toxic and can interfere with a range of biological processes, although many aspects of the mechanism of toxicity are still obscure or unknown. One especially intriguing property of Hg(II) is its ability to bind DNA directly, making interstrand cross-links between thymine nucleobases in AT-rich sequences. We have used a combination of small molecule X-ray diffraction, X-ray spectroscopies, and computational chemistry to study the interactions of Hg(II) with thymine. We find that the energetically preferred mode of thymine binding in DNA is to the N3 and predict only minor distortions of the DNA structure on binding one Hg(II) to two cross-adjacent thymine nucleotides. The preferred geometry is predicted to be twisted away from coplanar through a torsion angle of between 32 and 43°. Using 1-methylthymine as a model, the bis-thymine coordination of Hg(II) is found to give a highly characteristic X-ray spectroscopic signature that is quite distinct from other previously described biological modes of binding of Hg(II). This work enlarges and deepens our view of significant biological targets of Hg(II) and demonstrates tools that can provide a characteristic signature for the binding of Hg(II) to DNA in more complex matrices including intact cells and tissues, laying the foundation for future studies of mechanisms of mercury toxicity.

Molecular Dissection of dH3w, A Fluorescent Peptidyl Sensor for Zinc and Mercury

Previously, we reported that fluorescent peptide dansyl-HPHGHW-NH2 (dH3w), designed on the repeats of the human histidine-rich glycoprotein, shows a turn-on response to Zn(II) and a complex response to Hg(II) characterized by a turn-off phase at low Hg(II) concentrations and a turn-on phase at high concentrations. As Hg(II) easily displaces Zn(II), dH3w is a useful probe for the environmental monitoring of Hg(II). In order to investigate the molecular basis of the metal selectivity and fluorescence response, we characterized three variants, dH3w(H1A), dH3w(H3A), and dH3w(H5A), in which each of the three histidine residues was changed to alanine, and two variants with a single fluorescent moiety, namely dH3w(W6A), in which the tryptophan residue at the C-terminus was changed to alanine, and AcH3w, in which the N-terminal dansyl moiety was substituted by an acetyl group. These variants allowed us to demonstrate that all the histidine residues are essential for a strong interaction with Zn(II), whereas two histidine residues (in particular His5) and the dansyl group are necessary to bind Hg(II). The data reported herein shed light on the molecular behavior of dH3w, thus paving the way to the rational designing of further and more efficient fluorescent peptidyl probes for Hg(II).

Fluorescent peptide dH3w: A sensor for environmental monitoring of mercury (II)

Heavy metals are hazardous environmental contaminants, often highly toxic even at extremely low concentrations. Monitoring their presence in environmental samples is an important but complex task that has attracted the attention of many research groups. We have previously developed a fluorescent peptidyl sensor, dH3w, for monitoring Zn2+ in living cells. This probe, designed on the base on the internal repeats of the human histidine rich glycoprotein, shows a turn on response to Zn2+ and a turn off response to Cu2+. Other heavy metals (Mn2+, Fe2+, Ni2+, Co2+, Pb2+ and Cd2+) do not interfere with the detection of Zn2+ and Cu2+. Here we report that dH3w has an affinity for Hg2+ considerably higher than that for Zn2+ or Cu2+, therefore the strong fluorescence of the Zn2+/dH3w complex is quenched when it is exposed to aqueous solutions of Hg2+, allowing the detection of sub-micromolar levels of Hg2+. Fluorescence of the Zn2+/dH3w complex is also quenched by Cu2+ whereas other heavy metals (Mn2+, Fe2+, Ni2+, Co2+, Cd2+, Pb2+, Sn2+ and Cr3+) have no effect. The high affinity and selectivity suggest that dH3w and the Zn2+/dH3w complex are suited as fluorescent sensor for the detection of Hg2+ and Cu2+ in environmental as well as biological samples.

Transcriptional responses of Escherichia coli during recovery from inorganic or organic mercury exposure

Background

The protean chemical properties of mercury have long made it attractive for diverse applications, but its toxicity requires great care in its use, disposal, and recycling. Mercury occurs in multiple chemical forms, and the molecular basis for the distinct toxicity of its various forms is only partly understood. Global transcriptomics applied over time can reveal how a cell recognizes a toxicant and what cellular subsystems it marshals to repair and recover from the damage. The longitudinal effects on the transcriptome of exponential phase E. coli were compared during sub-acute exposure to mercuric chloride (HgCl₂) or to phenylmercuric acetate (PMA) using RNA-Seq.

Results

Differential gene expression revealed common and distinct responses to the mercurials throughout recovery. Cultures exhibited growth stasis immediately after each mercurial exposure but returned to normal growth more quickly after PMA exposure than after HgCl₂ exposure. Correspondingly, PMA rapidly elicited up-regulation of a large number of genes which continued for 30 min, whereas fewer genes were up-regulated early after HgCl₂ exposure only some of which overlapped with PMA up-regulated genes. By 60 min gene expression in PMA-exposed cells was almost indistinguishable from unexposed cells, but HgCl₂ exposed cells still had many differentially expressed genes. Relative expression of energy production and most metabolite uptake pathways declined with both compounds, but nearly all stress response systems were up-regulated by one or the other mercurial during recovery.

Conclusions

Sub-acute exposure influenced expression of ~45% of all genes with many distinct responses for each compound, reflecting differential biochemical damage by each mercurial and the corresponding resources available for repair. This study is the first global, high-resolution view of the transcriptional responses to any common toxicant in a prokaryotic model system from exposure to recovery of active growth. The responses provoked by these two mercurials in this model bacterium also provide insights about how higher organisms may respond to these ubiquitous metal toxicants.

Electronic supplementary material

The online version of this article (10.1186/s12864-017-4413-z) contains supplementary material, which is available to authorized users.

A modular platform to develop peptoid-based selective fluorescent metal sensors

Despite the reduction in industrial use of toxic heavy metals, there remain contaminated natural water sources across the world. Herein we present a modular platform for developing selective sensors for toxic metal ions using N-substituted glycine, or peptoid, oligomers coupled to a fluorophore. As a preliminary evaluation of this strategy, structures based on previously identified metal-binding peptoids were synthesized with terminal pyrene moieties. Both derivatives of this initial design demonstrated a turn-off response in the presence of various metal ions. A colorimetric screen was designed to identify a peptoid ligand that chelates Hg(ii). Multiple ligands were identified that were able to deplete Hg(ii) from a solution selectively in the presence of an excess of competing ions. The C-terminal fluoropeptoid derivatives demonstrated similar selectivity to their label-free counterparts. This strategy could be applied to develop sensors for many different metal ions of interest using a variety of fluorophores, leading to a panel of sensors for identifying various water source contaminants.

A turn on fluorescent sensor based on lanthanide coordination polymer nanoparticles for the detection of mercury(ii) in biological fluids

Im-quenched fluorescence of Eu/IPA CPNPs can be recovered upon the addition of Hg²⁺ through the formation of a Hg/Im complex.

C-Phycocyanin as a potential biosensor for heavy metals like Hg2+ in aquatic systems

Fluorescence quenching ability of C-phycocyanin (CPC) as a biosensor for detection of Hg²⁺ in lower concentrations (μM) in aquatic environment.

Organic and inorganic mercurials have distinct effects on cellular thiols, metal homeostasis, and Fe-binding proteins in Escherichia coli

The protean chemical properties of the toxic metal mercury (Hg) have made it attractive in diverse applications since antiquity. However, growing public concern has led to an international agreement to decrease its impact on health and the environment. During a recent proteomics study of acute Hg exposure in E. coli, we also examined the effects of inorganic and organic Hg compounds on thiol and metal homeostases. On brief exposure, lower concentrations of divalent inorganic mercury Hg(II) blocked bulk cellular thiols and protein-associated thiols more completely than higher concentrations of monovalent organomercurials, phenylmercuric acetate (PMA) and merthiolate (MT). Cells bound Hg(II) and PMA in excess of their available thiol ligands; X-ray absorption spectroscopy indicated nitrogens as likely additional ligands. The mercurials released protein-bound iron (Fe) more effectively than common organic oxidants and all disturbed the Na(+)/K(+) electrolyte balance, but none provoked efflux of six essential transition metals including Fe. PMA and MT made stable cysteine monothiol adducts in many Fe-binding proteins, but stable Hg(II) adducts were only seen in CysXxx(n)Cys peptides. We conclude that on acute exposure: (a) the distinct effects of mercurials on thiol and Fe homeostases reflected their different uptake and valences; (b) their similar effects on essential metal and electrolyte homeostases reflected the energy dependence of these processes; and (c) peptide phenylmercury-adducts were more stable or detectable in mass spectrometry than Hg(II)-adducts. These first in vivo observations in a well-defined model organism reveal differences upon acute exposure to inorganic and organic mercurials that may underlie their distinct toxicology.

Removal of Hg(II) by poly(1-vinylimidazole)-grafted Fe3O4@SiO2 magnetic nanoparticles

Fe3O4@SiO2 magnetic nanoparticles modified by grafting poly(1-vinylimidazole) oligomer (FSPV) was fabricated as a novel adsorbent to remove Hg(II) from water. Fourier transform infra-red spectroscopy confirmed the successful grafting of oligomer, and thermogravimetric analysis showed FSPV had a high grafting yield with organic content of 22.8%. Transmission electron microscopy image displayed that FSPV particles were polymer-coated spheres with size of 10-20 nm. With saturation magnetization of 44.7 emu/g, FSPV particles could be easily separated from water with a simple magnetic process in 5 min. The Hg(II) adsorption capacity of FSPV was found to be 346 mg/g at pH 7 and 25 °C in 10 mM NaCl. Moreover, the removal of Hg(II) by FSPV was not obviously affected by solution pH (from 4 to 10) or humic acid (up to 8 mg/L as TOC). The presence of seven common ions including Na(+), K(+), Ca(2+), Mg(2+), Cl(-), NO3(-), and SO4(2-) (up to 100 mM ionic strength) slightly increased the adsorption of Hg(II) by FSPV. X-ray photoelectron spectroscopy analysis revealed that the N atom of the imidazole ring was responsible for the bonding with Hg(II), whereas the bonding of Hg with N did not result in cleavage of Hg-Cl bond in HgCl2 and HgClOH. The regeneration of Hg(II)-loaded FSPV could be achieved with 0.5 M HCl rapidly in 10 min, and the removal of Hg(II) maintained above 94% in five consecutive adsorption-desorption cycles. Therefore, FSPV could serve as a promising adsorbent for Hg(II) removal from water.

catena-Poly[[[bis-(1H-imidazole-κN (3))zinc(II)]-μ2-imidazol-1-ido-κ(2) N:N'] nitrate]

The title compound, {[Zn(C3H3N2)(C3H4N2)2]NO3} n , is a one-dimensional coordination polymer along [01-1] with the Zn(II) atom coordinating to four imidazole/imidazolide rings. The Zn(II) atom has a regular tetra-hedral geometry with the planes of the two monodentate imidazole rings inclined to one another by 87.94 (17)°, while the planes of the bridging imidazolide rings are inclined to one another by 39.06 (17)°. In the crystal, the chains are linked via bifurcated N-H⋯(O,O) hydrogen bonds, forming sheets parallel to (001). These two-dimensional networks are linked via C-H⋯O hydrogen bonds and a C-H⋯π inter-action, forming a three-dimensional structure.

Synthesis, CP-MAS NMR Characterization, and Antibacterial Activities of Glycine and Histidine Complexes of Cd(SeCN) 2 and Hg(SeCN) 2

The synthesis and characterization of cadmium and mercury complexes of selenocyanate of the type [(L)M(SeCN)₂] are described, where L is L-Histidine (His) or L-Glycine (Gly) and M is Cd²⁺ or Hg²⁺. These complexes are obtained by the reaction of 1 equivalent of respective amino acids with metal diselenocyanate precursor in a mixture of solvents (methanol : water = 1 : 1). These synthesized compounds are characterized by analytical and various spectroscopic techniques such as elemental analysis (EA), IR, H,1 and C13 NMR in solution and in the solid state for C13 and N15. The in vitro antibacterial activities of these complexes have been investigated with standard type cultures of Escherichia coli (MTCC 443), Klebsiella pneumoniae (MTCC 109), Pseudomonas aeruginosa (MTCC 1688), Salmonella typhi (MTCC 733), and Staphylococcus aureus (MTCC 737).

N-(Imidazol-1-ylmethyl)phthalimide

The title compound [systematic name: 2-(imidazol-1-ylmeth­yl)isoindole-1,3-dione], C₁₂H₉N₃O₂, was prepared by reaction of N-(bromo­meth­yl)phthalimide and imidazole in chloro­form solution. The crystal structure is stabilized by weak inter­molecular C—H⋯π inter­actions and inter­molecular π–π inter­actions with centroid–centroid distances in the range 3.6469 (8)–3.8831 (9) Å.

Counterions influence reactivity of metal ions with cysteinyldopa model compounds

Cysteinyldopas are naturally occurring conjugates of cysteine and dopa (3,4-dihydroxy-l-phenylalanine) that are precursors to red pheomelanin pigments. Metal ions are known to influence pheomelanogenesis in vitro and may be regulatory factors in vivo. Cydo (3-[(2-amino-ethyl)sulfanyl]-4,6-di-tert-butylbenzene-1,2-diol) and CarboxyCydo (2-amino-3-(4,6-di-tert-butyl-2,3-dihydroxyphenylsulfanyl)-propionic acid) are model compounds of cysteinyldopa that retain its metal-binding functionalities but cannot polymerize due to the presence of blocking tert-butyl groups. Cydo reacts readily with zinc(II) acetate or nickel(II) acetate to form a cyclized 1,4-benzothiazine (zine) intermediate that undergoes ring contraction to form benzothiazole (zole) unless it is stabilized by coordination to a metal ion. The crystal structure of [Ni(zine)2] is reported. The acetate counteranion is required for the zinc-promoted reactivity, as neither zinc(II) sulfate nor zinc(II) chloride alone promotes the transformation. The counterion is less important for redox-active copper and iron, which both readily promote the oxidation of Cydo to zine and zole species; Cu(II) complexes of both zine and zole have been characterized by X-ray crystallography. In the case of CarboxyCydo, a 3-carboxy-1,4-benzothiazine intermediate decarboxylates to form [Cu(zine)2] under basic conditions, but in the absence of base forms a mixture of products that includes the carboxylated dimer 2,2'-bibenzothiazine (bi-zine). These products are consistent with species implicated in the pheomelanogenesis biosynthetic pathway and emphasize how metal ions, their counteranions, and reaction conditions can alter pheomelanin product distribution.

Synthesis and Crystal Structure of Hexakis(imidazole) nickel (II) O,O′-diphenyldithiophosphate [Ni(Im)6](Ph2O2PS2)2

The crystal and molecular structures of [Ni(Im)₆](dtp)₂ (Im = imidazole, dtp = O,O′-diphenyldithiophosphate) have been determined by X-ray crystallography. It crystallizes in the triclinic system, space group Pī, with cell parameters a = 9.375 (2), b = 12.324(3), c = 13.285(3) Å, α = 107.86(3), β = 102.28(3), γ = 109.24(3), and Z = 1. The crystal structure of the title compound is built up of discrete monomeric molecules of [Ni(Im)₆](dtp)₂. The nickel (II) ion is hexacoordinated by six imidazole molecules and the coordination environment of Ni (II) is of octahedral geometry. In the solid state, a network of N-H∙∙∙S intermolecular hydrogen bonds connect the Ni(Im)₆ moieties and O,O′-diphenyl- dithiophosphate molecules, forming a three-dimensional structure.

Proton magnetic resonance studies of carbonic anhydrase. II. Group controlling catalytic activity

The seven resonances observed in the histidine region of the proton magnetic resonance (pmr) spectrum of human carbonic anhydrase B and reported in the preceding paper are studied in the presence of sulfonamide, azide, cyanide, and chloride inhibitors and in metal-free, cadmium substituted, cobalt substituted, and carboxymethylated forms of the enzyme. Results indicate that the two resonances that move-downfield with increasing pH and the two that do not move with pH reflect residues located at the active site. The first two resonances are assigned to the same titratable histidine whose pK value of 8.24 corresponds to that of the group controlling catalytic activity. Addition of anions or sulfonamides, removal of zinc, or substitution of cadmium for zinc at the active site, procedures known to abolish enzymatic activity, prevent titration of this residue. Partial inhibition of carbonic anhydrase by chloride slectively increases the pK value of the group controlling catalytic activity and of the histidine with pK equals 8.24. Experiments with metal-free and cadmium carbonic anhydrases and comparisons with model systems suggest that this histidine is bound to the metal ion at high pH; at low pH this complex appears to dissociate as protons compete with the metal for the imidazole group. It is proposed that ionization of the group controlling catalytic activity represents loss of the pyrrole proton of this neutral ligand when it binds to Zn(II), forming an imidazolate anion and juxtaposing a strong base and a powerful Lewis acid at the active site. When bound to zinc as an anion, this histidine can act as a general base catalyst in the hydration of carbon dioxide and be replaced as a metal ligand by an oxygen of the substrate in the course of the reaction. The histidine-metal complex is thought to exist in a strained configuration in the active enzyme so that its imidazole-metal bond is readily broken on addition of substrates or inhibitors. This model is consistent with the available data on the enzyme and is discussed in relation to alternative proposals.