Protolytic cleavage of Hg-C bonds induced by 1-methyl-1,3-dihydro-2H-benzimidazole-2-selone: synthesis and structural characterization of mercury complexes

Emerging evidence on selenoneine and its public health relevance in coastal populations: a review and case study of dietary Se among Inuit populations in the Canadian Arctic

Selenium is an essential mineral yet both deficiency and excess are associated with adverse health effects. Dietary intake of Se in humans varies greatly between populations due to food availability, dietary preferences, and local geological and ecosystem processes impacting Se accumulation into agricultural products and animal populations. We argue there is a need to evaluate and reconsider the relevance of public health recommendations on Se given recent evidence, including the metabolic pathways and health implications of Se. This argument is particularly pertinent for Inuit populations in Northern Canada, who often exceed dietary tolerable upper intake levels and exhibit very high whole blood Se concentrations due to their dependence on local country foods high in the newly discovered Se compound, selenoneine. Since selenoneine appears to have lower toxicity compared to other Se species and does not contribute to the circulating pools of Se for selenoprotein synthesis, we argue that total dietary Se or total Se in plasma or whole blood are poor indicators of Se adequacy for human health in these populations. Overall, this review provides an overview of the current evidence of Se speciation, deficiency, adequacy, and excess and implications for human health and dietary recommendations, with particular reference to Inuit populations in the Canadian Arctic and other coastal populations consuming marine foods.

Chemical-Shift Standards for 199Hg NMR Spectroscopy, 25 Years Later

While 199Hg NMR is a well-established tool for elucidating details of coordination chemistry in biochemical and inorganic complexes, historically the technique has been associated with the use of an extremely toxic chemical, dimethylmercury [Me2Hg or (CH3)2Hg], as a reference standard. In the 25 years since an accidental exposure to Me2Hg led to the tragic death of Dr. Karen Wetterhahn, the community has learned a great deal about the insidious neurotoxicity of this compound as well as more appropriate ways to avoid exposure. Here, we track the general shift toward the use of alternative mercury reference standards and away from Me2Hg.

Concentrations of nucleophilic sulfur species in small Indian mongoose (Herpestes auropunctatus) in Okinawa, Japan

Reactive sulfur species (RSS), such as hydrogen per (poly)sulfide, cysteine per (poly)sulfide, glutathione per (poly)sulfide, and protein-bound per (poly)sulfides, can easily react with environmental electrophiles such as methylmercury (MeHg), because of their high nucleophilicity. These RSS are produced by enzymes such as cystathionine β-synthase (CBS) and cystathionine γ-lyase (CSE) and are found in mammalian organs. Organs of wildlife have not been analyzed for hydrogen sulfide, cysteine, glutathione, and RSS. In this study, low molecular weight nucleophilic sulfur substances, including RSS, were quantified by stable isotope dilution assay-based liquid chromatography-mass spectrometry using β-(4-hydroxyphenyl)ethyl iodoacetamide to capture the target chemicals in the small Indian mongoose which species possesses high mercury content as same as some marine mammals. Western blotting revealed that the mongoose organs (liver, kidney, cerebrum, and cerebellum) contained proteins that cross-reacted with anti-CBS and CSE antibodies. The expression patterns of these enzymes were similar to those in mice, indicating that mongoose organs contain CBS and CSE. Moreover, bis-methylmercury sulfide (MeHg)₂S, which is a low toxic compound in comparison to MeHg, was found in the liver of this species. These results suggest that the small Indian mongoose produces RSS and monothiols associated with detoxification of electrophilic organomercury. The animals which have high mercury content in their bodies may have function of mercury detoxification involved not only Se but also RSS interactions.

New insights into the biomineralization of mercury selenide nanoparticles through stable isotope analysis in giant petrel tissues

Tiemannite (HgSe) is considered the end-product of methylmercury (MeHg) demethylation in vertebrates. The biomineralization of HgSe nanoparticles (NPs) is understood to be an efficient MeHg detoxification mechanism; however, the process has not yet been fully elucidated. In order to contribute to the understanding of complex Hg metabolism and HgSe NPs formation, the Hg isotopic signatures of 40 samples of 11 giant petrels were measured. This seabird species is one of the largest avian scavengers in the Southern Ocean, highly exposed to MeHg through their diet, reaching Hg concentrations in the liver up to more than 900 µg g^-1. This work constitutes the first species-specific isotopic measurement (δ²⁰²Hg, Δ¹⁹⁹Hg) of HgSe NPs in seabirds and the largest characterization of this compound in biota. Similar δ²⁰²Hg values specifically associated to HgSe (δ²⁰²Hg_HgSe) and tissues (δ²⁰²Hg_bulk) dominated by inorganic Hg species were found, suggesting that no isotopic fractionation is induced during the biomineralization step from the precursor (demethylated) species. In contrast, the largest variations between δ²⁰²Hg_bulk and δ²⁰²Hg_HgSe were observed in muscle and brain tissues. This could be attributed to the higher fraction of Hg present as MeHg in these tissues. Hg-biomolecules screening highlights the importance of the isotopic characterization of these (unknown) complexes.

First Time Identification of Selenoneine in Seabirds and Its Potential Role in Mercury Detoxification

Birds are principally exposed to selenium (Se) through their diet. In long-lived and top predator seabirds, such as the giant petrel, extremely high concentrations of Se are found. Selenium speciation in biota has aroused great interest in recent years; however, there is a lack of information about the chemical form of Se in (sea)birds. The majority of publications focus on the growth performance and antioxidant status in broilers in relation to Se dietary supplementation. The present work combines elemental and molecular mass spectrometry for the characterization of Se species in wild (sea)birds. A set of eight giant petrels (Macronectes sp.) with a broad age range from the Southern Ocean were studied. Selenoneine, a Se-analogue of ergothioneine, was identified for the first time in wild avian species. This novel Se-compound, previously reported in fish, constitutes the major Se species in the water-soluble fraction of all of the internal tissues and blood samples analyzed. The levels of selenoneine found in giant petrels are the highest reported in animal tissues until now, supporting the trophic transfer in the marine food web. The characterization of selenoneine in the brain, representing between 78 and 88% of the total Se, suggests a crucial role in the nervous system. The dramatic decrease of selenoneine (from 68 to 3%) with an increase of Hg concentrations in the liver strongly supports the hypothesis of its key role in Hg detoxification.

In Vivo Formation of HgSe Nanoparticles and Hg-Tetraselenolate Complex from Methylmercury in Seabirds-Implications for the Hg-Se Antagonism

In vivo and in vitro evidence for detoxification of methylmercury (MeHg) as insoluble mercury selenide (HgSe) underlies the central paradigm that mercury exposure is not or little hazardous when tissue Se is in molar excess (Se:Hg > 1). However, this hypothesis overlooks the binding of Hg to selenoproteins, which lowers the amount of bioavailable Se that acts as a detoxification reservoir for MeHg, thereby underestimating the toxicity of mercury. This question was addressed by determining the chemical forms of Hg in various tissues of giant petrels Macronectes spp. using a combination of high energy-resolution X-ray absorption near edge structure and extended X-ray absorption fine structure spectroscopy, and transmission electron microscopy coupled to elemental mapping. Three main Hg species were identified, a MeHg-cysteinate complex, a four-coordinate selenocysteinate complex (Hg(Sec)₄), and a HgSe precipitate, together with a minor dicysteinate complex Hg(Cys)₂. The amount of HgSe decreases in the order liver > kidneys > brain = muscle, and the amount of Hg(Sec)₄ in the order muscle > kidneys > brain > liver. On the basis of biochemical considerations and structural modeling, we hypothesize that Hg(Sec)₄ is bound to the carboxy-terminus domain of selenoprotein P (SelP) which contains 12 Sec residues. Structural flexibility allows SelP to form multinuclear Hg_x(Se,Sec)_y complexes, which can be biomineralized to HgSe by protein self-assembly. Because Hg(Sec)₄ has a Se:Hg molar ratio of 4:1, this species severely depletes the stock of bioavailable Se for selenoprotein synthesis and activity to one μg Se/g dry wet in the muscle of several birds. This concentration is still relatively high because selenium is naturally abundant in seawater, therefore it probably does not fall below the metabolic need for essential selenium. However, this study shows that this may not be the case for terrestrial animals, and that muscle may be the first tissue potentially injured by Hg toxicity.

Demethylation of Methylmercury in Bird, Fish, and Earthworm

Toxicity of methylmercury (MeHg) to wildlife and humans results from its binding to cysteine residues of proteins, forming MeHg-cysteinate (MeHgCys) complexes that hinder biological functions. MeHgCys complexes can be detoxified in vivo, yet how this occurs is unknown. We report that MeHgCys complexes are transformed into selenocysteinate [Hg(Sec)₄] complexes in multiple animals from two phyla (a waterbird, freshwater fish, and earthworms) sampled in different geographical areas and contaminated by different Hg sources. In addition, high energy-resolution X-ray absorption spectroscopy (HR-XANES) and chromatography-inductively coupled plasma mass spectrometry of the waterbird liver support the binding of Hg(Sec)₄ to selenoprotein P and biomineralization of Hg(Sec)₄ to chemically inert nanoparticulate mercury selenide (HgSe). The results provide a foundation for understanding mercury detoxification in higher organisms and suggest that the identified MeHgCys to Hg(Sec)₄ demethylation pathway is common in nature.

Hg-C bond protonolysis by a functional model of bacterial enzyme organomercurial lyase MerB

Herein, we report a novel synthetic compound 1, having a highly nucleophilic selenolate (Se^-) moiety and a thiol (-SH) functional group, which showed efficient Hg-C bond protonolysis of various R-Hg-X molecules including neurotoxic methylmercury and thimerosal, via direct -SH proton transfer to the highly activated C-atom of a departed R group with low activation energy barrier at room temperature (21 °C), in the absence of any external proton source and, thus, acts as a functional model of MerB.

Homoleptic and heteroleptic Zn(ii) selone catalysts for thioetherification of aryl halides without scrubbing oxygen

Tetra-coordinated zinc imidazoline selone complexes were synthesized and utilized as potential catalysts in the thioetherification of aryl halides without scrubbing oxygen.

Exploiting the κ2 -Fashioned Coordination of [Se2 ]-Donor Ligand L3 Se for Facile Hg-C Bond Cleavage of Mercury Alkyls and Cytoprotection against Methylmercury-Induced Toxicity

The Hg-C bond of MeHgCl, a ubiquitous environmental toxicant, is notoriously inert and exceedingly difficult to cleave. The cleavage of the Hg-C bond of MeHgCl at low temperature, therefore, is of significant importance for human health. Among various bis(imidazole)-2-selones L_n Se (n=1-4, or 6), the three-spacer L₃ Se shows extraordinarily high reactivity in the degradation of various mercury alkyls including MeHgCl because of its unique ability to coordinate through κ² -fashion, in which both the Se atoms simultaneously attack the Hg center of mercury alkyls for facile Hg-C bond cleavage. It has the highest softness (σ) parameter and the lowest HOMO(L_n Se)-LUMO(MeHgX) energy gap and, thus, L₃ Se is the most reactive among L_n Se towards MeHgX (X=Cl or I). L₃ Se is highly efficient, more than L₁ Se, in restoring the activity of antioxidant enzyme glutathione reductase (GR) that is completely inhibited by MeHgCl; 80 % GR activity is recovered by L₃ Se relative to 50 % by L₁ Se. It shows an excellent cytoprotective effect in liver cells against MeHgCl-induced oxidative stress by protecting vital antioxidant enzymes from inhibition caused by MeHgCl and, thus, does not allow to increase the intracellular reactive oxygen species (ROS) levels. Furthermore, it protects the mitochondrial membrane potential (ΔΨ_m ) from perturbation by MeHgCl. Major Hg-responsive genes analyses demonstrate that L₃ Se plays a significant role in MeHg⁺ detoxification in liver cells.

Determinants of selenoneine concentration in red blood cells of Inuit from Nunavik (Northern Québec, Canada)

Selenium (Se) is a trace mineral essential to human health, and is especially abundant in marine foods consumed by Inuit populations in Nunavik (northern Quebec, Canada), leading to exceptionally high whole blood Se levels. While most epidemiological studies to date examine plasma or whole blood Se, little is known about the health implications of specific Se biomarkers (e.g. selenoproteins and small Se compounds). Selenoneine, a novel Se compound, is found in high concentrations in marine foods (and particularly beluga mattaaq) and the red blood cells (RBCs) of populations that consume them. We report here RBC selenoneine concentrations in a population of Inuit adults (n = 885) who participated in the Qanuippitaa? 2004 survey. Simple associations between RBC selenoneine and other Se and mercury (Hg) biomarkers were assessed using Spearman correlations and linear regressions. Wilcoxon ranksum tests were used to examine differences in biomarkers and characteristics between tertiles of RBC selenoneine concentration. A multiple linear regression analysis was used to determine factors (sociodemographic, lifestyle, and dietary) associated with RBC selenoneine concentrations. Selenoneine comprised a large proportion of whole blood Se and RBC Se in this population. Age and sex-adjusted geometric mean RBC selenoneine concentration was 118 μg/L (range: 1-3226 μg/L) and was much higher (p = 0.001) among women (150.3 μg/L) than men (87.6 μg/L) across all regions of Nunavik after controlling for age, region, and diet. RBC selenoneine was highly correlated with RBC Se (r_s = 0.96, p < 0.001) and whole blood Se (r_s = 0.89, p < 0.001), but only weakly correlated with plasma Se (r_s = 0.13, p < 0.001). Overall, increasing age (standardized β = 0.24), higher body-mass index (BMI; β = 0.08), female sex (β = 0.10), living in a Hudson Strait community (compared to Hudson Bay and Ungava Bay; β = 0.38), and consuming beluga mattaaq (g/day; β = 0.19) were positively associated with RBC selenoneine. Meanwhile, consumption of market meats (g/day; β = -0.07) was negatively associated with RBC selenoneine. RBC selenoneine is an important biomarker of Se dietary intake from local marine foods in Inuit populations. Further studies are needed to examine the health effects of selenoneine intake and the underlying mechanisms for sex differences among Inuit populations.

Coordination of 1-methyl-1,3-dihydro-2H-benzimidazole-2-selone to zinc and cadmium: Monotonic and non-monotonic bond length variations for [H(sebenzimMe)]2MCl2 complexes (M = Zn, Cd, Hg)

The reactions of 1-methyl-1,3-dihydro-2H-benzimidazole-2-selone, H(sebenzimMe), towards the zinc and cadmium halides, MX2 (M = Zn, Cd; X = Cl, Br, I), afford the adducts, [H(sebenzimMe)]2MX2, which have been structurally characterized by X-ray diffraction. The halide ligands of each of these complexes participate in hydrogen bonding interactions with the imidazole N-H moieties, although the nature of the interactions depends on the halide. Specifically, the chloride and bromide derivatives, [H(sebenzimMe)]2ZnX2 and [H(sebenzimMe)]2CdX2 (X = Cl, Br), exhibit two intramolecular N-H•••X interactions, whereas the iodide derivatives, [H(sebenzimMe)]2ZnI2 and [H(sebenzimMe)]2CdI2, exhibit only one intramolecular N-H•••I interaction. Comparison of the M-Se and M-Cl bond lengths of the chloride series, [H(sebenzimMe)]2MCl2 (M = Zn, Cd, Hg), indicates that while the average M-Cl bond lengths progressively increase as the metal becomes heavier, the variation in M-Se bond length exhibits a non-monotonic trend, with the Cd-Se bond being the longest. These different trends provide an interesting subtlety concerned with use of covalent radii in predicting bond length differences. In addition to tetrahedral [H(sebenzimMe)]2CdCl2, [H(sebenzimMe)]3,CdCl2•[H(sebenzim)Me]4CdCl2, which features both five-coordinate and six-coordinate coordinate centers, has also been structurally characterized. Finally, the reaction between CdI2 and H(sebenzimMe) at elevated temperatures affords the 1-methylbenzimidazole complex, [H(sebenzimMe)]-[H(benzimMe)]CdI2, a transformation that is associated with cleavage of the C-Se bond.

Luminescent zinc(ii) selone macrocyclic ring

The synthesis and photophysical properties of macrocyclic Zn(ii) selone molecule have been reported. The structural property of Zn(ii) selone was elucidated using single crystal X-ray diffraction study. The solid-state structure of zinc(ii) selone molecule exhibits a perfect zinc(ii) selone 28 membered ring system with tetra coordination geometry around zinc(ii) center. The zinc(ii) selone ring system can be considered as the largest zinc(ii) ring system known without any non-interacting centered guest moiety. Detailed trends in photophysical as well as thermal properties were probed. In photoluminescence study, the solid-state sample of zinc(ii) selone ring system emits the bluish-yellow color with considerable quantum yields, while the solution state sample of zinc(ii) selone ring system in DMSO emits bluish-yellow. The luminescence lifetime of zinc(ii) selone was measured using standard time-correlated single photon counting (TCSPC) technique.

Cytoprotective effects of imidazole-based [S1] and [S2]-donor ligands against mercury toxicity: a bioinorganic approach

Here we report the coordination behaviour of an imidazole-based [S1]-donor ligand, 1,3-dimethyl-imidazole-2(3H)-thione (L1), and [S2]-donor ligand, 3,3'-methylenebis(1-methyl-imidazole-2(3H)-thione) (L2) or 4,4'-(3,3'-methylenebis-(2-thioxo-2,3-dihydro-imidazole-3,1-diyl))dibutanoic acid (L3), with HgX2 (X = Cl, Br or I) in solution and the solid state. NMR, UV-Vis spectroscopic, and single crystal X-ray studies demonstrated that L1 or L2 coordinated rapidly and reversibly to the mercury center of HgX2 through the thione moiety. Treatment of L2 with HgCl2 or HgBr2 afforded 16-membered metallacycle k1-(L2)2Hg2Cl4 or k1-(L2)2Hg2Br4 where two Cl or Br atoms are located inside the ring. In contrast, treatment of L2 with HgI2 afforded a chain-like structure of k1-[L2Hgl2]n, possibly due to the large size of the iodine atom. Interestingly, [S1] and [S2]-donor ligands (L1, L2, and L3) showed an excellent efficacy to protect liver cells against HgCl2 induced toxicity and the strength of their efficacy is in the order of L3 > L2 > L1. 30% decrease of ROS production was observed when liver cells were co-treated with HgCl2 and L1 in comparison to those cells treated with HgCl2 only. In contrast, 45% and 60% decrease of ROS production was observed in the case of cells co-treated with HgCl2 and thiones L2 and L3, respectively, indicating that [S2]-donor ligands L2 and L3 have better cytoprotective effects against oxidative stress induced by HgCl2 than [S1]-donor ligand L1. Water-soluble ligand L3 with N-(CH2)3CO2H substituents showed a better cytoprotective effect against HgCl2 toxicity than L2 in liver cells.

Synthesis and characterization of fused imidazole heterocyclic selenoesters and their application for chemical detoxification of HgCl2

Selenoester derivatives of imidazo[1,2-a]pyridine/imidazo[1,2-a]pyrimidine has been synthesized by the reaction of sodium selenocarboxylates with 2-(chloromethyl)imidazo[1,2-a]pyridine/pyrimidine.

3,3′-[(2-Bromo-1,3-phenylene)bis(methylene)]bis(1-butyl-2,3-dihydro-1H-imidazole-2-selone)

In the title compound, C22H29BrN4Se2, the two Se atoms are directed in opposite directions with respect to the central benzene ring. The C=Se bond lengths at 1.848 (5) and 1.851 (5) Å are on the long side for a double bond but shorter than expected for a C—Se single bond. In the crystal, Br...Br intermolecular interactions [3.4685 (12) Å] link the molecules into a zigzag chain propagating along theb-axis direction. In addition, there are C—H...Se intermolecular interactions present, linking the chains to form slabs parallel to theabplane. One of the two butyl side chains is disordered over two conformations with occupancies of 0.777 (9) and 0.223 (9).

Molecular structures of tris(1-tert-butyl-2-mercaptoimidazolyl)hydroborate complexes of titanium, zirconium and hafnium

Cyclopentadienyl and tris(pyrazolyl)hydroborate have found much use as supporting ligands in the chemistry of titanium, zirconium and hafnium, especially with respect to applications involving olefin polymerization catalysis. In contrast, closely related tris(1-alkyl-2-mercaptoimidazolyl)hydroborate, [Tm^R], ligands have so far found little application to the chemistry of these elements, despite the fact that such ligands are currently used extensively in coordination chemistry. In view of the fact that a substituent in the 2-position exerts a direct influence on the steric environment of the metal center, we report here the application of the sterically demanding tris(1-tert-butyl-2-mercaptoimidazolyl)hydroborate [Tm^t-Bu] ligand to these metals. Dichlorido(η⁵-cyclopentadienyl)[tris(1-tert-butyl-2-sulfanylidene-2,3-dihydro-1H-imidazol-3-yl)borato-κ³S,S',H]zirconium(IV) benzene hemisolvate, [Zr(C₂₁H₃₄BN₆S₃)(C₅H₅)Cl₂]·0.5C₆H₆, (I), dichlorido(η⁵-cyclopentadienyl)[tris(1-tert-butyl-2-sulfanylidene-2,3-dihydro-1H-imidazol-3-yl)borato-κ³S,S',H]titanium(IV) benzene hemisolvate, [Ti(C₂₁H₃₄BN₆S₃)(C₅H₅)Cl₂]·0.5C₆H₆, (II), [bis(1-tert-butyl-2-sulfanylidene-2,3-dihydro-1H-imidazol-3-yl)borato-κ³S,S',H]dichlorido(η⁵-cyclopentadienyl)zirconium(IV), [Zr(C₁₄H₂₄BN₄S₂)(C₅H₅)Cl₂], (III), (1-tert-butyl-2,3-dihydro-1H-imidazole-2-thione-κS)(1-tert-butyl-2-sulfanylidene-1H-imidazol-3-ido-κ²N³,S)dichlorido(η⁵-cyclopentadienyl)zirconium(IV) benzene monosolvate, [Zr(C₇H₁₁N₂S)(C₇H₁₂N₂S)(C₅H₅)Cl₂]·C₆H₆, (IV), and tribenzyl[tris(1-tert-butyl-2-sulfanylidene-2,3-dihydro-1H-imidazol-3-yl)borato-κ³S,S',S'']hafnium(IV) benzene tetrasolvate, [Hf(C₇H₇)₃(C₂₁H₃₄BN₆S₃)]·4C₆H₆, (V), have been structurally characterized by X-ray diffraction. The [Tm^t-Bu] ligand coordinates to Ti and Zr in Cp[κ³S₂,H-Tm^t-Bu]MCl₂ [M = Zr, (I), and Ti, (II)] in a κ³S₂,H mode, while the benzyl compounds [Tm^t-Bu]M(CH₂Ph)₃ [M = Zr and Hf, (V)] exhibit κ³S₃ coordination.

Phenylselenolate Mercury Alkyl Compounds, PhSeHgMe and PhSeHgEt: Molecular Structures, Protolytic Hg-C Bond Cleavage and Phenylselenolate Exchange

The phenylselenolate mercury alkyl compounds, PhSeHgMe and PhSeHgEt, have been structurally characterized by X-ray diffraction, thereby demonstrating that both compounds are monomeric with approximately linear coordination geometries; the mercury centers do, nevertheless, exhibit secondary Hg•••Se intermolecular interactions that serve to increase the coordination number in the solid state. The ethyl derivative, PhSeHgEt, undergoes facile protolytic cleavage of the Hg-C bond to release ethane at room temperature, whereas PhSeHgMe exhibits little reactivity under similar conditions. Interestingly, the cleavage of the Hg-C bond of PhSeHgEt is also more facile than that of the thiolate analogue, PhSHgEt, which demonstrates that coordination by selenium promotes protolytic cleavage of the mercury-carbon bond. The phenylselenolate compounds PhSeHgR (R = Me, Et) also undergo degenerate exchange reactions with, for example, PhSHgR and RHgCl. In each case, the alkyl groups preserve coupling to the 199Hg nuclei, thereby indicating that the exchange process involves metathesis of the Hg-SePh/Hg-X groups rather than metathesis of the Hg-R/Hg-R groups.

Transition metal mediated formation of dicationic diselenides stabilised by N-heterocyclic carbenes: designed synthesis

Designed syntheses of dicationic diselenides have been developed by (i) reduction of dihaloselones (routes A and B) and (ii) by oxidation of selone adducts of metal halides (route C).