Nucleation of mercury sulfide by dealkylation

Abstract

Metal sulfide minerals are assumed to form naturally at ambient conditions via reaction of a metallic element with (poly)sulfide ions, usually produced by microbes in oxygen-depleted environments. Recently, the formation of mercury sulfide (β-HgS) directly from linear Hg(II)-thiolate complexes (Hg(SR)2) in natural organic matter and in cysteine solutions was demonstrated under aerated conditions. Here, a detailed description of this non-sulfidic reaction is provided by computations at a high level of molecular-orbital theory. The HgS stoichiometry is obtained through the cleavage of the S-C bond in one thiolate, transfer of the resulting alkyl group (R’) to another thiolate, and subsequent elimination of a sulfur atom from the second thiolate as a thioether (RSR’). Repetition of this mechanism leads to the formation of RS-(HgS)n-R chains which may self-assemble in parallel arrays to form cinnabar (α-HgS), or more commonly, quickly condense to four-coordinate metacinnabar (β-HgS). The mechanistic pathway is thermodynamically favorable and its predicted kinetics agrees with experiment. The results provide robust theoretical support for the abiotic natural formation of nanoparticulate HgS under oxic conditions and in the absence of a catalyst, and suggest a new route for the (bio)synthesis of HgS nanoparticles with improved technological properties.

Mercury(II) Binding to Metallothionein in Mytilus edulis revealed by High Energy‐Resolution XANES Spectroscopy

“…Mercuration, like auriphilic and argentophilic interactions, occurs in solution at room temperature because of the similarity in magnitude of metallophilic interactions with hydrogen bonding . A mercury atom bound to the two cysteines of the CXXC motif is therefore a nucleation center for the formation of Hg x S y oligomers through mercuration, similar to the formation of β‐HgS from Hg(SR) 2 complex in natural organic matter . Furthermore, the five CXC motifs from the α domain offer the correct spacing for the formation of the Hg 4 S 12 ‐α cluster, similarly to Cd 4 S 12 ‐α (Figure ).…”

“…What is the amino acid sequence of the MT‐10 units (primary structure) and how are they covalently bonded in MT‐40 (quaternary structure)? Do the monomeric units from MT‐40 contain 21 cysteines, as in canonical MT‐10, or 23 cysteines, as in the monomeric MT‐10 units of MT‐20? Lastly, are the biogenic MT clusters more stable than the metallothionein‐like β‐HgS nanoparticles formed naturally in natural organic matter and plant leaves? All these questions could be asked similarly to 4d 10 Ag I and 5d 10 Au I , which, like 5d 10 Hg II , form distorted thiolate clusters in metalloproteins and linear coordination with thiolate ligands when they are embedded in sterically complex and demanding macromolecules. All these important biometals and intriguing questions are now leading to investigations in living matter by HR‐XANES down to, and below, the ppm level …”

Biogenesis of Mercury–Sulfur Nanoparticles in Plant Leaves from Atmospheric Gaseous Mercury

“…The -HgSNP mercury species may result from the deposition and oxidation of Hg 0 g followed by the formation of a Hg(SR)2 complex with soil organic matter and its subsequent transformation into -HgSNP. 47,57 Mercury speciation in foliage. Principal component analysis (PCA) of the 22 native plants and the two plants exposed for 28 days to gaseous mercury in a growth chamber (corn C3 and P. vittata Pv2) shows that three abstract components are required to account for the variance in the dataset (SI Figure S4).…”

“…Cysteine was replaced by the model thiolate ligand methanethiolate (CH3S -), as in our previous computational studies. 46,47,52,53,57,106 This substitution does not change, or only marginally changes, the Hg-S bond length, the geometry of the complex, and the effective atomic charge on the sulfur (-0.63 e vs. -0.65 e) and Hg (+1.18 e vs. +1.16 e ) atoms (SI Figure S11). However, the predicted Hg-Hg distances and geometry will be somewhat affected in MTs by the enfolding protein backbone structure.…”

“…Lastly, sulfide sulfur could also be produced chemically by a dealkylation reaction between two Hg(SR)2 complexes. 47,57 This reaction can be written:…”