[show abstract][hide abstract] ABSTRACT: Mercury (Hg) is a major global pollutant arising from both natural and anthropogenic sources. Defining the factors that determine the relative affinities of different ligands for the mercuric ion, Hg2+, is critical to understanding its speciation, transformation, and bioaccumulation in the environment. Here, we use quantum chemistry to dissect the relative binding free energies for a series of inorganic anion complexes of Hg2+. Comparison of Hg2+–ligand interactions in the gaseous and aqueous phases shows that differences in interactions with a few, local water molecules led to a clear periodic trend within the chalcogenide and halide groups and resulted in the well-known experimentally observed preference of Hg2+ for soft ligands such as thiols. Our approach establishes a basis for understanding Hg speciation in the biosphere.
[show abstract][hide abstract] ABSTRACT: The bacterial organomercurial lyase, MerB, catalyzes the protonolysis of organomercurial compounds. MerB cleaves Hg-C bonds of various substrates ranging from the methylmercury cation (MeHg) to merbromin. Upon Hg-C bond cleavage, Hg2+ and an organic molecule are produced. For example, methane is the product resulting from the protonolysis of MeHg. The release pathway and mechanism of the organic product are unclear. Here, we have applied molecular dynamics and free energy simulations to study the dissociation of a series of organic molecules. The x-ray crystallographic structure of MerB with a bound Hg2+ cation was used as the starting model, and the organic products were manually placed in the active site. The umbrella sampling method was used to obtain free energy profiles for the dissociation pathways. Several hydrophobic sidechains of MerB were found to interact with the organic molecules and may have important roles in the dissociation processes. The relatively low free energy barriers of dissociation suggest that organic product dissociation is not rate limiting.