Modeling of the Passive Permeation of Mercury and Methylmercury Complexes Through a Bacterial Cytoplasmic Membrane.

Cellular uptake and export are important steps in the biotransformation of mercury (Hg) by microorganisms. However, the mechanisms of transport across biological membranes remain unclear. Membrane-bound transporters are known to be relevant, but passive permeation may also be involved. Inorganic Hg ^II and methylmercury ([CH ₃ Hg ^II ] ⁺ ) are commonly complexed with thiolate ligands. Here, we have performed extensive molecular dynamics simulations of the passive permeation of Hg ^II and [CH ₃ Hg ^II ] ⁺ complexes with thiolate ligands through a model bacterial cytoplasmic membrane. We find that the differences in free energy between the individual complexes in bulk water and at their most favorable position within the membrane are ∼2 kcal mol ^-1 . We provide a detailed description of the molecular interactions that drive the membrane crossing process. Favorable interactions with carbonyl and tail groups of phospholipids stabilize Hg-containing solutes in the tail-head interface region of the membrane. The calculated permeability coefficients for the neutral compounds CH ₃ S-Hg ^II -SCH ₃ and CH ₃ Hg ^II -SCH ₃ are on the order of 10 ^-5 cm s ^-1 . We conclude that small, nonionized Hg-containing species can permeate readily through cytoplasmic membranes.