Metallothionein-Like Multinuclear Clusters of Mercury(II) and Sulfur in Peat

Strong mercury(II)-sulfur (Hg-SR) bonds in natural organic matter, which influence mercury bioavailability, are difficult to characterize. We report evidence for two new Hg-SR structures using X-ray absorption spectroscopy in peats from the Florida Everglades with added Hg. The first, observed at a mole ratio of organic reduced S to Hg (S-red/Hg) between 220 and 1140, is a Hg4Sx, type of cluster with each Hg atom bonded to two S atoms at 2.34 angstrom and one Sat 2.53 angstrom, and all Hg atoms 4.12 angstrom apart. This model structure matches those of metal-thiolate clusters in metallothioneins, but not those of HgS minerals. The second, with one S atom at 2.34 angstrom and about six C atoms at 2.97 to 3.28 angstrom, occurred at S-red/Hg between 0.80 and 4.3 and suggests Hg binding to a thiolated aromatic unit. The multinuclear Hg cluster indicates a strong binding environment to cysteinyl sulfur that might impede methylation. Along with a linear Hg(SR)(2) unit with Hg-S bond lengths of 2.34 angstrom at S-red/Hg of about 10 to 20, the new structures support a continuum in Hg-SR binding strength in natural organic matter.