[4] Biochemical and spectroscopic probes of mercury(II) coordination environments in proteins

The high affinity of mercuric ion for thiolate ligands and the rapid ligand exchange rates of the resulting complexes make Hg(II) a relatively easy metal to bind to active sites of a variety of cysteine-containing enzymes. This feature has made mercuric ion a useful biochemical tool for selectively displacing one type of copper from a multicopper enzyme. It has become apparent that several spectroscopic features of Hg(II) complexes in their own right could be useful in distinguishing a variety of coordination environments. Although Hg(II) is a d10 metal, it would be a mistake to consider it spectroscopically silent. As described here, the interaction of Hg(II) with biopolymers can be probed using extended X-ray absorption fine structure (EXAFS), UV-Vis, 199Hg nuclear magnetic resonance (NMR), and circular dichroism (CD) spectroscopies. Several features in the spectra of structurally characterized model complexes have been correlated with the primary coordination number of the metal, aiding in the determination of coordination environments in proteins. This chapter discusses the advantages and limitations of using Hg(II) substitution in structure, function, and spectroscopic studies of proteins. Given these spectroscopic handles, determination of the coordination geometry, ligand identity, metal binding stoichiometry, dissociation rates, and binding constants are possible. Practical techniques, such as common methods for binding Hg(II) to proteins, are discussed. This chapter presents mercury chemistry, including structural and thermodynamic trends of biologically relevant mercury compounds.