In this study, ab initio and density functional theory methods have been used to understand the structures and thermodynamic stabilities of complexes formed between L-cysteine and mercury (II) ions in neutral aqueous solution. To better understand the interaction between sulfur and mercury (II) ion, the MP2, B3LYP, M06-2X, and TPSS methods have been used to optimize [HgSH x ] 22x , x5 1-4, complexes and compared to benchmark QCISD(T) structures. Furthermore, energies from these same methods are compared to CCSD(T)/CBS(2,3) energies. From these benchmark calculations, the M06-2X method was selected to optimize L-cysteinate-Hg(II) complexes and the MP2 method for estimating complex energies. L-cysteinate-mercury (II) ion complexes are formed primarily by forming a bond between cysteinate sulfur and the mercury ion. Stable complexes of L-cysteinate and mercury can be formed in 1:1, 2:1, 3:1, and 4:1 ratios. Each complex is stabilized further by interaction between carboxylate oxygen and mercury as well as hydrogen bonding among complex cysteinate ligands. The results indicate that at high cysteinate to Hg(II) ratios high-coordinate complexes can be present but at lower ratios the 2:1 complex should be dominant.

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Several mutants of the enzyme dihydrofolate reductase (DHFR) have been engineered by oligonucleotide-directed mutagenesis of the cloned E. coli gene. The mutations were designed to address specific questions about DHFR structure-function relations that arose from the analysis of the high-resolution structure. Mutations at the active site have revealed that the invariant residue aspartate-27 is involved in substrate protonation, and not in transition-state stabilization as previously thought. The 2.0 Å (1 Å = 10 -1 nm = 10 -10 m) refined structures of the Asn-27 and Ser-27 mutant enzymes reveal that the enhanced binding observed for the 2,4-diamino pteridine and pyrimidine inhibitors is probably not attributable to the charge interaction between Asp-27 and a protonated N-1 of the inhibitor. Substitution of a cysteine for a proline at position 39 places two sulphydryls within bonding distance, and under certain oxidation conditions they will quantitatively form a disulphide bond. The refined 2.0 Å structures of both reduced and oxidized forms of this mutant show that only minor conformational changes occur for disulphide bond formation. The crosslinked enzyme is significantly more conformationally stable to denaturants such as guanidine hydrochloride and urea.

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