NikR is a metal-responsive transcription factor that controls nickel uptake in Escherichia coli by regulating expression of a nickel-specific ATP-binding cassette (ABC) transporter. We have determined the first two structures of NikR: the full-length apo repressor at a resolution of 2.3 A and the nickel-bound C-terminal regulatory domain at a resolution of 1.4 A. NikR is the only known metal-responsive member of the ribbon-helix-helix family of transcription factors, and its structure has a quaternary arrangement consisting of two dimeric DNA-binding domains separated by a tetrameric regulatory domain that binds nickel. The position of the C-terminal regulatory domain enforces a large spacing between the contacts that each NikR DNA-binding domain can make with the nik operator. The regulatory domain of NikR contains four nickel-binding sites at the tetramer interface, each exhibiting a novel square-planar coordination by three histidines and one cysteine side chain.

A crystal structure of the anaerobic Ni-Fe-S carbon monoxide dehydrogenase (CODH) from Rhodospirillum rubrum has been determined to 2.8-Å resolution. The CODH family, for which the R. rubrum enzyme is the prototype, catalyzes the biological oxidation of CO at an unusual Ni-Fe-S cluster called the C-cluster. The Ni-Fe-S C-cluster contains a mononuclear site and a four-metal cubane. Surprisingly, anomalous dispersion data suggest that the mononuclear site contains Fe and not Ni, and the four-metal cubane has the form [NiFe3S4] and not [Fe4S4]. The mononuclear site and the four-metal cluster are bridged by means of Cys531 and one of the sulfides of the cube. CODH is organized as a dimer with a previously unidentified [Fe4S4] cluster bridging the two subunits. Each monomer is comprised of three domains: a helical domain at the N terminus, an α/β (Rossmann-like) domain in the middle, and an α/β (Rossmann-like) domain at the C terminus. The helical domain contributes ligands to the bridging [Fe4S4] cluster and another [Fe4S4] cluster, the B-cluster, which is involved in electron transfer. The two Rossmann domains contribute ligands to the active site C-cluster. This x-ray structure provides insight into the mechanism of biological CO oxidation and has broader significance for the roles of Ni and Fe in biological systems.

We have prepared ionic liquids by mixing either iron(II) chloride or iron(III) chloride with 1-butyl-3-methylimidazolium chloride (BMIC). Iron(II) chloride forms ionic liquids from a mole ratio of 1 FeCl(2)/3 BMIC to almost 1 FeCl(2)/1 BMIC. Both Raman scattering and ab initio calculations indicate that FeCl(4)(2-) is the predominant iron-containing species in these liquids. Iron(III) chloride forms ionic liquids from a mole ratio of 1 FeCl(3)/1.9 BMIC to 1.7 FeCl(3)/1 BMIC. When BMIC is in excess, Raman scattering indicates the presence of FeCl(4-). When FeCl(3) is in excess, Fe(2)Cl(7-) begins to appear and the amount of Fe(2)Cl(7-) increases with increasing amounts of FeCl(3). Ionic liquids were also prepared from a mixture of FeCl(2) and FeCl(3) and are discussed. Finally, we have used both Hartree-Fock and density functional theory methods to compute the optimized structures and vibrational spectra for these species. An analysis of the results using an all-electron basis set, 6-31G, as well as two different effective core potential basis sets, LANL2DZ and CEP-31G is presented.

A room-temperature molten salt has been prepared from AuCl3 and 1-ethyl-3-methylimidazolium chloride (EMIC). At a ratio of 1 mol of AuCl3 to 2 mol of EMIC, the salt is a bright yellow-orange and shows Raman spectral features at 170, 328, and 352 cm-1, indicating the presence of AuCl4-. Ab initio calculations indicate that a dinuclear Au2Cl7- species containing a bridging chlorine should be stable, but no such species has been observed.