A combination of solid-state 13C NMR, X-ray photoelectron spectroscopy (XPS) and sulfur X-ray absorption near edge structure (S-XANES) techniques are used to characterize organic oxygen, nitrogen, and sulfur species and carbon chemical/structural features in kerogens. The kerogens studied represent a wide range of organic matter types and maturities. A van Krevelen plot based on elemental H/C data and XPS derived O/C data shows the well established pattern for type I, type II, and type III kerogens. The anticipated relationship between the Rock−Eval hydrogen index and H/C is independent of organic matter type. Carbon structural and lattice parameters are derived from solid-state 13C NMR analysis. As expected, the amount of aromatic carbon, measured by both 13C NMR and XPS, increases with decreasing H/C. The correlation between aromatic carbon and Rock−Eval T max, an indicator of maturity, is linear for types II and IIIC kerogens, but each organic matter type follows a different relationship. The average aliphatic carbon chain length (Cn‘) decreases with an increasing amount of aromatic carbon in a similar manner across all organic matter types. The fraction of aromatic carbons with attachments (FAA) decreases, while the average number of aromatic carbons per cluster (C) increases with an increasing amount of aromatic carbon. FAA values range from 0.2 to 0.4, and C values range from 12 to 20 indicating that kerogens possess on average 2- to 5-ring aromatic carbon units that are highly substituted. There is basic agreement between XPS and 13C NMR results for the amount and speciation of organic oxygen. XPS results show that the amount of carbon oxygen single bonded species increases and carbonyl−carboxyl species decrease with an increasing amount of aromatic carbon. Patterns for the relative abundances of nitrogen and sulfur species exist regardless of the large differences in the total amount of organic nitrogen and sulfur seen in the kerogens. XPS and S-XANES results indicate that the relative level of aromatic sulfur increases with an increasing amount of aromatic carbon for all kerogens. XPS show that the majority of nitrogen exists as pyrrolic forms in comparable relative abundances in all kerogens studied. The direct characterization results using X-ray and NMR methods for nitrogen, sulfur, oxygen, and carbon chemical structures provide a basis for developing both specific and general average chemical structural models for different organic matter type kerogens.

No abstract

ACEl is the transcriptional activator of the metallothionein (CUP] locus) gene in Saccharomyces cerevisiae. Previous data had implicated the N-terminal domain of ACEl as responsible for the Cu-dependent specific DNA binding. An expression system in Escherichia coli was constructed to enable the isolation of an ACEl domain containing the DNA and Cu-binding regions. Here we report the purification and characterization of the Cu-ACE1 truncated molecule. Spectroscopic techniques showed that ACEl contains an unusual type of DNA binding structure that is based on a polynuclear Cu(I)-cysteinyl thiolate cluster. The cluster consists of six or seven Cu(I) ions coordinated to cysteinyl thiolates in a trigonal geometry distorted from planarity. The Cu(I)-cysteine cluster of Cu-ACE1 exhibits structural properties analogous to the Cu(I)-thiolate polynuclear cluster in yeast Cu-metallothionein itself, suggesting an unusual mechanism for the evolution ofthis regulatory factor. The Cu cluster organizes and stabilizes the conformation of the N-terminal domain of ACEl for specific DNA binding.Sequence-specific DNA-binding proteins play key roles in the expression and transmission of genetic material. Many of these proteins share common structural motifs-such as the helix-turn-helix, leucine zipper, and zinc finger-that position the specific DNA-binding residues in appropriate juxtaposition to the target DNA sequence (1). The ACE1 ¶ transcription factor, which activates transcription of the metallothionein (MT) gene in Saccharomyces cerevisiae in response to Cu ions, is not obviously homologous to any of the known DNA binding factors (2-4). Previous biochemical and genetic experiments implicated the N-terminal 122 amino acids of ACEl in specific DNA binding (2)(3)(4)(5)(6). This region of the protein bears several sequence similarities to yeast MT including the presence of 12 cysteinyl residues of which 10 are arranged in Cys-Xaa-Cys or Cys-Xaa-Xaa-Cys pairs and limited numbers of hydrophobic residues (2-4). Yeast MT contains a Cu(I)-cysteinyl thiol polynuclear cluster that provides the energy of stabilization for the tertiary fold (7, 8).The similarity of ACEl and MT led to the prediction that the active form of ACEl contained a Cu(I) cluster (2).The role of Cu in the function of ACEl was confirmed by studies on ACEl produced as a fusion protein in Escherichia coli or in an in vitro translation system. These studies suggested that the specific interaction of ACEl and DNA promoter sequences in the MT gene was dependent on the presence of Cu or Ag ions (2, 5). The addition of the Cu(I) chelator KCN, which depletes MT of bound Cu ions, abolished the specific interaction of Cu-ACE1 and DNA.To determine the nature of the Cu-ACE1 complex, an E. coli expression system was constructed for use in the purification of a truncated form of ACEl containing the N-terminal 122 amino acids. Here we provide biophysical evidence that Cu-ACE1 contains a Cu(I)-cysteinyl thiolate polynuclear cluster. MATERIALS AND METHODSExpression and Purific...

No abstract

The nature of the passive film grown on iron in a pH 8.4 borate buffer at a high potential [+0.4 V (MSE)] was investigated with high resolution in situ XANES (x-ray absorption near-edge spectroscopy). The pre-edge peak at the base of the Fe K edge showed no evidence of the splitting associated with structures such as c~-Fe2Q, a-FeOOH, and ~]-FeOOH which contain iron that is entirely octahedrally coordinated to oxygen. The single pre-edge peak is consistent with the proposed ~]-Fe2OJFe304 structure (in which one-third of the iron ions are tetrahedrally coordinated) or with a disordered structure with distorted coordination polyhedra.