Towards a custom chelator for mercury: evaluation of coordination environments by molecular modeling.

A chelator is a molecule which binds a metal or metalloid ion by two or more functional groups to form a stable ring complex known as a chelate. Despite the widespread clinical use of so-called chelation therapy to remove mercury, none of the drugs currently in use have been shown to chelate mercury. Mercury can adopt three common coordination environments: linear diagonal, trigonal planar, and tetrahedral. We have previously discussed some of the structural criteria for optimal binding of mercury in linear-diagonal coordination with thiolate donors (George et al. in Chem. Res. Toxicol. 17:999-1006, 2004). Here we employed density functional theory and X-ray absorption spectroscopy to evaluate the ideal chain length for simple alkane dithiolate chelators of Hg(2+). We have also extended our previous calculations of the optimum coordination geometries to the three-coordinate [Hg(SR)(3)](-) case. Finally, we propose a new chelator "tripod" molecule, benzene-1,3,5-triamidopropanethiolate, or "Trithiopod," which is expected to bind Hg(2+) in three-coordinate geometry with very high affinity.

Molybdenum X-ray absorption edges from 200 to 20,000eV: the benefits of soft X-ray spectroscopy for chemical speciation.

We have surveyed the chemical utility of the near-edge structure of molybdenum X-ray absorption edges from the hard X-ray K-edge at 20,000eV down to the soft X-ray M(4,5)-edges at approximately 230eV. We compared, for each edge, the spectra of two tetrahedral anions, MoO(4)(2-) and MoS(4)(2-). We used three criteria for assessing near-edge structure of each edge: (i) the ratio of the observed chemical shift between MoO(4)(2-) and MoS(4)(2-) and the linewidth, (ii) the chemical information from analysis of the near-edge structure and (iii) the ease of measurement using fluorescence detection. Not surprisingly, the K-edge was by far the easiest to measure, but it contained the least information. The L(2,3)-edges, although harder to measure, had benefits with regard to selection rules and chemical speciation in that they had both a greater chemical shift as well as detailed lineshapes which could be theoretically analyzed in terms of Mo ligand field, symmetry, and covalency. The soft X-ray M(2,3)-edges were perhaps the least useful, in that they were difficult to measure using fluorescence detection and had very similar information content to the corresponding L(2,3)-edges. Interestingly, the soft X-ray, low energy ( approximately 230eV) M(4,5)-edges had greatest potential chemical sensitivity and using our high-resolution superconducting tunnel junction (STJ) fluorescence detector they appear to be straightforward to measure. The spectra were amenable to analysis using both the TT-multiplet approach and FEFF. The results using FEFF indicate that the sharp near-edge peaks arise from 3d-->5p transitions, while the broad edge structure has predominately 3d-->4f character. A proper understanding of the dependence of these soft X-ray spectra on ligand field and site geometry is necessary before a complete assessment of the utility of the Mo M(4,5)-edges can be made. This work includes crystallographic characterization of sodium tetrathiomolybdate.

Effect of chain length and substrate temperature on the growth and morphology of n-alkane thin films.

Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and microscopy has been used to study the orientational morphology of thin films of the linear alkanes n-C36H74 and n-C60H122, prepared by vacuum deposition onto NaCl (001) surfaces at ambient and elevated substrate temperatures. The orientational morphology, specifically, the nature of domains with lateral and normal orientation, is explored as a function of the chain length and the substrate temperature. It is found that the longer n-C60H122 molecules are laterally oriented on the substrate surface within the investigated substrate temperatures but that the morphology of these thin films varies with substrate temperature. The shorter n-C36H74 molecules are partially laterally oriented at low substrate temperature and are completely normally oriented at high substrate temperature. The relative magnitude of "side-by-side" and "end-to-end" intermolecular interactions leads to the formation of highly ordered alkane structures with a high aspect ratio. The formation of complex, nanoscale orientational morphologies are rationalized by considering kinetic and thermodynamic effects, in particular, the relative enthalpic and entropic contributions to the free energy associated with the different molecular orientations.

Linear dichroism in the X-ray absorption spectra of linear n-alkanes.

The nature of the linear dichroism in the near-edge X-ray absorption fine structure (NEXAFS) spectra of linear n-alkanes is a matter of long-standing controversy. Linear dichroism in the carbon 1s --> sigma*(C-C) transition has been interpreted within a building block model and a molecular orbital model, leading to two different descriptions for the angular dependence of this feature. When used for measurement of molecular orientation, the application of these two different models will lead to different results. We have explored the linear dichroism in the carbon 1s NEXAFS spectra of single crystals of the linear n-alkane hexacontane (n-C60H122). An analysis of the angular dependence in this spectrum shows that the transition dipole moment associated with the carbon 1s --> sigma*(C-C) transition is oriented along the macromolecular chain axis, contradicting the predictions of the building block model. However, other transitions are observed in the sigma*(C-H) and the sigma*(C-C) bands that are orthogonal to the dominant transitions for each band. We also observe that radiation damage can be manifest in the form of molecular reorientation in highly ordered organic thin films.