| Third generation X-ray synchrotron sources provide brilliant X-ray light improving the performance in ultra-dilute samples (like proteins) of: a) well known techniques (EXAFS and XANES, XRF, microspectroscopies)* by several fold, and b) allow performing advanced experiments (high resolution emission and inelastic X-ray scattering) that were not possible before. a) For XANES, the availability of computer codes that include the recent progresses in solid state physics (FEFF8, FDMNES, MXAN, etc) [1-3] allow studying the electronic-structure-information-rich XANES region and get valuable information for the chemistry and biology [4,5]. However, for the analysis of highly covalent bonds, the full potential is needed, instead of the fast muffin-tin approximation. b) The so-called crossover peak in high-resolution X-ray emission (XES)* provides unique information about two otherwise unresolved problems in biology and coordination chemistry: protonation state of solvent molecules and identification of nitrogen vs. oxygen ligand to a metal. Using high-end calculations of the electronic structure as well as a comprehensive set of bioinorganic model compounds [6,7] it has been shown that the currently accepted fingerprinting of ligands is not possible for molecular complexes, and a detailed knowledge of the density of states is needed. Computational approach for this project: 1) Muffin tin XANES: a) This theory is enough for describing symmetrical structures of ligands binding to a metal by an O or N (like solvents or histidines). However, the intended project will cover several proteins (bearing Zn, Cu and Fe metal centers) and their model compounds. While the XANES of metal sites in proteins can be achieved with as few as 30 atoms [4], the multiple scattering from periodic lattices in model compounds requires clusters as big as 300 atoms [unpublished results, Univ. of Michigan, 2005]. Both, the huge number of samples to be analyzed as well as the escalation in cluster size for the biomimics, require the use of a supercomputer resource. 2) Full potential XANES: a) Sulfur XANES on a set of oil fractions and their model compounds will be studied with the full potential method. Test runs in a Pentium PC with 1 GB of ram are extremely lengthy, making the analysis of series of samples and scan of parameters impossible. The project would become feasible by the use of supercomputer resources. b) For the case of above mentioned protein studies, the High Resolution XANES experiments will provide detail on the edge structure, which appears integrated in the normal experiment. Thus, the more CPU costly full potential XANES calculations are needed for a correct interpretation of this data. References In Intended results section. *EXAFS: Extended X-ray Absorption Fine Structure, XANES: X-ray Absorption Near Edge Structure, XRF: X-ray fluorescence, XES: X-ray emission. |
