William O. Gillum

Bio: William O. Gillum is an academic researcher. The author has contributed to research in topics: Vanadium & Sulfur. The author has an hindex of 4, co-authored 4 publications receiving 510 citations.

The molybdenum site of nitrogenase. Preliminary structural evidence from x-ray absorption spectroscopy

Abstract: One of the two protein components of the nitrogenase enzyme system (the so-called MoFe protein) contains two Mo atoms and 24 to 32 Fe atoms per 220 to 270,000 molecular weight. Despite many hypotheses about the Mo site and its involvement in dinitrogen reduction, there has been no spectroscopic means of unambiguously probing the state of Mo. In this paper, the results of x-ray absorption spectroscopy studies are described and the analysis of these results provides the first direct evidence of the Mo coordination environment in the MoFe protein from Clostridium pasteurianum nitrogenase. The Mo is found to have primarily S ligation and in the resting state (''semireduced form'') of the protein there appears to be no Mo=O species present. On air oxidation, however, clear evidence is found for the presence of Mo=O. The interaction of the Mo with one other metal, most probably Fe and most certainly not Mo, is also seen and it is suggested that the Mo is present in a Mo--Fe--S cluster not yet adequately modeled by any synthetic inorganic system. 8 figures, 6 tables, 40 references.

Extended x-ray absorption fine structure—its strengths and limitations as a structural tool

Abstract: The authors review the development of extended x-ray absorption fine structure (EXAFS) within the last decade. Advances in experimental techniques have been largely stimulated by the availability of synchrotron radiation. The theory of EXAFS has also matured to the point where quantitative comparison with experiments can be made. The authors review in some detail the analysis of EXAFS data, starting from the treatment of raw data to the extraction of distances and amplitude information, and they also discuss selected examples of applications of EXAFS chosen to illustrate both the strength and limitations of EXAFS as a structural tool.

Assignment of pre‐edge peaks in K‐edge x‐ray absorption spectra of 3d transition metal compounds: electric dipole or quadrupole?

Abstract: The characteristics of pre-edge peaks in K-edge x-ray absorption near edge structure (XANES) spectra of 3d transition metals were reviewed from viewpoints of the selection rule, coordination number, number of d-electrons, and symmetry of the coordination sphere. The contribution of the electric dipole and quadrupole transition to the peaks was discussed on the basis of the group theory, polarized spectra, and theoretical calculations. The pre-edge peak intensity for Td symmetry is larger than those for Oh symmetry for all 3d elements. The intense pre-edge peak for tetrahedral species of 3d transition metals is not due to 1s–3d transition, but transition to the p component in d–p hybridized orbital. The mixing of metal 4p orbitals with the 3d orbitals depends strongly on the coordination symmetry, and the possibility is predictable by group theory. The transition of 1s electron to d orbitals is electric quadrupole component in any of the symmetries. The d–p hybridization does not occur with regular octahedral symmetry, and the weak pre-edge peak consists of 1s–3d electric quadrupole transition. The pre-edge peak intensity for a compound with a tetrahedral center changes as a function of the number of 3d electrons regardless of the kind of element; it is maximized at d0 and gradually decreases to zero at d10. The features of pre-edge peaks in K-edge XANES spectra for 4d elements and the L1-edge for 5d elements are analogous with those for 3d elements, but the pre-edge peak is broadened due to the wide natural width of the core level. Copyright © 2008 John Wiley & Sons, Ltd.