X-ray absorption spectroscopic investigations of non-heme iron active sites: Development of iron K- and L-edge multiplet interaction analyses as a probe of geometric and electronic structure.

摘要

Compared to heme systems, significantly less is known about the active sites of mononuclear non-heme iron enzymes, especially those in the ferrous oxidation state. The high-spin ferrous active sites are generally EPR silent systems, and lack spectroscopically accessible ligand-to-metal charge transfer transitions. In high-spin ferric systems, d→d transitions are spin forbidden. Nonetheless, an accurate knowledge of the catalytic mechanism of these enzymes is of paramount importance in order to understand their ability to biodegrade harmful toxins, their anti-tumor functionality, or the mechanism of disease-causing protein mutations. X-ray absorption spectroscopy (XAS) is an invaluable tool for determining the geometric and electronic structure of mononuculear non-heme iron enzyme active sites. In this thesis, Fe L-edge and K-edge multiplet analysis, and Fe K-edge EXAFS analysis is used to investigate the geometric and electronic structure of such sites.; In part I, a novel XAS L-edge methodology is developed in which the intensity of the multiplet transitions is used to determine the total covalency of the metal d-orbitals of a complex. Furthermore, it is found that ligand field theory does not accurately describe the electronic structure of the active site of most model complexes. For each set of symmetry related orbitals, the inclusion of differential orbital covalency (DOC) is necessary to simulate the data. Finally, a ground state projection method is developed for determining the DOC for the symmetry related metal d-orbitals in mononuclear non-heme iron active sites of varying geometry and spin state.; In part II, a combination of Fe K-edge multiplet and EXAFS analysis is used to develop a description of the iron active site in several mononuclear non-heme iron enzymes. In these systems, the EXAFS results present an accurate description of the geometric parameters of the active site, whereas the pre-edge multiplet analysis provides the coordination number and the degree of distortion at the active site. Used in combination with analyses of data from other spectroscopic techniques as well as molecular orbital calculations, these studies provide mechanistic insight into the reaction of this important class of enzymes and their reactions with dioxygen.