Characterization of biologically relevant metal sites by X-ray spectroscopic methods
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Authors
Keegan, Brenna C.
Issue Date
2019
Type
Dissertation
Language
Keywords
metalloenzymes , x-ray spectroscopy
Alternative Title
Abstract
In this research, X-ray spectroscopic techniques in combination with electronic structure calculations were used to characterize biologically relevant metal sites. A general goal was to determine the effects of electronic structure perturbations on the overall reactivity and physical properties for these coordination complexes. We begin by investigating a series of copper hexaethyltripyrrindione (Cu(H3TD1)) complexes inspired by urinary biopyrrins. In the monomeric form, there are two magnetically uncoupled, unpaired elections; one unpaired electron is localized on the Cu(II) center and the other is delocalized within the ligand -system. In solution, the monomers dimerize due to intermolecular antiferromagnetic coupling of the ligand-based spins. Two oxidized monomeric species exist after oxidation of the dimer by AgBF4. In solution, the species exists as [Cu(TD1)(H2O)]+, whereas in the solid state it forms [Cu(TD1)(H2O)(BF4)]. The latter species presents some curious findings in both the experimental and calculated results. We find that upon coordination of the anionic BF4 ligand, the traditionally metal dominated antibonding MOs become ligand dominating, indicating a ligand field inversion has occurred. We conclude that this inversion is the result of an electrostatic destabilization of the ligand-based MOs by the coordination of the BF4- anion. This study fundamentally expands our view of bonding in late first row transition metal complexes. We go on to observe similar ligand field inversion in the study of small molecule nickel thiolate complexes capable of reversible thiolate protonation. In an effort to correlate the electronic structure perturbations with coordinated thiolate sulfur-atom protonation as seen in biological systems (i.e. [NiFe] hydrogenase), we investigated a formal Ni2+P3S coordinated complex, [Ni(triphos)(SPh)]+, and its protonated form, [Ni(triphos)(S(H+)Ph)]2+. Using X-ray absorption and emission techniques in conjunction with electronic structure calculations, we find that [Ni(triphos)(SPh)]+ displays an inverted ligand field and upon protonation of the Ni-S bond, the electronic structure significantly changes such that [Ni(triphos)(S(H+)Ph)]2+ displays only a partially inverted ligand field. In both cases we find that the compounds are best described as Ni0(3d10) species, which highlights an inherent weakness in oxidation state formalisms. We conclude that the role of thiolate protonation in biological systems may include the gating of reactivity by altering the electronic structure of the metal site. Comparisons to [NiFe]-hydrogenase were made. Keeping in the theme of sulfur protonation, we used an altered nickel superoxide dismutase (NiSOD) peptide scaffold to produce a disulfide bridged bimetallic nickel species, {Ni2II(SODmds)}. We investigated this species at both physiological pH (7.4) and high pH (9.6). Herein, we find that at high pH the species contains a bimetallic nickel center, which becomes monomeric at physiological pH upon protonation of the terminal cysteine residues. We find that both species undergo oxidation when exposed to air, however, we find that the high pH species oxidizes more readily. This finding suggests that the protonated cysteinate may play a role in the protection of Ni-S bonds in the NiSOD active-site from oxidative damage. Lastly, we used K-edge XAS to define the structures of the zinc binding site in human calprotectin, a methyl ligated nickel azurin scaffold correlating to the active site of Acetyl Coenzyme A synthase, and an unusual Cobalt-oxo species that has implications in cobalt mediated biological processes. Herein, we correlate physical structure with the function of these metal sites.
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Creative Commons Attribution 4.0 United States