Synthesis, characterization and reactivity of (nitrogen,sulfur)M(II) complexes that model the active sites of peptide deformylase and superoxide reductase.

摘要

This thesis presents the synthesis, characterization and reactivity of nitrogen and sulfur containing metal complexes that model the active sites of two non-heme iron containing enzymes, peptide deformylase (PDF) and superoxide reductase (SOR). PDF is a hydrolytic metalloenzyme responsible for removing the N-terminal formyl group during bacterial protein synthesis. Its active site contains a tetrahedral (His2Cys)FeII-OH center from which the bound OH- serves as the active nucleophile. A zinc form of this enzyme was isolated and found to be less active than the FeII form, although zinc is usually the active cofactor in hydrolytic metalloenzymes. SOR catalyzes the reduction of superoxide to hydrogen peroxide via a one electron reduction mechanism. Its active site contains a square pyramidal (His4Cys)FeII center with the four histidines coordinated in one plane and an unusual cysteinate ligand bound trans to the substrate binding site. The reaction of SOR proceeds through a putative FeIII-OO(H) intermediate. One goal of this work was to propose a role of the cysteinate ligand and characterize the iron-peroxo intermediates in the mechanism of SOR using synthetic model complexes.;Chapter one reviews the biological relevance of PDF and SOR as well as the active site structures and the proposed mechanisms of reaction. The unusual metal dependent reactivity of PDF is described and the unknown Fe-OO(H) reaction intermediates in SOR are discussed. The known synthetic models that mimic the structure of the enzyme active sites as well as their function are also presented.;Chapter 2 addresses the unusual metal dependent reactivity of PDF. Our lab previously synthesized a functional model of Zn-PDF, (PATH)ZnOH and studied its hydrolytic activity towards a carboxylic ester. Here in, its hydrolytic activity towards a phosphate ester, tris(nitrophenyl)phosphate is presented. The differences in the reactivity of (PATH)ZnOH towards phosphate and carboxylic esters are interpreted in light of their relevance to the mechanistic differences between FeII-PDF and Zn-PDF.;Efforts to synthesize new models of PDF are described in chapter 3. The use of combinatorial peptide library technology and solid phase synthesis to synthesize and screen a small peptide library for hydrolytically active peptide sequences in the presence of divalent metal ions is described. This chapter also presents the rational design of hydrolytically active peptides that contain the His2Cys metal binding residues, which have been obtained by mutations in a Cys2His2 classical zinc finger peptide.;The synthesis and reactivity of new N4S(thiolate)Fe II models of SOR is the focus of chapter 4. The FeII complexes, [FeII([15]aneN4)(SAr)](BF4), where SAr represents arylthiolate ligands with varying electron donating abilities, were synthesized and fully characterized. The complexes react with alkylhydroperoxides at low temperature to generate metastable low-spin FeIII-OOR species with weak Fe-O and weak O-O bonds. The effect of varying the thiolate donor on the anodic potentials of the FeII complexes, and on the LMCT band and Fe-O bond strength of the FeIII-OOR complexes will be presented.;The structures of the low-spin FeIII-OOR complexes discussed in chapter 4 are elucidated by X-ray Absorption Spectroscopy (XAS) in chapter 5. The presence of a ligated thiolate in these complexes is confirmed and its effect on the Fe-O bond is emphasized by a combination of experimental (XAS) and theoretical (DFT) Fe-O bond distances. The reactivity of the Fe III-OOR complexes towards oxygen atom acceptors, hydrogen atom donors and as simple nucleophiles is discussed, and a mechanism of decay of these complexes, which is enhanced by proton donors, is proposed.;Chapter 6 presents [FeII(Me4[15]aneN4 )(SPh)(BPh4), another FeII complex synthesized to investigate the effect of alkylating amine donors on the properties of the corresponding FeIII-OOR complexes. Amine alkylation results in weak field ligands, which result in high-spin FeIII-OOR complexes. Studying another complex, [FeII(Me4[15]aneN 4)(OTf)](OTf), enables us to show the effect of changing axial ligands on the properties of high-spin FeIII-OOR complexes.