Protonation state of the bridging Ligand in the resting state of cytochrome c oxidase: investigations of synthetic Heme/Copper models
- UNCG Author/Contributor (non-UNCG co-authors, if there are any, appear on document)
- Maria C. Carrasco (Creator)
- Institution
- The University of North Carolina at Greensboro (UNCG )
- Web Site: http://library.uncg.edu/
- Advisor
- Shabnam Hematian
Abstract: Inspiration for our research stems from biological redox processes performed by metalloenzymes. Heme-containing enzymes are able to perform oxidation and reduction reactions, such as in cytochrome P450 and cytochrome c oxidase (CcO). The focus of the work presented in this thesis is on the resting state of the active site of CcO. The active site of CcO consists of an iron porphyrin (heme a3) and copper site (CuB) which perform the four electron, four proton reduction of dioxygen to generate two water molecules. The resting state at this active site contains a bridging oxygen-based ligand and is an intriguing example of a heme-containing oxygen-bridged assembly. We begin with a holistic review of the literature available on these complexes and provide a classification of all ??–(hydr)oxo heme complexes (Chapter I). The exact nature and protonation state of the oxygen-based bridging ligand of the resting state of CcO is not yet known and can have significant implications for its catalytic performance. Therefore, here, we utilize synthetic models to elucidate fundamental aspects of the acid-base chemistry, formation, correlation to structure, electronic-structure, and reactivity. For pKa measurements of our heme/copper ??–hydroxo complexes, we first prepare a series of weak acids, in the form of protonated nitrogen bases (Chapter II), to then use in titration experiments with our heme/copper ??–oxo complexes (Chapter III). Chapter I contains an overview of all published oxo- or hydroxo-bridged heme complexes. They are divided into three categories which are those with two identical metal sites and porphyrin ligands (homobinuclear homoleptic complexes), those with two identical metal centers and varying ligand environments (homobinuclear heteroleptic complexes) and those with two different metal centers and ligands (heterobinuclear heteroleptic complexes). A range of investigations have been performed on these (hydr)oxo-bridged heme constructs for the important roles that they play in many life processes or their different applications in catalysis and preparation of new functional materials. We compiled structural features from crystal structures to gain a better understanding of how modifications in the metal site or ligand environment result in remarkable changes in the reactivity of synthetic ??–(hydr)oxo complexes. Their use in other applications such as materials science and environmental sciences are also discussed. Chapter II describes the preparation and characterization of the protonated nitrogen bases, or weak acids, later used for titration experiments; these can be classified as alkyl ammoniums, pyridiniums, or anilinium. The anion used with the protonated nitrogen bases is the tetrakis(pentafluorophenyl)borate complex as it increases the solubility of complexes in non-aqueous solvents such as dichloromethane, tetrahydrofuran and toluene. Strikingly, the 1H-NMR spectra of all of these weak acids highlight a distinguished triplet splitting pattern for the coupling of the acidic proton to the quadrupolar 14N nuclei (I = 1). The results point to the relatively slow spin-lattice relaxation of the 14N nucleus in these systems which permits the observation of spin-spin coupling between the 14N and bound 1H nuclei. The proton chemical shifts and 14N-1H coupling constants (1JNH) of the acidic protons also correlate well with the known pKa values and hybridization of the nitrogen site in the respective protonated species. Chapter III details the pKa measurements for two heme/copper ??–hydroxo complexes as well as UV-vis and NMR studies of three heme/copper ??–hydroxo complexes. These were obtained through titration experiments with the protonated nitrogen bases described in Chapter II. While previous reports on the pKa of heme/copper ??–hydroxo complexes provided a range of values for each complex, we are able to provide a pKa value in a non-aqueous solvent for our heme/copper complexes that can be converted to an aqueous pKa value. This will be useful for future comparisons of the structural features of the enzyme and synthetic complexes, and how those correlate to a change in basicity of the bridging oxo moiety. [This abstract has been edited to remove characters that will not display in this system. Please see the PDF for the full abstract.]
Protonation state of the bridging Ligand in the resting state of cytochrome c oxidase: investigations of synthetic Heme/Copper models
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Created on 5/1/2021
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Additional Information
- Publication
- Thesis
- Language: English
- Date: 2021
- Keywords
- Cytochrome c Oxidase, pKa measurements, Protonated bases, Spin-spin coupling, Synthetic models
- Subjects
- Proton transfer reactions
- Cytochrome oxidase
- Nuclear magnetic resonance spectroscopy
- Acid-base chemistry