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Reactivity of metal (Co, Ni, Cu) bound peptides with organometallic fragments and small molecules

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Understanding the mechanisms of metalloproteins at the level necessary to engineer new functionalities is complicated by the need to parse the complex overlapping functions played by each amino acid without negatively impacting the host organism. Artificial or designed metallopeptides offer

Understanding the mechanisms of metalloproteins at the level necessary to engineer new functionalities is complicated by the need to parse the complex overlapping functions played by each amino acid without negatively impacting the host organism. Artificial or designed metallopeptides offer a convenient and simpler platform to explore metal-ligand interactions in an aqueous, biologically relevant coordination context. In this dissertation, the peptide SODA (ACDLPCG), a synthetic derivative of the nickel-binding pocket of nickel superoxide dismutase, is used as a scaffold to construct a variety of novel metallopeptides and explore their reactivity. In Chapter 2, I show that SODA binds Co(II) and the resulting peptide, CoSODA, reacts with oxygen in an unexpected two step process that models the biosynthesis of Co nitrile hydratase. First, the thiolate sulfur is oxidized and then the metallocenter is oxidized to Co(III). In Chapter 3, I show that both CoSODA and CuSODA form CN- adducts. Spectroscopic investigations of these metallopeptides are compared with data from NiSODA and Ni(CN)SODA to show the remarkable geometric versatility of SODA with respect to interactions with metallocenters. In Chapter 4, exploiting the propensity of sulfur ligands to form bridging structures, NiSODA is used as a metallosynthon to direct synthesis of hetero bi- and tri-metallic peptides as models for [NiFe]-hydrogenases and the A cluster of acetyl-CoA synthase carbon monoxide dehydrogenase. Building on this synthetic strategy, in Chapter 5, I demonstrate synthesis of NiRu complexes including a Ru(bipyridine)2 moiety and characterize their photochemistry.

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2012

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Mechanistic studies of one-electron reduced bipyridine reactions relevant to carbon dioxide sequestration

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Increasing concentrations of carbon dioxide in the atmosphere will inevitably lead to long-term changes in climate that can have serious consequences. Controlling anthropogenic emission of carbon dioxide into the atmosphere, however, represents a significant technological challenge. Various chemical approaches have

Increasing concentrations of carbon dioxide in the atmosphere will inevitably lead to long-term changes in climate that can have serious consequences. Controlling anthropogenic emission of carbon dioxide into the atmosphere, however, represents a significant technological challenge. Various chemical approaches have been suggested, perhaps the most promising of these is based on electrochemical trapping of carbon dioxide using pyridine and derivatives. Optimization of this process requires a detailed understanding of the mechanisms of the reactions of reduced pyridines with carbon dioxide, which are not currently well known. This thesis describes a detailed mechanistic study of the nucleophilic and Bronsted basic properties of the radical anion of bipyridine as a model pyridine derivative, formed by one-electron reduction, with particular emphasis on the reactions with carbon dioxide. A time-resolved spectroscopic method was used to characterize the key intermediates and determine the kinetics of the reactions of the radical anion and its protonated radical form. Using a pulsed nanosecond laser, the bipyridine radical anion could be generated in-situ in less than 100 ns, which allows fast reactions to be monitored in real time. The bipyridine radical anion was found to be a very powerful one-electron donor, Bronsted base and nucleophile. It reacts by addition to the C=O bonds of ketones with a bimolecular rate constant around 1* 107 M-1 s-1. These are among the fastest nucleophilic additions that have been reported in literature. Temperature dependence studies demonstrate very low activation energies and large Arrhenius pre-exponential parameters, consistent with very high reactivity. The kinetics of E2 elimination, where the radical anion acts as a base, and SN2 substitution, where the radical anion acts as a nucleophile, are also characterized by large bimolecular rate constants in the range ca. 106 - 107 M-1 s-1. The pKa of the bipyridine radical anion was measured using a kinetic method and analysis of the data using a Marcus theory model for proton transfer. The bipyridine radical anion is found to have a pKa of 40±5 in DMSO. The reorganization energy for the proton transfer reaction was found to be 70±5 kJ/mol. The bipyridine radical anion was found to react very rapidly with carbon dioxide, with a bimolecular rate constant of 1* 108 M-1 s-1 and a small activation energy, whereas the protonated radical reacted with carbon dioxide with a rate constant that was too small to measure. The kinetic and thermodynamic data obtained in this work can be used to understand the mechanisms of the reactions of pyridines with carbon dioxide under reducing conditions.

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2015