Ph.D. (Doctor of Philosophy)
Department of Chemistry and Biochemistry
Chemistry, Analytic||Chemistry, Physical and theoretical||Biochemistry
The investigation of gas-phase radical cations as a method of modeling and exploring the fundamental chemical characteristics of radicals found in biological systems has generated substantial interest over the last decade. Gas-phase techniques overcome the difficulties incurred when studying the highly reactive and often transient species in solution where they are subject to undesirable side reactions. This dissertation aims to expand the body of knowledge regarding biological free radicals through the study of their gas-phase analogues. Specifically, the studies undertaken focus on the elucidation of the intrinsic thermodynamic and kinetic characteristics unencumbered by solvation and counter-ion effects. The majority of the projects presented herein focus on radical migration in amino acid- and peptide-based systems. The effect of metal ion complexation, in comparison to protonation, on the propensity for thiyl radicals to undergo hydrogen atom transfer (HAT) and generate alpha-carbon (Calpha) radicals is investigated in depth. Two amino acids, cysteine (Cys) and its analogue homocysteine (Hcy), and the redox-active tripeptide glutathione (GSH) are evaluated using the combination of ion-molecule reactions (IMRs), infrared multiple photon dissociation (IRMPD) spectroscopy, and theoretical calculations. In all cases, species possessing Calpha-based radicals were found to be thermodynamically favorable compared to their thiyl-based radical counterparts due to the effect of captodative stabilization. Metal ion complexation was found, both experimentally and theoretically, to increase this effect in the order K⁺>Na⁺>Li⁺. Evaluation of the potential energy surface for alkali metal ion complexes of Cys and Hcy radicals revealed that the energy barrier towards S-to-Calpha radical migration followed the same trend. This explains the observation of radical migration via 1,4-HAT in metal-bound Hcy radicals. The more strained four-membered transition state did not allow for 1,3-HAT in Cys radical metal ion adducts. Although transition states were not calculated for radical migration within the GSH system, the experimental results indicate that alkali metal ions facilitate HAT also in the order K⁺>Na⁺>Li⁺ and all to a greater degree than seen in the protonated radical species. As a whole, these studies demonstrate that alkali metal ion complexation both stabilizes the radical ions and decreases the energy required for isomerization. In biological systems, radical migration is commonly observed between the side chains of redox-active amino acids tyrosine (Tyr) and Cys. This behavior is modeled both intramolecularly (via IMR of phenoxyl radical cations and a simple thiol neutral) and intermolecularly through analysis of the model peptide LysTyrCys. In both cases, Tyr-to-Cys radical migration was found to occur. The effect of hydrogen bonding and spin electron density on the phenoxyl radical (Tyr radical model) reactivity was evaluated using a series of aromatic model systems. As expected, IMRs revealed that higher radical delocalization and increased strength of hydrogen bonding decreased the reactivity of these oxygen-based radicals. Such observations agree with the notion that the kinetics of radical-initiated enzyme catalysis may be modulated by the local environment within the protein. The formation of radicals in building blocks of DNA is particularly concerning due to the severe consequences they can inflict (e.g., eventual mutagenesis and cancer). An initial attempt to study the fundamental chemistry of nucleobase radical cations is presented. Formation of the cytosine radical cation Cyt˙⁺ was achieved via oxidation of cytosine by Cu(II) in the gas phase, and the propensity of the ion to undergo several radical-driven reactions was screened using IMRs. Tautomer analysis was attempted using gas-phase infrared and ultraviolet spectroscopy and a mixture of isomers with similar energies were found. This study represents a proof-of-principle for investigating the radical cations of the constituents of DNA using a combination of gas-phase techniques and computation chemistry.
Lesslie, Michael, "Investigations of biologically relevant free radicals utilizing novel gas-phase analytical techniques" (2017). Graduate Research Theses & Dissertations. 3663.
xiv, 247 pages
Northern Illinois University
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