Coupled Electron Proton Transfer in Biological Redox Substrates Public Deposited

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  • March 22, 2019
  • Gagliardi, Christopher J.
    • Affiliation: College of Arts and Sciences, Department of Chemistry
  • The amino acids tyrosine, tryptophan and cysteine are used to carryout electron transfer within enzymes and proteins. The chemistry and oxidation mechanisms vary considerably between these molecules. Proton coupled electron transfer (PCET), where electrons and protons travel in sequence, or in concert is used by these amino acids in both proteins as well as free in aqueous solution in order to minimize energy expenditure and prevent build up of high-energy intermediates. The oxidation of these amino acids by a series of metal M(bpy)33+ M = Os, Fe, Ru complexes in aqueous solution at ITO electrodes as well as spectroscopically in stopped-flow mixing studies is reported. A common mechanism involving the pre-association of a base to form a hydrogen bound adduct prior to electron transfer was observed in all cases. Each of the three amino acids displayed various degrees of hydrogen bonding with bases, linked to their pKa values (cysteine 8.2, tyrosine, 10.1 and tryptophan 16.0). Base assisted oxidation pathways occur through two competing mechanisms; either proton transfer to a base followed electron transfer (PT-ET), or concerted electron proton transfer (EPT). The kinetics of these pathways as well as additional pathways such as outer-sphere electron transfer were studied using electrochemical, and spectroscopic methods, giving rate information for each pathway as well as kinetic isotope data which provided insight into how these molecules behave in enzymatic electron transfer reactions. We have found that for PCET reactions the driving force is the dominate factor dictating reactivity and can be influenced by both the pKa of the proton acceptor and the oxidation potential of the M(bpy)3III/II couple. This insight has provided a better understanding of how tyrosine, cysteine, and tryptophan are used to mediate biological electron transfer and allowed us to observe commonalities in redox mechanisms with respect to common bases and oxidants in biological PCET reactions.
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  • In Copyright
  • Meyer, Thomas
  • Doctor of Philosophy
Graduation year
  • 2012

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