Concerted proton-electron transfer reactions of small molecule iron-porphyrin complexes and organic substrates relevant to biological redox processes.

摘要

Any reaction where electron (e- ) transfer is modulated by protons (H+) is known as proton-coupled electron transfer (PCET). PCET reactions are important to many chemical and biological processes, such as the antioxidant activity of vitamins C and E. Thus, there is great interest in understanding PCET processes, from industrial syntheses to biochemical conversions. The work described here focuses on understanding PCET reactions of small molecule chemical models. The results from these studies are used to advance understanding of PCET in the broader context of chemistry and biology. Chapter 1 describes current terminology in the PCET field, and how that terminology relates to the mechanism of H +/e- transfer. Chapter 2 describes the thermochemical features of several organic and inorganic compounds, and how those thermochemical data are related to the mechanism of a PCET reaction. Chapter 3 develops a Marcus theory-based model for predicting PCET rate constants. The model specifically accounts for solvent effects on rate constants and equilibrium constants of organic PCET reactions. The combined model predicts rate constants to within a factor of 5 for organic PCET reactions, and to within a factor of 10 for inorganic PCET reactions. The success of the model indicates that the driving force and intrinsic barriers of PCET reactions are key determinants of the rate constants. Chapters 4 and 5 describe PCET reactions of ascorbate derivatives in acetonitrile. The product of PCET from ascorbate is the semidehydroascorbyl radical, which persists for hours in anhydrous acetonitrile, compared to its rapid decay in H2O. The stability of the ascorbyl radical allows for rate and equilibrium studies of PCET reactions of ascorbates, which show a unique local solvent effect. Chapters 6, 7 and 8 describe bis(imidazole)iron-porphyrin complexes that are chemical models for bis(histidine) ligated hemes. These models undergo facile PCET reactions with an ascorbate derivative, a hydroxylamine and hydroquinones. The roles of ligated imidazole and the heme-propionate as the proton accepting groups are discussed. PCET reactions can occur where the propionate accepts H+, even though it is distant from redox active iron. Chapter 9 summarizes the work described in the broader context of chemistry and biology.