The movement of protons and electrons in proteins drives many essential processes in biology, including photosynthesis and cellular respiration. When one proton and one electron move simultaneously, the overall transfer is usually faster and more efficient than cases where protons and electrons move alone or one at a time. Surprisingly, this increased efficiency is observed even when the proton and electron move in opposite directions. For this reason, multiple-site concerted proton electron transfer (MS-CPET) is prevalent in biological systems, for instance in tyrosine oxidation in photosystem II and in hydroquinone oxidation as part of mitochondrial respiratory chains. Even though MS-CPET is biologically ubiquitous and crucial, its mechanism and properties are poorly understood.
The aim of the proposed work is to develop a better understanding of how the rate and mechanism of MS-CPET are controlled. We propose to design and build a series of molecules that mimic MS-CPET from tyrosine. These synthetic substrates will contain a phenol to release the proton and electron, a photooxidant to receive the electron, and a base to receive the proton, all covalently tethered together. The synthetic systems will provide a unique opportunity to study MS-CPET at fast rates not limited by diffusion. The rates of MS- CPET will be measured by fluorescence quenching of the excited photooxidant moiety of the substrate. The rates for unimolecular MS-CPET will then be modeled using Marcus theory in order to understand how factors including thermodynamic driving force and molecular structure affect kinetic properties such as rate and whether the reaction involves more than one energy surface. Overall, the study of MS-CPET in unimolecular synthetic substrates will allow for the development of a fundamental and intuitive understanding of MS-CPET, which will provide insight into essential, complex biological systems.
The proposed research will provide a fundamental understanding of multiple-site concerted proton-electron transfer (MS-CPET) reactions by using simplified synthetic molecules to mimic complex biological systems. Many important processes in the human body employ MS-CPET, and malfunctions in these processes have been shown to contribute to diseases including mitochondrial myopathies, Lou Gehrig's disease, Wolfram Syndrome 2, and diseases of old age. A clearer description of the mechanism of MS-CPET will allow improved understanding of and treatment for these diseases.