Proton Transfer and Cofactor Function in Photosynthetic Reaction Centers

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All of life is based on a small subset of chemical reactions and processes, among the almost infinite number of possible ones. The selected reactions are catalyzed by biological macromolecules that speed up the reaction rates by very many orders of magnitude. The vast majority of these catalysts are proteins known as enzymes, and many of these have small molecules as cofactors that are intimately involved in the chemistry. In biological energy conversions, oxidation-reduction (redox) free energy is converted into transmembrane proton and electrical gradients. The redox chemistry is performed by diverse organic and metal cofactors that are invested with extraordinary properties by the proteins that bind them. The protein-cofactor interactions that yield these properties are the same as those that operate on a substrate in an enzyme active site. Because of the exceptional spectroscopic attributes of redox and photobiological cofactors, the membrane proteins of respiration, photosynthesis and methanogenesis provide ideal systems for studying the essential and defining quality of biochemistry - catalysis with astonishing specificity. This project focuses on the photosynthetic reaction center (RC), which catalyzes a light-driven electron transfer with almost 100% quantum efficiency. The primary aim is to elucidate the mechanisms whereby the photo-activated electron is delivered to the acceptor and stabilized against wasteful backreaction. In large part, this is due to accompanying charge redistributions within the RC, including internal proton transfers and net uptake from solution. The known structure of the RC, its rich spectroscopy, and the light-activatable nature of its reactions allow the origins of these cofactor properties to be characterized in terms of the dynamics and energetics of the molecular structure. In all projects, the impact of chemical and mutagenic perturbations on energetics and kinetics will be studied. Kinetic and equilibrium consequences will be determined by optical absorption spectroscopy, free energy levels by delayed fluorescence and by potentiometry, and structural implications by FTIR, X-ray diffraction and pulsed EPR spectroscopy.Broader impacts: The broader impacts of this research derive from the very diverse methodologies employed, which provide an exceptional training environment for young scientists, with a substantially multidisciplinary component. This work contributes to the practical understanding of processes with global and human impact, including alternative energy sources, biofuels and global warming. They form a strong basis for teaching, mentoring and motivating students in the classroom, at the high school as well as college level.