Structure and Dynamics of Radical Intermediates in Biological Electron Transfer Reactions

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In this project funded by the Chemical Structure, Dynamics, and Mechanisms-B program of the Chemistry Division, Professor Judy Kim of the University of California at San Diego is investigating the flow of charge in biological systems. This ability of proteins to separate charge is important for many chemical processes in nature, including photosynthesis and respiration. Her research group utilizes optical tools combined with molecular biology methods to investigate protein structures and rates associated with the flow of electrons in two model systems. Results from this project not only elucidate how nature controls the flow of charge over long distances, but these findings are also relevant to other disciplines, such as biology and physics. The broader impact of the research is significant because discoveries described here may inspire new strategies for directional control of long-range electron flow in man-made and biomimetic devices. In addition to the laboratory efforts, the research team participates in outreach programs that reinforce the significance of STEM topics to K-12 students. This project focuses on amino acids that participate in electron transfer (ET) reactions in proteins. Modern spectroscopic techniques are used to characterize tryptophan and tyrosine radicals in proteins, and to correlate protein environment with radical reactivity. Key topics of investigation are (1) the optical, magnetic, and electronic properties of biological tryptophan and tyrosine radicals; (2) kinetics and structural changes associated with ET reactions of amino acid radicals; (3) effects of hydrogen-bonding and local dielectric on radical stability and dynamics; and (4) multi-step ET reactions along amino acid chains in a protein. Spectroscopic techniques are employed, including time-resolved and steady-state resonance Raman, transient absorption, and electron paramagnetic resonance spectroscopy. These tools, combined with protein mutagenesis efforts, provide insights on the structures and dynamics of the cation and neutral radical species.