CAREER: Time-resolved Studies of the Oxygen Evolving Complex of Photosystem II

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The Chemistry of Life Processes Program and the Molecular Biophysics Program are funding Dr. Yulia Pushkar of Purdue University for research towards a detailed mechanism for photosynthesis, the process by which plants produce substances such as sugars from carbon dioxide, water and sunlight. The understanding of this key life process can advance the development of alternative energy sources through the design of better water-splitting catalysts that enable artificial photosynthesis. The work employs various physical measurements to determine the molecular details of this important process in real time and, as such, is at the interface of physics, chemistry and the biological sciences. Consequently, the broader impact of the research is also on several scientific areas including bioinorganic chemistry and molecular biophysics. Additionally, the work has consequences on science education at the high school level through the involvement of the research team in the design of lesson modules about photosynthesis that high school science teachers can adopt and use. Inquiry-based educational modules that incorporate the results of the research are developed for the "Physics of the Green Leaf" program. The research is focused on the study of the light-induced function of the oxygen evolving complex (OEC) in Photosystem II. The light-induced water splitting by the Mn4Ca cluster of the oxygen evolving complex is a fundamental biological process that sustains our biosphere. Laser pump X-ray probe time-resolved X-ray emission and X-ray absorption spectroscopy are used to monitor changes in the electronic structure of the OEC in real time during catalysis. Transient intermediates in Photosystem II protein are trapped by ultra-fast freeze-quenching and their structure is characterized using the extended X-ray absorption fine structure (EXAFS) method. The research aims are to determine aspects of both the dynamics of the change in electronic structure and the geometry of the manganese cluster and, when combined with DFT calculations, to provide details of the mechanism of catalytic water splitting.