Structural and Electron Dynamics of the O-O bond Formation in Photosystem II

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With this award, the Chemistry of Life Processes Program of the Chemistry Division is supporting Dr. Yulia Pushkar of the Department of Physics and Astronomy at Purdue University to study the mechanism of dioxygen (O2) formation in the naturally-occurring photosynthetic protein Photosystem II. This protein uses the energy of sunlight to split water molecules into protons, electrons and oxygen molecules, providing the majority of the oxygen in the Earth’s atmosphere. Plants subsequently use the electrons and protons in chemical processes resulting in CO2 fixation. The chemical center of Photosystem II responsible for water-splitting contains manganese and calcium ions, that work together in the crucial chemical catalysis process that sustains our biosphere. Establishing the fundamental mechanism of this process in molecular detail could enable the design of new methods for CO2-neutral sunlight-powered energy production. The planned educational activities focus on increasing opportunities for and the participation in research activities for students from backgrounds underrepresented in STEM careers. Dr. Pushkar advises in research students who participate in the Minority Access to Research Careers and Access Internally for Minorities, the Louise Stokes Alliance for Minority Participation, and Research Experiences for Undergraduate Programs. She also mentors student recipients of Summer Undergraduate Research Fellowships, Discovery Park Undergraduate Research Internships, and the Edward S. Akeley Award and Lijuan Wang Award for Women in Physics and contributes to the training of young women scientists through research and presentations in Purdue’s Women in Physics and Women in Science Programs. Research in this project focuses on the use of laser pump X-ray probe time-resolved X-ray emission, absorption and scattering to study the oxygen-evolving complex (OEC) of Photosystem II, which participates in the O-O bond formation and O2 evolution, the critical step of photosynthesis. The analysis of the electronic structure of the Mn4Ca cluster is conducted with microsecond (µsec) time resolution and the structural changes in the cluster are determined with ~0.02 Å precision. Critical technology developments are made in high-resolution, high-sensitivity X-ray spectrometers, nanoliter drop-on demand sample delivery systems, and biological time-resolved X-ray spectroscopy at newly developed high-flux synchrotron beamlines. The experimental results are interpreted in the framework of currently available structural models of the OEC. Complementary DFT calculations are conducted to monitor the energy profile of the proposed chemical transformations in the OEC. Elucidation of the structural changes in the OEC that accompany the critical catalytic step of O-O bond formation and elucidation of the nature of the species activating water by combined experimental data and DFT modeling are stepping stones for understanding the details of the mechanism of catalytic water-splitting.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.