Spin Coherences in Photosystem I Reaction Center Proteins and Model Systems

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This award in the Chemistry of Life Processes (CLP) program supports work by Professor Michael R. Wasielewski at Northwestern University to carry out fundamental studies on the nature of multiple pathways for light-induced charge separation in one of the key light transduction proteins in green plants, the Photosystem I (PSI) reaction center (RC), which is responsible for providing the chemical potential necessary to reduce carbon dioxide to carbohydrates. The PSI RC in oxygenic organisms is unique because it is the only RC for which there is evidence that both sets of redundant cofactors engage in electron transport. The electron transfer cofactors in the PSI RC protein are arranged in two symmetric pathways, A and B, which results in charge transport to a common electron acceptor, and suggests the possibility that the PSI protein may function as an interferometer in which electrons traversing the two pathways are in a quantum superposition state. This project will address two key questions regarding the role of quantum superposition in PSI function: 1) Does charge transport within PSI involve independent spin-coherent radical ion pairs using either the A and B pathways or a quantum superposition state involving both pathways simultaneously? 2) Does the inherent quantum coherent nature of the dual-pathway charge separation sense and control excess excitation energy dissipation in the PSI RC? Time-resolved electron paramagnetic resonance spectroscopy will be used to probe these mechanisms, while theory and synthetic donor-acceptor systems will be used to model this behavior.Understanding excess energy dissipation in photosynthetic organisms is critical to ensuring the viability and sustainability of photosynthetic energy transduction in nature, a process essential to life on Earth. Improved knowledge of this process provides an important conceptual basis for the design of artificial photosynthetic systems for solar energy conversion. The results of this research will be communicated to the public through the extensive outreach network of the Northwestern-funded Initiative for Sustainability and Energy at Northwestern (ISEN).