Ultrafast Studies of Electron Transfer in Photosystem II

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9727948 Sension In this research project state-of-the-art ultrafast spectroscopic techniques will be used to study the primary electron transfer processes in photosystem II. Three specific lines of attack will be pursued in this series of investigations. (1) Ultrafast fluorescence detection will be used to study the excited state population dynamics directly in the D1-D2-Cyt b559 reaction center complex. A streak camera of novel design, which provides greater flexibility, superior signal averaging and sub five picosecond time resolution, will be used. These characteristics are necessary for the study of energy transfer and change separation in PSII. (2) Larger PSII complexes will be prepared in order to study the electron transfer rate in active centers and set to rest any speculation as to whether or not the D1-D2-Cyt b559 complex retains the native reaction center structure and dynamics. The studies will investigate the D1-D2-Cyt b559 complex, the CP47-D1-D2-Cyt b559 complex, and the reaction center core under consistent conditions permitting direct and quantitative comparisons. (3) Finally, the addition of external replacement quinones will be used to study the primary and secondary electron transfer processes in PSII. These measurements will be performed on the D1-D2-Cyt b559 complex and the CP47-D1-D2-Cyt b559 complex. These studies will explore the effect of the presence of a secondary electron acceptor on the population equilibrium distributed over the locally excited fluorescent and charge separated dark states. A rigorous understanding of the primary electron transfer reactions in biological photosynthetic systems is one of the keys to a complete understanding of natural solar energy conversion. The protein complex referred to as photosystem II is responsible for the ability of green plants to utilize light energy to fuel the production of oxygen from water. The PSII protein complex works in conjunction with a second complex, photosystem 1, to form carbohydrates from carbon dioxide. From a chemical point of view, the ability of green plants to produce oxygen and carbohydrates from light, water and carbon dioxide represents an amazing achievement. The overall goal of this investigation is to understand the factors that control the efficiency of photosynthesis in green plants.