CHAPTER 6. Elucidating Ultrafast Structural Motions in Photosynthetic Reaction Centers with XFEL Radiation

Abstract

Photosynthetic reaction centers are integral membrane proteins found in photosynthetic plants and bacteria and are the work-horses of photosynthesis. Within these proteins, the energy of sunlight is directed to a special pair of closely spaced chlorophyll molecules that become photo-oxidized as an electron is dispelled to the opposite side of the biological membrane. Evolution has optimized the charge separation reactions of photosynthetic reaction centers so as to achieve a remarkably high quantum yield and energy efficiency. Whether or not ultrafast structural changes play any functional role in guiding the primary charge separation reactions of photosynthesis has been debated for two decades but is not yet resolved. Here, we review progress towards visualizing structural changes in photosynthetic reaction centers using synchrotron radiation. We further describe the revolutionary potential of X-ray free electron lasers (XFELs) for shedding new light on ultrafast structural changes in biomolecules. We emphasize milestones towards the goal of observing functionally important ultrafast motions in photosynthetic reaction centers in real time and anticipate that novel high-resolution structural insights are now technically within reach.