The electron-proton bottleneck of photosynthetic oxygen evolution

Photosynthesis fuels life on Earth by storing solar energy in chemical form, inspiring technological schemes for sustainable fuel production. Today’s oxygen-rich atmosphere results from photosynthetic O2-production during water-splitting at the protein-bound manganese cluster of photosystem II. Formation of the O2 molecule starts from a state with four accumulated electron holes, the S4-state, postulated half a century ago1 and remaining enigmatic ever since. Here we resolve this missing key element in photosynthetic O2-formation and its crucial mechanistic role. We tracked 230,000 excitation cycles of dark-adapted photosystems with microsecond infrared spectroscopy. Combing these results with computational chemistry reveals that in S4 not only are four electron holes accumulated by metal ion and protein sidechain oxidation, but also a crucial proton vacancy is created through gated sidechain deprotonation. Subsequently, a reactive oxygen radical is formed in an astonishing single-electron multi-proton transfer event. This is the slowest step in photosynthetic O2-formation – despite its low energetic barrier – due to entropic slowdown. In conjunction with previous breakthroughs in experimental and computational investigations, a compelling atomistic picture of photosynthetic O2-formation emerges. Our results provide insight into a biological process that has probably operated in the same unique way for three billion years and are expected to support the knowledge-based design of artificial water-splitting systems.