Analysis of the radiation-damage-free x-ray structure of photosystem ii in light of exafs and QM/MM data

A recent femtosecond X-ray diffraction study produced the first high-resolution structural model of the oxygen-evolving complex of photosystem II that is free of radiation-induced manganese reduction (Protein Data Bank entries 4UB6 and 4UB8). We find, however, that the model does not match extended X-ray absorption fine structure and QM/MM data for the S₁ state. This is attributed to uncertainty about the positions of oxygen atoms that remain partially unresolved, even at 1.95 Å resolution, next to the heavy manganese centers. In addition, the photosystem II crystals may contain significant amounts of the S₀ state, because of extensive dark adaptation prior to data collection.