Abstract
Photosystem II (PSII) is a huge membrane-protein complex which catalyzes light-driven water-oxidation
reaction at its catalytic center, the oxygen evolving complex (OEC). The crystal structure of PSII has
been determined at 1.9 Å resolution using synchrotron radiation which revealed that the OEC is a
Mn4CaO5 cluster. Some of the manganese atoms of the OEC are, however, rapidly reduced by X-ray
irradiation which results in slight elongation of the distances between manganese cations. Furthermore,
high-resolution 3D structural information is only limited to the dark-stable S1 state and the structures in
the other intermediate states are missing. X-ray free electron laser (XFEL) has the potential to address
these unsolved problems, and unveil the water-splitting reaction mechanism of PSII. In this review, we
will introduce the analyses of the damage-free structure and an intermediate structure of PSII using
XFEL, demonstrating the potential to obtain high spatial resolution of biological samples by XFEL.
