Two Distinct Oxygen-Radical Conformations in the X‑ray Free Electron Laser Structures of Photosystem II

We report the existence
of two distinct oxygen-radical-containing
Mn4CaO5/6 conformations with short O···O
bonds in the crystal structures of the oxygen-evolving enzyme photosystem
II (PSII), obtained using an X-ray free electron laser (XFEL). A short
O···O distance of <2.3 Å between the O4 site
of the Mn4CaO5 complex and the adjacent water
molecule (W539) in the proton-conducting O4-water chain was observed
in the second flash-induced (2F) XFEL structure (2F-XFEL), which may
correspond to S3. By use of a quantum mechanical/molecular
mechanical approach, the OH• formation at W539 and
the short O4···OW539 distance (<2.3 Å)
were reproduced in S2 and S3 with reduced Mn1­(III),
which lacks the additional sixth water molecule O6. As the O•– formation at O6 and the short O5···O6 distance (1.9
Å) have been reported in another 2F-XFEL structure with reduced
Mn4­(III), two distinct oxygen-radical conformations exist in the 2F-XFEL
crystals.