Catalytic reactions of oxygen-evolving complex during the S2 → S3 transition are investigated.
Two reaction pathways, R- and L-reactions are elucidated.
Both reactions start by moving the Ca-bound water (W3) to the vacant Mn(III) coordination.
The L-reaction is more energetically favored than the R-reaction.
Proton transfer occurs through O5, W2(OH) and the active site water molecules (W5–W7).