QM/MM study of the S2 to S3 transition reaction in the oxygen-evolving complex of photosystem II

- 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).