Fig. 1.
Photosynthetic water oxidation and protonation of a water cluster during the S1-to-S2 transition. (A) S-state cycle of photosynthetic water oxidation (7). (A, Inset) Predicted hydrogen-bond network of water molecules in the OEC of PSII (5). Amino acids are shown as sticks. Oxygen atoms of water molecules are shown in blue, with hydrogen bonds to peptide carbonyl groups shown as dashed lines. (B) Mechanism proposed for deprotonation of a terminal water ligand during the S1-to-S2 transition in PSII (14). Hydrogen bonds are shown as dashed lines. Putative substrate water molecules are shown in blue. (C) Schematic diagram showing the formation of a cationic water cluster, W5+, on the S1 to S2 transition in PSII. (D) Diagram showing method generating the reaction-induced FTIR spectrum, corresponding to the S2-minus-S1 spectrum. Difference FTIR spectra for the other S-state transitions are produced with two (S3-minus-S2), three (S0-minus-S3), or four (S1–S0) flashes (see cycle in A).