Fig. 1. Reaction cycle of photosynthetic oxygen evolution.

a, Model of the S-state cycle with sequential electron and proton removal from the oxygen-evolving site^10,11,50. Starting in the dark-stable S₁ state, each laser flash initiates oxidation of the primary chlorophyll donor (P680⁺ formation) followed by electron transfer from a tyrosine sidechain (Tyr_Z oxidation) and—in three of the four S-state transitions—manganese oxidation, until four electron holes (oxidizing equivalents) are accumulated by the Mn₄Ca-oxo cluster in its S₄ state. b, Example of tracing S-state transitions using IR absorption changes after excitation with visible-wavelength laser flashes (at zero on the time axis). The absorption changes (ΔA) are provided in optical density (OD) units. The IR transients at 1,384 cm⁻¹ reflect symmetric stretching vibrations of carboxylate protein sidechains that sense changes in the oxidation state of manganese in the microsecond and millisecond time domain (coloured lines are simulations with time constants provided in Supplementary Table 2). Note that the scale on the x axis is linear below t = 0 and logarithmic above t = 0. c, The Mn₄Ca cluster (Mn, violet; Ca, pink) in the S₃ state with six bridging oxygens, the redox-active tyrosine (Tyr_Z), and further selected protein sidechains as well as water molecules (red spheres), based on crystal structures²⁵. Assignment to polypeptide chains, numbering of the atoms of Mn₄Ca-oxo and water molecules and hydrogen-bond distances are indicated in Supplementary Fig. 1. The two oxygens atoms that form the O–O bond in the oxygen-evolving S₃ → S₀ transition are indicated by red arrows.

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