Fig. 3. Proton and electron transfer steps of the oxygen-evolution transition.

a, Schematic summary of experimental findings on reaction intermediates, time constants (reciprocal rate constants), enthalpic and entropic contribution of the activation energy (see Extended Data Fig. 5), and S-state assignment. The entropic contribution is the product of the entropy of activation (S_act) and the absolute temperature that corresponds to 20 °C (T₀ = 293.15 K). Key features are highlighted with orange circles, the two oxygen atoms from ‘substrate water’ are shown in red, and charge-transfer events are indicated with arrows (red, electron transfer; blue, proton transfer). Out of the four Mn ions of the Mn₄Ca cluster, only the three Mn ions (Mn1, Mn3 and Mn4) that have accumulated oxidizing equivalents (holes) in preceding S-state transitions and are ‘discharged’ concomitantly with O₂ formation are shown. The fourth hole transiently residing on the oxidized Tyr_Z (denoted here as ${\mathrm{Y}}_{\mathrm{Z}}^{+}$) is filled by electron transfer from a substrate-water oxygen in the ${\mathrm{S}}_3^{\prime }\to {\mathrm{S}}_4$ transition. b,c, The rate-constant processes depicted in a, ${\mathrm{S}}_3^{+}\to {\mathrm{S}}_3\to {\mathrm{S}}_3^{\prime }$ (b) and ${\mathrm{S}}_3^{\prime }\to {\mathrm{S}}_4$ (c), are assigned to atomistic events, facilitated by computational results. The dotted blue circles highlight the creation of a proton vacancy induced by Tyr_Z oxidation and activating Asp61 as a proton acceptor in the ${\mathrm{S}}_3^{\prime }\to {\mathrm{S}}_4$ transition via movement of the Lys317 sidechain. d, Energy values associated with MEP calculations and internuclear distances (top graph, dashed lines) as well as spin populations of the Mn ions, atoms and residues species  (bottom graph, solid lines) that characterize the peroxide formation. See Extended Data Figs. 7 and 8, for structures complementing d and describing the complete oxygen-evolution transition.