Fig. 3.

(A) Computed free energy surface for the O$\to$O* transition as a function of the D85C$\gamma$–D212C$\gamma$ distance and the PT reaction coordinate $\zeta$. (B) The same surface as a function of the net charge on the four QM water molecules $q_{\mathrm{n}\mathrm{e}\mathrm{t}}$ and $\zeta$. There are three distinct pathways, which differ in the barrier heights. (C) The geometries of the upper active site found in the transition structures marked by colored dots. Black, minimum free energy path, direct proton transfer; red, proton hole transfer (OH^–); blue, excess proton transfer (H₃O⁺); green spheres, protons bound to an Asp or Glu; orange molecules, charged water species (OH^– or H₃O⁺).