The computational, structural and spectroscopic evidence above all points to an S 3 state involving an equilibrium between an O5 O6H oxo-hydroxo and an [O5O6] 3-species. The most recent XFEL structures for the S 3 state also reveal a very short O6 to Oε2Glu189 distance of 2.4 -2.5 Å suggesting a low barrier hydrogen bond between the two atoms. This strongly indicates that the S 3 state equilibrium is established by proton sharing between these two atoms as illustrated in Figure . Based on our combined computational, spectroscopic and structural analysis, we demonstrate that O-O bond formation has begun between the O5 and O6 atoms in the S 3 state with the generation of the [O5O6] 3-ion. This is the dominant species present in the S 3 state. Figure shows that this provides a low barrier pathway to subsequent formation of the peroxo form.