Fig. 6: Suggested missing link and isomerism in Kok’s cycle.
From: Closing Kok’s cycle of nature’s water oxidation catalysis
The missing link during the S4 → S0 transition embedded in the full catalytic cycle is highlighted in red, and the isomerism in green. O5 in S1B is at a non-bonding distance with respect to both Mn1 and Mn467. 
in the S3 state denotes the unidirectional isomerization because the open-cubane structure is largely stabilized12,91,92; however, our recent study suggests the isomerization becomes reversible again in the ensuing S3YZ• state after H+ release93. O–O bond formation in the S4 state (peroxo in S4’) for either open-11,94,95,96,97 or closed-cubane27,31,98,99,100 structure (with O5 and Ox as substrates) has been theoretically supported, and the \({{{\mbox{Im}}}0}^{{-{{\mbox{O}}}}_{2}}\) structure formed after O2 release would be basically the same; thus the missing link would apply in either case. Note that the protonation states for some certain ligands are still debatable, such as W2 = OH−/H2O34,35,36,37,38,101,102, O4/O5 = O2−/OH− (for S0)34,37,38,64,103, and Ox = OH−/O2−/O•− (for S3/S3YZ•)4,5,12,38,97,104, etc.
