Catalytic Cooperativity of Mono-Manganese and Tri-Manganese Clusters for Water-Splitting and Oxygen-Evolving Reaction in Photosystem II: Chemical Mechanistic Insight

Applying the UDFT/B3LYP/(lacvp**, lacv3p**) geometry optimization method together with a Poisson-Boltsman equation solver in the e = 4 dielectric medium to a version-upped “truncated-OEC-cluster” model of MT-type, we found that (1) Upon the Si-state transitions (I = 0 — 4) in a cyclic change of the most-stable tautomer(s), a proton release pattern of 1:0:1:2 has been derived with use of calculated exothermic vs endothermic energies, to yield the oxidation states: S0Mna III; Mnb III, Mnc III, Mnd IV, S1Mna III; Mnb IV, Mnc III, Mnd IV, S2 +Mna IV; Mnb IV, Mnc III, Mnd IV, S3 +Mna IV; Mnb IV, Mnc IV, Mnd IV and S4aMna IV; Mnb IV, Mnc IV, Mnd IV, (2) The redox potential for the last S3/S4a + oxidation step has been evaluated to be ca.1.07 V, a significantly-reduced value due to the H-bonding network between YZ, H190 and Ca 2+-binding site in the Mn4Ca cluster, (3) The S4a-intermediate contains the catalytic Mna IV ion binding two adjoining substrate derivatives, a hydroxyl anion (W1 = HO−) and an oxo radical (W2 = O−·), and (4) The O-O bond formation is thermally induceable by a proton-coupled electron transfer (PCET) via a transition state with an activation energy of ca. 11.2 kcal/mol and a small exothermicity of ca. −4.5 kcal/mol, to yield a side-on superoxo anion radial bound to the Mna III ion in the second intermediate, formulated as S4bMna III:O2−·(W1 = W2); Mnb IV, Mnc III, Mnd IV, where the third Mnc III ion is in a low-spin state of S c=1.