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

摘要

Applying the UDFT/B3LYP/(lacvp**, Iacv3p**) 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 MTtype, 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: S0{MnaⅢ; MnbⅢ, MncⅢ, MndⅣ}, S1{MnaⅢ; MnbⅣ, MncⅢ, MndⅣ}, S2+{MnaⅣ; MnbⅣ, MncⅢ, MnⅣ}, S3+ {MnaⅣ; MnbⅣ, MacⅣ, MndⅣ} and S4a{MnaⅣ; MnbⅣ, MncⅣ, MndⅣ}, (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 Ca2+-binding site in the MruCa cluster, (3) The S4a-intermediate contains the catalytic MnaⅣ 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 MnaⅢ ion in the second intermediate, formulated as S4b{MnaⅢ:O2-(W1 = W2); MnbⅣ, MncⅢ, MndⅣ}, where the third MncⅢ ion is in a low-spin state of Sc=l.