The oxygen-evolving complex (OEC) of photosystem II — a Mn4CaO5 cluster — forms the O–O bond of O2 in the short-lived S4 state of the Kok cycle (the light-driven sequence of oxidation states S0–S4 through which the OEC advances), which has never been structurally observed. Against the consensus that all manganese remain at oxidation state IV or lower, with O–O bond formation by coupling of an oxyl radical to an oxo bridge, Zhang and Sun (2018) proposed that the dangling manganese becomes a closed-shell, permanganate-like Mn(VII) dioxo species whose two cis oxo ligands couple directly. Li, Li and Siegbahn (2020) found this route energetically prohibitive (overall barriers of 78–81 kcal/mol versus 11–13 kcal/mol for the Mn(IV) route), but with acknowledged caveats: the Mn(VII) transition states were approximated by reaction-coordinate scans rather than saddle-point optimizations, and the S-state energetics of the two mechanisms — carried by models of different total charge — cannot be compared for transitions that release only an electron. No subsequent computational study (full transition-state optimizations, higher-level electronic-structure methods, larger models) and no experimental characterization of the S4 state has appeared; the proposing group itself moved to Mn(V)-based alternatives. The problem is to deliver a definitive, quantitatively supported verdict — exclusion or validation — on the Mn(VII) dioxo mechanism, by computations meeting those standards and/or by structural or spectroscopic characterization of the S4 state's manganese oxidation states.

Background

Photosystem II (PSII) is the membrane protein complex that powers oxygenic photosynthesis; it oxidizes water to molecular oxygen, the reaction that sustains essentially all aerobic life. The catalyst is an inorganic Mn4CaO5 cluster, the oxygen-evolving complex (OEC), held in the protein by amino-acid ligands. Each absorbed photon advances the OEC one step through the five oxidation states S0 to S4 of the Kok cycle, removing one electron per step (via the redox-active tyrosine YZ) and, in alternating steps, one proton. The O–O bond of O2 is formed in the transient S4 state, which has never been trapped for structural characterization: serial femtosecond crystallography at X-ray free-electron lasers (XFELs — facilities whose femtosecond pulses record diffraction before radiation damage sets in) has resolved only the observable S0–S3 states.

For decades two families of O–O-forming mechanisms competed: nucleophilic attack of a water or hydroxide ligand on a high-valent terminal Mn=O, and radical coupling between an oxyl radical (an oxygen ligand carrying one unpaired electron, formally O(−1) with radical character) and an adjacent oxo bridge. The present consensus, built on density functional theory (DFT) energetics and XFEL structures, favors the second: in S4 all four manganese remain Mn(IV), the additional oxidizing equivalent is stored as an oxyl radical on a Mn-bound oxygen ligand, and this oxyl couples with the bridging oxo O5, with computed barriers of roughly 11–13 kcal/mol, compatible with the millisecond kinetics of O2 evolution.

In 2018 Zhang and Sun proposed a radical alternative: during the S3→S4 transition a charge rearrangement converts the dangling manganese (Mn4, the manganese atom outside the Mn3CaO4 cubane portion of the cluster) into a closed-shell, permanganate-like Mn(VII) bearing two cis oxo ligands (a Mn(VII)-dioxo site, oxidation patterns Mn(III,III,IV,VII) or Mn(III,IV,III,VII)), and the O–O bond forms between those two oxo groups.

Li, Li and Siegbahn (2020) tested that proposal with their calibrated cluster-DFT protocol based on the B3LYP hybrid density functional (a widely used exchange–correlation functional mixing 15% exact exchange, the fraction that reproduces the experimental redox energetics of the OEC), with Poisson–Boltzmann solvation (a continuum-electrostatics treatment of the surroundings) and protein backbone constraints — the same methodology that had previously predicted the observable S-state structures. Because the Mn(VII) cycle shifts one proton, they compared two ligand models for the cluster — W1(H2O)+ (W1 is a terminal water ligand bound to the dangling manganese) for the Mn(IV) mechanism and W1(OH), with one proton fewer, for the Mn(VII) mechanism. They found that reaching the Mn(VII) S4 state costs +33.8 kcal/mol from the S3 reference (versus +5.7 kcal/mol for the Mn(IV) S4) and that the overall O–O-formation barriers are 78–81 kcal/mol (local coupling barriers 36–44 kcal/mol) versus overall barriers of 11.3–13.4 kcal/mol for the oxyl–oxo route — far too high for the observed kinetics. They nevertheless flagged three limitations. First, the Mn(VII) transition states were approximated by reaction-coordinate scans, not full saddle-point optimizations. Second, for transitions that release only an electron (such as S1→S2), the two models differ in total charge, so their energies cannot be directly compared; a fully quantitative S-state energy diagram for the Mn(VII) proposal is therefore not available. Third, any reshuffling of protons and oxidation states before O–O bond formation would simply convert the Mn(VII) transition state into that of the Mn(IV) mechanism, so Mn(VII) could only be competitive if such rearrangements are avoided. The paper's own conclusion is that Mn(VII) oxidation states are 'very unlikely' to be significant, but the field has never mounted the higher-rigor computational or experimental effort that a definitive exclusion would require.

Problem Statement

Determine definitively whether the Mn(VII) dioxo mechanism of Zhang and Sun (2018) is a viable O–O bond-forming pathway in the natural oxygen-evolving complex of photosystem II, or can be rigorously excluded. All requested results below serve this single objective; a complete answer establishes a quantified verdict by one or both of the following routes, and either route alone suffices if its evidentiary standard is met.

(1) Computational route: a reaction-energy and barrier analysis of the Mn(VII) pathway across the catalytic cycle (S0 through S4 and O–O bond formation) in which (a) the O–O coupling transition states are true saddle points, verified by harmonic frequency analysis showing exactly one imaginary mode, rather than reaction-coordinate scans; (b) the energetics of transitions that release only an electron (such as S1→S2) are placed on a consistent reference across models of different total charge — for example by explicit thermodynamic cycles or matching electrostatic embedding — so that a full, quantitative S-state energy diagram for the Mn(VII) proposal is obtained and compared with the Mn(IV) mechanism; and (c) the sensitivity of the verdict to the electronic-structure treatment is assessed, including at minimum the dependence on the fraction of exact exchange and at least one check beyond the original B3LYP cluster protocol (a larger structural model, explicit treatment of the protein electrostatic environment, or a correlated wavefunction benchmark). A verdict of 'excluded' requires showing that the Mn(VII) route's overall barrier and S4-formation energy remain incompatible with the observed kinetics of O2 evolution by margins exceeding the demonstrated method uncertainty; a verdict of 'viable' requires showing a competitive pathway, which would additionally demand reconciliation with existing spectroscopic constraints on the manganese oxidation states.

(2) Experimental route: structural and/or spectroscopic characterization of the S4 state (or the S3→S4 transient) — for example by XFEL serial femtosecond crystallography or time-resolved manganese K-edge X-ray absorption or Kβ emission spectroscopy — that directly constrains the manganese oxidation states (all-Mn(IV) with an oxyl radical versus Mn(III,III,IV,VII) or Mn(III,IV,III,VII)) and the presence or absence of a permanganate-like, closed-shell Mn(VII) dioxo site on the dangling manganese, with controls for radiation damage and S-state purity.

References

  1. Li, Xi-Chen; Li, Jing; Siegbahn, Per E. M. (2020). A Theoretical Study of the Recently Suggested Mn(VII) Mechanism for O–O Bond Formation in Photosystem II. The Journal of Physical Chemistry A 124 (39), 8011–8018. DOI: 10.1021/acs.jpca.0c05135. https://doi.org/10.1021/acs.jpca.0c05135
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