Density-functional simulations with Mn spin flips used as markers under the photoelectric polarization field (Huo, Jiang, Yang and Zhang, Phys. Rev. Applied 21, 024024 (2024)) predicted two classes of S2→S3_2\to S_3 evolution pathways at the Mn4_4CaO5_5 oxygen-evolving complex (OEC) of photosystem II — one via closed-cubane S2_2 isomerization with hydroxyl insertion between Mn4_4 and O5_5, one via direct W3 water insertion between Mn1_1 and O5_5 with a spontaneous-dissociation branch yielding two geometrically distinct S3_3 isomers sharing the same spin configuration — while noting that the model excludes Tyr161 from the quantum region and that dioxygen generation at the S4_4 state remains unexplained. All three components remain open: the predicted branching scenarios, transient intermediates, and degenerate S3_3 isomers have not been experimentally discriminated (room-temperature XFEL studies constrain only the main stages, and even the insertion of the O6 water is still debated); no published work has extended the spin-marker polarization-field methodology to a Tyr161-explicit quantum region; and the mechanism of O–O bond formation and O2_2 release at S4_4, though strongly constrained by the 2023 capture of a peroxide-like intermediate, remains unresolved and actively contested through 2025–2026.

Background

Photosystem II (PSII) is the membrane-protein machine of oxygenic photosynthesis that oxidizes water to molecular oxygen. The catalyst is the oxygen-evolving complex (OEC), a Mn4_4CaO5_5 cluster in which four manganese ions and one calcium ion are bridged by five oxygen atoms. Water oxidation proceeds through the Kok cycle, a sequence of five oxidation states S0S_0–S4S_4 defined in 1970 by flash experiments: each of four absorbed photons creates at the reaction-center chlorophyll P680 a strong oxidant (a photoelectric 'hole') that is passed, via the redox-active tyrosine TyrZ (Tyr161 of the D1 polypeptide), to the Mn4_4CaO5_5 cluster; four successive oxidations of the cluster, coupled to proton release, accumulate enough oxidizing power to form O2_2 at the transient S4S_4 state and return the cluster to S0S_0.

Each S-state advance couples the removal of one electron from the cluster with the removal of a proton and, at some steps, the insertion of a substrate water — a proton-coupled electron transfer (PCET) process whose event ordering is hard to observe because the intermediates live on microsecond-to-millisecond timescales. The S2S_2 state is special: it exists as two interconvertible geometric isomers, the open-cubane form (seen in electron paramagnetic resonance, EPR, a magnetic-resonance technique sensitive to unpaired electron spins, as the low-spin signal at g = 2) and the closed-cubane form (high-spin signal at g = 4.1), in which the bridging oxygen O5_5 occupies different positions relative to Mn1_1 and Mn4_4. Three water molecules bound at the cluster — W1 and W2 on Mn4_4, W3 on Ca/Mn — are candidate substrates. The S2→S3S_2\to S_3 transition is the most contested step of the cycle: it requires one deprotonation, one electron removal, and the insertion of a new substrate water, and different orderings of these events, different substrate waters, and different S2S_2 isomer entry points have been proposed.

Huo, Jiang, Yang and Zhang (Phys. Rev. Applied 21, 024024 (2024)) approached this problem with spin-polarized all-electron density-functional theory (DFT, a first-principles electronic-structure method; here the DMol3 code with the PBE exchange-correlation functional and a DNP basis set) on a 112-atom model of the OEC. They represented the photoelectric hole at Tyr161 by effective point charges (Qh≈0.1eQ_h\approx 0.1e screened, or 1.0e1.0e for oxidized Tyr161) — the electric field of this hole is the photoelectric polarization field of the title — and tracked how the localized spins of the four Mn ions — each Mn carries an unpaired-electron spin drawn as ↑\uparrow or ↓\downarrow, and each S state has a characteristic pattern known from EPR spectroscopy — flip as electrons, protons, and waters move. Using these spin flips as markers, they reconstructed two viable S2→S3S_2\to S_3 pathway classes. Path-1: the open-cubane S2S_2 first isomerizes to the closed-cubane form; proton removal from W1 opens a coordination site; a hydroxyl derived from W2 inserts between Mn4_4 and O5_5; final electron transfer yields the high-spin S3S_3 pattern ↑↑↑↓\uparrow\uparrow\uparrow\downarrow. Path-2: the open-cubane S2S_2 evolves directly; the substrate water W3 inserts between Mn1_1 and O5_5, either (branch I) spontaneously dissociating into OH−^- and H+^+ — after which the two alternative proton routes (removal of Ha versus Hb) lead to two geometrically distinct S3S_3 isomers that nevertheless share the same spin configuration ↑↑↓↑\uparrow\uparrow\downarrow\uparrow, a spin-degeneracy prediction — or (branch II) inserting intact with proton transfer to W1 before the final electron transfer.

The authors themselves state the limits of this picture: the predictions about intermediate states 'remain to be verified by experiments in the future'; the medium-size model treats Tyr161 only as a point charge, omitting the hydrogen-bond switch that accompanies Tyr161 oxidation (its deprotonation toward His190, the neighboring D1-polypeptide histidine that accepts the proton), so the full PCET dynamics are not captured; and the mechanism of dioxygen generation at the elusive S4S_4 state — the step that closes the Kok cycle — remains unexplained. Room-temperature serial femtosecond crystallography at X-ray free-electron lasers (XFEL-SFX, which collects diffraction from thousands of microcrystals with femtosecond pulses before radiation damage develops) has meanwhile constrained the main stages: a new bridging oxygen (O6) appears between Mn1_1 and Ca during S2→S3S_2\to S_3, proton removal precedes electron transfer, and in 2023 a peroxide-like transient was caught between S3S_3 and S0S_0. However, the fine structure of the S2→S3S_2\to S_3 pathway — which water inserts, whether branching and spin-degenerate S3S_3 isomers occur — and the atomistic mechanism of O–O bond formation at S4S_4 are not settled.

Problem Statement

The problem, as posed by the outlook of Huo et al. (Phys. Rev. Applied 21, 024024 (2024)), is to complete the dynamic mechanism of the OEC Kok cycle from the S2→S3S_2\to S_3 transition through dioxygen release by adjudicating the predictions of that model against the experimental constraints already reported in the literature and described in the Background. The three components jointly serve this single objective — the S2→S3S_2\to S_3 sequence fixes the substrate-water inventory and spin configuration that the S4S_4 step consumes — and each is an independently refutable sub-claim; the objective is resolved only when all three are settled:

(1) Verify or refute the predicted S2→S3S_2\to S_3 branching scenarios against the model and the reported constraints: determine which sequence of proton removal, water insertion or dissociation, and electron transfer is compatible with the available structural, spectroscopic and computational evidence, and in particular whether (a) the transition proceeds via isomerization of the open-cubane S2S_2 to the closed-cubane form followed by hydroxyl insertion between Mn4_4 and O5_5 (path-1), or directly from the open-cubane form with W3 insertion between Mn1_1 and O5_5 (path-2); (b) the inserted water spontaneously dissociates into OH−^- and H+^+; and (c) the S3S_3 ensemble contains the predicted two geometrically distinct isomers that share the same Mn spin configuration ↑↑↓↑\uparrow\uparrow\downarrow\uparrow.

(2) Realize, within the same spin-marker polarization-field framework, the more complete model the source calls for: a quantum-mechanical treatment in which Tyr161 (with its hydrogen-bond switch and the coupled His190 deprotonation) is included explicitly rather than as a point charge, so that the proton-coupled electron transfer dynamics during the S0S_0–S3S_3 evolution are captured under the photoelectric polarization field and can be compared with the S0S_0–S3S_3 signatures of the native system.

(3) Explain the mechanism of dioxygen generation at the S4S_4 state: how the O–O bond is formed (which oxygen pair couples, in which electronic/spin configuration of the cluster) and how O2_2 is released, closing the Kok cycle — for instance, though not necessarily, by extending the spin-flip-marker polarization-field approach to a full model that includes S4S_4 — with the resulting mechanism required to be compatible with the intermediates characterized between S3S_3 and S0S_0.

References

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