{"id":"3bc2300e-4437-4b4c-836c-ffa50b0f6786","arxiv_id":"1908.05056","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":5.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":5,"one_line_summary":"A computational study proposes that Photosystem II's S2-to-S3 transition proceeds via the g=4.1 isomer, with W3 hydroxide insertion at Mn4 and a second hydroxide, O7, displacing D1-E189 at Mn1.","lead":"This paper models the step in photosynthesis where the oxygen-making machine grabs a new water molecule and prepares to split it. It proposes that the process starts from a specific intermediate state and suggests how two conflicting X-ray structures can both be right.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The g=2 vs g=4.1 water-binding conclusion depends on an unspecified bulk water/hydroxide chemical potential, so the central claim is not well-posed.","rationale":"The reader identified the classical electrostatics model, fixed protein conformation, and limited rotamer sampling as the weakest assumption. My concern is more specific: even within the stated model, the absolute water/hydroxide binding free energies are not well-defined because the bulk reservoir chemical potential is not specified. This directly undermines the central claim that addition is favorable in S2 g=4.1 but not S2 g=2. The 25% occupancy in S2 g=4.1 is particularly fragile, since it corresponds to a small positive ΔG for O6 relative to W3. This is an addressable issue, not a fatal flaw: a sensitivity analysis over the missing reference, or an explicit-solvent free-energy calculation, could settle it. The paper also honestly reports that its S3 model fits EXAFS less well than the prior open form, which supports treating the structure as an intermediate but does not by itself invalidate the initiation mechanism. I retain the reader's CONDITIONAL verdict rather than moving to ACCEPT or REJECT, because the missing reference is a well-posed but fixable gap.","tokens_in":23093,"tokens_out":8310,"duration_ms":85587,"concrete_test":"Recompute the MCCE/GCMC occupancies of O6/W3 (and O7) in S2 g=2, S2 g=4.1, and S2 g=4.1/YZ•H190+ while varying the bulk water/hydroxide chemical potential over a physically reasonable ±2 kcal/mol range and the protein dielectric over 2–8. If S2 g=2 remains unoccupied and S2 g=4.1 O6 occupancy remains ≥50% across the whole range, the central claim survives; if the occupancy ordering inverts or S2 g=2 becomes populated, the paper's conclusion fails. A complementary check is explicit-solvent free-energy perturbation of one water from bulk to the O6 site in both isomers.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The load-bearing result is a free-energy comparison: water/hydroxide addition is said to be unfavorable in S2 g=2 but favorable in S2 g=4.1, initiating the S3 transition. However, the MC/MCCE protocol as described never defines the chemical potential (or standard-state concentration) of the bulk water/hydroxide reservoir. The Methods state only that each of the 451 grid oxygens can be water, hydroxide, or \"moved out of the protein into solvent\"; the energy of that solvent conformer and the pH-7 reference are not specified, and no calibration or sensitivity analysis is reported. The occupancy numbers are therefore not anchored: in S2 g=4.1, O6 is occupied in only 25% of microstates and W3 in 75%, which corresponds to ΔG ≈ +0.65 kcal/mol for O6 relative to W3, i.e. O6 binding alone is not thermodynamically favorable on an absolute scale. A shift of ~1 kcal/mol in the bulk reference could turn the 25% occupancy into a dominant state or eliminate it, and could also populate sites in S2 g=2. Since the abstract's mechanistic conclusion is built on this comparison, the missing reference makes the load-bearing result underdetermined. The paper's own EXAFS comparison, which shows the resulting S3 structure fits worse than the earlier open form, further weakens the endpoint, though the initiation claim is the primary issue here.","agreement_with_reader":"partial"},"referee_report":{"model":"deepseek-v4-flash","summary":"This paper presents a computational model of the S2-to-S3 transition in the oxygen-evolving complex of photosystem II. The authors use QM/MM-optimized structures of the S2 g=2 and g=4.1 redox isomers, embed them in a Monte Carlo / continuum-electrostatics (MCCE/APBS) framework, and sample water/hydroxide positions on a 1 Å grid together with side-chain rotamers, protonation states, and Mn oxidation states. They report that water/hydroxide binding is thermodynamically unfavorable in the S2 g=2 state but favorable in the S2 g=4.1 state, leading to the proposal that the S2-to-S3 transition begins with a g=2-to-g=4.1 isomerization, followed by deprotonation of W3 and transfer of the resulting hydroxide to O6 at Mn4, and eventually binding of a second hydroxide (O7) at Mn1 with a conformational change of D1-E189. The resulting S3 structures are compared with experimental EXAFS and with XFEL structures from Suga et al. and Kern et al., and the paper argues that the computed S3 structure is an intermediate preceding the open S3 form.","tokens_in":23401,"tokens_out":4808,"duration_ms":49935,"significance":"The paper addresses a central open question in photosynthesis research: the molecular mechanism of the S2-to-S3 transition, and specifically the role of the redox isomer equilibrium and water/hydroxide insertion. The methodological approach, combining MCCE sampling of many oxygen positions, rotamers, and protonation states, is more comprehensive than standard single-structure QM/MM studies and provides a concrete, falsifiable proposal about the order of events (isomerization, deprotonation, insertion, E189 conformational change). The authors are transparent in providing optimized coordinates and spin densities in the Supporting Information, and they honestly report that their final S3 model fits EXAFS less well than an earlier open-form model. However, the central thermodynamic comparison lacks a well-defined bulk water/hydroxide reference state, the key free-energy differences are small (sub-kcal/mol to a few kcal/mol), and no sensitivity or error analysis is provided.","major_comments":[{"comment":"The central conclusion that water/hydroxide addition is unfavorable in S2 g=2 but favorable in S2 g=4.1 depends on the free energy of the \"moved out of the protein into solvent\" conformer that serves as the reference for binding. This free energy, and the standard-state concentration (or chemical potential) of the bulk water/hydroxide reservoir, is never defined in the manuscript. The occupancy numbers in the S2 g=4.1 state (O6 at 25% vs W3 at 75%) imply a free-energy difference of only about 0.65 kcal/mol, so a shift of roughly 1 kcal/mol in the bulk reference would reverse the predicted ordering. Please specify the exact energy expression for the solvent conformer, the standard-state convention, and whether the same reference is used for all four states; in addition, report a sensitivity analysis (e.g., varying the bulk reference energy, the protein dielectric constant, and the grid spacing) to demonstrate that the \"unfavorable in g=2, favorable in g=4.1\" result is not an artifact of the unspecified reference.","section":"Computational Methods, paragraph on Monte Carlo sampling"},{"comment":"The paper acknowledges that the EXAFS spectrum of the proposed S3 model matches the experimental data worse than the previously published open-form S3 model, and interprets this as evidence that the new model is an intermediate on the S2-to-S3 pathway. However, no free-energy or kinetic calculation is provided to show that this structure lies on the reaction path; a worse fit is equally consistent with the model being incorrect. The claim that this is \"an intermediate structure during the S2-to-S3 state transition\" is therefore a post hoc interpretation. Please state explicitly that this conclusion is not supported by the EXAFS comparison alone, and either add a genuine pathway calculation (e.g., minimum-energy path between S2 g=4.1 and the open S3 form) or soften the claim to a speculation.","section":"Figure 2 and the paragraph beginning \"Figure 2 compares...\""},{"comment":"The DFT (B97D) energy difference between the Mn1-OH7 and Mn1-E189 forms of the S3 state is reported as 2.4 kcal/mol, and the two states are called \"isoenergetic.\" Given that B97D with a modest basis set (LanL2DZ for metals, 6-31G** for light atoms) is generally accurate to only a few kcal/mol for transition-metal redox and isomerization energetics, this 2.4 kcal/mol difference cannot be used to support a robust preference for O7 binding. The claim that O7 replaces E189 as a ligand of Mn1 should be supported by tests with other functionals, larger basis sets, or a clear statement that the two structures are degenerate within the accuracy of the method and that the proposal rests on the broader electrostatic sampling rather than this single DFT energy difference.","section":"Supporting Information, Table S1"}],"minor_comments":[{"comment":"The last sentence contains a grammatical error: \"In the S3 state in the population of protonated D1-E189 increases\" should be \"In the S3 state, the population of protonated D1-E189 increases.\"","section":"Abstract"},{"comment":"The manuscript states that \"the parameters for the OEC and ligands used here as reported previously in Amin et al.\" but does not specify which parameters are transferred and whether any were re-optimized or re-fitted for the S2/S3 states. A short table or list of transferred parameters and their references would improve reproducibility.","section":"Computational Methods"},{"comment":"The manuscript does not state the number of Monte Carlo steps, convergence criteria, or whether the reported occupancies are averages over multiple independent runs. Reporting these details would allow readers to assess the statistical uncertainty in the key 25%/75% occupancies and the 0% occupancy in S2 g=2.","section":"Computational Methods, Monte Carlo sampling"},{"comment":"The sentence \"All the Mn(IV) ions were defined in the high spin state\" should be justified, because different spin states of the Mn ions can alter the electrostatics and ligand-binding energetics. Please provide a reference or a test for the spin-state assumption.","section":"Computational Methods, DFT settings"},{"comment":"The phrase \"reduced distance space\" in the caption is unusual; the text refers to the \"Fourier Transformed spectrum.\" Please use consistent terminology and define χ(R).","section":"Figure 2 caption and text"}],"recommendation":"major_revision","confidential_remarks":"The paper is within scope and addresses an important question, but the central claim hinges on a free-energy comparison whose reference state is not specified. This is fixable in revision, so I recommend major rather than minor. The authors should also be encouraged to be more cautious in interpreting the worse EXAFS fit as evidence of an intermediate, since the current wording overstates what the data show. The manuscript shares methodology and parameters with several earlier papers from the same group; while this is not a problem per se, the novelty claim should clearly state what is new beyond those prior studies."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Short version: this is a serious computational study of the S2-to-S3 transition, and the main mechanistic claim (g=4.1 first, then W3 deprotonation and translation to Mn4, plus a second hydroxide at Mn1) is plausible and consistent with earlier work. The genuinely new pieces are the large Monte Carlo sampling of 451 oxygen positions with explicit proton conformers, and the identification of O7 as a second binding site that offers a natural way to reconcile the Kern and Suga XFEL structures. The paper is also honest: it credits Ugur, Narzi, Bovi and others for the g=4.1 idea, and it explicitly says its own S3 model fits EXAFS worse than the earlier open form, interpreting itself as an intermediate. That transparency is worth encouraging.\n\nThe soft spots are real but addressable. The biggest is the missing bulk water/hydroxide chemical potential. The GCMC protocol has a \"moved to solvent\" conformer for each oxygen, but the paper never specifies the standard-state reference or how that energy is calibrated. So the absolute statement \"binding is unfavorable in g=2, favorable in g=4.1\" is underdetermined. The 25% O6 occupancy in g=4.1 is, on its own, about +0.65 kcal/mol relative to W3, so \"favorable\" is doing a lot of work. A shift of about 1 kcal/mol in the bulk reference could change the conclusion. This is fixable with calibration or a sensitivity analysis, but without it the central comparison is not anchored.\n\nOther concerns are minor-to-moderate: no error bars on the MC occupancies, no code or input files, and the DFT energy difference for the E189 states is 2.4 kcal/mol, within typical DFT error. The model assumes high-spin Mn centers and one functional (B97D) without testing those choices. The self-citation pattern is less concerning than it may look; the parameters come from earlier work by the same group, and the new binding and protonation results are not fitted to the target XFEL structures.\n\nWho is this for? Anyone working on PSII water oxidation or on interpreting XFEL S-state structures. It deserves a serious referee, not a desk reject. A referee should push for the chemical-potential calibration and for sensitivity tests on dielectric constant and grid spacing, but the core hypothesis is worth engaging with.","headline":"A plausible and honestly presented computational model of the S2-to-S3 transition, but the central g=2 vs g=4.1 water-binding claim lacks an anchored bulk reference and needs sensitivity analysis before it can be taken as quantitative.","tokens_in":23972,"tokens_out":4962,"would_cite":true,"duration_ms":49197,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"This paper claims that the S2-to-S3 transition of Photosystem II's oxygen-evolving complex is gated by a switch from the S2 g=2 to the S2 g=4.1 redox isomer, after which a hydroxide from W3 inserts at Mn4 and a second hydroxide can…","keywords":["photosystem II","oxygen-evolving complex","S2-to-S3 transition","S2 redox isomers","water oxidation","Monte Carlo sampling","continuum electrostatics","D1-E189"],"falsifier":"A concrete test is to drive the S2-to-S3 transition under conditions where the g=2 isomer is the only populated state, for example at low temperature or with modified calcium, and measure with time-resolved Mn EXAFS whether any water insertion or S3 signature appears; if S3 forms without any detectable g=4.1 population, the gating claim fails. A second test is to trap the oxidized-intermediate state and look for the W3 proton: if W3 is still bound as water and no proton has been released to the lumen before manganese oxidation, the proposed deprotonation-before-oxidation sequence is wrong.","tokens_in":22899,"feed_emoji":"💧","tokens_out":9415,"duration_ms":85167,"temperature":0.7,"pith_summary":"This paper tries to establish a thermodynamic ordering for the S2-to-S3 step of the water-oxidation clock in Photosystem II, the step in which an extra oxygen is added to the Mn4O5Ca cluster. Using Monte Carlo sampling of hundreds of possible water and hydroxide positions against a continuum-electrostatics energy function, the authors find that water/hydroxide addition is not favorable in the common S2 g=2 state but is favorable in the S2 g=4.1 redox isomer. The transition therefore begins with an isomer interconversion, then deprotonation of the calcium-ligated water W3, then insertion of that hydroxide at Mn4 to complete Mn4's coordination shell and enable its oxidation. The model also has a second hydroxide replace the side chain D1-E189 as a ligand of Mn1, which explains the multiple E189 conformations seen in S3 X-ray free-electron laser structures. If correct, it resolves the disagreement among S3 structures by assigning the competing oxygen positions to two distinct sites, O6 and O7.","feed_headline":"Water joins the oxygen-evolving complex only after an S2 isomer flip","feed_subtitle":"A thermodynamics model explains why the extra oxygen appears only after the g=4.1 redox-isomer switch.","key_machinery":"The machinery is a Boltzmann/Monte Carlo sampling of the oxygen cavity: 451 oxygen atoms placed on a 1 Å grid inside the QM/MM-optimized protein model around the Mn4O5Ca cluster, each oxygen allowed to be water, hydroxide, or exit to solvent, together with proton conformers and sidechain rotamers of D1-E189. Energies come from solving the Poisson-Boltzmann equation for each conformer, and Metropolis Monte Carlo generates the grand-canonical Boltzmann distribution at pH 7. The discriminating variable is the S2 redox isomer: in the g=2 isomer Mn1 is oxidized and the cavity is closed to new ligands, while in the g=4.1 isomer Mn4 is oxidized, the Mn4-Ca2+ distance opens by 0.27 Å, and the O6 site between Mn4 and Ca2+ becomes occupiable. Density-functional optimization of the sampled structures then refines O6 into a 1.91 Å Mn4 ligand with a hydrogen bond to the mu-oxo O5, and the EXAFS comparison against experimental data places the computed structure as a pre-open intermediate.","core_discovery":"The central claim is that S3-state formation cannot start from the S2 g=2 state: in that state the extra water/hydroxide is thermodynamically rejected, and the computed oxidation potential of Mn1 (1.4 V) is above the potential available from the photooxidized chlorophyll P680. Instead, the S2 g=2 state first converts to the S2 g=4.1 redox isomer, where the Mn4-Ca2+ distance is lengthened by 0.27 Å and a hydroxide (O6) bridging Mn4 and Ca2+ is occupied with about 25% probability, at the expense of the W3 water ligand of Ca2+. When the electron acceptor YZ is oxidized and D1-H190 becomes protonated, O6 is always present and W3 is never found, implying W3 deprotonates and releases a proton to the lumen before manganese oxidation. The resulting hydroxide completes the octahedral coordination of Mn4, which is then oxidized to Mn(IV) to form S3. Independently, sampling of more than 50 conformers of D1-E189 shows E189 moving away from Mn1 when a second hydroxide (O7) binds Mn1, an isoenergetic substitution in the S2 g=4.1 and S3 states; the computed EXAFS of the resulting S3 model is an intermediate between the closed and open S3 forms, with the final metastable S3 better described by the earlier open form.","pith_inferences":["If the g=2-to-g=4.1 interconversion is the gate, the rate of S3 formation should track the equilibrium population of the g=4.1 isomer; a quantitative EPR-visible correlation between isomer population and S3 yield would be a direct test the paper does not perform.","The model's O6 and O7 sites are independent, so a transient S3 intermediate could carry two added hydroxides; a two-oxygen intermediate would appear as an extra Mn-Mn or Mn-Ca scattering feature in time-resolved EXAFS delayed relative to the first insertion.","Because the computed structure sits between the closed and open S3 forms, the relaxation from intermediate to open form is a distinct physical step that could show up as a slow phase in time-resolved X-ray or EXAFS measurements after the fast electron-transfer phase.","A testable extension would be to run the same Boltzmann sampling on a g=4.1 structure with Mn1 reduced rather than oxidized, predicting whether O7 binding is driven purely by electrostatics or requires the oxidized state."],"forward_implications":["The S2 g=4.1 isomer is not a side population but a required gateway: conditions that destabilize it should block S3 formation, while conditions that populate it should accelerate the transition.","Proton release precedes manganese oxidation in this step, with W3 as the deprotonated group; the proton goes to the lumen upon formation of the YZ-centered oxidized intermediate with protonated D1-H190.","The two conflicting XFEL oxygen positions are compatible: one site (O6) completes Mn4 and the other (O7) replaces E189 at Mn1, so different crystals can resolve different parts of the same insertion process.","The final S3 state is the open form; the structure produced by the S2-to-S3 insertion is an intermediate that relaxes afterward, which is why EXAFS of the computed model does not fully match the experimental S3 spectrum.","E189's apparent indifference to mutation is explained by an isoenergetic alternative structure in which a hydroxide replaces it as the Mn1 ligand."],"supporting_citations":[{"why":"The QM/MM-optimized structures of the S2 isomers that are the starting point for all Monte Carlo sampling.","marker":"[7]"},{"why":"Showed the S2 state has two accessible spin/metastable forms and proposed a path toward S3 that this mechanism elaborates.","marker":"[8]"},{"why":"Established the two interconvertible S2 structures behind the g=2 and g=4.1 EPR signals, defining the redox-isomer pair.","marker":"[9]"},{"why":"Calculated the YZ-oxidation pathway that stabilizes the open-to-closed S2 isomer conversion prior to S3 formation.","marker":"[19]"},{"why":"The 2.07 Å S3 XFEL structure with the extra oxygen 2.08 Å from D1-E189, the main experimental observation the model must account for.","marker":"[28]"},{"why":"The earlier QM/MM open-form S3 model and its EXAFS spectrum, used as the reference for judging the new structure.","marker":"[10]"},{"why":"Proposed W3 deprotonation and translation to complete the Mn coordination shell, the insertion mechanism adopted and refined here.","marker":"[52]"},{"why":"Earlier simulation of water delivery during the S2-to-S3 transition supporting insertion from the closed g=4.1 form.","marker":"[53]"},{"why":"Experimental Mn EXAFS data against which the new S3 model and the open-form model are compared.","marker":"[65]"},{"why":"The electrostatic model of proton-coupled electron transfer in the S-state cycle that the present calculations extend.","marker":"[35]"}],"fun_headline_variants":["S3 state requires S2 g=4.1 isomer before water joins","Water joins OEC only after S2 g=4.1 redox switch","Photosystem II: S2 g=2 rejects water, g=4.1 accepts it","OEC's S2-to-S3 transition demands a prior isomer flip","S2 g=4.1 isomer is gatekeeper for water insertion"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The load-bearing premise is that a classical continuum-electrostatics model with a fixed protein backbone, discrete sidechain rotamers, and high-spin manganese centers reliably predicts which water and hydroxide positions are thermodynamically favored; if protein dynamics open alternative channels or move residues outside the sampled conformations, the preference for O6/O7 and the order of proton release could change.","fun_headline_variants_meta":{"raw":{"variants":["S3 state requires S2 g=4.1 isomer before water joins","Water joins OEC only after S2 g=4.1 redox switch","Photosystem II: S2 g=2 rejects water, g=4.1 accepts it","OEC's S2-to-S3 transition demands a prior isomer flip","S2 g=4.1 isomer is gatekeeper for water insertion"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000764,"raw_usage":{"total_tokens":3525,"prompt_tokens":1218,"completion_tokens":2307,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":834,"completion_tokens_details":{"reasoning_tokens":2204}},"tokens_in":834,"tokens_out":2307,"duration_ms":13553,"temperature":1.0,"reasoning_tokens":2204,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-14T13:24:38.821599+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"A concrete test is to drive the S2-to-S3 transition under conditions where the g=2 isomer is the only populated state, for example at low temperature or with modified calcium, and measure with time-resolved Mn EXAFS whether any water insertion or S3 signature appears; if S3 forms without any detectable g=4.1 population, the gating claim fails. A second test is to trap the oxidized-intermediate state and look for the W3 proton: if W3 is still bound as water and no proton has been released to the lumen before manganese oxidation, the proposed deprotonation-before-oxidation sequence is wrong.","supporting_citations":[],"review_version":1}