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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchA 2015 study argued that the manganese cluster in photosystem II follows a high-valent oxidation-state scheme as it advances through photosynthesis’s water-splitting cycle. Its calculations fit the full sequence of catalytic states more consistently under that scheme than under the competing low-valent model. That was the authors’ conclusion—not proof that the debate ended: scientists quoted at the time disagreed, and the available sources do not establish today’s consensus.
What question did the study address?
Photosystem II uses a four-manganese catalyst, the oxygen-evolving complex, to oxidize water. During this process the catalyst moves through five states, S0 to S4. The question is how to assign oxidation states to the manganese ions in each state—and whether one coherent structural and chemical model can account for the cycle.
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The 2015 paper by Vera Krewald and colleagues compared two rival descriptions. In the high-valent scheme, the manganese ions reach higher oxidation states during the cycle. In the low-valent scheme, some remain at lower states. These are proposed assignments for the catalyst, not two different forms of photosynthesis.
How did the S2 assignments differ?
The S2 state provides a clear example of the disagreement. The two schemes assign different oxidation states to three of the four manganese ions:
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| Scheme | S2 manganese oxidation-state assignment |
|---|---|
| High-valent | Mn(III, IV, IV, IV) |
| Low-valent | Mn(III, III, III, IV) |
These formulas describe the oxidation states assigned across the four manganese ions; they do not, by themselves, settle the cluster’s structure or explain how it changes during catalysis.
Why did the authors favor the high-valent model?
Krewald and colleagues tested models within a shared structural and theoretical framework rather than judging the S2 assignment alone. They compared how well each scheme could describe the complete S-state cycle, including its structures and protonation pattern, and assessed the models against experimental observations from EXAFS, XFEL-XRD, EPR, ENDOR and manganese K pre-edge XANES. The study also included new low-temperature 55Mn ENDOR data for S2.
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In their model, the high-valent assignments form a consistent progression from S0, Mn(III, III, III, IV), to S3, Mn(IV, IV, IV, IV). The authors reported that the observations could be accommodated by this scheme, whereas their low-valent models did not produce a consistent description of the full cycle. The paper’s conclusion is therefore about which model best fit the combined constraints they tested, not a claim that every mechanistic detail of water oxidation was settled.
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The paper, “Metal oxidation states in biological water splitting,” was first published on 9 January 2015 in Chemical Science. Read the open-access paper at the Royal Society of Chemistry.
Why was the interpretation contested?
Chemistry World’s report on 4 February 2015 presented the result alongside objections. Rob Stranger of the Australian National University argued that “the argument for the low-valent model is by no means dead”. He pointed to alternative interpretations of spectroscopy, disagreements over crystal structures and substrate-water exchange kinetics as reasons the low-valent view remained viable.
Marcel Swart of the University of Girona was more persuaded by the paper, praising its systematic combination of theory and experiment as a convincing case for high-valent assignments along the catalytic cycle. He also noted that the oxygen-forming step and the return to S0 still needed further work. Dimitrios A. Pantazis, the corresponding author, called the study’s conclusion a “definitive answer”; that phrase was his characterization of the team’s result, not evidence that other researchers accepted it as final.
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These are positions reported at the time of publication. The paper and 2015 news report establish what the study argued and why it drew disagreement; they do not establish whether later work has resolved the issue.
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1Fix the driver behind crashes, sound loss and screen glitches2Repair Windows errors before they cause bigger problems3Scan for outdated or missing drivers - takes under a minuteWhat the 2015 result does—and does not—show
- It supports: within the models and evidence considered by the authors, the high-valent scheme offered a consistent account of the full S-state sequence.
- It does not establish: that the low-valent interpretation was disproved across every possible model or that the field has reached a lasting consensus.
- It leaves open: mechanistic questions including how oxygen forms and how the catalyst returns to S0.
For the original report and its attributed reactions, see Chemistry World’s “Photosynthesis takes the high road”.
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