Photosynthesis Research

, Volume 84, Issue 1–3, pp 325–331 | Cite as

Considerations on the mechanism of photosynthetic water oxidation – dual role of oxo-bridges between Mn ions in (i) redox-potential maintenance and (ii) proton abstraction from substrate water

  • Holger Dau
  • Michael Haumann


Two mechanistic problems of photosynthetic water oxidation at the Mn complex of Photosystem II (PS II) are considered. (I) In the four Mn-oxidizing transitions, any pure Mn oxidation is predicted to cause an increase in redox potential that renders subsequent oxidation steps impossible (redox-potential problem). Formation of unprotonated oxo-bridges may counteract the potential increase. (II) The O–O formation step without any high-pK bases acting as proton acceptors is energetically unfavorable (acceptor-base problem). The pK of oxides in a bridging position between Mn ions may increase drastically upon reduction of Mn in the water-oxidation step (>10 units), thus rendering them favorable proton acceptors. It is proposed that in PS II, in the course of the four oxidizing transitions at least two unprotonated oxo-bridges are formed. Thereby (i) a redox potential increase is prevented and (ii) proton acceptors are prepared for the O–O formation step. Water oxidation in the O–O bond formation step is facilitated by simultaneous Mn reduction and proton transfer to bridging oxides amounting to hydrogen atom or hydride transfer from substrate water to the Mn-oxo core of the Mn complex of PS II.


manganese complex oxygen evolution Photosystem II X-ray absorption spectroscopy 



electron paramagnetic resonance


extended X-ray absorption fine-structure




Tyr 161 of the D1 protein of PS II


X-ray absorption near-edge structure


X-ray absorption spectroscopy


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Copyright information

© Springer 2005

Authors and Affiliations

  1. 1.Freie Universität BerlinBerlinGermany

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