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Keywords:

  • density functional calculations;
  • EPR spectroscopy;
  • oxygen evolving complex;
  • photosystem II;
  • water splitting
Thumbnail image of graphical abstract

Using models derived from the X-ray structure of photosystem II, it is shown that the oxygen evolving complex in the S2 state exists in two energetically similar and interconvertible forms. A longstanding question regarding the spectroscopy of the catalyst is thus answered: one form corresponds to the multiline g=2.0 EPR signal (see picture, right; O red, Mn purple, Ca yellow), and the other to the g≥4.1 signals (left).