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Open Access Publications from the University of California

Calcium EXAFS establishes the Mn-Ca cluster in the oxygen-evolving complex of Photosystem II

  • Author(s): Cinco, Roehl M.;
  • Holman, Karen L. McFarlane;
  • Robblee, John H.;
  • Yano, Junko;
  • Pizarro, Shelly A.;
  • Bellacchio, Emanuele;
  • Sauer, Kenneth;
  • Yachandra, Vittal K.
  • et al.

The proximity of Ca to the Mn cluster of the photosynthetic water-oxidation complex is demonstrated by X-ray absorption spectroscopy. We have collected EXAFS data at the Ca K-edge using active PS II membrane samples that contain approximately 2 Ca per 4 Mn. These samples are much less perturbed than previously investigated Sr-substituted samples, which were prepared subsequent to Ca depletion. The new Ca EXAFS clearly shows backscattering from Mn at 3.4 angstroms, a distance that agrees with that surmised from previously recorded Mn EXAFS. This result is also consistent with earlier related experiments at the Sr K-edge, using samples that contained functional Sr, that show Mn is ~ 3.5 angstroms distant from Sr. The totality of the evidence clearly advances the notion that the catalytic center of oxygen evolution is a Mn-Ca heteronuclear cluster.

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