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

Roehl M Cinco¹, Karen L McFarlane Holman, John H Robblee

¹Physical Biosciences Division, Melvin Calvin Laboratory, Lawrence Berkeley National Laboratory, Berkeley, California 94720-5230, USA.

|October 23, 2002

Abstract:

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 after Ca depletion. The new Ca EXAFS clearly shows backscattering from Mn at 3.4 A, 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 approximately 3.5 A 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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Abstract:

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