PARENT SESSION
Posters P7A Mechanisms of water oxidation. Abstracts (347-381)

Modelling the Mn₄Ca cluster of the water-oxidizing complex of Photosystem II. Kenneth Sauer^{*,1, 2}, Junko Yano^{1, 2}, Vittal Yachandra¹, ¹ Lawrence Berkeley National Laboratory, Berkeley, CA, USA² Department of Chemistry, University of California, Berkeley, CA, USA

ABSTRACT- The water-oxidation center is a metallo-enzyme complex that is closely associated with the PS II reaction center. The metal cluster contains 4 Mn and 1 Ca atom (replaceable by Sr). In the absence of a high-resolution structure of PS II, information about the cluster arrangement has come largely from EPR and from Extended X-ray Absorption Fine Structure (EXAFS) measurements. Analysis of these results provides evidence for the presence of Mn-O(N) bonds (vectors) of 1.8-2.0 length, two or three Mn-Mn vectors in the range 2.7-2.8 , one or two Mn-Mn vectors at 3.3 and Mn-Ca(Sr) at 3.4(3.5) . The 2.7-2.8 Mn-Mn vectors are in the range seen for di--oxo bridged pairs of Mn and the 3.3 Mn-Mn vectors are in the range seen in mono--oxo bridged pairs of Mn in model compounds. Information relevant to vector orientations within PS II is available from polarized X-ray measurements (dichroism). Replacement of the cofactor Ca by Sr in PS II preparations from spinach, produced membranes that could be layered so as to bring the membranes nearly parallel. Samples prepared in this fashion exhibit pronounced Sr (K-edge) EXAFS dichroism at 3.5 associated with one or two Sr-Mn vectors in the cluster. Assuming that Sr replaces physiological Ca, the displacement of the Ca-Mn vectors from the thylakoid membrane normal is at 0° or 23°. Because significant information about Mn-Mn and Mn-Ca vector angles is now available, topological models previously discussed can be refined to include the presence of Ca and account for the dichroism data. A survey of MnO minerals has provided not only a possible clue as to the evolutionary origin of the Mn₄CaO cluster, but also a rich assortment of possible cluster arrangements. Several likely candidates have emerged, which will be presented along with criteria for deciding among them.

KEY WORDS: Mn Complex, Oxygen evolution, X-ray Spectroscopy, EXAFS