PARENT SESSION

Symposium S7A Mechanisms of water oxidation
Thursday September 2nd, 2004 2:40 PM-4:40 PM Room 210A
Chair: Stenbjörn Styring
Co-Chair: Ron Pace

Structural and oxidation state changes of the PSII manganese complex in its S-state cycle identified by XAFS at 20 K and at room temperature. Holger Dau^*,1, Michael Haumann¹, Claudia Mueller¹, Peter Liebisch¹, Markus Grabolle¹, Marcos Barra¹, Thomas Neisius⁷, Wolfram Meyer-Klaucke⁸, ¹ Free Univ. Berlin, D-14185 Berlin, Germany⁷ ESRF, BP. 220, 38043 Grenoble cedex, France⁸ EMBL Outstation Hamburg, Notkestr. 85, D-22603 Hamburg, Germany

ABSTRACT- The structural and electronic changes (oxidation states) of the Mn4Ca-complex of photosystem II (PSII) in the water-oxidation cycle are of prime interest. For the first time, for all four transitions between semistable S-states (S0=>S1, S1=>S2, S2=>S3, S3=>S4=>S0), oxidation state and structural changes of the Mn complex are investigated by X-ray absorption spectroscopy (XAS) not only at 20 K, but also at room temperature (RT). Three distinct experimental approaches were used: (1) illumination-freeze approach (XAS at 20 K); (2) flash-and-rapid-scan approach (RT); (3) the novel timescan/sampling-XAS method (RT). The rate of X-ray photoreduction was quantitatively assessed, and it was thus verified that the Mn ions remained in their initial oxidation state throughout the data collection period (>90%, at 20 K and at RT, for all S-states). Analysis of the complete XANES and EXAFS data sets (20 K and RT data, S0 to S3, XANES and EXAFS) obtained by the three approaches leads to the following conclusions: (i) In all S-states the gross structural and electronic features of the Mn complex are similar at 20 K and at room temperature. There are no indications for significant temperature-dependent variations in structure, protonation state, or charge localization. (ii) Mn-centered oxidation likely occurs on each of the three S-transitions leading to the S3-state as indicated by XANES simulations. (iii) Significant structural changes are coupled to the S0=>S1 and the S2=>S3 transition which are identified as changes in the Mn-Mn bridging mode. We propose that in the S2=>S3 transition a third di-mue-oxo bridge is formed, whereas the S0=>S1 transition involves deprotonation of a mue-hydroxo bridge. In the light of these results, the mechanism of accumulation of four oxidation equivalents by the Mn complex and its relation/coupling to proton movements is considered and compared to insights obtained by model chemistry. Furthermore, mechanistic hypotheses on the O-O bond formation step are (re)evaluated.

KEY WORDS: oxygen evolution, Photosystem II, water oxidation, S-state cycle