PARENT SESSION
Posters P6A Type II reaction centres: Excited state dynamics and donor side. Abstracts (313-346)

The manganese complex of oxygenic photosynthesis studied by time-resolved X-ray absorption spectroscopy at room temperature. Michael Haumann^*,1, Claudia Mueller¹, Peter Liebisch¹, Thomas Neisius², Holger Dau¹, ¹ Freie Universitaet Berlin, Physik, Arnimallee 14, 14195 Berlin, Germany² ESRF, BP. 220, 38043 Grenoble, Cedex, France

ABSTRACT- Elucidation of the mechanism of water oxidation at the 4Mn-Ca complex of photosystem II (PSII) requires precise knowledge of the structure and oxidation state in all intermediates (S-states; S1, S2, S3, S4) of the photocycle under conditions where catalysis is active (at room temperature, RT). So far, the Mn complex has mainly been studied by X-ray absorption spectroscopy (XAS) and protein crystallography at cryogenic temperatures (below 100 K); effects of radiation damage and Mn photoreduction due to X-ray irradiation may have sometimes been important. Novel approaches for time-resolved XAS at room temperature on biological metal centers to follow state transitions were developed [1,2,3]: (1) Experimental protocols were established where photoreduction in all S-states becomes negligible in XAS measurements so that the unperturbed structure of the Mn complex is accessible. (2) Monitoring of the oxidation/reduction reactions in micro- to milliseconds in XAS experiments at RT revealed that the Mn complex is kinetically fully competent in PSII samples used for XAS [4]. (3) Structural and oxidation state changes upon the S-transitions were monitored at RT within milliseconds after the initiating Laser flash [4,5]. (4) Time-resolved XAS experiments with microsecond-resolution will possibly allow for tracking intermediates formed during the oxygen-evolving transition and for characterization of the crucial S4-state [4,5]. These experiments open the road to investigate structural and oxidation state changes at biological metal centers in real time during the catalytic cycle. (Financial support from the DFG (projects C6 and C8) and from the BMBF (grant 05KS1KEA-6) is gratefully acknowledged.) References: 1. M. Haumann, P. Pospisil, M. Grabolle, C. Mueller, P. Liebisch, A. Sole, T. Neisius, J. Dittmer, L. Iuzzolino, H. Dau (2002) J. Synchrotron Rad. 9, 304-308. 2. M. Haumann, M. Grabolle, T. Neisius, H. Dau (2002) FEBS lett. 512, 116-120. 3. H. Dau, M. Haumann (2003) J. Synchrotron. Rad. 10, 76-85. 4. M. Haumann, C. Mueller, P. Liebisch, T. Neisius, H. Dau (2004) J. Synchrotron Rad., submitted for publication. 5. M. Haumann, C. Mueller, P. Liebisch, L. Iuzzolino, J. Dittmer, M. Grabolle, T. Neisius, W. Meyer-Klaucke, H. Dau (2004) J. Am. Chem. Soc., submitted for publication.

KEY WORDS: photosystem II, time resolution, BioXAS, water oxidation