PARENT SESSION
Posters P5A Type II reaction centres : Structure. Abstracts (289-312)

New X-ray spectroscopy techniques to study the Mn complex in Photosystem II. JUNKO YANO^*,1, Johannes Messinger², Pieter Glatzel³, Kenneth Sauer^{1, 4}, Stephen Cramer^{1, 3}, Uwe Bergmann⁵, Vittal Yachandra¹, ¹ Lawrence Berkeley National Laboratory, Berkeley, CA, USA² Max-Planck Institut fur Bioanorganische Chemie, Mulheim, Germany³ Department of Applied Science, University of California, Davis, CA, USA⁴ Department of Chemistry, University of California, Berkeley, CA, USA⁵ Stanford Synchrotron Radiation Laboratory, Stanford University, Menlo Park, CA, USA

ABSTRACT- Mn K-edge (1s to 4p) absorption spectroscopy, and more recently, Mn K (3p to 1s) emission spectroscopy have been used to determine the oxidation states of the Mn complex in the various S-states. We are now using the new technique of resonant Raman (1s to 3d, 2p to 1s) or resonant inelastic X-ray scattering spectroscopy (RIXS). We have collected data from PS II samples in the various S-states and compared the spectra to those of oxides, and molecular coordination complexes. Data reduction yields two-dimensional plots that can be interpreted along the incident (absorption) energy or the energy transfer axis. The RIXS final state electron configuration along the energy transfer is identical to conventional L-edge absorption spectroscopy (2p to 3d) and RIXS spectra are therefore sensitive to the Mn spin state. Ligand field multiplet theory is used to qualitatively describe the spectra. We propose to explain the spectral changes upon Mn oxidation in terms of the effective number of 3d electrons. The spectra show strong covalency for the electron configuration in the OEC, and the spectral changes between the PS II S₁ and S₂ states are different compared to those of model compounds. The results indicate that the electron in the S₁-S₂ step is transferred from a strongly delocalized orbital. By using a high resolution crystal analyzer that has a resolution of 1 eV, tuned to the K (2p to 1s) emission, we can collect EXAFS spectra well beyond the Fe K-edge, that has until now been a serious limitation. Using this new technique, we have improved the resolution from the current 0.14 to about 0.10 . This becomes especially important for the S₃ and S₀ states that both exhibit distance heterogeneity that is at the limit of the present resolution. The higher resolution EXAFS data show that there is heterogeniety in the Mn-Mn distances even in S₁ and S₂ states. The orientation of the Mn-Mn vectors is being studied using layered membrane samples. We have made progress with these two new methods and 1) the relevance of RIXS data to the oxidation states and the electronic structure of the Mn complex in the S-states and 2) the high resolution EXAFS data of the S-states will be presented.

KEY WORDS: X-ray Spectroscopy, Photosystem II, EXAFS, Oxygen evolution