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

Characterization of the "Split" EPR-signal from Tyr-Z-S1 in PSII with Tyr-D reduced. Kajsa Sigfridsson^*,1, Yashar Feyziyev^*,2, Stenbjörn Styring¹, ¹ Molecular Biomimetics, Uppsala, Sweden² Laboratory for Photobiology, Photodynamic Research Center, Sendai, Japan

ABSTRACT- Illumination of photosystem II at 5 K generates "split" EPR-signals from centers in the S₁ and the S₀ states of the Ca/Mn-cluster. The signals, preliminary assigned to a magnetic interaction between Tyr-Z^⋅ and the respective S-state, are observed in the g2 region, and are 80 and 160 Gauss wide in the S₁ and S₀ state, respectively. Here the signal originating from PS II in the S₁ state is characterised in BBY-preparations with Tyr-D reduced by treatment with Ascorbate/DAD. The reduction protocol eliminated the contribution from the big radical from Tyr-D in the g2 region (1-4% remaining). This allowed us to obtain the detailed spectral shape of the "Split S₁" signal from decay associated kinetic spectra. The "clean" signal had a peak at g=2.04 (the low field side), a "radical contribution" at g=2.01 and a "deep and wide shoulder" on the high field side of the radical. This signal decayed with the halftime 3-5 min at 5 K. Investigation of the microwave power dependence of the "Split S₁" EPR-signal revealed that it is quiet fast relaxing, with P of 1mW at 5 K. The size of the signal also depended on the intensity of the inducing light. After illumination with 300 W/m² the maximum signal size was reached. The dependence on temperature for the induction and measurement of the "Split S₁" EPR-signal was studied. The signal intensity peaked at 8 K (nonsaturating conditions). The pH dependence of the signal is also presented.

KEY WORDS: S1-state, reduced Tyr-D, Tyr-Z, Split EPR-signal