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

Flash-number dependent oscillation of the light-induced (at 5 K) split EPR signals from OEC. Jihu Su^*,1, Kajsa Sigfridsson^*,1, Yashar Feyziyev ^*,1,2, Stenbjörn Styring^*,1, ¹ Molecular Biomimetics, Uppsala University, Villavägen 6, 752 36 Uppsala, Sweden² Molecular Biomimetics, Uppsala University, Villavägen 6, 752 36 Uppsala, Sweden

ABSTRACT- PSII membrane particles were used to investigate the split radical EPR signals induced by illumination at 5 K. Tyrosine D was reduced by sodium ascorbate and DAD, and then the EPR samples were given 0-9 flashes at 0 degree and then rapidly frozen. Two split signals around g=2.0, originating from the S0 and S1 states (Nugent et al., 2002 Biochemistry 41: 4117-26; Zhang & Styring, Biochemistry 42: 8066-76), were followed over 9 flashes and thereby demonstrated to oscillate through two entire S-cycles. We could also distinguish what seems to be a new signal from the S3 state with a different high-field peak. The split signal from the S0 state was studied with respect to microwave power and light intensity saturation in the sample given 3 flashes. The intensity of the new signal oscillates with flash number (period of 4) and has a clear maximum intensity in the samples trapped in the S3-state. The decay kinetics of the split signals from the different S-states were scrutinized. During 40 min dark-adaptation, the low-field side of the S0- and S1-split signals decayed with half-times of 3-4 min, while the middle part and the high-field side of the signals only decayed about 50% with this half time. The new split signal is also very stable and oscillates with the S3-state in the samples. Based on these decay kinetic studies, we differentiated the light-induced signals into two different components. One component decayed completely within 40 min dark adaptation. This represents the "split" signal and originates from the interactions between YZ and the S0 and S1 states. Presumably the same holds for the signal in the S3-state. The non-decaying component contained a radical signal and its induction is not S-state dependent. Most probably this signal involves the oxidation of beta-carotene.

KEY WORDS: split radical EPR signal, PSII, S-states