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

Origin of the g=2 broad EPR signal in the Ca²⁺-depleted PS II in S₁ state. Hiroyuki Mino^*,1, Shigeru Itoh¹, ¹ Grad. School of Sci, Nagoya University, Nagoya, JAPAN

ABSTRACT- Upon illuminating the Ca²⁺-depleted PS II in the S₂ state, an EPR signal with a 130-160 G splitting linewidth appears around g = 2. The signal has been called the split signal, and investigated as the clue to elucidate the mechanism of water-oxidation. The similar signals have been observed in Cl^--depleted, NH₄Cl-treated and acetate-treated PS II. Recently, Nugent et al. have reported the similar EPR signal upon illumination of the untreated PS II in S₁ state at 4 K. We have reported that the split signal involves two signals that overlap at the g=2 region. Short illumination of the Ca²⁺-depleted PSII at 273 K in the S₂ state results in the formation of a doublet signal with splitting of about 150 G at g=2. Further illumination at 273 K led to the formation of a singlet-like signal. Pulsed ENDOR-induced EPR indicates that Y_Z^* is associated with the doublet, but not with the singlet-like signal. We will report a new g=2 broad ESR signal with a 130 G linewidth produced by 245 K illumination for a short period in the Ca²⁺-depleted PS II in S₁ state. A normal Y_Z^* radical signal was efficiently trapped at the same time. Both the g=2 broad signal and trapped normal Y_Z^* radical disappeared and S₂ multiline signal appeared in the same time course by 245 K annealing. Therefore, the g=2 broad signal state is suggested to be an intermediate state of S₁-S₂ transition that is in equilibrium with trapped Y_Z^* radical. Assuming that the g=2 broad signal arise from the magnetic dipole-dipole interaction of S₁ state and X^* (S₁X^*), the distance between spins on them was estimated as about 6 A. We will discuss the relevance of these split signals in the Ca²⁺-depleted PS II.

KEY WORDS: split signal, ESR, EPR, YZ