EPR properties of a g=2 broad signal trapped in an S1 state in Ca2+-depleted Photosystem II

We investigated a new EPR signal that gives a broad line shape around g=2 in Ca2+-depleted Photosystem (PS) II. The signal was trapped by illumination at 243 K in parallel with the formation of YZ. The ratio of the intensities between the g=2 broad signal and the YZ signal was 1:3, assuming a Gaussian line shape for the former. The g=2 broad signal and the YZ signal decayed together in parallel with the appearance of the S2 state multiline at 243 K. The g=2 broad signal was assigned to be an intermediate S1X state in the transition from the S1 to the S2 state, where X represents an amino acid radical nearby manganese cluster, such as D1-His337. The signal is in thermal equilibrium with YZ. Possible reactions in the S state transitions in Ca2+-depleted PS II were discussed.