The origin of split EPR signals in the Ca2+-depleted Photosystem II

A light-driven reaction model for the Ca2+-depleted Photosystem (PS) II is proposed to explain the split signal observed in electron paramagnetic resonance (EPR) spectra based on a comparison of EPR assignments with recent x-ray structural data. The split signal has a splitting linewidth of 160 G at around g = 2 and is seen upon illumination of the Ca2+-disappearance of the S-2 state multiline signal. Another g=2 broad ESR signal with a 110 G linewidth was produced by 245 K illumination for a short period in the Ca2+-depleted PS II in S-1 state. At the same time a normal Y-Z(center dot) radical signal was also efficiently trapped. The g = 2 broad signal is attributed to an intermediate S1Xcenter dot state in equilibrium with the trapped Y-Z(center dot) radical. Comparison with x-ray structural data suggests that one of the split signals (doublet signal) is attributable to interaction between His 190 and the Y-Z(center dot) radical, and other signals is attributable to interaction between His 337 and the manganese cluster, providing further clues as to the mechanism of water oxidation in photosynthetic oxygen evolution.