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+-depleted PS II in the S2 state associated with complete or partial disappearance of the S2 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 S1 state. At the same time a normal YZ . radical signal was also efficiently trapped. The g=2 broad signal is attributed to an intermediate S1X. state in equilibrium with the trapped YZ . 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 YZ . 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.