Formation of Split Electron Paramagnetic Resonance Signals in Photosystem II Suggests That TyrosineZ Can Be Photooxidized at 5 K in the S0 and S1 States of the Oxygen-Evolving Complex

The effect of illumination at 5 K of photosystem II in different S-states was investigated with EPR spectroscopy. Two split radical EPR signals around g 2.0 were observed from samples given 0 and 3 flashes, respectively. The signal from the 0-flash sample was narrow, with a width of ~80 G, in which the low-field peak can be distinguished. This signal oscillated with the S1 state in the sample. The signal from the 3-flash sample was broad, with a symmetric shape of ~160 G width from peak to trough. This signal varied with the concentration of the S0 state in the sample. Both signals are assigned to arise from the donor side of PSII. Both signals relaxed fast, were formed within 10 ms after a flash, and decayed with half-times at 5 K of 3-4 min. The signal in the S0 state closely resembles split radical signals, originating from magnetic interaction between YZ and the S2 state, that were first observed in Ca2+-depleted photosystem II samples. Therefore, we assign this signal to YZ in magnetic interaction with the S0 state, YZS0. The other signal is assigned to the magnetic interaction between YZ and the S1 state, YZS1. An important implication is that YZ can be oxidized at 5 K in the S0 and S1 states. Oxidation of YZ involves deprotonation of the tyrosine. This is restricted at 5 K, and we therefore suggest that the phenolic proton of YZ is involved in a low-barrier hydrogen bond. This is an unusually short hydrogen bond in which proton movement at very low temperatures can occur.