The EPR signals from the S0 and S2 states of the Mn cluster in photosystem II relax differently

The oxygen evolving complex (OEC) of photosystem II (PSII) gives rise to manganese-derived electron paramagnetic resonance (EPR) signals in the S0 and S2 oxidation states. These signals exhibit different microwave power saturation behavior between 4 and 10 K. Below 8 K, the S0 state EPR signal is a faster relaxer than the S2 multiline signal, but above 8 K, the S0 signal is the slower relaxer of the two. The different temperature dependencies of the relaxation of the S0 and S2 ground-state Mn signals are due to differences in the spin-lattice relaxation process. The dominating spin-lattice relaxation mechanism is concluded to be a Raman mechanism in the S0 state, with a T(4.1) temperature dependence of the relaxation rate. It is proposed that the relaxation of the S2 state arises from a Raman mechanism as well, with a T(6.8) temperature dependence of the relaxation rate, although the data also fit an Orbach process. If both signals relax through a Raman mechanism, the different exponents are proposed to reflect structural differences in the proteins surrounding the Mn cluster between the S0 and S2 states. The saturation of SII(slow) from the Y(D)(ox) radical on the D2 protein was also studied, and found to vary between the S0 and the S2 states of the enzyme in a manner similar to the EPR signals from the OEC. Furthermore, we found that the S2 multiline signal in the second turnover of the enzyme is significantly more difficult to saturate than in the first turnover. This suggests differences in the OEC between the first and second cycles of the enzyme. The increased relaxation rate may be caused by the appearance of a relaxation enhancer, or it may be due to subtle structural changes as the OEC is brought into an active state.