The EPR signals from the S-0 and S-2 states of the Mn cluster in photosystem II relax differently

The oxygen evolving complex (OEC) of photosystem II (PSII) gives rise to ma nganese-derived electron paramagnetic resonance (EPR) signals in the S-0 an d S-2 oxidation states. These signals exhibit different microwave power sat uration behavior between 4 and 10 K. Below 8 K, the S-0 state EPR signal is a faster relaxer than the S-2 multiline signal, but above 8 K, the So sign al is the slower relaxer of the two. The different temperature dependencies of the relaxation of the S-0 and S-2 ground-state Mn signals are due to di fferences in the spin-lattice relaxation process, The dominating spin-latti ce relaxation mechanism is concluded to be a Raman mechanism in the So slat e, with a T-4.1 temperature dependence of the relaxation rate. It is propos ed that the relaxation of the St State arises from a Raman mechanism as wel l, 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 mec hanism, the different exponents are proposed to reflect structural differen ces in the proteins surrounding the Mn cluster between the S-0 and S-2 stat es. The saturation of SIIslow from the Y-D(OX) radical on the D2 protein wa s also studied, and found to vary between the S-0 and the S-2 states of the enzyme in a manner similar to the EPR signals from the OEC, Furthermore, w e found that the S-2 multiline signal in the second turnover of the enzyme is significantly more difficult to saturate than in the first turnover. Thi s suggests differences in the OEC between the first and second cycles of th e 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.