Split EPR signals from photosystem II are modified by methanol, reflecting S state-dependent binding and alterations in the magnetic coupling in the CaMn4 cluster

Methanol binds to the CaMn4 cluster in photosystem II ( PSII). Here we report the methanol dependence of the split EPR signals originating from the magnetic interaction between the CaMn4 cluster and the Y(Z)(.)radical in PSII which are induced by illumination at 5 K. We found that the magnitudes of the split S-1 and split S-3 signals induced in the S-1 and S-3 states of PSII centers, respectively, are diminished with an increase in the methanol concentration. The methanol concentrations at which half of the respective spectral changes had occurred ([MeOH](1/2)) were 0.12 and 0.57%, respectively. By contrast, the split S-0 signal induced in the S-0 state is broadened, and its amplitude is enhanced. [MeOH](1/2) for this change was found to be 0.54%. We discuss these observations with respect to the location and nature of the methanol binding site. Furthermore, by comparing this behavior with methanol effects reported for other EPR signals in the different S states, we propose that the observed methanol-dependent changes in the split S-1 and split S-0 EPR signals are caused by an increase in the extent of magnetic coupling within the cluster.