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 S₁" and "split S₃" signals induced in the S₁ 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 S₀" signal induced in the So 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 S₁ and split So EPR signals are caused by an increase in the extent of magnetic coupling within the cluster.