The S₁ to S₂ and S₂ to S₃ state transitions in plant photosystem II: relevance to the functional and structural heterogeneity of the water oxidizing complex

In Photosystem II, light-induced water splitting occurs via the S state cycle of the CaMn₄O₅-cluster. To understand the role of various possible conformations of the CaMn₄O₅-cluster in this process, the temperature dependence of the S₁ → S₂ and S₂ → S₃ state transitions, induced by saturating laser flashes, was studied in spinach photosystem II membrane preparations under different conditions. The S₁ → S₂ transition temperature dependence was shown to be much dependent on the type of the cryoprotectant and presence of 3.5% methanol, resulting in the variation of transition half-inhibition temperature by 50 K. No similar effect was observed for the S₂ → S₃ state transition, for which we also show that both the low spin g = 2.0 multiline and high spin g = 4.1 EPR configurations of the S₂ state advance with similar efficiency to the S₃ state, both showing a transition half-inhibition temperature of 240 K. This was further confirmed by following the appearance of the Split S₃ EPR signal. The results are discussed in relevance to the functional and structural heterogeneity of the water oxidizing complex intermediates in photosystem II.