Abstract
In Photosystem II, light-induced water splitting occurs via the S state cycle of the CaMn(4)O(5)-cluster. To understand the role of various possible conformations of the CaMn(4)O(5)-cluster in this process, the temperature dependence of the S(1) → S(2) and S(2) → S(3) state transitions, induced by saturating laser flashes, was studied in spinach photosystem II membrane preparations under different conditions. The S(1) → S(2) 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 S(2) → S(3) 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 S(2) state advance with similar efficiency to the S(3) state, both showing a transition half-inhibition temperature of 240 K. This was further confirmed by following the appearance of the Split S(3) EPR signal. The results are discussed in relevance to the functional and structural heterogeneity of the water oxidizing complex intermediates in photosystem II.