Electron transfer pathways from the S2-states to the S3-states either after a Ca2+/Sr2+ or a Cl-/I- exchange in Photosystem II from

The site for water oxidation in Photosystem II (PSII) goes through five sequential oxidation states (S-0 to S-4) before O-2 is evolved. It consists of a Mn4CaO5-cluster close to a redox-active tyrosine residue (Y-z). Cl is also required for enzyme activity. By using EPR spectroscopy it has been shown that both Ca2+/Sr2+ exchange and Cl-/I- exchange perturb the proportions of centers showing high (S = 5/2) and low spin (S = 1/2) forms of the Systate. The S-3-state was also found to be heterogeneous with: i) a S = 3 form that is detectable by EPR and not sensitive to near-infrared light; and ii) a form that is not EPR visible but in which Mn photochemistry occurs resulting in the formation of a (S2Yz)' split EPR signal upon near-infrared illumination. In Sr/Cl-PSII, the high spin (S = 5/2) form of S-2 shows a marked heterogeneity with a g = 4.3 form generated at low temperature that converts to a relaxed form at g = 4.9 at higher temperatures. The high spin g = 4.9 form can then progress to the EPR detectable form of S-3 at temperatures as low as 180 K whereas the low spin (S = 1/2) S-2-state can only advance to the S-3 state at temperatures >= 235 K. Both of the two S-2 configurations and the two S-3 configurations are each shown to be in equilibrium at >= 235K but not at 198 K. Since both S-2 configurations are formed at 198 K, they likely arise from two specific populations of Si. The existence of heterogeneous populations in Si, Sy and S-3 states may be related to the structural flexibility associated with the positioning of the oxygen O-5 within the cluster highlighted in computational approaches and which has been linked to substrate exchange. These data are discussed in the context of recent in silico studies of the electron transfer pathways between the S-2-state(s) and the S-3-state(s).