States during flash illumination

To account for the periodicity of four in the yield of O2 in a series of flashes (12-13), Kok and coworkers (14) proposed that photosystem II cycles through five states during flash illumination. These intermediate oxidation states are referred to as states (i = 0-4) with the subscript denoting the number of oxidizing equivalents accumulated. The sequential advancement of the S states occurs via the light-induced charge separation in photosystem II. 

Figure 7 Several manganese C IV-EPR signals from the OEC in PS II, labelled by S-state. The S0 multiline EPR signal was generated by three-flash illumination of spinach thylakoid membranes. The S, multiline EPR signal was observed by parallel mode EPR detection of dark-adapted Synechocystis PS II particles. The S2 multiline and g = 4.1 EPR signals were generated by 195 K illumination of spinach thylakoid membranes. The S2-Yz EPR signal was generated in acetate inhibited spinach thylakoid membranes frozen during illumination. For further details and references see reference 449. Reproduced with permission from reference 449.

A theoretical analysis of charge distribution within supercomplexes (or clusters in which the movement of diffusible carriers is restricted) has been developed by Lavergne et al [4]. This theory predicts the evolution of the redox state of the carriers under continuous illumination or flash excitation for any cluster stoichiometry. The predictive power of this treatment is illustrated by the analysis of the light-induced oxidation of primary and secondary donors in isolated centers of Rhodopseudomonas viridis (Fig. 3). In this case, it is definitely established that the secondary donors (cytochromes) are irreversibly bound to the reaction center. In the absence of mediators, no electron exchange is expected to occur between photocenters. In the presence of 200yM ascorbate, only two of the four cytochromes (cyt 556 and cyt 559) are in their reduced state prior to the illumination. As expected, the apparent equilibrium constant between P and the cytochromes measured during the course of illumination is much lower than that computed from the value of the redox potentials (K = 50 for cyt 559 and K 1500 for cyt 556). The fit between the experimental data and the theoretical simulation (dashed lines) is excellent and clearly demonstrates that the measurement of electron transfer reactions under weak illumination is a powerful tool to characterize the degree of structuration of a photosynthetic electron transfer chain. 

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