Evidence for Intermediate S-States as Initial Phase in the Process of Oxygen-Evolving Complex Oxidation

We have analyzed flash-induced period-four damped oscillation of oxygen evolution and chlorophyll fluorescence with the aid of a kinetic model of photosystem II. We have shown that, for simulation of the period-four oscillatory behavior of oxygen evolution, it is essential to consider the so-called intermediate S-state as an initial phase of each of the Sn-Sn+1, (n = 0, 1, 2, 3) transitions. The intermediate S-states are defined as [SnYZox]-states (n = 0, 1, 2, 3) and are formed with rate constant kiSn ∼1.5 × 106 s−1, which was determined from comparison of theoretical predictions with experimental data. The assumed intermediate S-states shift the equilibrium in reaction P680+YZ↔P680YZox more to the right and we suggest that kinetics of the intermediate S-states reflects a relaxation process associated with changes of the redox equilibrium in the above reaction. The oxygen oscillation is simulated without the miss and double-hit parameters, if the intermediate S-states, which are not the source of the misses or the double-hits, are included in the simulation. Furthermore, we have shown that the intermediate S-states, together with S2QA− charge recombination, are prerequisites for the simulation of the period-four oscillatory behavior of the chlorophyll fluorescence.