Evidence for intermediate S-states as initial phase in the process of Oxygen Evolving Complex oxidation

¹ Palacky University, Faculty of Science, Laboratory of Biophysics

^* To whom correspondence should be addressed. E-mail: lazard{at}seznam.cz.

Submitted on September 27, 2007
Revised on October 26, 2007
Accepted on 27 November 2007

	Abstract

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 S_n-S_n+1, (n = 0, 1, 2, 3) transitions. The intermediate S-states are defined as [S_nY_Z^ox]-states (n = 0, 1, 2, 3) and are formed with rate constant k_iSn ~ 1.5 x 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⁺Y_Z P680Y_Z^ox 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, as defined by Bessel Kok, if the intermediate S-states, which however 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 S₂Q_A charge recombination are prerequisites for the simulation of the period-four oscillatory behavior of the chlorophyll fluorescence.

Key Words: Kok model, charge recombination, photosystem II, tyrosine, water splitting

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