Evidence for intermediate S-states as initial phase in the process of oxygen-evolving complex oxidation

Area of Science:

• Photosynthesis Research
• Plant Physiology
• Biophysical Chemistry

Background:

• Photosystem II (PSII) governs oxygen evolution and chlorophyll fluorescence.
• Oscillatory behavior in oxygen evolution has been observed but not fully explained.
• Previous models often relied on 'miss' and 'double-hit' parameters.

Purpose of the Study:

• To analyze flash-induced oscillations in oxygen evolution and chlorophyll fluorescence.
• To develop a kinetic model of PSII that accurately simulates observed oscillatory patterns.
• To elucidate the role of intermediate states in PSII reactions.

Main Methods:

• Utilized a kinetic model of Photosystem II.
• Analyzed flash-induced period-four damped oscillations.
• Compared theoretical predictions with experimental data to determine rate constants.
• Incorporated intermediate S-states into the kinetic model.

Main Results:

• Identified essential intermediate S-states ([S(n)Y(Z)(ox)]) for simulating oxygen evolution oscillations.
• Determined the formation rate constant for these intermediate states (k(iSn) ≈ 1.5 x 10^6 s⁻¹).
• Demonstrated that intermediate S-states, not miss/double-hit parameters, enable accurate simulation of oxygen oscillations.
• Showed intermediate S-states and S(2)Q(A)(-) recombination are necessary for simulating chlorophyll fluorescence oscillations.

Conclusions:

• Intermediate S-states are critical for understanding the period-four oscillations in oxygen evolution.
• The kinetics of intermediate S-states likely reflect redox equilibrium relaxation in PSII.
• Accurate modeling of PSII function requires the inclusion of these intermediate states and specific recombination events.