Electron paramagnetic resonance kinetic studies of the S states in spinach PSII membranes

Abstract

The decay kinetics of Y ⋅ Z have been detected using time-resolved EPR to obtain the half-times of the S-state transitions in the oxygen-evolving complex of spinach PSII membranes. The half-times were determined by deconvoluting the signal II vf kinetics for the individual S-state contributions observed in a series of single turnover flashes. The Kok parameters required for deconvolution were estimated from the oxygen flash pattern measured in situ using nitroxide-based EPR oximetry. The half-times of the individual S-state transitions in PSII membranes were found to be: 70 μs for S 0→S 1, 110 μs for S l→S 2, 180 μs for S 2→S 3, and 1400 μs for S 3→S 0. Comparison of the S-state transition kinetics in PSII membranes with those of thylakoids shows that the S-state transitions are slowed in PSII membranes with the largest effect being observed for the S 3→S 0 transition. Suitable conditions for applying EPR oximetry to PSII membranes are described.