ELECTRON-PARAMAGNETIC-RESONANCE KINETIC-STUDIES OF THE S-STATES IN SPINACH PSII MEMBRANES

The decay kinetics of Y-Z(.) have been detected using time-resolved EP R to obtain the half-times of the S-state transitions in the oxygen-ev olving complex of spinach PSII membranes. The half-times were determin ed by deconvoluting the signal IIvf kinetics for the individual S-stat e contributions observed in a series of single turnover flashes. The K ok parameters required for deconvolution were estimated from the oxyge n flash pattern measured in situ using nitroxide-based EPR oximetry. T he half-times of the individual S-state transitions in PSII membranes were found to be: 70 mu s for S-0 --> S-1, 110 mu s for S-1-->S-2, 180 mu s for S-2 --> S-3, and 1400 mu s for S-3 --> S-0. Comparison of th e S-state transition kinetics in PSII membranes with those of thylakoi ds shows that the S-state transitions are slowed in PSII, membranes wi th the largest effect being observed for the S-3 --> S-0 transition. S uitable conditions for applying EPR oximetry to PSII membranes are des cribed. (C) 1997 Elsevier Science B.V.