Electron Paramagnetic Resonance Kinetic Studies of the S States in Spinach Thylakoids

The Tyrz+ decay kinetics have been analyzed by using time-resolved EPR to determine the half-time of each Si-->S(i + 1) transition in the O2-evolving complex of spinach thylakoids under physiological conditions. Using dark-adapted thylakoids and appropriate single-turnover flash sequences, we were able to detect the signal IIvf kinetics of the Tyrz+ S0-->Tyrz S1, Tyrz+ S1-->Tyrz S2, Tyrz+ S2-->Tyrz S3, and Tyrz+ S3-->(S4)-->Tyrz S0 transitions. To correct for damping of the S state synchronization during the flash sequence, the Kok parameters were estimated by measuring the oxygen flash pattern in situ using nitroxide-based EPR oximetry. Following deconvolution of the individual S state contributions, the signal IIvf decay kinetics yield the following half-times for the S state transitions: S0-->S1 in 40-60 microseconds, S1-->S2 in 85 microseconds, S2-->S3 in 140 microseconds, and S3-->(S4)-->S0 in 750 microseconds. Preliminary results with detergent-solubilized PSII membranes suggest that the S3-->S0 transition at least is slowed by a factor of approximately 2 in this system. Ramifications of these half-times in terms of electron transfer events on the donor site of PSII are discussed.