EPR kinetic studies of the S_-1 state in spinach thylakoids

The Y_Z^• decay kinetics in a formal S _-1 state, regarded as a reduced state of the oxygen evolving complex, was determined using time-resolved EPR spectroscopy. This S_-1 state was generated by biochemical treatment of thylakoid membranes with hydrazine. The steady-state oxygen evolution of the sample was used to optimize the biochemical procedure for performing EPR experiments. A high yield of the S _-1 state was generated as judged by the two-flash delay in the first maximum of oxygen evolution in Joliot flash-type experiments. We have shown that the Y_Z^• re-reduction rate by the S_-1 state is much slower than that of any other S-state transition in hydrazine-treated samples. This slow reduction rate in the S_-1 to S₀ transition, which is in the order of the S₃ to S ₀ transition rate, suggests that this transition is accompanied by some structural rearrangements. Possible explanations of this unique, slow reduction rate in the S_-1 to S₀ transition are considered, in light of earlier observations by others on hydrazine/hydroxylamine reduced PS II samples.