EPR kinetic studies of the S−1 state in spinach thylakoids

The YZ 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 YZ 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 S0 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.