Total recovery of O2 evolution and nanosecond reduction kinetics of chlorophyll-a II (+) (P-680 (+)) after inhibition of water cleavage with acetate.

Oxygen evolution and reduction kinetics of the photooxidized Chl-aII (+) have been measured in oxygen-evolving complexes from the thermophilic cyanobacterium Synechococcus sp. 1. Incubation of PS II particles with acetate resulted in an inhibition of oxygen evolution and a retardation of the Chl-aII (+)=reduction kinetics from the nanosecond range to the microsecond range, indicating a modification of the donor side of photosystem II (PS II). 2. After the first two flashes given to a dark-adapted, acetate treated sample, Chl-aII (+) was re-reduced with a half-life time of 160 μs by a component of the donor side of PS II. Under repetitive excitation Chl-aII (+) was re-reduced in 500 μs by electron back reaction from the primary acceptor QA (-) (X-320(-)). Obviously, in the presence of acetate only two electrons are available from the donor side. 3. Both oxygen evolution and nanosecond reduction kinetics of Chl-aII (+) were restored to the control level when acetate was removed. 4. The results indicate a tight coupling between O2 evolution and nanosecond reduction kinetics of Chl-aII (+). 5. The reversible inhibition is probably due to a replacement of Cl(-) by acetate within the water splitting enzyme. 6. Due to its strongly retarded kinetics, the reversibly modified system may facilitate investigations of the mechanism of the donor side.