P680(+) reduction in oxygen-evolving photosystem II core complexes

The kinetics of P680(+) reduction in oxygen-evolving spinach Photosystem II (PS II) core particles were studied using both repetitive and single-flash 830 nm transient absorption. From measurements on samples in which PS II turnover is blocked, we estimate radical-pair lifetimes of 2 ns and 19 ns. Nanosecond single-flash measurements indicate decay times of 7 ns, 40 ns and 95 ns. Both the longer 40 ns and 95 ns components relate to the normal S-state controlled Y-z --> P680(+) electron transfer dynamics. Our analysis indicates the existence of a 7 ns component which provides evidence for an additional process associated with modified interactions involving the water-splitting catalytic site. Corresponding microsecond measurements show decay times of 4 mu s and 90 mu s with the possibility of a small component with a decay time of 20-40 mu s. The precise origin of the 4 mu s component remains uncertain but appears to be associated with the water-splitting center or its binding site while the 90 mu s component is assigned to P680(+)-Q(A)(-) recombination. An amplitude and kinetic analysis of the hash dependence data gives results that are consistent with the current model of the oxygen-evolving complex.