Effects of selective destruction of iron-sulfur center B on electron transfer and charge recombination in Photosystem I.

Incubation of spinach thylakoids with HgCl2 selectively destroys Fe-S center B (FB). The function of electron acceptors in FB-less PS I particles was studied by following the decay kinetics of P700(+) at room temperature after multiple flash excitation in the absence of a terminal electron acceptor. In untreated particles, the decay kinetics of the signal after the first and the second flashes were very similar (t 1/2∼2.5 ms), and were principally determined by the concentration of the artificial electron donor added. The decay after the third flash was fast (t 1/2∼0.25 ms). In FB-less particles, although the decay after the first flash was slow, fast decay was observed already after the second flash. We conclude that in FB-less particles, electron transfer can proceed normally at room temperature from FX to FA and that the charge recombination between P700(+) and FX (-)/A1 (-) predominated after the second excitation. The rate of this recombination process is not significantly affected by the destruction of FB. Even in the presence of 60% glycerol, FB-less particles can transfer electrons to FA at room temperature as efficiently as untreated particles.