KINETIC-STUDIES ON THE STABILIZATION OF THE PRIMARY RADICAL PAIR P680(+) PHEO(-) IN DIFFERENT PHOTOSYSTEM-II PREPARATIONS FROM HIGHER-PLANTS

The stabilization of the primary radical pair P680(+) pheophytin (Pheo)(-) through rapid electron transfer from Pheo(-) to the special plastoquinone of photosystem II (PS II), Q(A), was analyzed on the basis of time-resolved (40 ps) UV-absorption changes detected in different PS II preparations from higher plants. Lifetime measurements of (1)Chl* fluorescence by single photon counting and a numerical analysis of the redox reactions revealed (1) at exciton densities required for light saturation of the stable charge separation, annihilation processes dominate the excited state decay leading to very similar lifetimes of (1)Chl* in systems with open and closed reaction centers and (2) the difference of absorption changes induced by actinic flashes of comparatively high photon density in samples with open and photochemically closed reaction centers, respectively, provides a suitable measure of the rate constant of Q(A)(-) formation. Conclusion 2 was confirmed in PS II membrane fragments by measurements at three wavelengths (280 nm, 292 nm and 325 nm) where the difference spectrum of Q(A)(-) formation exhibits characteristic features. The numerical evaluation of the experimental data led to the following results: (1) the rate constant of Q(A)(-) formation was found to be (300 +/- 100 ps)(-1) in PS II membrane fragments and PS II core complexes deprived of the distal and proximal antenna and (2) an iron depletion treatment of membrane fragments does not affect these kinetics. The implications of these results are briefly discussed in terms of the PS II reaction pattern.