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

The stabilization of the primary radical pair P680(+) pheophytin (Pheo )(-) through rapid electron transfer from Pheo(-) to the special plast oquinone 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 f luorescence by single photon counting and a numerical analysis of the redox reactions revealed (1) at exciton densities required for light s aturation of the stable charge separation, annihilation processes domi nate the excited state decay leading to very similar lifetimes of (1)C hl in systems with open and closed reaction centers and (2) the diffe rence of absorption changes induced by actinic flashes of comparativel y high photon density in samples with open and photochemically closed reaction centers, respectively, provides a suitable measure of the rat e constant of Q(A)(-) formation. Conclusion 2 was confirmed in PS II m embrane fragments by measurements at three wavelengths (280 nm, 292 nm and 325 nm) where the difference spectrum of Q(A)(-) formation exhibi ts characteristic features. The numerical evaluation of the experiment al 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 frag ments and PS II core complexes deprived of the distal and proximal ant enna and (2) an iron depletion treatment of membrane fragments does no t affect these kinetics. The implications of these results are briefly discussed in terms of the PS II reaction pattern.