FLUORESCENCE AND SPECTROSCOPIC STUDIES OF EXCITON TRAPPING AND ELECTRON-TRANSFER IN PHOTOSYSTEM-II OF HIGHER-PLANTS

Measurements of time-resolved fluorescence decay, laser-flash-induced absorption changes in the UV and at 820 nm and of the relative fluores cence quantum yield in different preparations (thylakoids, photosystem II (PSII) membrane fragments and PSII core complexes) from spinach le d to a number of conclusions. (I) Light is transformed into Gibbs ener gy with trapping times of 250 ps and 130 ps in open reaction centres o f PSII membrane fragments and PSII core complexes, respectively. Assum ing rapid Boltzmann distribution of excitation energy and taking into account the antenna properties (size and spectral distribution), the m olecular rate constant of primary charge separation is estimated to be about (3 ps)(-1). (2) The electron transfer from Pheo(-) to Q(A) is c haracterised by a rate constant of (300 ps)-1. (3) The Q(A)(-) reoxida tion kinetics are significantly retarded in D2O suspensions. These H/D isotope effects are interpreted as to reflect hydrogen-bond dependent changes in the protein dynamics that are relevant to electron transfe r. (4) In PSII reaction centres closed for photochemical trapping the yield of a primary radical pair with lifetimes exceeding 1 ns is compa ratively small (< 30%) at room temperature. Short illumination in the presence of Na2S2O4 changes the radical pair dynamics. (5) Photoinhibi tion under aerobic conditions impairs the primary charge separation an d leads to formation of quencher(s) of excitation energy.