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 fluorescence quantum yield in different preparations (thylakoids, photosystem II (PSII) membrane fragments and PSII core complexes) from spinach led to a number of conclusions. (1) Light is transformed into Gibbs energy with trapping times of 250 ps and 130 ps in open reaction centres of PSII membrane fragments and PSII core complexes, respectively. Assuming rapid Boltzmann distribution of excitation energy and taking into account the antenna properties (size and spectral distribution), the molecular rate constant of primary charge separation is estimated to be about (3 ps)-1. (2) The electron transfer from Pheo- to QA is characterised by a rate constant of (300 ps)-1. (3) The QA- reoxidation 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 transfer. (4) In PSII reaction centres closed for photochemical trapping the yield of a primary radical pair with lifetimes exceeding 1 ns is comparatively small (c 30%) at room temperature. Short illumination in the presence of Na2S2O4 changes the radical pair dynamics. (5) Photoinhibition under aerobic conditions impairs the primary charge separation and leads to formation of quencher(s) of excitation energy.