~u t h o r for correspondence, facsimile: +30 314 2 1122.Abstract. 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 11 (PSII) membrane fragments and PSII core complexes) from spinach led to a number of conclusions. (I) 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)-'. (2) The electron transfer from Pheo-to Q, is characterised by a rate constant of (300 p -' . (3) The Q i reoxidation kinetics are significantly retarded in D20 suspensions. These HID 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 Na2S204 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.
To analyze a possible correlation between the extent of QA-* reoxidation and protein dynamics, fluorometric and Mössbauer spectroscopic measurements were performed in photosystem II membrane fragments from spinach. Numerical evaluation of the flash-induced change of the normalized fluorescence quantum yield revealed that the extent of reoxidation starts to decrease below 275 K and is almost completely suppressed at 230 K. Detailed analyses of Mössbauer spectra measured at different temperatures in 57Fe-enriched material indicate that the onset of fluctuations between conformational substates of the protein matrix occurs also at around 230 K. Based on this correspondence, protein flexibility is inferred to play a key role for QA-* reoxidation in photosystem II. Taking into account the striking similarities with purple bacteria and the latest structural information on these reaction centers [Stowell, M. H. B., McPhillips, T. M., Rees, D. C., Soltis, S. M., Abresch, E., and Feher, G. (1997) Science 276, 812-816], it appears most plausible that also the headgroup of plastoquinone-9 bound to the QB-site in PSII requires a structural reorientation for its reduction to the semiquinone.
The origin of the`35-W Ws kinetics' of P680 c reduction in photosystem II (PS II) with an intact water oxidising complex has been analysed by comparative measurements of laser flash induced changes of the 830-nm absorption and the relative quantum yield of chlorophyll (Chl) fluorescence. The latter parameter was monitored at a time resolution of 500 ns by using newly developed home built equipment [Reifarth, F., Christen, G. and Renger, G. (1997)
Studies on thermodynamics and kinetics of electron transfer from QA- to QB(QB-) were performed by monitoring laser flash induced changes of the relative fluorescence emission as a function of temperature (220 K < T < 310 K) in isolated thylakoids and PS II membrane fragments.In addition, effects of bivalent metal ions on PS II were investigated by measuring conventional fluorescence induction curves, oxygen evolution, manganese content and atrazine binding mostly in PS II membrane fragments. It was found: a) the normalized level of the fluorescence remaining 10 s after the actinic flash (Ft/F0) steeply increases at temperatures below -10 to - 20 °C, b) the fast phase of the transient fluorescence change becomes markedly retarded with decreasing temperatures, c) among different cations (Cu2+, Zn2+, Cd2+, Ni2+, Co2+) only Cu2+ exhibits marked effects in the concentration range below 100 μᴍ and d) Cu2+ decreases the normalized variable fluorescence, inhibits oxygen evolution and diminishes the affinity to atrazine binding without affecting the number of binding sites. The content of about four manganeses per functionally competent oxygen evolving complex is not changed by [Cu2+] < 70 μᴍ.Based on these findings it is concluded: i) a temperature dependent equilibrium between an inactive (I) and active (A) state of QA- reoxidation by QB(QB- ) is characterized by standard enthalpies ΔH° of 95 kJ mol-1 and 60 kJ mol-1 and standard entropies ΔS° of 370 kJ K-1 mol-1 and 240 kJ K-1 mol-1 in isolated thylakoids and PS II membrane fragments, respectively, ii) the activation energies of QA- reoxidation by plastoquinone bound to the QB site are about 30 kJ mol-1 (thylakoids) and 40 kJ mol-1 (PS II membrane fragments) in 220 K < T < 300 K, and iii) Cu2+ causes at least a two-fold effect on PS II by modifying the atrazine binding affinity at lower concentrations ( ~ 5 μᴍ) and interference with the redox active tyrosine Yz at slightly higher concentration ( ~ 10 μᴍ) leading to blockage of oxygen evolution.
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