Photoelectric study on the kinetics of trapping and charge stabilization in oriented PS II membranes

Abstract

Excitation energy trapping and charge separation in Photosystem II were studied by kinetic analysis of the fast photovoltage detected in membrane fragments from peas with picosecond excitation. With the primary quinone acceptor oxidized the photovoltage displayed a biphasic rise with apparent time constants of 100-300 ps and 550±50 ps. The first phase was dependent on the excitation energy whereas the second phase was not. We attribute these two phases to trapping (formation of P-680(+) Phe(-)) and charge stabilization (formation of P-680(+) QA (-)), respectively. A reversibility of the trapping process was demonstrated by the effect of the fluorescence quencher DNB and of artificial quinone acceptors on the apparent rate constants and amplitudes. With the primary quinone acceptor reduced a transient photoelectric signal was observed and attributed to the formation and decay of the primary radical pair. The maximum concentration of the radical pair formed with reduced QA was about 30% of that measured with oxidized QA. The recombination time was 0.8-1.2 ns.The competition between trapping and annihilation was estimated by comparison of the photovoltage induced by short (30 ps) and long (12 ns) flashes. These data and the energy dependence of the kinetics were analyzed by a reversible reaction scheme which takes into account singlet-singlet annihilation and progressive closure of reaction centers by bimolecular interaction between excitons and the trap. To put on firmer grounds the evaluation of the molecular rate constants and the relative electrogenicity of the primary reactions in PS II, fluorescence decay data of our preparation were also included in the analysis. Evidence is given that the rates of radical pair formation and charge stabilization are influenced by the membrane potential. The implications of the results for the quantum yield are discussed.

Excitation energy transfer in native and unstacked thylakoid membranes studied by low temperature and ultrafast fluorescence spectroscopy

“…The relative increase of LHCII fluorescence with respect to PSII is interpreted as a decrease in the energy transfer from LHCII to PSII. This observed reduction in connectivity between LHCII and PSII upon unstacking of the thylakoid membrane, may be caused by a release of LHCII from PSII and/or by a decrease of transversal energy transfer between the membrane layers in a stack (Leibl et al 1989;Kirchhoff et al 2004). At the same time, unstacking of the thylakoid membrane is seen to result in an increase of the PSI fluorescence emission.…”

Modelling the fast fluorescence rise of photosynthesis

“…Some light may be thrown onto the conflicting qualitative ideas by modelling the proposed mechanisms and comparing them with the experimental data quantitatively. Previous models of fluorescence kinetics have either focused on electron transport through the two-electron-gate on the millisecond timescale (Renger and Schulze, 1985), or on the fast (nano-microsecond timescale) processes of light absorption, transfer of excited states, and charge separation (Schatz et al, 1988;Leibl et al, 1989). In this paper, whose main results are taken from Baake (1989), we modify these and bring them together in a consistent way, which yields our "core model" of fluorescence induction.…”

“…We propose the reaction scheme shown in Fig. 3, which relies on the results of Schatz et al (1988) and Leibl et al (1989), and additionally includes the possibility of excitation energy transfer between neighbouring units.…”

“…QA can carry at most one electron (in contrast to QB which can carry two of them), so with QA reduced ("closed" reaction centre), the Phe-charge cannot be stabilized and recombines with the electron hole, whereby the excited state is (g-l) or is not (~2) repopulated [it is still controversial which of these (or both?) possibilities is actually realized; see Leibl et al, 1989].…”

“…We additionally assume ~3 = ~c3 (found experimentally in Leibl et al, 1989) and K s = ~%, q = q (which can hardly be conceived not to hold) and rescale the parameters:…”