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• Rate of Carotenoid Triplet Formation in Solubilized Light-Ha...

  Rate of Carotenoid Triplet Formation in Solubilized Light-Harvesting Complex II (LHCII) from Spinach Biophysical Journal, Volume 75, Issue 6, 1 December 1998, Pages 3143-3153 René Schödel, Klaus-D. Irrgang, Joachim Voigt and Gernot Renger Abstract In the present study the rate of triplet transfer from chlorophyll to carotenoids in solubilized LHCII was investigated by flash spectroscopy using laser pulses of ∼2ns for both pump and probe. Special attention has been paid to calibration of the experimental setup and to avoid saturation effects. Carotenoid triplets were identified by the pronounced positive peak at ∼507nm in the triplet-singlet difference spectra. Δ (507nm) exhibits a monoexponential relaxation kinetics with characteristic lifetimes of 2–9s (depending on the oxygen content) that was found to be independent of the pump pulse intensity. The rise of Δ (507nm) was resolved via a pump probe technique where an optical delay of up to 20ns was used. A thorough analysis of these experimental data leads to the conclusion that the kinetics of carotenoid triplet formation in solubilized LHCII is almost entirely limited by the lifetime of the excited singlet state of chlorophyll but neither by the pulse width nor by the rate constant of triplet-triplet transfer. Within the experimental error the rate constant of triplet-triplet transfer from chlorophyll to carotenoids was estimated to be >(0.5ns). This value exceeds all data reported so far by at least one order of magnitude. The implications of this finding are briefly discussed. Abstract | Full Text | PDF (193 kb)

• Modulation of Primary Radical Pair Kinetics and Energetics i...

  Modulation of Primary Radical Pair Kinetics and Energetics in Photosystem II by the Redox State of the Quinone Electron Acceptor QA Biophysical Journal, Volume 80, Issue 4, 1 April 2001, Pages 1617-1630 Krzysztof Gibasiewicz, Andrzej Dobek, Jacques Breton and Winfried Leibl Abstract Time-resolved photovoltage measurements on destacked photosystem II membranes from spinach with the primary quinone electron acceptor Q either singly or doubly reduced have been performed to monitor the time evolution of the primary radical pair P680Pheo. The maximum transient concentration of the primary radical pair is about five times larger and its decay is about seven times slower with doubly reduced compared with singly reduced Q. The possible biological significance of these differences is discussed. On the basis of a simple reversible reaction scheme, the measured apparent rate constants and relative amplitudes allow determination of sets of molecular rate constants and energetic parameters for primary reactions in the reaction centers with doubly reduced Q as well as with oxidized or singly reduced Q. The standard free energy difference Δ° between the charge-separated state P680Pheo and the equilibrated excited state (ChlP680)* was found to be similar when Q was oxidized or doubly reduced before the flash (∼−50 meV). In contrast, single reduction of Q led to a large change in Δ° (∼+40 meV), demonstrating the importance of electrostatic interaction between the charge on Q and the primary radical pair, and providing direct evidence that the doubly reduced Q is an electrically neutral species, i.e., is doubly protonated. A comparison of the molecular rate constants shows that the rate of charge recombination is much more sensitive to the change in Δ° than the rate of primary charge separation. Abstract | Full Text | PDF (218 kb)

• Decay Kinetics and Quantum Yields of Fluorescence in Photosy...

  Decay Kinetics and Quantum Yields of Fluorescence in Photosystem I from Synechococcus elongatus with P700 in the Reduced and Oxidized State: Are the Kinetics of Excited State Decay Trap-Limited or Transfer-Limited? Biophysical Journal, Volume 79, Issue 2, 1 August 2000, Pages 992-1007 Martin Byrdin, Ingo Rimke, Eberhard Schlodder, Dietmar Stehlik and Theo A. Roelofs Abstract Transfer and trapping of excitation energy in photosystem I (PS I) trimers isolated from have been studied by an approach combining fluorescence induction experiments with picosecond time-resolved fluorescence measurements, both at room temperature (RT) and at low temperature (5K). Special attention was paid to the influence of the oxidation state of the primary electron donor P700. A fluorescence induction effect has been observed, showing a ∼12% increase in fluorescence quantum yield upon P700 oxidation at RT, whereas at temperatures below 160K oxidation of P700 leads to a decrease in fluorescence quantum yield (∼50% at 5K). The fluorescence quantum yield for open PS I (with P700 reduced) at 5K is increased by ∼20-fold and that for closed PS I (with P700 oxidized) is increased by ∼10-fold, as compared to RT. Picosecond fluorescence decay kinetics at RT reveal a difference in lifetime of the main decay component: 34±1ps for open PS I and 37±1ps for closed PS I. At 5K the fluorescence yield is mainly associated with long-lived components (lifetimes of 401ps and 1.5ns in closed PS I and of 377ps, 1.3ns, and 4.1ns in samples containing ∼50% open and 50% closed PS I). The spectra associated with energy transfer and the steady-state emission spectra suggest that the excitation energy is not completely thermally equilibrated over the core-antenna-RC complex before being trapped. Structure-based modeling indicates that the so-called red antenna pigments (A708 and A720, i.e., those with absorption maxima at 708nm and 720nm, respectively) play a decisive role in the observed fluorescence kinetics. The A720 are preferentially located at the periphery of the PS I core-antenna-RC complex; the A708 must essentially connect the A720 to the reaction center. The excited-state decay kinetics turn out to be neither purely trap limited nor purely transfer (to the trap) limited, but seem to be rather balanced. Abstract | Full Text | PDF (236 kb)

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Abstract

The study of the fluorescence of chlorophyll a offers a useful approach toward better understanding of the primary act of photosynthesis. This paper describes new measurements of the decay of chlorophyll a fluorescence in vivo, made with a considerably improved oscilloscopic-display technique. The main result is the identification of two decay periods both of the order of a few nanoseconds. Possible interpretations of this phenomenon are discussed.