Winfried Leibl

Electron transfer in photosystem I

This mini-review focuses on recent experimental results and questions, which came up since the last more comprehensive reviews on the subject. We include a brief discussion of the different techniques used for time-resolved studies of electron transfer in photosystem I (PS I) and relate the kinetic results to new structural data of the PS I reaction centre.

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

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.

A systematic survey of conserved histidines in the core subunits of Photosystem I by site-directed mutagenesis reveals the likely axial ligands of P700

The Photosystem I complex catalyses the transfer of an electron from lumenal plastocyanin to stromal ferredoxin, using the energy of an absorbed photon. The initial photochemical event is the transfer of an electron from the excited state of P700, a pair of chlorophylls, to a monomer chlorophyll serving as the primary electron acceptor. We have performed a systematic survey of conserved histidines in the last six transmembrane segments of the related polytopic membrane proteins PsaA and PsaB in the green alga Chlamydomonas reinhardtii. These histidines, which are present in analogous positions in both proteins, were changed to glutamine or leucine by site‐directed mutagenesis. Double mutants in which both histidines had been changed to glutamine were screened for changes in the characteristics of P700 using electron paramagnetic resonance, Fourier transform infrared and visible spectroscopy. Only mutations in the histidines of helix 10 (PsaA‐His676 and PsaB‐His656) resulted in changes in spectroscopic properties of P700, leading us to conclude that these histidines are most likely the axial ligands to the P700 chlorophylls.

Artificial photosynthetic systems. Using light and water to provide electrons and protons for the synthesis of a fuel

This review covers the progress achieved in the synthesis and characterization of different metal based catalysts designed for the photocatalytic oxidation of water with special focus on molecular designed systems. In those cases where the mechanism of the light-driven catalytic activity of these complexes is not known, we have looked to published mechanisms related to chemically activated catalysts. We also discuss the choice of the chromophores used to sensitize these reactions as well as the recovery of the reaction products and nature of the electron acceptors used.

Primary electrogenic reactions of Photosystem II as probed by the light-gradient method

The amplitude and the kinetics of the fast photovoltage from pea chloroplasts, which arise from the light-gradient effect, were reexamined with respect to the relative contributions of the two photosystems. Chloroplasts prepared under stacking conditions displayed no photovoltage due to Photosystem II. The photovoltage observed originated exclusively from Photosystem I. This conclusion was drawn from various experimental assays that have been chosen to separate the two photosystems: (i) reduction of the quinone acceptor, QA, of Photosystem II either by 3-(3,4-dichlorophenyl)-1,1-dimethylurea and preillumination or by dithionite; (ii) kinetic analysis of the fast photovoltage elicited by 30-ps flashes; (iii) oxidation of Photosystem I by ferricyanide and continuous far-red light; (iv) the effect of the artificial quencher of the excited state, dinitrobenzene; and (v) the use of Photosystem-I-deficient mutants of the alga Chlamydomonas reinhardtii and barley chloroplasts. We ascribe the lack of electrogenicity of Photosystem II to a delocalization of the excitation energy within the grana stacks, such that the anisotropy of the light-gradient is equilibrated before trapping takes place (‘excitonic short-circuit’ of the light-gradient effect). This conclusion is substantiated by the observation that chloroplasts prepared under destacking conditions (‘blebs’ or EDTA-vesicles) did display a photovoltage due to Photosystem II. In this case we could show (i) that the primary charge separation proceeds in two kinetically distinct steps, the first occurring in less than 50 ps and the second in 300–450 ps; (ii) that the charge separation between the primary donor, P-680, and the intermediary acceptor, pheophytin, accounts for approx. 12 to 23 of the total charge separation; (iii) that the trapping kinetics is on the order of 200 ps at 50 μJ/cm2; (iv) that no charge separation occurs when QA is reduced; and (v) that there is no electrogenicity connected with the rereduction of P-680 by the water-splitting enzyme.

Electron transfer in the heliobacterial reaction center: evidence against a quinone-type electron acceptor functioning analogous to A1 in photosystem I.

Membrane fragments from Heliobacillus mobilis were characterized using time resolved optical spectroscopy and photovoltage measurements in order to detect a possible participation of menaquinone (MQ), functioning analogous to the phylloquinone A1 in photosystem I, as intermediate in electron transfer from the primary acceptor A0 to the iron-sulfur cluster FX in the photosynthetic reaction center. The spectroscopic data obtained exclude that electron transfer from a semiquinone anion MQ- to FX occurred in the time window from 2 ns to 4 micros, where it would be expected in analogy to photosystem I. In the case of a prereduction of FX, only the primary pair P798+A0- was formed. The photovoltage data yielded a single kinetic phase with a time constant of 700 ps for the transmembrane electron transfer beyond A0; the relative amplitude of this phase suggests that it reflects electron transfer from A0- to FX.

Trapping and annihilation in the antenna system of photosystem I

The primary charge separation in Photosystem I of pea chloroplasts was measured as a photovoltage in the pico- and nanosecond time range by applying laser flashes at 532 nm of variable energy and different duration (12 ns and 30 ps, respectively). Contributions to the photovoltage from Photosystem II was eliminated by addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea and preillumination. The dependence of the photovoltage amplitude on the excitation energy could be described by an exponential saturation law when the excitation flash had a duration of 12 ns. Nearly the same dependence was found when the excitation source was the train of a mode-locked laser (approx. ten 30-ps flashes spaced by 7 ns; highest energy of a single flash, 80 μJ / cm−2). Even with single 30-ps flashes the photovoltage was only slightly smaller than the one elicited by 12-ns flashes of the same energy. These findings demonstrate that trapping of excitation energy by the reaction center of Photosystem I is much more effective than losses by annihilation and other loss processes. The photovoltage yield was nearly independent of the fraction of closed traps, thus demonstrating that the absorption cross section of Photosystem I is not altered by the closing of its reaction centers. By recording the rise time of the photovoltage with our highest time resolution we found that the trapping rate of the excitation energy in Photosystem I depended on the energy of the 30-ps flashes: at low excitation energies (less than 1014 photons / cm2 per pulse) trapping occurred within 90 ± 15 ps and at high excitation energy (1015 photons / cm2 per pulse) trapping and charge stabilization occurred within the time resolution of the apparatus, i.e., up to 50 ps. The trapping rate at low energies is in agreement with the one determined by fluorescence decay kinetics. Up to 50 ns there was no further detectable electrogenic phase (neither forward nor backward reactions). This demonstrates that all the electrogenicity, produced by the charge separation, takes place in less than 50 ps.

Modulation of Primary Radical Pair Kinetics and Energetics in Photosystem II by the Redox State of the Quinone Electron Acceptor QA

Time-resolved photovoltage measurements on destacked photosystem II membranes from spinach with the primary quinone electron acceptor Q(A) either singly or doubly reduced have been performed to monitor the time evolution of the primary radical pair P680(+)Pheo(-). 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(A). 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(A) as well as with oxidized or singly reduced Q(A). The standard free energy difference DeltaG degrees between the charge-separated state P680(+)Pheo(-) and the equilibrated excited state (Chl(N)P680)* was found to be similar when Q(A) was oxidized or doubly reduced before the flash (approximately -50 meV). In contrast, single reduction of Q(A) led to a large change in DeltaG degrees (approximately +40 meV), demonstrating the importance of electrostatic interaction between the charge on Q(A) and the primary radical pair, and providing direct evidence that the doubly reduced Q(A) 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 DeltaG degrees than the rate of primary charge separation.

Click chemistry on a ruthenium polypyridine complex. An efficient and versatile synthetic route for the synthesis of photoactive modular assemblies.

In this Communication, we present the synthesis and use of [Ru(bpy)(2)(bpy-CCH)](2+), a versatile synthon for the construction of more sophisticated dyads by means of click chemistry. The resulting chromophore-acceptor or -donor complexes have been studied by flash photolysis and are shown to undergo efficient electron transfer to/from the chromophore. Additionally, the photophysical and chemical properties of the original chromophore remain intact, making it a very useful component for the preparation of visible-light-active dyads.

Efficient electron transfer through a triazole link in ruthenium(II) polypyridine type complexes

Spectroscopic, electrochemical and theoretical characterisations of photoactive systems readily assembled via click-chemistry show an efficient bi-directional charge shift through the triazole link.