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The oxidation potential of the primary electron donor P740 of photosystem I (PS I) of the chlorophyll d dominated cyanobacterium, Acaryochloris marina, has been reinvestigated. We find a midpoint potential of about 450 mV in a photosystem I-enriched membrane fraction as well as in thylakoids, which is significantly higher than that originally reported by Hu et al. (1998). This value, which is, indeed, very similar to that found for P700 in chlorophyll a dominated organisms, is in agreement with results of Bailleul et al. (this Congress). In addition, the extinction difference coefficient for the oxidation of the primary donor has been determined and a value of 45 (±4) mM−1 cm−1 at 740 nm was obtained.

Transient absorbance difference spectroscopy has been used to study the reduction kinetics of P680+ after the first flash given to darkadapted oxygen-evolving PS II complexes from Thermosynechococcus elongatus as a function of temperature between 80 and 320 K. The half-life of P680+ reduction by TyrZ increases from 20 ns at 300 K to about 4 μs at 150 K corresponding to an activation energy of (122 ± 3) meV. Analyzing the temperature dependence of the rate in terms of nonadiabatic electron transfer theory, one obtains a reorganization energy of about 700 meV and an edge-to-edge distance of about 8.5 A which is in good agreement with the distance between PD1 and TyrZ in the recent structural model of PS II at 3.0 A resolution (Loll et al. 2005). In the range from 260 to 150 K, the re-reduction of P680+ by TyrZ is increasingly replaced by the charge recombination between P680+ and QA −. It is proposed that reorganization processes which are required for the stabilization of the state P680TyrZ ox become arrested around 200 K. Freezing out of the electron donation from TyrZ to P680+ is the result. The yield of TyrZ oxidation at low temperature is

At low temperatures (

Summary In cyanobacteria, long wavelength chlorophylls (LWC) are located in the Photosystem I (PS I) core complex, whereas in plants and algae they are distributed between the PS I core and the light-harvesting complexes (LHC I). LWC are most probably aggregates of (excitonically) coupled chlorophylls, mainly dimers or trimers. The total number of LWC is rather small (≤10% of the total chlorophylls). Depending on their location in the PS I antenna and their distance from P700, they can play a crucial role in the kinetics of energy transfer and in the trapping of the excitation energy by charge separation. Energy absorbed by LWC is transferred uphill to P700 with high efficiency at room temperature, thereby increasing the cross-section for the absorption of red light. LWC are involved also in the dissipation of excess energy, thus protecting the reaction center. Under physiological conditions, the excitations within the PS I antenna are nearly thermally equilibrated over the different spectral forms and the excitation energy is efficiently trapped via charge separation in the reaction center. When the photochemistry in the reaction center is blocked, the excitations migrate to the LWC and are quenched either by P700 + or by the P700 triplet state depending on the state of P700.

Photosystem I (PS I) is a large, membrane protein complex, consisting of 12–15 proteins and more than 100 bound cofactors, that catalyzes light-driven electron transfer across the photosynthetic membrane in cyanobacteria, green algae and plants. Photosystem I is unique compared to other photosynthetic systems because the majority of the antenna pigments and the cofactors of the electron transport system are bound to the same protein subunits. Photosystem I can therefore be regarded as a joint reaction center-core antenna system. In this chapter, PS I is described with respect to both structure and function. The kinetics of energy transfer and trapping are discussed in the light of the structural information. We also discuss models for the interaction of PS I with the external antenna complexes in cyanobacteria and plants.

The interaction of the primary donor (P), a dimer of bacteriochlorophylls (BChls), denoted PL and PM, with the protein environment in reaction centers (RCs) of Rhodobacter (Rb.) sphaeroides is studied in a series of mutants, in which His M202 is exchanged to either Glu, Asn, Ser, as well as in HL(M202) [1] and HG(M202) [2]. Mutant RCs were isolated and investigated by optical spectroscopy, redox titrations of P and EPR/ENDOR spectroscopy of the light-induced radical cation P*+. The aim of this study is to assess the ability of different amino acid residues to serve as axial ligand to PM and to influence the pigment composition of the RCs.

The primary processes of excitation transfer, charge separation and charge stabilization in Photosystem II (PS II) are commonly described by the exciton-radical-pair-equilibrium model [1]. In this model the primary charge separation is thought to take place after complete equilibration of the excitation energy among the spectrally heterogeneous antenna and the nearly isoenergetic primary donor P680 (i.e. trap-limited) and incorporates the reversible character of the primary charge separation, thus predicting bi-exponential decay kinetics for the equilibrated excited states in PS II. The trap-limited character of the model implies that the trapping rate should be independent of the spectral position of the initially excited pigment pool. To our knowledge, there exists only one report on such a systematic study on the excitation wavelength dependence of the excited state kinetics in PS II. Performing one-color pump-probe absorption experiments on PS II core particles from spinach, Schelvis et al. [2] found the apparent trapping time to vary from ~200 ps at 670 nm to ~80 ps at >700 nm. In order to clarify this discrepancy, we decided to examine the wavelength dependence of the excited state dynamics in PS II core particles more closely. Here we report on the ps fluorescence kinetics in open (F0) and closed (Fmax) PS II from cyanobacteria upon various excitation wavelengths (670–700 nm). Our results show the trapping time to be independent of the wavelength used for excitation, supporting the exciton-radical-pair-equilibrium model for cyanobacterial PS II core complexes.

Photosystem I (PS I) is a membrane-bound pigment-protein complex, consisting of at least 11 subunits (1). The cofactors which are involved in the electron transfer are located in the PsaA and PsaB subunits (P700, a chlorophyll a dimer, A0 a monomeric chlorophyll a, Al, a phylloquinone and FX, a [4Fe-4S] cluster) and in the PsaC subunit (two additional [4Fe-4S] clusters, FA and FB) (for a recent review see (2)). In this work, the recombination of the flash-induced charge separated pair was studied by transient absorbance spectroscopy under conditions of oxidized and stepwise prereduced [4Fe-4S] cluster to clarify the kinetics of the different recombination reactions. Measurements of the amplitudes of the recombination phases as a function of the potential offer the possibility to determine the midpoint potentials of the first and second reduction of the iron-sulphur clusters at room temperature. In the past, midpoint potentials were mostly measured by EPR at cryogenic temperatures (e.g. 3,4). In addition, the activation energy of the charge recombination between P700+ and (FAFB) has been measured since values described in the literature are contradictory (200 meV in (5) and 460 meV in (6)).

Photosystem I (PS I) is a pigment protein complex, consisting of at least 11 subunits, that mediates the light-driven electron transfer from reduced plastocyanin or cytochrome c6 to ferredoxin. The PS I complex contains two large subunits (A, B of about 83 kDa), which bind about 100 antenna chlorophyll a molecules and the following redox centers: P700 (presumably a chlorophyll a dimer), Ao (a monomeric chlorophyll a), A1 (a phylloquinone) and Fx (a [4Fe-4S] iron-sulfur-cluster). The structure of PS I has been determined at 4.0 A resolution in a recent X-ray crystallographic study using crystals of trimeric PS I of Synechococcus elongatus (1).

The aim of this study was to explore the electronic structure of the radical cation of the primary electron donor in the PSII reaction centre, P680, using ENDOR spectroscopy. This technique has been used to elucidate the electronic structure of the oxidised primary donors in purple bacteria (1) and P700 (2,3). However, there has only been one report of similar ENDOR studies on P680+· (2). Study of P680+· is hampered by the fact that its very high redox potential results in oxidation of secondary electron donors (4–6). Here we have compared untreated PSII RC with samples in which secondary donors were inactivated.

Photosystem II (PS II) is a membrane-bound pigment-protein complex that uses light energy to drive the transfer of electrons from water to plastoquinone. When the electron transfer to the quinone acceptors is blocked, the primary radical pair, P680+Pheo-, can decay by three pathways: (i) by charge recombination to the singlet ground state, (ii) by an activated back reaction to the excited singlet state of P680 and (iii) after singlet-triplet mixing in the radical pair by recombination to the lowest excited triplet state of P680, 3P680. In PS II centers, in which the first quinone acceptor, QA, is removed or doubly reduced (QAH2), the formation of 3P680 can be monitored by the characteristic spin-polarized triplet EPR spectrum. The lifetime of 3P680 is similar to that of the chlorophyll a triplet state in solvents (τ ≈ 1.4 ms (70%)/7 ms (30%) at 5 K and τ ≈ 1.4 ms at T > 20 K under anaerobic conditions). In the presence of singly reduced QA, however, the P680 triplet state with ms lifetime could not be detected, neither by transient absorption spectroscopy nor by cw-EPR [1]. In more recent work, transient absorption spectroscopy at low temperature (T = 25 K) revealed that the primary radical pair decays to an intermediate with μs lifetime (τ ≈ 2 μs (50%)/20 μs (50%) at 25 K). From its spectral features in the QY-region, this intermediate has been identified with the triplet state of P680 [2,3].

Evidence has been presented in the last years that there is a specific requirement for calcium and chloride in the process of water oxidation [1]. Most of these investigations have been performed on PSII from higher plants, where this ion requirement has been found to be dependent on the presence, or rather absence, of the extrinsic 17 and 23 kDa proteins. PSII from cyanobacteria lacks these proteins. Therefore it is of interest to investigate the effect of various salts on photosynthetic water oxidation in PSII-complexes from Synechococcus Sp. This was performed by measuring flash induced oxygen yield. Significant differences compared to higher plants have been observed.

The reaction sequence between the primary electron acceptor, the oxidized Chlorophyll-aII, and the terminal electron donor, the water splitting enzyme system S, is being described in the range from nanoseconds to milliseconds. For the cleavage of water Chlorophyll-a II + extracts four electrons in four turnovers from the enzyme system S responsible for the water oxidation. For each extraction the electron is moved step by step along the chain that connects the Chlorophy11-aII center with that of S. Beginning with the transfer from the immediate donor, D1, to Chl-a II + , the subsequent transfer from D2 to D I + ends in the electron transfer from S to D II + . This final act establishes in S the oxidizing equivalent, probably in the form of oxidized manganese. Coupled with these acts is an intrinsic proton release and a surplus charge formation. After the generation of the 4th oxidizing equivalent in a concerted final action the evolution of O2 from water takes place. Correlations between the events are described quantitatively.