Showing papers on "Photosynthetic reaction centre published in 2001"
TL;DR: The effects in vivo of ROS were investigated to clarify the nature of the damage caused by such excess light energy to the photosynthetic machinery in the cyanobacterium Synechocystis sp.
Abstract: Absorption of excess light energy by the photosynthetic machinery results in the generation of reactive oxygen species (ROS), such as H2O2. We investigated the effects in vivo of ROS to clarify the nature of the damage caused by such excess light energy to the photosynthetic machinery in the cyanobacterium Synechocystis sp. PCC 6803. Treatments of cyanobacterial cells that supposedly increased intracellular concentrations of ROS apparently stimulated the photodamage to photosystem II by inhibiting the repair of the damage to photosystem II and not by accelerating the photodamage directly. This conclusion was confirmed by the effects of the mutation of genes for H2O2-scavenging enzymes on the recovery of photosystem II. Pulse labeling experiments revealed that ROS inhibited the synthesis of proteins de novo. In particular, ROS inhibited synthesis of the D1 protein, a component of the reaction center of photosystem II. Northern and western blot analyses suggested that ROS might influence the outcome of photodamage primarily via inhibition of translation of the psbA gene, which encodes the precursor to D1 protein.
484 citations
TL;DR: In this paper, the X-ray structure of photosystem I at a resolution of 2.5 A O shows the location of the individual subunits and cofactors and provides new information on the protein-cofactor interactions.
477 citations
TL;DR: An extremely long-lived charge-separated state has been achieved successfully using a ferrocene-zincporphyrin-freebaseporphyrin -fullerene tetrad which reveals a cascade of photoinduced energy transfer and multistep electron transfer within a molecule in frozen media as well as in solutions.
Abstract: An extremely long-lived charge-separated state has been achieved successfully using a ferrocene−zincporphyrin−freebaseporphyrin−fullerene tetrad which reveals a cascade of photoinduced energy transfer and multistep electron transfer within a molecule in frozen media as well as in solutions. The lifetime of the resulting charge-separated state (i.e., ferricenium ion−C60 radical anion pair) in a frozen benzonitrile is determined as 0.38 s, which is more than one order of magnitude longer than any other intramolecular charge recombination processes of synthetic systems, and is comparable to that observed for the bacterial photosynthetic reaction center. Such an extremely long lifetime of the tetrad system has been well correlated with the charge-separated lifetimes of two homologous series of porphyrin−fullerene dyad and triad systems.
459 citations
TL;DR: The present system provides the first example of an artificial photosynthetic system, which not only mimics light-harvesting and charge separation processes in photosynthesis but also acts as an efficient light-to-current converter in molecular devices.
Abstract: Three different kinds of mixed self-assembled monolayers have been prepared to mimic photosynthetic energy and electron transfer on a gold surface. Pyrene and boron−dipyrrin were chosen as a light-harvesting model. The mixed self-assembled monolayers of pyrene (or boron−dipyrrin) and porphyrin (energy acceptor model) reveal photoinduced singlet−singlet energy transfer from the pyrene (or boron−dipyrrin) to the porphyrin on the gold surface. The boron−dipyrrin has also been combined with a reaction center model, ferrocene−porphyrin−fullerene triad, to construct integrated artificial photosynthetic assemblies on a gold electrode using mixed monolayers of the respective self-assembled unit. The mixed self-assembled monolayers on the gold electrode have established a cascade of photoinduced energy transfer and multistep electron transfer, leading to the production of photocurrent output with the highest quantum yield (50 ± 8%, based on the adsorbed photons) ever reported for photocurrent generation at monolay...
376 citations
TL;DR: A model is presented that gives a quantitative picture of the distribution of the picturesynthetic components in the photosynthetic membrane of higher plants, finding that most of the pigments are located in the grana where photosystem I and II carry out linear electron transport, whereas the stroma lamellae, which harbour <20% of the Pigments.
360 citations
TL;DR: A direct comparison of the energy transfer and trapping properties of the PS I core complexes of cyanobacteria bear a large resemblance to the core complex of plants, and this results in a transmembrane electrochemical gradient that can be used to produce ATP.
349 citations
TL;DR: In vivo kinetics of electron transfer from the quinone in mutant PS I reaction centers are examined, and it is concluded that both electron transfer branches in PS I are active.
Abstract: All photosynthetic reaction centers share a common structural theme. Two related, integral membrane polypeptides sequester electron transfer cofactors into two quasi-symmetrical branches, each of which incorporates a quinone. In type II reaction centers [photosystem (PS) II and proteobacterial reaction centers], electron transfer proceeds down only one of the branches, and the mobile quinone on the other branch is used as a terminal acceptor. PS I uses iron-sulfur clusters as terminal acceptors, and the quinone serves only as an intermediary in electron transfer. Much effort has been devoted to understanding the unidirectionality of electron transport in type II reaction centers, and it was widely thought that PS I would share this feature. We have tested this idea by examining in vivo kinetics of electron transfer from the quinone in mutant PS I reaction centers. This transfer is associated with two kinetic components, and we show that mutation of a residue near the quinone in one branch specifically affects the faster component, while the corresponding mutation in the other branch specifically affects the slower component. We conclude that both electron transfer branches in PS I are active.
301 citations
TL;DR: In this paper, the charge transfer absorption and emission in porphyrin-linked fullerene where the C60 moiety is closely located on the porphrin plane was analyzed.
Abstract: Both charge-transfer absorption and emission have been observed in porphyrin-linked fullerene where the C60 moiety is closely located on the porphyrin plane. Electron-transfer parameters including reorganization energies, free energy changes, and electronic coupling matrix elements were determined by analyzing the charge-transfer absorption and emission in benzene. The reorganization energy is estimated as 0.23 ± 0.11 eV, which is the smallest value among inter- and intramolecular donor−acceptor systems ever reported and is comparable to the smallest ones for the primary charge separation in the photosynthetic reaction center. The results clearly show that fullerenes combined with porphyrins are potential components for constructing artificial photosynthetic systems.
260 citations
TL;DR: EPR studies of P700(+*) in frozen solution and single crystals indicate a large asymmetry in the electron spin and charge distribution towards one Chl of the dimer, suggesting that the unpaired electron would predominantly reside on the Chl a.
231 citations
TL;DR: It is proposed that the monomeric accessory chlorophyll, B(A), is a long-wavelength trap located at 684 nm at 5 K and can be appreciably modulated both positively and negatively by ligand replacement at D1-198 but somewhat less so at D2-197.
Abstract: Site-directed mutations were introduced to replace D1-His198 and D2-His197 of the D1 and D2 polypeptides, respectively, of the photosystem II (PSII) reaction center of Synechocystis PCC 6803. These residues coordinate chlorophylls PA and PB which are homologous to the special pair Bchlorophylls of the bacterial reaction centers that are coordinated respectively by histidines L-173 and M-200 (202). PA and PB together serve as the primary electron donor, P, in purple bacterial reaction centers. In PS II, the site-directed mutations at D1 His198 affect the P + -P-absorbance difference spectrum. The bleaching maximum in the Soret region (in WT at 433 nm) is blue-shifted by as much as 3 nm. In the D1 His198Gln mutant, a similar displacement to the blue is observed for the bleaching maximum in the Qy region (672.5 nm in WT at 80 K), whereas features attributed to a band shift centered at 681 nm are not altered. In the YZ¥-YZ-difference spectrum, the band shift of a reaction center chlorophyll centered in WT at 433-434 nm is shifted by 2-3 nm to the blue in the D1-His198Gln mutant. The D1-His198Gln mutation has little effect on the optical difference spectrum, 3 P- 1 P, of the reaction center triplet formed by P + Pheo - charge recombination (bleaching at 681-684 nm), measured at 5-80 K, but becomes visible as a pronounced shoulder at 669 nm at temperatures g150 K. Measurements of the kinetics of oxidized donor-QA - charge recombination and of the reduction of P + by redox active tyrosine, YZ, indicate that the reduction potential of the redox couple P + /P can be appreciably modulated both positively and negatively by ligand replacement at D1-198 but somewhat less so at D2-197. On the basis of these observations and others in the literature, we propose that the monomeric accessory chlorophyll, BA, is a long-wavelength trap located at 684 nm at 5K. B A* initiates primary charge separation at low temperature, a function that is increasingly shared with PA* in an activated process as the temperature rises. Charge separation from BA* would be potentially very fast and form PA + BA - and/or BA + Pheo - as observed in bacterial reaction centers upon direct excitation of BA (van Brederode, M. E., et al. (1999) Proc. Natl. Acad Sci. 96, 2054-2059). The cation, generated upon primary charge separation in PSII, is stabilized at all temperatures primarily on P A, the absorbance spectrum of which is displaced to the blue by the mutations. In WT, the cation is proposed to be shared to a minor extent (20%) with PB, the contribution of which can be modulated up or down by mutation. The band shift at 681 nm, observed in the P + -P difference spectrum, is attributed to an electrochromic effect of PA + on neighboring BA. Because of its low-energy singlet and therefore triplet state, the reaction center triplet state is stabilized on B A at e80 K but can be shared with PA at >80 K in a thermally activated process.
228 citations
01 Jun 2001
TL;DR: The strong reductant produced by photosystem I has a central role in chloroplast metabolism, and thus photosystem II has a critical role in the metabolic networks and physiological responses in plants.
Abstract: ▪ Abstract Photosystem I is the light-driven plastocyanin-ferredoxin oxidoreductase in the thylakoid membranes of cyanobacteria and chloroplasts. In recent years, sophisticated spectroscopy, molecular genetics, and biochemistry have been used to understand the light conversion and electron transport functions of photosystem I. The light-harvesting complexes and internal antenna of photosystem I absorb photons and transfer the excitation energy to P700, the primary electron donor. The subsequent charge separation and electron transport leads to the reduction of ferredoxin. The photosystem I proteins are responsible for the precise arrangement of cofactors and determine redox properties of the electron transfer centers. With the availability of genomic information and the structure of photosystem I, one can now probe the functions of photosystem I proteins and cofactors. The strong reductant produced by photosystem I has a central role in chloroplast metabolism, and thus photosystem I has a critical role in...
TL;DR: It is proposed that the plasma membrane, and not the thylakoid membrane, is the site for a number of the early steps of biogenesis of the photosynthetic reaction center complexes in these cyanobacterial cells.
Abstract: During oxygenic photosynthesis in cyanobacteria and chloroplasts of plants and eukaryotic algae, conversion of light energy to biologically useful chemical energy occurs in the specialized thylakoid membranes. Light-induced charge separation at the reaction centers of photosystems I and II, two multisubunit pigment-protein complexes in the thylakoid membranes, energetically drive sequential photosynthetic electron transfer reactions in this membrane system. In general, in the prokaryotic cyanobacterial cells, the thylakoid membrane is distinctly different from the plasma membrane. We have recently developed a two-dimensional separation procedure to purify thylakoid and plasma membranes from the genetically widely studied cyanobacterium Synechocystis sp. PCC 6803. Immunoblotting analysis demonstrated that the purified plasma membrane contained a number of protein components closely associated with the reaction centers of both photosystems. Moreover, these proteins were assembled in the plasma membrane as chlorophyll-containing multiprotein complexes, as evidenced from nondenaturing green gel and low-temperature fluorescence spectroscopy data. Furthermore, electron paramagnetic resonance spectroscopic analysis showed that in the partially assembled photosystem I core complex in the plasma membrane, the P700 reaction center was capable of undergoing light-induced charge separation. Based on these data, we propose that the plasma membrane, and not the thylakoid membrane, is the site for a number of the early steps of biogenesis of the photosynthetic reaction center complexes in these cyanobacterial cells.
TL;DR: In this minireview, an overview of the electron-transfer reactions in photosystem II is presented, with an emphasis on those involving carotenoids.
01 Jan 2001
TL;DR: Photosystem II is frequently undergoing photoinduced damages targeted to its reaction center in a process normally referred to as photoinhibition, and Photosynthesis is maintained through an intricate repair mechanism involving degradation of the damaged D1 reaction center protein and insertion of a new protein copy into the photosystem.
Abstract: Photosystem II is frequently undergoing photoinduced damages targeted to its reaction center in a process normally referred to as photoinhibition. Photosynthesis is maintained through an intricate repair mechanism involving degradation of the damaged D1 reaction center protein and insertion of a new protein copy into the photosystem. The photoinhibition process is induced by inoptimal electron transfer at the acceptor or donor side of Photosystem II. Photoinhibition induced from the acceptor side is caused by a stepwise accumulation of stably reduced abnormal QA species that lead to formation of chlorophyll triplets and production of singlet oxygen, resulting in oxidative damage to the D1 protein. Photoinhibition induced from the donor side involves formation of long-lived highly oxidizing P680+ and finally D1 protein damage. A damaged D1 protein is triggered to be degraded via a multistep proteolytic reaction requiring GTP and ATP and catalyzed by chloroplast Deg P2 and FtsH, homologues to known bacterial proteases. Synthesis and assembly of the new D1 copy into PS II is also a multistep process. After targeting of the psbA mRNA ribosome complex to the thylakoid membrane, the elongating D1 protein is cotranslationally inserted into the thylakoid membrane and concomitantly assembled with the D2 protein. Both the cotranslational and post-translational assembly steps of the D1 protein into PS II are under strict redox control.
TL;DR: Results indicate that the degradation of the photodamaged D1 protein proceeds through membrane-bound proteases and stromal proteases, thus contributing to the control of the quality of photosystem II under light stress conditions.
Abstract: Photosystem II is particularly vulnerable to excess light. When illuminated with strong visible light, the reaction center D1 protein is damaged by reactive oxygen molecules or by endogenous cationic radicals generated by photochemical reactions, which is followed by proteolytic degradation of the damaged D1 protein. Homologs of prokaryotic proteases, such as ClpP, FtsH and DegP, have been identified in chloroplasts, and participation of the thylakoid-bound FtsH in the secondary degradation steps of the photodamaged D1 protein has been suggested. We found that cross-linking of the D1 protein with the D2 protein, the alpha-subunit of cytochrome b(559), and the antenna chlorophyll-binding protein CP43, occurs in parallel with the degradation of the D1 protein during the illumination of intact chloroplasts, thylakoids and photosystem II-enriched membranes. The cross-linked products are then digested by a stromal protease(s). These results indicate that the degradation of the photodamaged D1 protein proceeds through membrane-bound proteases and stromal proteases. Moreover, a 33-kDa subunit of oxygen-evolving complex (OEC), bound to the lumen side of photosystem II, regulates the formation of the cross-linked products of the D1 protein in donor-side photoinhibition of photosystem II. Thus, various proteases and protein components in different compartments in chloroplasts are implicated in the efficient turnover of the D1 protein, thus contributing to the control of the quality of photosystem II under light stress conditions.
TL;DR: In this article, all rates for the inter-complex excitation transfer processes on the basis of the atomic level structures of the pigment−protein complexes and of an effective Hamiltonian, established previously, for intracomplex excitations were calculated.
Abstract: Purple bacteria have developed an efficient apparatus to harvest sunlight. The apparatus consists of up to four types of pigment−protein complexes: (i) the photosynthetic reaction center surrounded by (ii) the light-harvesting complex LH1, (iii) antenna complexes LH2, which are replaced under low-light conditions by (iv) antenna complexes LH3 with a higher absorption maximum. Following absorption of light anywhere in the apparatus, electronic excitation is transferred between the pigment−protein complexes until it is used for the primary photoreaction in the reaction center. We calculate, using Forster theory, all rates for the inter-complex excitation transfer processes on the basis of the atomic level structures of the pigment−protein complexes and of an effective Hamiltonian, established previously, for intracomplex excitations. The kinetics of excitation migration in the photosynthetic apparatus is described through a master equation which connects the calculated transfer rates to the overall archite...
TL;DR: In this paper, the P840 reaction center complex (RC) of green sulfur bacteria is compared to the PsaAB/PsaC-core of the P700 reaction center in Photosystem I.
TL;DR: In this paper, a photoreactive heteropoly acid (HPA) was incorporated into TiO2 colloids in aqueous polyvinyl alcohol (PVA) (0.1%) solution, and two light reactions appeared to operate in a series.
Abstract: TiO2 colloids are very useful photocatalytic systems, capable of converting solar energy to chemical or electrical energy and environmental cleaning. The key step in enhancing photocatalytic efficiency is improving photoinduced interfacial electron transfer like plant photosynthesis. It remains difficult to modify TiO2 particles as a real analogue of the photosynthetic reaction centers of green plants. We attempted to incorporate a photoreactive heteropoly acid (HPA) such as H3PW12O40 into TiO2 colloids in aqueous polyvinyl alcohol (PVA) (0.1%) solution, and found that two light reactions appear to operate in a series. Upon illumination of the HPA/TiO2 system with near-UV light (300−375 nm), interfacial electron transfer takes place from the conduction band of TiO2 to the incorporated HPA, which is also excited to catalyze photoreduction of Methyl Orange. The extent of the photoinduced reduction of the HPA adsorbed on TiO2 particles depends on the concentration ratio of the HPA and TiO2 colloids, irradiat...
TL;DR: Scholes et al. as discussed by the authors applied the theory to wild-type and mutant photosynthetic reaction centers (RCs) from Rb. sphaeroides, as well as to the wild type RC from Rps viridis.
Abstract: In the accompanying paper (Scholes, G. D.; Jordanides, X. J.; Fleming, G. R. J. Phys. Chem. 2001, 105, 1640), a generalization of Forster theory is developed to calculate electronic energy transfer (EET) in molecular aggregates. Here we apply the theory to wild-type and mutant photosynthetic reaction centers (RCs) from Rb. sphaeroides, as well as to the wild-type RC from Rps. viridis. Experimental information from the X-ray crystallographic structure, resonance Raman excitation profiles, and hole-burning measurements are integrated with calculated electronic couplings to model the EET dynamics within the RC complex. Optical absorption and circular dichroism spectra are calculated at various temperatures between 10 K and room temperature, and compare well with the experimentally observed spectra. The calculated rise time of the population of the lower exciton state of P, P-, as a result of energy transfer from the accessory bacteriochlorophyll, B, to the special pair, P, in Rb. sphaeroides (Rps. viridis) w...
TL;DR: A kinetic model for PSII, based on the x-ray crystal structure coordinates of 37 antenna and reaction center pigment molecules, allows us to map the major energy transfer routes from the antenna chlorophylls to the reaction center chromophores and predicts a value for the intrinsic photochemical rate constant that is 4 times that found in bacterial reaction centers.
Abstract: The heart of oxygenic photosynthesis is photosystem II (PSII), a multisubunit protein complex that uses solar energy to drive the splitting of water and production of molecular oxygen. The effectiveness of the photochemical reaction center of PSII depends on the efficient transfer of excitation energy from the surrounding antenna chlorophylls. A kinetic model for PSII, based on the x-ray crystal structure coordinates of 37 antenna and reaction center pigment molecules, allows us to map the major energy transfer routes from the antenna chlorophylls to the reaction center chromophores. The model shows that energy transfer to the reaction center is slow compared with the rate of primary electron transport and depends on a few bridging chlorophyll molecules. This unexpected energetic isolation of the reaction center in PSII is similar to that found in the bacterial photosystem, conflicts with the established view of the photophysics of PSII, and may be a functional requirement for primary photochemistry in photosynthesis. In addition, the model predicts a value for the intrinsic photochemical rate constant that is 4 times that found in bacterial reaction centers.
TL;DR: In this paper, a mutant lacking TyrZ was used to investigate electron donation from TyrD by using EPR and time-resolved absorption spectroscopy, and it was shown that reduced TyrD is capable of donating an electron to P680+ with t1/2 ≈ 190 ns at pH 85 in approximately half of the centers.
Abstract: Two symmetrically positioned redox active tyrosine residues are present in the photosystem II (PSII) reaction center One of them, TyrZ, is oxidized in the ns–μs time scale by P680+ and reduced rapidly (μs to ms) by electrons from the Mn complex The other one, TyrD, is stable in its oxidized form and seems to play no direct role in enzyme function Here, we have studied electron donation from these tyrosines to the chlorophyll cation (P680+) in Mn-depleted PSII from plants and cyanobacteria In particular, a mutant lacking TyrZ was used to investigate electron donation from TyrD By using EPR and time-resolved absorption spectroscopy, we show that reduced TyrD is capable of donating an electron to P680+ with t1/2 ≈ 190 ns at pH 85 in approximately half of the centers This rate is ≈105 times faster than was previously thought and similar to the TyrZ donation rate in Mn-depleted wild-type PSII (pH 85) Some earlier arguments put forward to rationalize the supposedly slow electron donation from TyrD (compared with that from TyrZ) can be reassessed At pH 65, TyrZ (t1/2 = 2–10 μs) donates much faster to P680+ than does TyrD (t1/2 > 150 μs) These different rates may reflect the different fates of the proton released from the respective tyrosines upon oxidation The rapid rate of electron donation from TyrD requires at least partial localization of P680+ on the chlorophyll (PD2) that is located on the D2 side of the reaction center
TL;DR: This work has found that the chlorophyll-binding proteins in a deep-living strain of this oxyphotobacterium form a ring around a trimer of the photosystem I (PS I) photosynthetic reaction centre, a clever arrangement that maximizes the capture of light energy in such dim conditions.
Abstract: The oceanic picoplankton Prochlorococcus - probably the most abundant photosynthetic organism on our planet - can grow at great depths where light intensity is very low. We have found that the chlorophyll-binding proteins in a deep-living strain of this oxyphotobacterium form a ring around a trimer of the photosystem I (PS I) photosynthetic reaction centre, a clever arrangement that maximizes the capture of light energy in such dim conditions.
TL;DR: The structure of the higher plant PSII core dimer is reviewed and evidence for the tentative assignment of the low molecular weight subunits is provided.
TL;DR: Comparison of the subunit organization of the higher plant photosystem II core dimer reported here with that of its thermophilic cyanobacterial counterpart recently determined by X-ray crystallography shows significant similarities, indicative of a common evolutionary origin.
TL;DR: Gobets et al. as discussed by the authors studied energy transfer and trapping processes in trimeric PSI complexes of this species at femtosecond resolution by means of the fluorescence-upconversion technique.
Abstract: The photosystem I (PSI) core complex of oxygenic photosynthesis is an integral pigment-protein complex that incorporates both the antenna and the reaction center (RC). It binds about 100 Chl a and 20 ‚-carotene molecules. In the PSI core complex of the cyanobacterium Synechococcus elongatus, a total of about 9 antenna Chl a molecules are red-shifted with respect to the primary electron donor, which absorbs at 700 nm. We have studied energy transfer and trapping processes in trimeric PSI complexes of this species at femtosecond resolution by means of the fluorescence-upconversion technique. By simultaneously analyzing the fluorescence upconversion results and those obtained with a streak camera with picosecond resolution and multichannel detection (Gobets, B.; et al. Biophys. J., in press), we have mapped out the energy transfer processes that follow immediately after photon absorption. Equilibration among Chl a pigments in the bulk antenna was found to occur with a time constant of 360 fs. A major energy equilibration phase between bulk Chl a and the red-shifted antenna Chls occurs in 3.6 ps. A slow phase in energy equilibration takes place in 9.8 ps, after which the excitations are trapped by the RC in 38 ps. Fluorescence anisotropy measurements indicated an initial anisotropy of 0.30, which decayed biphasically with a major fast phase of 160 fs and a minor slow phase of 1.8 ps to a final anisotropy of 0.06. The 160 fs phase is assigned to elementary energy transfer steps in the bulk Chl a antenna, and the 1.8 ps phase to further equilibration processes, possibly involving energy transfer to or among red-shifted Chls. Energy transfer from ‚-carotene to Chl a was found to proceed both from the S2 state and the S1 state, with the majority of transferred excitations (60%) originating from the S 2 state, resulting in an estimated overall yield of 90%. A comparison is made with the PSII core antenna protein CP47, which binds the same pigments but has a substantially lower carotenoid-Chl a energy transfer yield of 35% (van Dorssen R. J.; et al. Biochim. Biophys. Acta1987, 893, 267).
TL;DR: This work emphasize the flexibility of cyanobacterial light-harvesting systems in response to the lowering of phycobilisome and PSI levels under iron-deficient conditions, but it has implications for understanding the organization of the related chlorophyll a/b-binding Pcb proteins of oxychlorobacteria, formerly known as prochlorophytes.
TL;DR: A mathematical model explaining fluorescence was developed, suggesting antenna quenching as the predominant mechanism of dissipation of light energy in the moss Rhytidiadelphus, whereas reaction-center quenched appeared to be important in spinach and Arabidopsis.
Abstract: Dissipation of light energy was studied in the moss Rhytidiadelphus squarrosus (Hedw.) Warnst., and in leaves of Spinacia oleracea L. and Arabidopsis thaliana (L.) Heynh., using chlorophyll fluorescence as an indicator reaction. Maximum chlorophyll fluores cence of 3-(3,4-dichlorophenyl)-l,l-dimethylurea (DCMU)-treated spinach leaves, as produced by satu rating light and studied between + 5 and -20 °C, revealed an activation energy AE of 0.11 eV. As this suggested recombination fluorescence produced by charge recombination between the oxidized primary donor of photosystem II and reduced pheophytin, a mathematical model explaining fluorescence, and based in part on known characteristics of primary electron transport reactions, was developed. The model permit ted analysis of different modes of fluorescence quench ing, two localized in the reaction center of photosystem II and one in the light-harvesting system of the antenna complexes. It predicted differences in the relationship between quenching of variable fluorescence Fv and quenching of basal, so-called F0 fluorescence depending on whether quenching originated from antenna com plexes or from reaction centers. Such differences were found experimentally, suggesting antenna quenching as the predominant mechanism of dissipation of light energy in the moss Rhytidiadelphus, whereas reaction center quenching appeared to be important in spinach and Arabidopsis. Both reaction-center and antenna quenching required activation by thylakoid protonation but only antenna quenching depended on or was strongly enhanced by zeaxanthin. De-protonation per mitted relaxation of this quenching with half-times below 1 min. More slowly reversible quenching, tenta tively identified as so-called q\ or photoinhibitory quenching, required protonation but persisted for pro longed times after de-protonation. It appeared to originate in reaction centers.
TL;DR: High-temperature effects on Photosystem II and plasma membranes, temperature dependence of growth, and acclimation to the growth temperature were studied in a mesophilic cyanobacterium, Synechocystis sp.
Abstract: High-temperature effects on Photosystem II and plasma membranes, temperature dependence of growth, and acclimation to the growth temperature were studied in a mesophilic cyanobacterium, Synechocystis sp. PCC6803. The following results were obtained. (1) Small but distinct temperature acclimation of the PSII reaction center activity was shown for the first time when the activity was measured at inhibitory high temperatures. However, the reaction center activity showed no apparent acclimation when it was measured at growth temperatures after heat stress. (2) Oxygen-evolving activity and the permeability of plasma membranes showed higher resistance to heat when PCC6803 cells were grown at higher temperatures. (3) Acclimation of photosynthesis to the growth temperature seemed to occur so as to maintain photosynthesis activity not at a maximum level but in a certain range at the growth temperatures. (4) Neither sensitivity to high-temperature-induced dissociation of phycobilisomes from the PSII reaction center complexes nor degradation of phycocyanin were altered by changes in the environmental temperature. (5) A close relationship between the viability of cells and the structural changes of plasma membranes (but not the inactivation of photosynthesis) was observed. The denaturation process of PSII complexes and the relationship between the temperature dependence of the growth of Synechocystis PCC6803 cells and that of the photosynthetic activity are also discussed.
TL;DR: The photosystem (PS) I photosynthetic reaction center was modified thorough the selective extraction and exchange of chlorophylls and quinones to enable accurate detection of the primary photoreactions with little disturbance from the absorbance changes of the bulk pigments.
TL;DR: This Review discusses energy transfer pathways in Photosystem I (PS I) from oxygenic organisms and spectrally distinct red pigments in the LHC I are suggested to function largely as photoprotective excitation sinks in the peripheral antenna of PS I.
Abstract: This Review discusses energy transfer pathways in Photosystem I (PS I) from oxygenic organisms. In the trimeric PS I core from cyanobacteria, the efficiency of solar energy conversion is largely determined by ultrafast excitation transfer processes in the core chlorophyll a (Chl a) antenna network and efficient photochemical trapping in the reaction center (RC). The role of clusters of Chl a in energy equilibration and photochemical trapping in the PS I core is discussed. Dimers of the longest-wavelength absorbing (red) pigments with strongest excitonic interactions localize the excitation in the PS I core antenna. Those dimers that are located closer to the RC participate in a fast energy equilibration with coupled pigments of the RC. This suggests that the function of the red pigments is to concentrate the excitation near the RC. In the PS I holocomplex from algae and higher plants, in addition to the red pigments of the core antenna, spectrally distinct red pigments are bound to the peripheral Chl a/b-binding light-harvesting antenna (LHC I), specifically to the Lhca4 subunit of the LHC I-730 complex. Intramonomeric energy equilibration between pools of Chl b and Chl a in Lhca1 and Lhca4 monomers of the LHC I-730 heterodimer are as fast as the energy equilibration processes within the PS I core. In contrast to the structural stability of the PS I core, the flexible subunit structure of the LHC I would probably determine the observed slow excitation energy equilibration processes in the range of tens of picoseconds. The red pigments in the LHC I are suggested to function largely as photoprotective excitation sinks in the peripheral antenna of PS I.