Light Harvesting Complexes

Stark spectroscopy (electroabsorption) is used to study the variation of electronic properties with the size of helical H-aggregates that are formed by the spontaneous noncovalent assembly of co-facial dimers of the cyanine dye (DiSC 2 (5)) into the minor groove of double-helical DNA. The unique and important property of these aggregates, first synthesized and characterized by Armitage and co-workers (J. Am. Chem. Soc. 1999, 121, 2987), is that their size is controlled by the properties of the DNA template. Specifically, the length of the aggregate formed is determined by the length of the DNA template and its width along the π stacking dimension is restricted to that of the dye dimer due to steric constraints in the minor groove. Results for aggregates consisting of 1, 2, 5, and ∼35 adjacent dimers bound to DNA are presented here. The absorption maxima of these species exhibit a large blue shift (1750 cm -1 ) from that of the monomer due to the faceto-face interactions within the dimers. Relatively weak (330-650 cm -1 ) secondary splittings are also seen that arise from end-to-end interactions between adjacent dimers on the chain. The average change in polarizability on excitation (〈∆R〉) is found to double when two dyes form a stacked dimer whereas no further increase in 〈∆R〉 is seen as the chain length is increased. Semiempirical (INDO-SCI) calculations yield exciton coupling energies that are consistent with experiment. However, 〈∆R〉 is predicted to increase toward more positive values on dimerization while the reverse trend is seen experimentally. Nonetheless, both experiment and theory find that 〈∆R〉 is unaffected by higher aggregation.

It has already been established that the quaternary structure of the main light-harvesting complex (LH2) from the photosynthetic bacterium Rhodopseudomonas palustris is a nonameric 'ring' of PucAB heterodimers and under low-light culturing conditions an increased diversity of PucB synthesis occurs. In this work, single molecule fluorescence emission studies show that different classes of LH2 'rings' are present in "low-light" adapted cells and that an unknown chaperon process creates multiple sub-types of 'rings' with more conformational sub-states and configurations. This increase in spectral disorder significantly augments the cross-section for photon absorption and subsequent energy flow to the reaction centre trap when photon availability is a limiting factor. This work highlights yet another variant used by phototrophs to gather energy for cellular development.

Light-harvesting pigment-protein complexes of photosystem II of plants have a dual function: they efficiently use absorbed energy for photosynthesis at limiting sunlight intensity and dissipate the excess energy at saturating intensity for photoprotection. Recent single-molecule spectroscopy studies on the trimeric LHCII complex showed that environmental control of the intrinsic protein disorder could in principle explain the switch between their light-harvesting and photoprotective conformations in vivo. However, the validity of this proposal depends strongly on the specificity of the protein dynamics. Here, a similar study has been performed on the minor monomeric antenna complexes of photosystem II (CP29, CP26, and CP24). Despite their high structural homology, similar pigment content and organization compared to LHCII trimers, the environmental response of these proteins was found to be rather distinct. A much larger proportion of the minor antenna complexes were present in permanently weakly fluorescent states under most conditions used; however, unlike LHCII trimers the distribution of the single-molecule population between the strongly and weakly fluorescent states showed no significant sensitivity to low pH, zeaxanthin, or low detergent conditions. The results support a unique role for LHCII trimers in the regulation of light harvesting by controlled fluorescence blinking and suggest that any contribution of the minor antenna complexes to photoprotection would probably involve a distinct mechanism.

Significance To optimize photosynthetic performance and minimize photooxidative damage, photosynthetic organisms evolved to efficiently balance light energy absorption and electron transport with cellular energy requirements under constantly changing light conditions. The regulation of linear electron flow (LEF) and cyclic electron flow (CEF) contributes to this fine-tuning. Here we present a model of the formation and structural molecular organization of a CEF-performing photosystem I (PSI)–light harvesting complex I (LHCI)–cytochrome (cyt) b 6 f supercomplex from the green alga Chlamydomonas reinhardtii . Such a structural arrangement could modulate the distinct operation of LEF and CEF to optimize light energy utilization, despite the same individual structural units contributing to these two different functional modes.

The largest light-harvesting antenna in nature, the chlorosome, is a heterogeneous helical BChl self-assembly that has evolved
in green bacteria to harvest light for performing photosynthesis in low-light environments. Guided by NMR chemical shifts
and distance constraints for Chlorobaculum tepidum wild-type chlorosomes, the two contrasting packing modes for syn-anti
parallel stacks of BChl c to form polar 2D arrays, with dipole moments adding up, are explored. Layered assemblies were
optimized using local orbital density functional and plane wave pseudopotential methods. The packing mode with the low-
est energy contains syn-anti and anti-syn H-bonding between stacks. It can accommodate R and S epimers, and side chain
variability. For this packing, a match with the available EM data on the subunit axial repeat and optical data is obtained with
multiple concentric cylinders for a rolling vector with the stacks running at an angle of 21° to the cylinder axis and with
the BChl dipole moments running at an angle ß ∼ 55° to the tube axis, in accordance with optical data. A packing mode
involving alternating syn and anti parallel stacks that is at variance with EM appears higher in energy. A weak cross-peak at
-6 ppm in the MAS NMR with 50 kHz spinning, assigned to C-181, matches the shift of antiparallel dimers, which possibly
reflects a minor impurity-type fraction in the self-assembled BChl c.

A quantitative understanding of the photosynthetic machinery depends largely on quantities, such as concentrations, sizes, absorption wavelengths, redox potentials, and rate constants. The present contribution is a collection of numbers and quantities related mainly to photosynthesis in higher plants. All numbers are taken directly from a literature or database source and the corresponding reference is provided. The numerical values, presented in this paper, provide ranges of values, obtained in specific experiments for specific organisms. However, the presented numbers can be useful for understanding the principles of structure and function of photosynthetic machinery and for guidance of future research.

Light-harvesting complex II (LHCII) from the marine green macroalga Bryopsis corticulans is spectroscopically characterized to understand the structural and functional changes resulting from adaptation to intertidal environment. LHCII is homologous to its counterpart in land plants but has a different carotenoid and chlorophyll (Chl) composition. This is reflected in the steady-state absorption, fluorescence, linear dichroism, circular dichroism and anisotropic circular dichroism spectra. Time-resolved fluorescence and two-dimensional electronic spectroscopy were used to investigate the consequences of this adaptive change in the pigment composition on the excited-state dynamics. The complex contains additional Chl b spectral forms-absorbing at around 650 nm and 658 nm-and lacks the red-most Chl a forms compared with higher-plant LHCII. Similar to plant LHCII, energy transfer between Chls occurs on timescales from under hundred fs (mainly from Chl b to Chl a) to several picoseconds (mainly between Chl a pools). However, the presence of long-lived, weakly coupled Chl b and Chl a states leads to slower exciton equilibration in LHCII from B. corticulans. The finding demonstrates a trade-off between the enhanced absorption of blue-green light and the excitation migration time. However, the adaptive change does not result in a significant drop in the overall photochemical efficiency of Photosystem II. These results show that LHCII is a robust adaptable system whose spectral properties can be tuned to the environment for optimal light harvesting.

Photosynthetic organisms possess a highly efficient photo-protective apparatus respon-sible for non-photochemical quenching (NPQ) of the excess excitation energy that helps them to minimize the harmful effects of excess light. The rapidly-reversible part of NPQ, termed qE, is associated with a number of different factors: the pH gradient across the thylakoid membrane, the action of the PsbS protein, the xanthophyll cycle conversion, i.e. de-epoxidation of violaxanthin into zeaxanthin (Zx), conformational changes in the light-harvesting complexes of PS II (LHC II). The exact mechanism of qE is however not known, the quenching sites, their location and molecular origin remain questionable. These questions are addressed in the thesis by time-resolved fluorescence spectroscopy performed on different levels of organization and physiological state of the photosyn-thetic apparatus – from isolated pigment-protein complexes to intact leaves. The single photon counting technique was utilized ...

A key step in the photosynthetic reactions in photosystem 11 of green plants is the transfer of an electron from the singlet-excited chlorophyll molecule called P680 to a nearby pheophytin molecule. The free energy difference of this primary charge separation reaction is determined in isolated photosystem 11 reaction center complexes as a function of temperature by measuring the absolute quantum yield of P680 triplet formation and the time-integrated fluorescence emission yield. The total triplet yield is found to be 0.83 ± 0.05 at 4 K, and it decreases upon raising the temperature to 0.30 at 200 K. It is suggested that the observed triplet states predominantly arise from P680 but to a minor extent also from antenna chlorophyll present in the photosystem 11 reaction center. No carotenoid triplet states could be detected, demonstrating that the contamination of the preparation with CP47 complexes is less than 1/100 reaction centers. The fluorescence yield is 0.07 ± 0.02 at 10 K, and ...

On page 147 of the original publication, last paragraph, it is incorrectly stated that PYP has been found so far only in photosynthetic bacteria. Genome sequencing has revealed the presence of PYP genes in various non-photosynthetic bacteria (see for example the review by Van der Horst et al. 2007).

Photosynthetic eukaryotes show a remarkable variability in photosynthesis, including large differences in light-harvesting proteins and pigment composition. In vivo circular spectropolarimetry enables us to probe the molecular architecture of photosynthesis in a non-invasive and non-destructive way and, as such, can offer a wealth of physiological and structural information. In the present study, we have measured the circular polarizance of several multicellular green, red, and brown algae and higher plants, which show large variations in circular spectropolarimetric signals with differences in both spectral shape and magnitude. Many of the algae display spectral characteristics not previously reported, indicating a larger variation in molecular organization than previously assumed. As the strengths of these signals vary by three orders of magnitude, these results also have important implications in terms of detectability for the use of circular polarization as a signature of life.

Photosystem II (PSII) uses light energy to split water into protons, electrons, and oxygen, ultimately sustaining heterotrophic life on Earth. The major light harvesting complex in plants (LHCII) is packed with chlorophylls and carotenoids and is the main supplier of excitation energy to PSII reaction centers. The protein scaffold acts as a programmed solvent for the pigments in LHCII, tuning their orientations while at the same time impeding concentration quenching to ensure efficient storage of excitation energy by chlorophylls. However, under stress, the very fuel of PSII, solar photons, can damage its delicate inner components and hamper photosynthesis. In a crucial regulatory strategy in plants, LHCII evolved a flexible design that allows it to switch between light-harvesting and dissipative conformations, thereby safely releasing the excess energy that is absorbed into heat. Several mechanisms have been proposed to explain chlorophyll de-excitation pathways in LHCII, such as c...

To maintain high photosynthetic rates plants must adapt to their light environment on timescale of seconds to minutes. Therefore the light harvesting antenna system of photosystem II (LHCII) in thylakoid membranes has a feedback mechanism, which determines the proportion of absorbed energy dissipated as heat: non-photochemical chlorophyll fluorescence quenching (NPQ). This is crucial to prevent photo-oxidative damage to photosystem II and is controlled by the transmembrane pH differences (ΔpH). High ΔpH activates NPQ by protonation of the protein PsbS and the enzymatic de-epoxidation of LHCII bound violaxanthin to zeaxanthin. But the precise role of PsbS and its interactions with different LHCII complexes remain uncertain. We have investigated PsbS-LHCII interactions in native thylakoid membranes using magnetic bead-linked antibody pull-downs. The interaction of PsbS with the antenna system is affected both by ΔpH and the level of zeaxanthin. In the presence of ΔpH alone PsbS is found mainly associated with the trimeric LHCII protein polypeptides, lhcb1, lhcb2 and lhcb3. However a combinations of ΔpH and zeaxanthin increases the proportion of PsbS bound to the minor LHCII antenna complex proteins lhcb4, 5 and 6. This pattern of interactions was not influenced by the presence of PSII reactions centers. Like LHCII particles in the photosynthetic membrane, PsbS protein forms clusters in the NPQ state. NPQ recovery in the dark required uncoupling of PsbS. We suggest that PsbS acts as a 'seeding' center for the LHCII antenna rearrangement involved in NPQ. complexes. PSII light-harvesting complexes (LHCIIs) include the major LHCII trimers (comprising of lhcb1, lhcb2 and lhcb3 polypeptides) and minor LHCII monomers: CP29 (lhcb4), CP26 (lhcb5) and CP24 (lhcb6) complexes 2. Chlorophylls and carotenoids bound to LHCs absorb light energy and then transfer it to the PSII reaction centers (RCII), where charge separation occurs. However, in high light conditions the amount of light absorbed by the antenna exceeds that which can be used by RCII in photosynthesis. This excess absorbed energy can cause photo-oxidative damage to RCII, a condition known as photoinhibition 3-7 , which significantly decreases photosynthetic productivity. NPQ is the most rapid and efficient mechanism of dissipation of excess absorbed light energy into heat 8-11. It is a complex process and a complete understanding of its mechanism has not yet been achieved. In recent years, several crucial factors were found to trigger NPQ formation. The primary and fastest NPQ component, called qE (energy-dependent quenching), depends on the generation of a pH gradient across the thylakoid membranes 8,9,11. The acidification of the thylakoid lumen is essential for NPQ induction 8,12 , most probably by facilitating protonation of certain aspartate and glutamate residues within LHCII complexes 8,10,13 , as well as the conversion of violaxanthin into zeaxanthin 14,15. In addition, a third essential component, the PsbS protein, was discovered by Niyogi and co-workers and shown to respond to proton gradient changes 16,17. The recently published PsbS crystal structure suggests that this four transmembrane helix member of the light-harvesting complex (LHC) superfamily is a non-pigment binding protein, taking it off the list of potential candidates for a direct NPQ quencher 18 .

To maintain high photosynthetic rates plants must adapt to their light environment on timescale of seconds to minutes. Therefore the light harvesting antenna system of photosystem II (LHCII) in thylakoid membranes has a feedback mechanism, which determines the proportion of absorbed energy dissipated as heat: non-photochemical chlorophyll fluorescence quenching (NPQ). This is crucial to prevent photo-oxidative damage to photosystem II and is controlled by the transmembrane pH differences (ΔpH). High ΔpH activates NPQ by protonation of the protein PsbS and the enzymatic de-epoxidation of LHCII bound violaxanthin to zeaxanthin. But the precise role of PsbS and its interactions with different LHCII complexes remain uncertain. We have investigated PsbS-LHCII interactions in native thylakoid membranes using magnetic bead-linked antibody pull-downs. The interaction of PsbS with the antenna system is affected both by ΔpH and the level of zeaxanthin. In the presence of ΔpH alone PsbS is found mainly associated with the trimeric LHCII protein polypeptides, lhcb1, lhcb2 and lhcb3. However a combinations of ΔpH and zeaxanthin increases the proportion of PsbS bound to the minor LHCII antenna complex proteins lhcb4, 5 and 6. This pattern of interactions was not influenced by the presence of PSII reactions centers. Like LHCII particles in the photosynthetic membrane, PsbS protein forms clusters in the NPQ state. NPQ recovery in the dark required uncoupling of PsbS. We suggest that PsbS acts as a 'seeding' center for the LHCII antenna rearrangement involved in NPQ. complexes. PSII light-harvesting complexes (LHCIIs) include the major LHCII trimers (comprising of lhcb1, lhcb2 and lhcb3 polypeptides) and minor LHCII monomers: CP29 (lhcb4), CP26 (lhcb5) and CP24 (lhcb6) complexes 2. Chlorophylls and carotenoids bound to LHCs absorb light energy and then transfer it to the PSII reaction centers (RCII), where charge separation occurs. However, in high light conditions the amount of light absorbed by the antenna exceeds that which can be used by RCII in photosynthesis. This excess absorbed energy can cause photo-oxidative damage to RCII, a condition known as photoinhibition 3-7 , which significantly decreases photosynthetic productivity. NPQ is the most rapid and efficient mechanism of dissipation of excess absorbed light energy into heat 8-11. It is a complex process and a complete understanding of its mechanism has not yet been achieved. In recent years, several crucial factors were found to trigger NPQ formation. The primary and fastest NPQ component, called qE (energy-dependent quenching), depends on the generation of a pH gradient across the thylakoid membranes 8,9,11. The acidification of the thylakoid lumen is essential for NPQ induction 8,12 , most probably by facilitating protonation of certain aspartate and glutamate residues within LHCII complexes 8,10,13 , as well as the conversion of violaxanthin into zeaxanthin 14,15. In addition, a third essential component, the PsbS protein, was discovered by Niyogi and co-workers and shown to respond to proton gradient changes 16,17. The recently published PsbS crystal structure suggests that this four transmembrane helix member of the light-harvesting complex (LHC) superfamily is a non-pigment binding protein, taking it off the list of potential candidates for a direct NPQ quencher 18 .

Bacteriophytochromes are red/far-red photoreceptors that bacteria use to mediate sensory responses to their light environment. Here, we show that the photosynthetic bacterium Rhodopseudomonas palustris has two distinct types of bacteriophytochrome-related protein (RpBphP4) depending upon the strain considered. The first type binds the chromophore biliverdin and acts as a light-sensitive kinase, thus behaving as a bona fide bacteriophytochrome. However, in most strains, RpBphP4 does not to bind this chromophore. This loss of light sensing is replaced by a redox-sensing ability coupled to kinase activity. Phylogenetic analysis is consistent with an evolutionary scenario, where a bacteriophytochrome ancestor has adapted from light to redox sensing. Both types of RpBphP4 regulate the synthesis of light harvesting (LH2) complexes according to the light or redox conditions, respectively. They modulate the affinity of a transcription factor binding to the promoter regions of LH2 complex genes by controlling its phosphorylation status. This is the first complete description of a bacteriophytochrome signal transduction pathway involving a two-component system.

investigate the presence of quantum entanglement in the Fenna-Matthews-Olson complex (FMO), a protein complex in the photosynthetic pathway of green sulfur bacteria which is involved in exciton transport at nearly 100% efficiency. We present a novel optimization algorithm for calculating entanglement in open systems, and apply it to 5-site entanglement calculations in FMO simulations. We find that significant entanglement exists if exactly one exciton is assumed to reside in the FMO at all times, and that this entanglement can be described almost exclusively using bipartite entanglement monogamy, without resort to multipartite entanglement measures. Our results support the hypothesis that entanglement occurs primarily along the transport pathways in the FMO. However, we also find that the entanglement quickly goes to zero if one includes non-zero amplitudes in the two-exciton subspace, indicating that further work is required to understand the mechanism by which excitons enter the FMO.

When cyanobacteria are grown under iron-limited or other oxidative stress conditions the iron stress inducible pigment-protein IsiA is synthesized in variable amounts. IsiA accumulates in aggregates inside the photosynthetic membrane that strongly dissipate chlorophyll excited state energy. In this paper we applied Stark fluorescence (SF) spectroscopy at 77K to IsiA aggregates to gain insight into the nature of the emitting and energy dissipating state(s). Our study shows that two emitting states are present in the system, one emitting at 684nm and the other emitting at about 730nm. The new 730nm state exhibits strongly reduced fluorescence (F) together with a large charge transfer character. We discuss these findings in the light of the energy dissipation mechanisms involved in the regulation of photosynthesis in plants, cyanobacteria and diatoms. Our results suggest that photosynthetic organisms have adopted common mechanisms to cope with the deleterious effects of excess light un...

Photosynthesis is the process conducted by plants and algae to capture photons and store their energy in chemical forms. The light-harvesting, excitation transfer, charge separation and electron transfer in photosystem II (PSII) are the critical initial reactions of photosynthesis and thereby largely determine its overall efficiency. In this review, we outline the rapidly accumulating knowledge about the architectures and assemblies of plant and green algal PSII–light harvesting complex II (LHCII) supercomplexes, with a particular focus on new insights provided by the recent high-resolution cryo-electron microscopy map of the supercomplexes from a green alga Chlamydomonas reinhardtii. We make pair-wise comparative analyses between the supercomplexes from plants and green algae to gain insights about the evolution of the PSII–LHCII supercomplexes involving the peripheral small PSII subunits that might have been acquired during the evolution and about the energy transfer pathways that...

In green algae, light-harvesting complex stress-related 3 (LHCSR3) is responsible for the pH-dependent dissipation of absorbed light energy, a function vital for survival under highlight conditions. LHCSR3 binds the photosystem II and lightharvesting complex II (PSII-LHCII) supercomplex and transforms it into an energy-dissipative form under acidic conditions, but the molecular mechanism remains unclear. Here we show that in the green alga Chlamydomonas reinhardtii, LHCSR3 modulates the excitation energy flow and dissipates the excitation energy within the light-harvesting complexes of the PSII supercomplex. Using fluorescence decay-associated spectra analysis, we found that, when the PSII supercomplex is associated with LHCSR3 under highlight conditions, excitation energy transfer from light-harvesting complexes to chlorophyllbinding protein CP43 is selectively inhibited compared with that to CP47, preventing excess excitation energy from overloading the reaction center. By analyzing femtosecond up-conversion fluorescence kinetics, we further found that pH-and LHCSR3dependent quenching of the PSII-LHCII-LHCSR3 supercomplex is accompanied by a fluorescence emission centered at 684 nm, with a decay time constant of 18.6 ps, which is equivalent to the rise time constant of the lutein radical cation generated within a chlorophyll-lutein heterodimer. These results suggest a mechanism in which LHCSR3 transforms the PSII supercomplex into an energy-dissipative state and provide critical insight into the molecular events and characteristics in LHCSR3-dependent energy quenching.

An intriguing molecular architecture called the “semi-crystalline photosystem II (PSII) array” has been observed in the thylakoid membranes in vascular plants. It is an array of PSII–light-harvesting complex II (LHCII) supercomplexes that only appears in low light, but its functional role has not been clarified. Here, we identified PSII–LHCII supercomplexes in their monomeric and multimeric forms in low light–acclimated spinach leaves and prepared them using sucrose-density gradient ultracentrifugation in the presence of amphipol A8-35. When the leaves were acclimated to high light, only the monomeric forms were present, suggesting that the multimeric forms represent a structural adaptation to low light and that disaggregation of the PSII–LHCII supercomplex represents an adaptation to high light. Single-particle EM revealed that the multimeric PSII–LHCII supercomplexes are composed of two (“megacomplex”) or three (“arraycomplex”) units of PSII–LHCII supercomplexes, which likely cons...

We analyze a system of two coupled cavities, where one of them is leaky, namely, is interacting with the environment. We derive the master equation for the system, and show that the relaxation term, in the case of strongly coupled cavities, is not simply the standard one, which is obtained by neglecting the interaction between the two cavities, when calculating the effect of the bath. The relaxation term is calculated here by first diagonalizing the Hamiltonian of the coupled modes of the cavities, and then it is properly evaluated by eliminating the degrees of freedom of the reservoir, maintaining only the energy-conserving terms. Only by following this prescription one can guarantee that the coupled system approaches thermal equilibrium. We study both the steady-state solutions of the master equation, and the time evolution of the system from given initial conditions.

We review recent theoretical calculations of quantum entanglement in photosynthetic light harvesting complexes. These works establish, for the first time, a manifestation of this characteristically quantum mechanical phenomenon in biologically functional structures. We begin by summarizing calculations on model biomolecular systems that aim to reveal non-trivial characteristics of quantum entanglement in non-equilibrium biological environments. We then discuss and compare several calculations performed recently of excitonic dynamics in the Fenna-Matthews-Olson light harvesting complex and of the entanglement present in this widely studied pigment-protein structure. We point out the commonalities between the derived results and also identify and explain the differences. We also discuss recent work that examines entanglement in the structurally more intricate light harvesting complex II (LHCII). During this overview, we take the opportunity to clarify several subtle issues relating to entanglement in such biomolecular systems, including the role of entanglement in biological function, the complexity of dynamical modeling that is required to capture the salient features of entanglement in such biomolecular systems, and the relationship between entanglement and other quantum mechanical features that are observed and predicted in light harvesting complexes. Finally, we suggest possible extensions of the current work and also review the options for experimental confirmation of the predicted entanglement phenomena in light harvesting complexes.

Prasiola crispa, an aerial green alga, forms layered colonies under the severe terrestrial conditions of Antarctica. Since only far-red light is available at a deep layer of the colony, P. crispa has evolved a molecular system for photosystem II (PSII) excitation using far-red light with uphill energy transfer. However, the molecular basis underlying this system remains elusive. Here, we purified a light-harvesting chlorophyll (Chl)-binding protein complex from P. crispa (Pc-frLHC) that excites PSII with far-red light and revealed its ringshaped structure with undecameric 11-fold symmetry at 3.13 Å resolution. The primary structure suggests that Pc-frLHC evolved from LHCI rather than LHCII. The circular arrangement of the Pc-frLHC subunits is unique among eukaryote LHCs and forms unprecedented Chl pentamers at every subunit-subunit interface near the excitation energy exit sites. The Chl pentamers probably contribute to far-red light absorption. Pc-frLHC's unique Chl arrangement likely promotes PSII excitation with entropy-driven uphill excitation energy transfer. The capture of light energy and its transfer to photosynthetic reaction centers are the primary photosynthetic processes performed in lightharvesting proteins with photosynthetic pigments such as chlorophylls and carotenoids. To adapt to the various light conditions that exist in diverse micro-environments on Earth, pigmented species and their binding proteins became highly diversified during their evolution 1,2. Eukaryotic photoautotrophs commonly use chlorophyll a (Chl a)based light-harvesting complexes (LHCs) 2,3. Subunits of LHCs each

A mathematical modeling is applied to illuminate and predict the mechanism of electron transfer in algae photosynthesis. A magnetic field can accelerate its reaction by decreasing the frequency of reverse reactions in the radical pair mechanism. The effect of magnetic field (MF) changes the rotational dynamic of the radical pair and in general alters the yield and lifetimes of its product. The external magnetic field applied to the system causes a radical pair reaction change. This phenomenon can be explained by a rate constant K 3 for dual-reaction coordinate movement or triplet state formulation and according to the effect of the function rate dependent on magnetic K ISC. To achieve a desired magnetic effect on the system, state that the of K 3 and rates must be equally large to observe the effect of the magnetic field on the reaction rate. In systems with forward reaction rate at 3 >> , the modeling result indicates that the maximum rate of effectiveness, () is likely to occur at ≈7.85 militesla with () value at 2.63x10-10 second. As simulated into the system, the MF influences reach at peak values of between 7.25mT (6.24 x 10-9 second, reaction rate) to 8.25mT (3.94 x 10-10 second, reaction rate). The significant result of the MF influences towards the microalgae growth production is indicated the range of 110-10 to 7.75 militesla is observed to have influence on the rate of ().

Photosynthetic light-harvesting antennae are pigment-binding proteins that perform one of the most fundamental tasks on Earth, capturing light and transferring energy that enables life in our biosphere. Adaptation to different light environments led to the evolution of an astonishing diversity of light-harvesting systems. At the same time, several strategies have been developed to optimize the light energy input into photosynthetic membranes in response to fluctuating conditions. The basic feature of these prompt responses is the dynamic nature of antenna complexes, whose function readily adapts to the light available. High-resolution microscopy and spectroscopic studies on membrane dynamics demonstrate the crosstalk between antennae and other thylakoid membrane components. With the increased understanding of light-harvesting mechanisms and their regulation, efforts are focusing on the development of sustainable processes for effective conversion of sunlight into functional bio-prod...

Chlorophylls (Chl) are important pigments in plants that are used to absorb photons and release electrons. There are several types of Chls but terrestrial plants only possess two of these: Chls a and b. The two pigments form light-harvesting Chl a/b-binding protein complexes (LHC), which absorb most of the light. The peak wavelengths of the absorption spectra of Chls a and b differ by c. 20 nm, and the ratio between them (the a/b ratio) is an important determinant of the light absorption efficiency of photosynthesis (i.e., the antenna size). Here, we investigated why Chl b is used in LHCs rather than other light-absorbing pigments that can be used for photosynthesis by considering the solar radiation spectrum under field conditions. We found that direct and diffuse solar radiation (PARdir and PARdiff, respectively) have different spectral distributions, showing maximum spectral photon flux densities (SPFD) at c. 680 and 460 nm, respectively, during the daytime. The spectral absorban...

Photosynthetic light-harvesting antennae are pigment-binding proteins that perform one of the most fundamental tasks on Earth, capturing light and transferring energy that enables life in our biosphere. Adaptation to different light environments led to the evolution of an astonishing diversity of light-harvesting systems. At the same time, several strategies have been developed to optimize the light energy input into photosynthetic membranes in response to fluctuating conditions. The basic feature of these prompt responses is the dynamic nature of antenna complexes, whose function readily adapts to the light available. High-resolution microscopy and spectroscopic studies on membrane dynamics demonstrate the crosstalk between antennae and other thylakoid membrane components. With the increased understanding of light-harvesting mechanisms and their regulation, efforts are focusing on the development of sustainable processes for effective conversion of sunlight into functional bio-prod...

In the present paper, we construct QMC (Quantum Markov Chains) associated with Open Quantum Random Walks such that the transition operator of the chain is defined by OQRW and the restriction of QMC to the commutative subalgebra coincides with the distribution of OQRW. Furthermore, we first propose a new construction of QMC on trees, which is an extension of QMC considered in Ref. [9]. Using such a construction, we are able to construct QMCs on tress associated with OQRW. Our investigation leads to the detection of the phase transition phenomena within the proposed scheme. This kind of phenomena appears first time in this direction. Moreover, mean entropies of QMCs are calculated.

Chlorophylls (Chl) are important pigments in plants that are used to absorb photons and release electrons. There are several types of Chls but terrestrial plants only possess two of these: Chls a and b. The two pigments form light-harvesting Chl a/b-binding protein complexes (LHC), which absorb most of the light. The peak wavelengths of the absorption spectra of Chls a and b differ by c. 20 nm, and the ratio between them (the a/b ratio) is an important determinant of the light absorption efficiency of photosynthesis (i.e., the antenna size). Here, we investigated why Chl b is used in LHCs rather than other light-absorbing pigments that can be used for photosynthesis by considering the solar radiation spectrum under field conditions. We found that direct and diffuse solar radiation (PARdir and PARdiff, respectively) have different spectral distributions, showing maximum spectral photon flux densities (SPFD) at c. 680 and 460 nm, respectively, during the daytime. The spectral absorban...

Photosynthetic light-harvesting antennae are pigment-binding proteins that perform one of the most fundamental tasks on Earth, capturing light and transferring energy that enables life in our biosphere. Adaptation to different light environments led to the evolution of an astonishing diversity of light-harvesting systems. At the same time, several strategies have been developed to optimize the light energy input into photosynthetic membranes in response to fluctuating conditions. The basic feature of these prompt responses is the dynamic nature of antenna complexes, whose function readily adapts Update

Photosystem II (PSII) is the pigment–protein complex driving the photoinduced oxidation of water and reduction of plastoquinone in all oxygenic photosynthetic organisms. Excitations in the antenna chlorophylls are photochemically trapped in the reaction center (RC) producing the chlorophyll–pheophytin radical ion pair P+ Pheo−. When electron donation from water is inhibited, the oxidized RC chlorophyll P+ acts as an excitation quencher, but knowledge on the kinetics of quenching is limited. Here, we used femtosecond transient absorption spectroscopy to compare the excitation dynamics of PSII with neutral and oxidized RC (P+). We find that equilibration in the core antenna has a major lifetime of about 300 fs, irrespective of the RC redox state. Two-dimensional electronic spectroscopy revealed additional slower energy equilibration occurring on timescales of 3–5 ps, concurrent with excitation trapping. The kinetics of PSII with open RC can be described well with previously proposed m...

Light-harvesting complex II (LHCII) from the marine green macroalga Bryopsis corticulans is spectroscopically characterized to understand the structural and functional changes resulting from adaptation to intertidal environment. LHCII is homologous to its counterpart in land plants but has a different carotenoid and chlorophyll (Chl) composition. This is reflected in the steady-state absorption, fluorescence, linear dichroism, circular dichroism and anisotropic circular dichroism spectra. Time-resolved fluorescence and two-dimensional electronic spectroscopy were used to investigate the consequences of this adaptive change in the pigment composition on the excited-state dynamics. The complex contains additional Chl b spectral forms-absorbing at around 650 nm and 658 nm-and lacks the red-most Chl a forms compared with higher-plant LHCII. Similar to plant LHCII, energy transfer between Chls occurs on timescales from under hundred fs (mainly from Chl b to Chl a) to several picoseconds (mainly between Chl a pools). However, the presence of long-lived, weakly coupled Chl b and Chl a states leads to slower exciton equilibration in LHCII from B. corticulans. The finding demonstrates a trade-off between the enhanced absorption of blue-green light and the excitation migration time. However, the adaptive change does not result in a significant drop in the overall photochemical efficiency of Photosystem II. These results show that LHCII is a robust adaptable system whose spectral properties can be tuned to the environment for optimal light harvesting.

The carotenoid triplet populations associated with the fluorescence emission chlorophyll forms of Photosystem II have been investigated in isolated spinach thylakoid membranes by means of fluorescence detected magnetic resonance in zero field (FDMR). The spectra collected in the 680-690 nm emission range, have been fitted by a global analysis procedure. At least five different carotenoid triplet states coupled to the terminal emitting chlorophyll forms of PS II, peaking at 682 nm, 687 nm and 692 nm, have been characterised. The triplets associated with the outer antenna emission forms, at 682 nm, have zero field splitting parameters |D| = 0.0385 cm)1 , |E| = 0.00367 cm)1 ; |D| = 0.0404 cm)1 , |E| = 0.00379 cm)1 and |D| = 0.0386 cm)1 , |E| = 0.00406 cm)1 which are very similar to those previously reported for the xanthophylls of the isolated LHC II complex. Therefore the FDMR spectra recorded in this work provide insights into the organisation of the LHC II complex in the unperturbed environment represented by thylakoid membranes. The additional carotenoid triplet populations, detected by monitoring the chlorophyll emission at 687 and 692 nm, are assigned to carotenoids bound to inner antenna complexes and hence attributed to b-carotene molecules.

Photosynthetic light-harvesting antennae are pigment-binding proteins that perform one of the most fundamental tasks on Earth, capturing light and transferring energy that enables life in our biosphere. Adaptation to different light environments led to the evolution of an astonishing diversity of light-harvesting systems. At the same time, several strategies have been developed to optimize the light energy input into photosynthetic membranes in response to fluctuating conditions. The basic feature of these prompt responses is the dynamic nature of antenna complexes, whose function readily adapts Update

The effects of a five-day treatment with low light intensity on tomato plants—Ailsa Craig and tangerine mutant—at normal and low temperatures and after recovery for three days under control conditions were investigated. The tangerine tomato, which has orange fruits, yellowish young leaves, and pale blossoms, accumulates prolycopene rather than all-trans lycopene. We investigated the impact of low light at normal and low temperatures on the functioning and effectiveness of photosynthetic apparatuses of both plants. The photochemical activities of Photosystem I (PSI) and Photosystem II (PSII) were assessed, and the alterations in PSII antenna size were characterized by evaluating the abundance of PSII-associated proteins Lhcb1, Lhcb2, CP43, and CP47. Alterations in energy distribution and interaction of both photosystems were analyzed using 77K fluorescence. In Aisla Craig plants, an increase in thylakoid membrane fluidity was detected during treatment with low light at a low temperat...

Fluorescence detected magnetic resonance (FDMR) of various chlorophyll a triplets in PS II large particles was studied with the aim of identifying the nature of the different components underlying the complex line shape of the signals. The latter are deconvoluted into sums of Gaussians in different experimental conditions, and the charge recombination triplet is unequivocally identified as the source of two lines with negative signs at 722.5 and 992 MHz (I D [ = 0.0286 cm-1, I E I = 0.0043 cm-1). The origin of the other triplet signals from the integrated particles is discussed in terms of functional antenna components and products from alteration of the native environment of pigments. FDMR signals also arise from isolated reaction centre complexes and are all to be attributed to different triplet populations of the primary electron donor P680. The results are compared with those obtained by other authors on ADMR. The heterogeneity of P680 pigments in D1D2-cyt-bs59 complexes is shown to arise during preparation procedures.

A photosystem II (PSII) core complex lacking the internal antenna CP43 protein was isolated from the photosystem II of Synechocystis PCC6803, which lacks photosystem I (PSI). CP47-RC and reaction centre (RCII) complexes were also obtained in a single procedure by direct solubilization of whole thylakoid membranes. The CP47-RC subcore complex was characterized by SDS/PAGE, immunoblotting, MALDI MS, visible and fluorescence spectroscopy, and absorption detected magnetic resonance. The purity and functionality of RCII was also assayed. These preparations may be useful for mutational analysis of PSII RC and CP47-RC in studying primary reactions of oxygenic photosynthesis.

The carotenoid triplet populations associated with the fluorescence emission chlorophyll forms of Photosystem II have been investigated in isolated spinach thylakoid membranes by means of fluorescence detected magnetic resonance in zero field (FDMR). The spectra collected in the 680-690 nm emission range, have been fitted by a global analysis procedure. At least five different carotenoid triplet states coupled to the terminal emitting chlorophyll forms of PS II, peaking at 682 nm, 687 nm and 692 nm, have been characterised. The triplets associated with the outer antenna emission forms, at 682 nm, have zero field splitting parameters |D| = 0.0385 cm)1 , |E| = 0.00367 cm)1 ; |D| = 0.0404 cm)1 , |E| = 0.00379 cm)1 and |D| = 0.0386 cm)1 , |E| = 0.00406 cm)1 which are very similar to those previously reported for the xanthophylls of the isolated LHC II complex. Therefore the FDMR spectra recorded in this work provide insights into the organisation of the LHC II complex in the unperturbed environment represented by thylakoid membranes. The additional carotenoid triplet populations, detected by monitoring the chlorophyll emission at 687 and 692 nm, are assigned to carotenoids bound to inner antenna complexes and hence attributed to b-carotene molecules.

Chlorophylls (Chls) are the most abundant plant pigments on Earth. Chls are located in the membrane of thylakoids where they constitute the two photosystems (PSII and PSI) of terrestrial plants, responsible for both light absorption and transduction of chemical energy via photosynthesis. The high efficiency of photosystems in terms of light absorption correlates with the need to protect themselves against absorption of excess light, a process that leads to the so-called photoinhibition. Dynamic photoinhibition consists of the downregulation of photosynthesis quantum yield and a series of photo-protective mechanisms aimed to reduce the amount of light reaching the chloroplast and/or to counteract the production of reactive oxygen species (ROS) that can be grouped in: (i) the first line of chloroplast defence: non-photochemical quenching (NPQ), that is, the dissipation of excess excitation light as heat, a process that takes place in the external antennae of PSII and in which other pigments, that is carotenoids, are directly involved; (ii) the second line of defence: enzymatic antioxidant and antioxidant molecules that scavenge the generated ROS; alternative electron transport (cyclic electron transport, pseudo-cyclic electron flow, chlororespiration and water-water cycle) can efficiently prevent the over-reduction of electron flow, and reduced ferredoxin (Fd) plays a key role in this context.

The review summarizes results concerning photosynthetic systems with chlorophylls and carotenoids obtained by means of spectral methods such as polarized radiation, photoacoustic spectroscopy, delayed luminescence, thermal deactivation, and leading to construction of model systems.

Laser-flash-induced transient absorption measurements were performed on trimeric light-harvesting complex 11 to study carotenoid (Car) and chlorophyll (Chi) triplet states as a function of temperature. In these complexes efficient transfer of triplets from Chl to Car occurs as a protection mechanism against singlet oxygen formation. It appears that at room temperature all triplets are being transferred from Chi to Car; at lower temperatures (77 K and below) the transfer is less efficient and chlorophyll triplets can be observed. In the presence of oxygen at room temperature the Car triplets are partly quenched by oxygen and two different Car triplet spectral species can be distinguished because of a difference in quenching rate. One of these spectral species is replaced by another one upon cooling to 4 K, demonstrating that at least three carotenoids are in close contact with chlorophylls. The triplet minus singlet absorption (T-S) spectra show maxima at 504-506 nm and 517-523 nm, respectively. In the Chi Qy region absorption changes can be observed that are caused by Car triplets. The T-S spectra in the Chl region show an interesting temperature dependence which indicates that various Car's are in contact with different Chl a molecules. The results are discussed in terms of the crystal structure of light-harvesting complex 11.

A spectral and functional assignment of the xanthophylls in monomeric and trimeric lightharvesting complex II of green plants has been obtained using HPLC analysis of the pigment composition, laser-flash induced triplet-minus-singlet, fluorescence excitation, and absorption spectra. It is shown that violaxanthin is not present in monomeric preparations, that it has most likely a red-most absorption maximum at 510 nm in the trimeric complex, and that it is involved in both light-harvesting and Chltriplet quenching. Two xanthophylls (per monomer) have an absorption maximum at 494 nm. These play a major role in both singlet and triplet transfer. These two are most probably the two xanthophylls resolved in the crystal structure, tentatively assigned to lutein, that are close to several chlorophyll molecules [Kühlbrandt, W., Wang, N. D., & Fujiyoshi, Y. (1994) Nature 367, 614-621]. A last xanthophyll contribution, with an absorption maximum at 486 nm, does not seem to play a significant role in lightharvesting or in Chl-triplet quenching. On the basis of the assumption that the two structurally resolved xanthophylls are lutein, this 486 nm absorbing xanthophyll should be neoxanthin. The measurements demonstrate that violaxanthin is connected to at least one chlorophyll a with an absorption maximum near 670 nm, whereas the xanthophylls absorbing at 494 nm are connected to at least one chlorophyll a with a peak near 675 nm.

Time-resolved fluorescence of the LHC I-730 complex and its monomeric subunits of the light-harvesting complex (LHC) of photosystem I was studied in complexes reconstituted from Lhca1 and Lhca4 apoproteins and HPLC purified chlorophyll a, chlorophyll b, and carotenoids [