Pigment-protein Complexes Research Articles

Exploiting Structured Environments for Efficient Energy Transfer: The Phonon Antenna Mechanism.

A nontrivial interplay between quantum coherence and dissipative environment-driven dynamics is becoming increasingly recognized as the key for efficient energy transport in photosynthetic pigment-protein complexes, and converting these biologically inspired insights into a set of design principles that can be implemented in artificial light-harvesting systems has become an active research field. Here we identify a specific design principle, the phonon antenna, by which interpigment coherence is able to modify and optimize the way that excitations spectrally sample their local environmental fluctuations. We provide numerical simulations that suggest that the Fenna-Matthews-Olson complex of green sulfur bacteria has an excitonic structure that is close to such an optimal operating point, and place the phonon antenna concept into a broader context that leads us to conjecture that this general design principle might well be exploited in other biomolecular systems.

Carotenoid–Chlorophyll Coupling and Fluorescence Quenching Correlate with Protein Packing Density in Grana-Thylakoids

The regulation of light-harvesting in photosynthesis under conditions of varying solar light irradiation is essential for the survival and fitness of plants and algae. It has been proposed that rearrangements of protein distribution in the stacked grana region of thylakoid membranes connected to changes in the electronic pigment-interaction play a key role for this regulation. In particular, carotenoid-chlorophyll interactions seem to be crucial for the down-regulation of photosynthetic light-harvesting. So far, it has been difficult to determine the influence of the dense protein packing found in native photosynthetic membrane on these interactions. We investigated the changes of the electronic couplings between carotenoids and chlorophylls and the quenching in grana thylakoids of varying protein packing density by two-photon spectroscopy, conventional chlorophyll fluorometry, low-temperature fluorescence spectroscopy, and electron micrographs of freeze-fracture membranes. We observed an increasing carotenoid-chlorophyll coupling and fluorescence quenching with increasing packing density. Simultaneously, the antennas size and excitonic connectivity of Photosystem II increased with increasing quenching and carotenoid-chlorophyll coupling whereas isolated, decoupled LHCII trimers decreased. Two distinct quenching data regimes could be identified that show up at different protein packing densities. In the regime corresponding to higher protein packing densities, quenching is strongly correlated to carotenoid-chlorophyll interactions whereas in the second regime, a weak correlation is apparent with low protein packing densities. Native membranes are in the strong-coupling data regime. Consequently, PSII and LHCII in grana membranes of plants are already quenched by protein crowding. We concluded that this ensures efficient electronic connection of all pigment-protein complexes for intermolecular energy transfer to the reaction centers and allows simultaneously sensitive regulation of light harvesting by only small changes in the protein packaging.

Mobilization of Photosystem II-Light Harvesting Complex II Supercomplexes during High Light Illumination and State Transitions

The photosystem II (PSII) light harvesting complex (LHC) consists of a variety of pigment protein complexes which are involved in structural organization and regulation of photosynthetic unit. These LHC proteins encoded by a group of Lhcb genes are essential for the structural integrity of PSII supercomplex, the channeling the excitation energy to the reaction center of PSII and its redistribution to photosystem I by state transitions. Numerous studies with the help of recent technological advancements have enabled a significant progress in our understanding on the structure of PSII-LHCII supercomplexes and their mobilization under various light conditions. Here, we present a mini-review on the latest concepts and models depicting the structure of PSII-LHCII supercomplexes and the role of Lhcb proteins in their supra-molecular organization. Also we will review on the current understandings and remaining problems involved in the mobilization of the supercomplexes during state transitions and during high light illumination for controlling light energy distribution between the two photosystems.

Structural and functional reorganization of the photosynthetic apparatus in adaptation to cold of wheat plants

In seedlings of a cold-resistant wheat variety, the dynamics was studied of the main structural-functional parameters of the photosynthetic apparatus (PSA) and of cold resistance of leaf cells in low-temperature plant adaptation. It has been established that a complex of structural-functional PSA changes takes place in seedling leaves under the influence of cold. As a result, as early as in the first hours of hardening, the formation of chloroplasts begins to occur in mesophyll cells of larger sizes and with a thylakoid system of the “sun type.” Owing to structural and functional readjustment (a change of content of pigments, stabilization of pigment-protein complexes, and enhancement of nonphotochemical quenching of excess energy) in the process of cold adaptation, the rate of photosynthesis stabilizes. It is suggested that the observed structural-functional PSA rearrangement is a necessary condition for formation of increased cold resistance of leaf cells; this, alongside with other physiological-biochemical changes occurring in parallel in cells and tissues of the plants, provides their survival under conditions of low temperatures.

Trehalose Preserves the Integrity of Lyophilized Phycoerythrin–AntiHuman CD8 Antibody Conjugates and Enhances their Thermal Stability in Flow Cytometric Assays

An increasing number of publications report on the efficacy of trehalose in preserving organisms, cells, and macromolecules from adverse environmental conditions such as extreme temperatures and dryness. Although the mechanism by which this disaccharide exerts its protection is still debated, the implementation of trehalose as stabilizer is becoming a praxis in several preparative protocols from the pharmaceutical industry. We tested the ability of trehalose in protecting R-Phycoerythrin (R-PE), a pigment-protein complex widely used as fluorescent marker, from thermal denaturation. Once embedded into a dried trehalose matrix, R-PE retains its optical absorption-emission characteristics even when exposed to 70°C for h or when subjected to freeze-drying. We subsequently examined the protection exerted by trehalose on freeze-dried antihuman CD8-RPE (CD8-RPE) conjugated antibodies. Flow cytometric analysis showed that colyophilized trehalose-CD8-RPE preparations can be exposed for 4 weeks at 45°C without significant loss of functionality. Remarkably, even following 4 weeks incubation at 70°C, the preparations are still able to specifically recognize CD8(+) lymphocyte populations. These results show that colyophilization with trehalose makes possible the preparation of antibody-based diagnostic kits which can withstand breaks in the "cold chain" distribution, particularly suited for use in less-developed countries of the tropical areas.

Spatial and Electronic Correlations in the PE545 Light-Harvesting Complex.

The recent discovery of long-lasting quantum coherence effects in photosynthetic pigment-protein complexes has challenged our view of the role that protein motions play in light-harvesting processes. Several groups have suggested that correlated fluctuations involving the pigments site energies and couplings could be at the origin of such unexpected behavior. Here we combine molecular dynamics simulations with quantum mechanics/molecular mechanics calculations to analyze the degree of correlated fluctuations in the PE545 complex of Rhodomonas sp. strain CS24. We find that correlations between the motions of the chromophores, which are significantly assisted by the water solvent, do not translate into appreciable site energy correlations but do lead to significant cross-correlations of energies and couplings. Such behavior, not observed in a recent study on the Fenna-Mathews-Olson complex, seems to provide phycobiliproteins with an additional fundamental mechanism to control quantum coherence and light-harvesting efficiency compared with chlorophyll-containing complexes.

Reassociation of All-trans-3,4-Dihydroanhydrorhodovibrin with LH1 Subunits Isolated from Rhodospirillum rubrum: Selective Binding of All-trans Isomer from Mixture of cis- and trans-Isomers

Abstract The reassociation of carotenoids with the photosynthetic light-harvesting 1 (LH1) pigment–protein complex has proven to be a powerful technique to investigate the function of carotenoids. We have recently investigated the reassociation of the carotenoids, spirilloxanthin (number of conjugated C=C bonds (n) = 13), anhydrorhodovibrin (n = 12), rhodopin (n = 11), and spheroidene (n = 10)) with carotenoid-depleted LH1 subunits isolated from Rhodospirillum rubrum. In the case of the rhodopin complex, small anomalies were observed in the absorption maximum of the Qy band of bacteriochlorophyll a (BChl a Qy) and the singlet–singlet energy-transter efficiency of 1carotenoid* → 1BChl a* within the LH1–carotenoid complex. The reason for these anomalies could probably be ascribed to the rhodopin hydroxy group. To investigate this further, reassociation experiments of the LH1 complex with hydroxy-protected rhodopin, all-trans-3,4-dihydroanhydrorhodovibrin (1), were conducted. We observed a normal absorption maximum (883 nm) of the BChl a Qy band in the LH1–1 complex. The energy-transter efficiency of 1carotenoid* (11*) → 1BChl a* was found to be 53%, giving a good correlation with the other carotenoids. Furthermore, we found a preferential complexation of all-trans-1 to carotenoid-depleted LH1 from a 1:1 mixture of all-trans-1 and cis-1 (cis-1 consists of a 1:1 mixture of 5-cis-5′-trans-1 and 5-trans-5′-cis-1).

Correlated driving and dissipation in two-dimensional spectroscopy

The correlation between coherent driving and non-Markovian dissipation plays a vital role in optical processes. To exhibit its effect on the simulation of optical spectroscopy, we explore the correlated driving-dissipation equation (CODDE) [R. X. Xu and Y. J. Yan, J. Chem. Phys. 116, 9196 (2002)], which modifies the conventional Redfield theory with the inclusion of correlated driving-dissipation effect at the second-order system-bath coupling level. With an exciton model mimicking the Fenna-Matthews-Olson pigment-protein complex, we compare between the Redfield theory, CODDE, and exact hierarchical dynamics, for their results on linear absorption and coherent two-dimensional spectroscopy. We clarify that the failure of Redfield approach originates mainly from the neglect of driving-dissipation correlation, rather than its second-order nature. We further propose a dynamical inhomogeneity parameter to quantify the applicable range of CODDE. Our results indicate that CODDE is an efficient and quantifiable theory for many light-harvesting complexes of interest. To facilitate the evaluation of multi-dimensional spectroscopy, we also develop the mixed Heisenberg-Schrödinger picture scheme that is valid for any dynamics implementation on nonlinear response functions.

The role of non-equilibrium vibrational structures in electronic coherence and recoherence in pigment–protein complexes

Photosynthesis is remarkably efficient. The transport of optically generated excitons from absorbing pigments, through protein complexes, to reaction centres is nearly perfect. Simulations now uncover the microscopic mechanism that drives this coherent behaviour: interactions between the excitons and the vibrational modes of the pigment-protein complex.

Understanding photosynthetic light-harvesting: a bottom up theoretical approach

We discuss a bottom up approach for modeling photosynthetic light-harvesting. Methods are reviewed for a full structure-based parameterization of the Hamiltonian of pigment-protein complexes (PPCs). These parameters comprise (i) the local transition energies of the pigments in their binding sites in the protein, the site energies; (ii) the couplings between optical transitions of the pigments, the excitonic couplings; and (iii) the spectral density characterizing the dynamic modulation of pigment transition energies and excitonic couplings by protein vibrations. Starting with quantum mechanics perturbation theory, we provide a microscopic foundation for the standard PPC Hamiltonian and relate the expressions obtained for its matrix elements to quantities that can be calculated with classical molecular mechanics/electrostatics approaches including the whole PPC in atomic detail and using charge and transition densities obtained with quantum chemical calculations on the isolated building blocks of the PPC. In the second part of this perspective, the Hamiltonian is utilized to describe the quantum dynamics of excitons. Situations are discussed that differ in the relative strength of excitonic and exciton-vibrational coupling. The predictive power of the approaches is demonstrated in application to different PPCs, and challenges for future work are outlined.

Theoretical Study of Electron Transfer Rate from Phylloquinones to Iron-Sulfur Cluster (Fx) in Photosystem I

Photosystem I (PSI) is one of the two multi-subunit, pigment-protein complexes that drive the initial processes of light utilization in oxygenic photosynthesis. The PSI complex has approximately 100 packed chlorophyll (Chl) molecules that form the antenna and functional redox cofactors comprising the reaction center (RC). The latter cofactors are arranged in two pseudo-symmetric branches, starting from P700, and then split into two branches with the ec2 and ec3 chlorophylls and phylloquinone (A1) on each branch, and then rejoin at the iron-sulfur cluster FX. All of these are bound by the heterodimer of the PsaA and PsaB subunits, while the terminal FA and FB clusters are bound by PsaC. Electron transfer (ET) along the two branches has been studied by various investigators, and it was established that both branches are active in electron transfer, but that electron transfer to FX was ∼10-fold faster from PhQB than from PhQA.New results from electron paramagnetic resonance showed that the orientations of the PhQ radicals are different which affect the strength of hydrogen bonding between PhQ and the protein backbone. We are investigating these different rates of electron transfer and orientation of PhQ radicals through molecular dynamics simulation and time dependent density functional theory (TD-DFT) methods.

Research on Entanglement and Coherence in Light Harvesting Complex during Photosynthesis

Photosynthesis is a wonderful phenomenon which is present in green plant. In recent years, it has been discovered that there is entanglement in the biological pigment protein complexes, and that may be the reason of high transport efficiency. And coherence also plays an important role during the process of this efficiency energy transport. However, some scientists consider that it is not at all clear entanglement exists in the FMO complex, or unlike coherence, its role for the transport efficiency seems to be irrelevant. This paper mainly introduces what progress have scientists made during the past few years.

Steps of the coupled charge translocation in the catalytic cycle of cytochrome c oxidase

Cytochromecoxidase (COX) terminates the respiratory chain in mitochondria and in plasmatic membrane of many aerobic bacteria. The enzyme reduces dioxygen molecule into water and the reaction is accompanied with generation of transmembrane difference of electric potentials. The energy conservation by COX is based on the vectorial organization of the chemical reaction due to substrate protons transfer from the negative phase into the active site to meet the electrons, coming from opposite side of the membrane. In addition, a half of free energy of redox-chemistry reaction is conserved through transmembrane proton pumping. Each of the reaction steps in the catalytic cycle of COX involves a sequence of coupled electron and proton transfer reaction. The time-resolved study of the coupled charge translocation during catalytic cycle of cytochromecoxidase is reviewed. Based on many fascinating parallels between the mechanisms of COX and pigment-protein complex of photosystem II from oxygenic photosynthetic organisms, the data described could be used in the light-driven water-splitting process.

Switching light harvesting complex II into photoprotective state involves the lumen-facing apoprotein loop

In higher plants, high light conditions trigger the activation of non-photochemical quenching (NPQ), a process of photoprotective light energy dissipation, via acidification of the chloroplast lumen. Spectral changes occurring in the neoxanthin domain of the major light harvesting antenna complex (LHCII) have previously provided indirect evidence of a protein conformational switch during NPQ. We report here of two recombinant LHCII complexes mutated at the level of lumenal loop with altered quenching capacity with respect to the control. Replacement of the acidic lumenal-facing residue aspartate 111 (D111) with neutral valine (V111) yielded a recombinant complex with increased quenching capacity under low pH, due to a shift of the pK by 1 pH unit. The increase in total quenching was consistent with 40% reduction in the relative chlorophyll fluorescence lifetime and was accompanied by a lower energy emitting state of the mutant, as demonstrated by 77 K fluorescence spectroscopy. On the other hand, replacement of acidic glutamate 94 (E94) with glycine (G94) resulted in reduction of the fluorescence quenching yield attained at low pH. These results show for the first time that a subtle change in the LHCII apoprotein structure at the level of the lumenal loop induced by single aminoacid mutagenesis can affect protein sensitivity to pH leading to the establishment of NPQ. This work opens a potential avenue for manipulation of light harvesting efficiency in the natural antenna pigment-protein complexes that can be used for the creation of hybrid light energy conversion systems in future.

Probing the picosecond kinetics of the photosystem II core complex in vivo

Photosystems I (PSI) and II (PSII) are two major pigment-protein complexes of photosynthetic organisms that function in series to convert sunlight energy into chemical energy. We have studied the picosecond fluorescence behaviour of the core of both photosystems in vivo by using two Synechocystis PCC 6803 mutants: BE cells contain PSI but are lacking both PSII and the light-harvesting complexes called phycobilisomes (PBs) whereas PAL cells contain both PSI and PSII but lack the PBs. Measurements were performed at room temperature and at 77 K. The fluorescence kinetics of PSI and PSII can nicely be separated, en passant providing the PSI/PSII ratio. At room temperature, the PSI kinetics are identical to those of isolated PSI whereas the PSII kinetics can equally well be described by the in vitro trap-limited model of Y. Miloslavina, M. Szczepaniak, M. G. Muller, J. Sander, M. Nowaczyk, M. Rogner and A. R. Holzwarth, Biophys J., 2009, 96(2), 621-631, and by the transfer-to-the-trap-limited model of C. D. van der Weij-de Wit, J. P. Dekker, R. van Grondelle and I. H. M. van Stokkum, J. Phys. Chem. A, 2011, 115(16), 3947-3956, albeit that the in vivo kinetics turn out to be somewhat slower. At 77 K several low-energy pigments are observed in both photosystems which complicate the overall dynamics but the PSII kinetics can still be described by both a trap-limited and a transfer-to-the-trap-limited model.

Understanding desiccation tolerance using the resurrection plant Boea hygrometrica as a model system.

Vegetative tissues of Boea hygrometrica, a member of the Gesneriaceae family, can tolerate severe water loss to desiccated state and fully recover upon rehydration. Unlike many other so called “resurrection plants,” the detached leaves of B. hygrometrica also possess the same level of capacity for desiccation tolerance (DT) as that of whole plant. B. hygrometrica is distributed widely from the tropics to northern temperate regions in East Asia and grows vigorously in areas around limestone rocks, where dehydration occurs frequently, rapidly, and profoundly. The properties of detached B. hygrometrica leaves and relative ease of culture have made it a useful system to study the adaptive mechanisms of DT. Extensive studies have been conducted to identify the physiological, cellular, and molecular mechanisms underlying DT in the last decade, including specific responses to water stress, such as cell wall folding and pigment-protein complex stabilizing in desiccated leaves. In this review, the insight into the structural, physiological, and biochemical, and molecular alterations that accompany the acquisition of DT in B. hygrometrica is described. Finally a future perspective is proposed, with an emphasis on the emerging regulatory roles of retroelements and histone modifications in the acquisition of DT, and the need of establishment of genome sequence database and high throughput techniques to identify novel regulators for fully understanding of the matrix of DT.

The effect of water stress and salinity on growth and physiology of tomato (Lycopersicon esculentum Mil.)

Carotenoids like lycopene are important pigments found in photosynthetic pigment-protein complexes in plants. They are responsible for the bright colors of fruits and vegetables and perform various functions in photosynthesis. Our research has shown that the application of moderate salt stress on tomato plants can enhance lycopene and potentially other antioxidant concentrations in fruits. The increase in lycopene in response to salt stress in the tomato fruits varied from 20% to 80%. Although the specific biological mechanisms involved in increasing fruit lycopene deposition has not been clearly elucidated, evidence suggests that increasing antioxidant concentrations is a primary physiological response of the plant to salt stress. Additionally drought stress during cultivation increased the antioxidant capacity of tomato fruit while maintaining the lycopene concentration. In addition, the effects of silicium were investigated, added to the nutrient solution either at low concentration or at an increased concentration. The present study clearly indicates that an enhanced silicium supply to tomato increases markedly the lycopene contents, irrespective of the salinity status in the tomato fruit.

Effects of 24-epibrassinolide Pre-treatment on UV-B-Induced Changes in the Pigment Content of Pea Leaves

In the present work, the effects of 24-epibrassinolide (EBR) on the UV-Binduced changes in the pigment content of pea leaves were studied. Control (non-EBR-treated) and EBR-treated plants were irradiated with UV-B for 3 h and pigment analysis was performed after 24 and 48 h. The results show that EBR spraying of plants 48 h prior to UV-B exposure alleviates its detrimental effect on chlorophyll a and b (Chl a and Chl b) content in comparison with control pea leaves. An increase in carotenoids (Car) and UV-B absorbing compounds was also observed at low dose of UV-B radiation. For the first time, it is shown that UV-B damage effect on control leaves is accompanied by a significant (more than 50%) increase in their pheophytin a (Pheo a) content 48 h after the UV-B exposure and that the EBR pre-treatment prevents the increase of Pheo a content in UV-B irradiated leaves. In addition, it is demonstrated that EBR application modifies UV-B-induced alterations of energy distribution between the main pigment-protein complexes in pea thylakoid membranes.

Sensitivity of two Ecotypes of Arabidopsis thaliana (Cvi and Te) towards UV‑B Irradiation

The susceptibility of Arabidopsis thaliana towards the detrimental effect of UV-B irradiation was investigated using two ecotypes, Cvi and Te. The effect of UV-B treatment on primary photosynthetic reactions – energy interaction between the main pigment-protein complexes and oxygen evolution, was evaluated at low (4 ◦ C) and at room (22 ◦ C) temperature. UV-B-induced alterations of investigated photosynthetic reactions are better expressed at 22 ◦ C than at 4 ◦ C for Cvi. For Te ecotype the energy interaction was suppressed to higher extent at 22 ◦ C, while oxygen evolving activity was affected similarly at both temperatures. At low and room temperature, the energy interaction in the complex PSII-core antenna is affected stronger by UV-B treatment than the energy distribution between both photosystems, as revealed by fluorescence ratios of 77 K spectra. The results presented indicate that the Arabidopsis thaliana ecotype Cvi (Cape Verde Islands) is less affected by UV-B irradiation in respect to the investigated primary photosynthetic reactions than the ecotype Te (Finland).

Two-Dimensional Electronic Spectroscopy Using Incoherent Light: Theoretical Analysis

Electronic energy transfer in photosynthesis occurs over a range of time scales and under a variety of intermolecular coupling conditions. Recent work has shown that electronic coupling between chromophores can lead to coherent oscillations in two-dimensional electronic spectroscopy measurements of pigment-protein complexes measured with femtosecond laser pulses. A persistent issue in the field is to reconcile the results of measurements performed using femtosecond laser pulses with physiological illumination conditions. Noisy-light spectroscopy can begin to address this question. In this work we present the theoretical analysis of incoherent two-dimensional electronic spectroscopy, I((4)) 2D ES. Simulations reveal diagonal peaks, cross peaks, and coherent oscillations similar to those observed in femtosecond two-dimensional electronic spectroscopy experiments. The results also expose fundamental differences between the femtosecond-pulse and noisy-light techniques; the differences lead to new challenges and new opportunities.