Photosynthetic Center Research Articles

Bridge effect on charge transfer and energy transfer in fullerene-chromophore dyads.

Fullerene-chromophore dyads have attracted a great deal of research interest because these complexes can be potentially designed as nanoscale artificial photosynthetic centers, in which the chromophore and fullerene function as the electron donor and acceptor, respectively. The basic operation of this dyad-type artificial reaction center is photoinduced electron transfer from the donor to the acceptor. The fullerene and chromophore are usually covalently linked so that sufficient electronic coupling between these two moieties can facilitate the electron transfer. However, other deactivation pathways for the chromophore excited state, such as energy transfer to the fullerene, may reduce the quantum yield of the photoinduced electron transfer. Here, a series of C60-perylene dyads is exploited to interrogate the effect of the linkage on deactivation mechanisms of the chromophore excited state. For the C60-perylene dyads with a single or double bond bridge, we find that the decay of the singlet state of the chromophore is dominated by the electron transfer, and the corresponding time constant is determined to be 45ps. On the other hand, for the dyad with a triple bond bridge, the singlet state of the chromophore is quickly quenched through energy transfer to fullerene, and the time constant is as short as 7.9ps. Our finding suggests that the bond order of the bridge in the fullerene-chromophore dyads can be utilized to control the deactivation pathways of the excited state.

Photosynthetic Responses of Racomitrium japonicum L. to Strontium Stress Evaluated through Chlorophyll a Fluorescence OJIP Transient Analysis.

Nuclides pollution and its biological effects are of great concern, especially for bryophytes during their terrestrial adaptation. Understanding PSII activity and electron transport response is vital for comprehending moss abiotic stress reactions. However, little is known about the photosynthetic performance of moss under nuclide treatment. Therefore, this study aimed to evaluate the chlorophyll fluorescence of Racomitrium japonicum L. The moss was subjected to Sr2+ solutions at concentrations of 5, 50, and 500 mg/L to evaluate chlorophyll a fluorescence using the OJIP test. Moderate and high Sr2+ stress led to inner cell membrane dissolution and reduced chlorophyll content, indicating impaired light energy absorption. At 5 mg/L Sr2+, fluorescence kinetics showed increased light energy capture, energy dissipation, and total photosynthetic driving force, thus stimulating transient photosynthetic activity of PSII and improving PSI reduction. Linear electron transfer and PSII stability significantly decreased under moderate and high Sr2+ stress, indicating potential photosynthetic center damage. Cyclic electron transfer (CEF) alleviated photosynthetic stress at 5 mg/L Sr2+. Thus, low Sr2+ levels stimulated CEF, adjusting energy flux and partitioning to protect the photosynthetic apparatus. Nevertheless, significant damage occurred due to inefficient protection under high Sr2+ stress.

Morpho-Physiological Responses of Three Italian Olive Tree (Olea europaea L.) Cultivars to Drought Stress

Water scarcity in agriculture can limit crop production and trigger the need for more effective water resource management. As a result, it is critical to identify new crop genotypes that are more drought tolerant and perform better under low irrigation or even rain-fed conditions. The olive tree is a high-value crop that is well adapted to dry Mediterranean conditions. However, different genotypes may have developed different mechanisms of tolerance to water stress. To investigate such mechanisms, we examined three Italian olive cultivars (‘Giarraffa’, ‘Leccino’, and ‘Maurino’) grown in a greenhouse under drought stress. We found that single genotypes responded differently to the drought, though not all parameters revealed significant differences. The first major difference among the cultivars was in transpiration: the lower stomatal density and stomatal conductance of ‘Giarraffa’ allow this cultivar to use water more conservatively. In parallel with the reduction in stomatal and mesophyll conductance, the drought-stressed group of ‘Giarraffa’ maintained the electron transport rate and effective efficiency levels of photosystem II similar to those of the control until the fourth week of stress. The fluorescence parameters revealed the earlier closure of reaction photosynthetic centres in ‘Leccino’. Finally, the higher rate of electrolyte leakage in ‘Maurino’ indicated a significant ions loss in this cultivar when it was subjected to the drought. Both water management under stress conditions and the effect of drought on photosynthesis make ‘Giarraffa’ interesting to researchers studying its use in breeding or water-saving programmes.

Characterization and Detection of Leaf Photosynthetic Response to Citrus Huanglongbing from Cool to Hot Seasons in Two Orchards

HighlightsAn abnormal accumulation of sucrose and glucose was found in HLB-infected leaves, and it presented a similar pattern in different orchards from cool to hot seasons.A decreasing value of the actual quantum yield of PSII (FPSII) in HLB-infected leaves was mainly related to an increase of non-regulated energy quenching (FNO) due to the irreversible damage of PSII.Chlorophyll fluorescence imaging combined with a random forest was able to identify HLB at the asymptomatic stage.Abstract Citrus Huanglongbing (HLB) poses a serious threat to citrus production. This research aimed to explore chlorophyll fluorescence imaging for characterizing the photosynthetic response to HLB-infected citrus leaves in different orchards and seasons. Chlorophyll fluorescence images of citrus leaves were acquired with an in-house chlorophyll fluorescence imaging system. It was found that sucrose and glucose accumulated earlier than starch in HLB-infected leaves, and a similar carbohydrate metabolic pattern was observed in HLB-infected leaves grown in different orchards from cool to hot seasons. The pathogen damaged the thylakoid structure of chloroplasts with a higher value of Fo. It decreased photosynthetic activity of the host by reducing the number of active photosynthetic centers and the maximum quantum yield of PSII (Fv/Fm) with lower values of Fv/Fo and Fv/Fm. Additionally, the pathogen modified the allocation of excitation energy in citrus leaves by reducing the actual quantum yield of PSII (FPSII) due to an increase of non-regulated energy quenching (FNO), which indicated irreversible PSII damage before symptom development. Moreover, photosynthetic signatures combined with the random forest method were able to identify HLB in the asymptomatic stage with an overall accuracy of 91.8%. These results demonstrated the potential of chlorophyll fluorescence imaging for evaluating the photosynthetic response to HLB as well as disease diagnosis. Keywords: Carbohydrate metabolism, Chlorophyll fluorescence imaging, Citrus Huanglongbing, Photosynthetic efficiency, Random forest model.

The Green Box: An Electronically Versatile Perylene Diimide Macrocyclic Host for Fullerenes.

The powerful electron accepting ability of fullerenes makes them ubiquitous components in biomimetic donor-acceptor systems that model the intermolecular electron transfer processes of Nature's photosynthetic center. Exploiting perylene diimides (PDIs) as components in cyclic host systems for the noncovalent recognition of fullerenes is unprecedented, in part because archetypal PDIs are also electron deficient, making dyad assembly formation electronically unfavorable. To address this, we report the strategic design and synthesis of a novel large, macrocyclic receptor composed of two covalently strapped electron-rich bis-pyrrolidine PDI panels, nicknamed the "Green Box" due to its color. Through the principle of electronic complementarity, the Green Box exhibits strong recognition of pristine fullerenes (C60/70), with the noncovalent ground and excited state interactions that occur upon fullerene guest encapsulation characterized by a range of techniques including electronic absorption, fluorescence emission, NMR and time-resolved EPR spectroscopies, cyclic voltammetry, mass spectrometry, and DFT calculations. While relatively low polarity solvents result in partial charge transfer in the host donor-guest acceptor complex, increasing the polarity of the solvent medium facilitates rare, thermally allowed full electron transfer from the Green Box to fullerene in the ground state. The ensuing charge separated radical ion paired complex is spectroscopically characterized, with thermodynamic reversibility and kinetic stability also demonstrated. Importantly, the Green Box represents a seminal type of C60/70 host where electron-rich PDI motifs are utilized as recognition motifs for fullerenes, facilitating novel intermolecular, solvent tunable ground state electronic communication with these guests. The ability to switch between extremes of the charge transfer energy continuum is without precedent in synthetic fullerene-based dyads.

Selective immobilization of bacterial light-harvesting proteins and their photoelectric responses

Abstract

Towards first-principles calculation of electronic excitations in the ring of the protein-bound bacteriochlorophylls

Modeling electronic excitation of bacteriochlorophyll (BChl) molecules in light-harvesting (LH) antennae from photosynthetic centers presents a challenge for the quantum theory. We report on a quantum chemical study of the ring of 32 BChl molecules from the bacterial core complex LH1-RC. Diagonal and off-diagonal elements of the excitonic Hamiltonian matrices are estimated in quantum chemical calculations of relevant fragments using the TD-DFT and CIS approaches. The deviation of the computed excitation energy of this BChl system from the experimental data related to the Qy band maximum of this LH1-RC complex is about 0.2 eV. We demonstrate that corrections due to improvement in modeling of an individual BChl molecule and due to contributions from the protein environment are in the range of the obtained discrepancy between theory and experiment. Differences between results of the excitonic model and direct quantum chemical calculations of BChl aggregates fall in the same range.

Light-harvesting and Electron Transfer in a Structure-controlled Polymer for Artificial Photosynthetic Antenna-reaction Centers

A branched polymer was prepared by pseudo-living addition–condensation polymerization. The polymer features a large amount of energy donors in the branched parts and a small amount of energy accept...

Effects of Dihydroartemisinin and Artemether on the Growth, Chlorophyll Fluorescence, and Extracellular Alkaline Phosphatase Activity of the Cyanobacterium Microcystis aeruginosa.

Increased eutrophication in the recent years has resulted in considerable research focus on identification of methods for preventing cyanobacterial blooms that are rapid and efficient. The objectives of this study were to investigate the effects of dihydroartemisinin and artemether on the growth of Microcystis aeruginosa and to elucidate its mode of action. Variations in cell density, chlorophyll a, soluble protein, malondialdehyde, extracellular alkaline phosphatase activity (APA), and chlorophyll fluorescence parameters (Fv/Fm, ΦPSII, ETR, rapid light curves, fast chlorophyll fluorescence curves on fluorescence intensity, and relative variable fluorescence) were evaluated by lab-cultured experiments. Our results demonstrated that both dihydroartemisinin and artemether inhibited the growth of M.aeruginosa by impairing the photosynthetic center in photosystem II and reducing extracellular APA, with a higher sensitivity exhibited toward artemether. The inhibitory effects of dihydroartemisinin on M.aeruginosa increased with concentration, and the maximum growth inhibitory rate was 42.17% at 24 mg·L-1 after 120h exposure, whereas it was 55.72% at 6 mg·L-1 artemetherafter 120h exposure. Moreover, the chlorophyll fluorescence was significantly inhibited (p<0.05) after 120h exposure to 12 and 24 mg·L-1 dihydroartemisinin. Furthermore, after 120h exposure to 6 mg·L-1 artemether, Fv/Fm, ΦPSII, ETR and rETRmax showed a significant decrease (p<0.01) from initial values of 0.490, 0.516, 17.333, and 104.800, respectively, to 0. One-way analysis of variance showed that 6 mg·L-1 artemether and 24 mg·L-1 dihydroartemisinin had significant inhibitory effects on extracellular APA (p<0.01). The results of this study would be useful to further studies to validate the feasibility of dihydroartemisinin and artemether treatment to inhibit overall cyanobacterial growth in water bodies, before this can be put into practice.

Conformation-Controlled Diplatinum(II)-Ferrocene Dyads to Achieve Long-Lived Charge-Separated States.

Square-planar polypyridyl platinum(II) complexes possess a rich range of structural and spectroscopic properties that are ideal for designing artificial photosynthetic centers. Taking advantage of the directionality in the charge-transfer excitation from the metal to the polypyridyl ligand, we describe here diplatinum(II)-ferrocene dyads, open-butterfly-like dyad 1 and closed-butterfly-like dyad 2, which were designed to understand the conformation and orientation effects to prolong the lifetime of charge-separated state. In contrast to the open-butterfly-like dyad 1, the closed-butterfly-like dyad 2 shows three-times long lifetime of charge separated state upon photoexcitation, demonstrating that the orientation in the rigid structure of dyad 2 is a very important issue to achieve long-lived charge separated state.

G-Quadruplex Supramolecular Assemblies in Photochemical Upconversion.

Parallel, tetramolecular G-quadruplex (G4) DNA possessing TINA monomer, (R)-1-O-[4-(1-pyrenylethynyl)phenylmethyl]glycerol, were synthesised and evaluated in complexes with tris(2,2'-bipyridine)ruthenium(II), [Ru(bpy)3 ](2+) , and the Zn(2+) derivative of 5,10,15,20-tetrakis-(1-methyl-4-pyridyl)-21 H,23H-porphine, ZnTMpyP4. UV/Vis, fluorescence, and circular dichroism (CD) spectroscopy showed that the use of G4-DNA as a template resulted in the effective communication between the ligands and the TINA molecule that was covalently attached to the 5'-end and between T and dG at the 5'-end of the dTG4 T sequence. Only one G4-DNA possessing the TINA molecule at the 5'-end of the dTG4 T sequence was able to yield a green-to-blue photochemical upconversion (PUC, λem =420 nm) in the presence of [Ru(bpy)3 ](2+) upon excitation at 500 nm. Different DNA secondary structures can thus be used in DNA-based assemblies for PUC and the way of attachment of chromophores to DNA plays a pivotal role for the creation of a photosynthetic centre.

Multi-Scale Supercomputing of Large Molecular Aggregates: A Case Study of the Light-Harvesting Photosynthetic Center

Numerical solution of the quantum mechanical Schrodinger equation is required to model electronic excitations in the light-harvesting photosynthetic complexes composed of up to millions of atoms. We demonstrate that the modern supercomputers can be used to treat electronic structure calculations in such large molecular aggregates if proper multi-scale massive-parallel approaches are applied. We show that the three-level parallelization scheme based on the novel numerical algorithms assuming fragmentation of a light-harvesting complex allows us to reduce considerably the high scaling of ab initio quantum chemistry methods. More specifically we applied the time-dependent density functional theory based upon the fragment molecular orbital presentation FMO-TDDFT implemented at the modern supercomputers to obtain a realistic estimate of the electronic excitation in the complex. The application shows a good overall scaling.

Effects of garlic and diallyl trisulfide on the growth, photosynthesis, and alkaline phosphatase activity of the toxic cyanobacterium Microcystis aeruginosa.

To identify a botanical algicide and elucidate the response of cyanobacteria to the extract from higher plants, the effects of garlic and garlic-derived diallyl trisulfide on Microcystis aeruginosa were studied. Effects were evaluated by changes in cell density, chlorophyll a, maximum effective quantum yield (Fv/Fm), effective quantum yield (YII), non-photochemical quenching (NPQ), and rapid light curves of M. aeruginosa. In addition, alkaline phosphatase activity (APA) was measured when M. aeruginosa was incubated with diallyl trisulfide. Results indicated that the inhibition by garlic and diallyl trisulfide was significant. The 120-h 50 % effective concentrations of garlic and diallyl trisulfide (EC50) were 0.75 g L(-1) and 2.84 mg L(-1), respectively. Moreover, the inhibitory rate increased with increasing concentration and the growth of M. aeruginosa was inhibited by 90.0 % at the highest concentrations. We also show that the response of M. aeruginosa to stress could involve both impairment of the photosynthetic center PSII and alteration of APA. For example, at high garlic concentration (2.0 g L(-1)), Fv/Fm significantly decreased from 0.501 to 0.084 (p < 0.05) after 120 h of exposure. Furthermore, the total APA was significantly decreased by exposure to a high diallyl trisulfide concentration after 24 h exposure. As new algal inhibitors, there are several advantages for their utilization, such as being common, cheap, non-toxic, and with high efficiency. It would be meaningful to further research on garlic as an environmentally friendly algicide.

ChemInform Abstract: Photobiocatalysis: The Power of Combining Photocatalysis and Enzymes

AbstractReview: 98 refs.

Photobiocatalysis: the power of combining photocatalysis and enzymes.

Photobiocatalysts are constituted by a semiconductor with or without a light harvester that activates an enzyme. A logical source of inspiration for the development of photobiocatalysts has been natural photosynthetic centers. In photobiocatalysis, the coupling of the semiconductor and the enzyme frequently requires a natural cofactor and a relay transferring charge carriers from the semiconductor. The most widely studied photobiocatalysts so far make use of conduction band electrons of excited semiconductors to promote enzymatic reductions mediated by NAD(+) /NADH and an electron relay. The present review presents the state of the art in the field and has been organized based on the semiconductor and the reaction type including oxidations, hydrogen generation, and CO2 reduction. The possibility of direct enzyme activation by the semiconductor and the influence of the nature of mediator are also discussed as well as the use of mimics of the enzyme active center in combination with the semiconductor. The final section summarizes the state of the art of photobiocatalysis and comments on our view on future developments of the field.

Computer modeling of components of photoreceptor systems

Computational methods of molecular modeling, including quantum chemistry, molecular dynamics, as well as multilevel quantum mechanics/molecular mechanics (QM/MM) approaches, were applied to study fragments of selected photoreceptor systems. Large-scale molecular systems modeling natural photosynthetic centers and components of prospective solar cells containing titanium dioxide and organic dye molecules were chosen as the most important representatives. It is shown that the modern computational approaches are capable of predicting structures and spectra of such complicated molecular aggregates.

Femto-picosecond relaxation of triazole-bridged bis(zinc porphyrin)

The femtosecond dynamics of excitation relaxation has been revealed for the zinc porphyrin dimer using the pump-probe technique. The data obtained have been analyzed with the use of quantum-chemical calculations. The excitation relaxation dynamics shows that systems of this kind hold promise as models for investigation of photosystems and development of artificial analogues of natural photosynthetic centers. A model has been proposed to explain the found coherent dynamics of exciton bands.

Optical transitions in the light-harvesting complexes of bacterial photosynthetic centers

The geometric parameters of the model structure of the bacterial photosynthetic center, which consists of a light-harvesting antenna complex, a reaction center, lipid layers, and the solvation shells of water molecules, were optimized using quantum mechanics-molecular mechanics (QM/MM). The optical absorption spectra of the bacteriochlorophyll were evaluated in the quantum subsystem. It was shown that the shift of the absorption bands towards longer wavelengths as compared to the spectrum of the individual bacteriochlorophyll molecules is characteristic of the cluster of pigment molecules that are found in the antennas.

Chlorella triggers stomatal closure mediated by NADPH oxidase and improves instantaneous water use efficiency in Vicia faba

Stomatal closure induced by live microbes and microbe-associated molecular patterns (MAMPs) has been associated with early defense responses of plants. Chlorella is a unicellular autotrophic microorganism that can synthesize many bioactive substances with positive effects on humans, animals and plants. However, its effects on stomatal movement and instantaneous intrinsic water use efficiency (WUEi) in plants have been not explored yet. Our present work showed that application of Chlorella to isolated epidermal peels of Vicia faba induced stomatal closure in a dose-and time-dependent manner. Pharmacological study revealed that the Chlorella-triggered stomatal closure was mainly mediated by reactive oxygen species (ROS) production via NADPH oxidase. Additionally, exogenous application of optimal concentrations of Chlorella suspension caused an obvious reduction in leaf transpiration rate (E) without a parallel reduction in net photosynthetic rate (Pn), favoring the improvement of WUEi in Vicia faba. The chlorophyll fluorescence and content analysis further indicated that Chlorella had no effects on plant photosynthetic reactions center after short-term foliar application.

A Conversation with J. Craig Venter, PhD

A Conversation with J. Craig Venter, PhD