State Photosynthesis Research Articles

Charge-transfer states in photosynthesis and organic solar cells

Light-induced charge-transfer mechanisms are at the heart of both photosynthesis and photovoltaics. The underlying photophysical mechanisms occurring within photosynthesis and organic photovoltaics in particular show striking similarities. However, they are studied by distinct research communities, often using different terminology. This contribution aims to provide an introductory review and comparison of the light-induced charge-transfer mechanisms occurring in natural photosynthesis and synthetic organic photovoltaics, with a particular focus on the role of so-called charge-transfer complexes characterized by an excited state in which there is charge-transfer from an electron-donating to an electron-accepting molecular entity. From light absorption to fully separated charges, it is important to understand how a charge-transfer complex is excited, forming a charge-transfer state, which can decay to the ground state or provide free charge carries in the case of photovoltaics, or radicals for photochemistry in photosynthetic complexes. Our motivation originates from an ambiguity in the interpretation of charge-transfer states. This review attempts to standardize terminology between both research fields with the general aim of initiating a cross-fertilization between the insights and methodologies of these two worlds regarding the role of charge-transfer complexes, inspiring the cross-disciplinary development of next-generation solar cells. Likewise, we hope to encourage photosynthesis researchers to collaborate with the photovoltaics field, thereby gaining further knowledge of the charge-transfer process in natural light-harvesting systems.

Site Characteristics Mediate the Relationship Between Forest Productivity and Satellite Measured Solar Induced Fluorescence

Solar-Induced Chlorophyll Fluorescence (SIF) can provide key information about the state of photosynthesis and offers the prospect of defining remote sensing-based estimation of Gross Primary Production (GPP). There is strong theoretical support for the link between SIF and GPP and this relationship has been empirically demonstrated using ground-based, airborne, and satellite-based SIF observations, as well as modeling. However, most evaluations have been based on monthly and annual scales, yet the GPP:SIF relations can be strongly influenced by both vegetation structure and physiology. At the monthly timescales, the structural response often dominates but short-term physiological variations can strongly impact the GPP:SIF relations. Here, we test how well SIF can predict the inter-daily variation of GPP during the growing season and under stress conditions, while taking into account the local effect of sites and abiotic conditions. We compare the accuracy of GPP predictions from SIF at different timescales (half-hourly, daily, and weekly), while evaluating effect of adding environmental variables to the relationship. We utilize observations for years 2018–2019 at 31 mid-latitudes, forested, eddy covariance (EC) flux sites in North America and Europe and use TROPOMI satellite data for SIF. Our results show that SIF is a good predictor of GPP, when accounting for inter-site variation, probably due to differences in canopy structure. Seasonally averaged leaf area index, fraction of absorbed photosynthetically active radiation (fPAR) and canopy conductance provide a predictor to the site-level effect. We show that fPAR is the main factor driving errors in the linear model at high temporal resolution. Adding water stress indicators, namely canopy conductance, to a multi-linear SIF-based GPP model provides the best improvement in the model precision at the three considered timescales, showing the importance of accounting for water stress in GPP predictions, independent of the SIF signal. SIF is a promising predictor for GPP among other remote sensing variables, but more focus should be placed on including canopy structure, and water stress effects in the relationship, especially when considering intra-seasonal, and inter- and intra-daily resolutions.

Inspecting S states in photosynthesis

Water Oxidation Oxygenic photosynthesis uses a Mn4CaO5 cluster in the oxygen-evolving complex to extract electrons from water and produce dioxygen. Visualizing each of the chemical states in this process, S to S4, and assigning chemical identities and mechanisms on the basis of structures has been a challenge addressed recently by work at x-ray free-electron lasers. Suga et al. used serial crystallography at cryogenic temperatures to trap and determine the structures of several stable states during photosystem II water oxidation (see the Perspective by Britt and Marchiori). Changes around the water cluster already happen in the S2 state and set the stage for water insertion that occurs during transition to the S3 state. A short 1.9-angstrom distance between the two oxygen atoms in the S3 state is consistent with theoretical studies supporting an oxyl/oxo mechanism for oxygen-oxygen coupling. Science , this issue p. [334][1]; see also p. [305][2] [1]: /lookup/doi/10.1126/science.aax6998 [2]: /lookup/doi/10.1126/science.aaz4522

Electron-vibrational coupling decreases trapping by low-energy states in photosynthesis

In photosynthetic light harvesting, states with energy well below that needed for charge separation can be found in abundance. They do not hinder the quantum efficiency of the primary processes; on the contrary, they can be highly functional, extending the absorption towards the red. Although many properties of these states are well described based on spectroscopic and theoretical studies, the physical mechanisms underlying their working are not known. Here we propose a mechanism which utilizes high-frequency vibrations of the photosynthetic pigments and the combined spatio-energetic aspect of the excitation dynamics. We present numerical calculations of the excitation dynamics in explicit electron-vibrational basis, with parameters based on photosynthetic complexes such as the Lhca4 complex of higher plants. The electron-vibrational states have two roles. For the trapped, low-energy excitation they provide a thermally populated ladder out of the trap. And for the high-energy excitation they provide local-bath states, effectively forming a bridge over the trap.

Photosynthetic rapid light curves (RLC) of Thalassia testudinum exhibit diurnal variation

The use of pulse-amplitude-modulated fluorometry (PAM) and rapid light curves (RLC) were evaluated for monitoring the physiological condition of the seagrass, Thalassia testudinum, at the landscape scale in Florida Bay, USA. PAM fluorometry provides rapid, non-invasive, and quantitative physiological information on the state of photosynthesis. Yet, previous studies of effective and maximum quantum yields have shown that problems arise when expanding measurements from the organismal scale to the landscape scale, mainly due to temporal and irradiance-induced changes in photophysiology. Here, the magnitude of diurnal and spatial variation of photosynthetic characteristics among 10 sample basins and between two sample years was investigated using RLCs. Because RLCs measure effective quantum yields over a range of changing actinic irradiances, we hypothesized that the response parameters might be less sensitive to diurnal light history effects. Our results indicate that the RLC parameters, alpha and ETR max, significantly changed diurnally, as was previously found for both maximum and effective quantum yields, but the diurnal patterns were variable among the 10 basins. Both among-basin and between-year comparisons were confounded by diurnal variation and statistical analyses comparing morning, mid-day, and afternoon time periods were unable to definitively discern which time of day was best suited for assessing the relative photophysiological status of T. testudinum. However, pooling RLC data at the basin scale revealed among-basin differences and landscape scale trends that were consistent with basin-level morphometric variation in this seagrass. Thus, PAM fluorometry may be useful as a landscape scale monitoring tool within certain constraints. When using this approach over large spatial and temporal scales, diurnal variability must be considered.

Single-cell chlorophyll fluorescence kinetic microscopy of Pylaiella littoralis (Phaeophyceae) infected by Chytridium polysiphoniae (Chytridiomycota)

Chlorophyll fluorescence imaging is a non-invasive method to monitor the metabolic state of photosynthetic organisms. We used spatially resolved (imaging) microscopic measurements of chlorophyll fluorescence kinetics to follow the fate of individual cells of the filamentous brown alga Pylaiella littoralis infected by the biotrophic parasite Chytridium polysiphoniae (Chytridiomycota). These measurements showed strong differences between individual parameters of the inhibition of photosynthesis, revealing important details about the mechanism of damage. The dark-adapted photochemistry of photosystem II itself (measured as Fv/Fm, where Fv = Fm − F0) remained unaffected until a very late stage of damage to the cell, while the light-adapted efficiency of PSII electron transport decreased earlier. A particularly complex pattern was found for the changes in nonphotochemical quenching (NPQ). The shape of the fluorescence transients suggests that the changes in NPQ during the actinic light period are caused by changes in F0. The infection affected NPQ directly after the onset of the actinic light period more than in the steady state of photosynthesis. These results indicate that the infection affects the regulation of energy dissipation (e.g. by changes in antenna coupling). In early infection stages, NPQ increased, which is reflected by an increase in the ‘vitality parameter’ (relative fluorescence decrease, defined as (Fp − Fs)/Fs). In the second half of the infection process, all photosynthetic parameters declined, including the efficiency of photosystem II as measured by Fv/Fm, and NPQ.

ChemInform Abstract: Radicals, Radical Pairs and Triplet States in Photosynthesis

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Radicals, radical pairs and triplet states in photosynthesis.

The investigation of radical ions, radical pairs, and triplet states that occur in the primary processes of photosynthesis by various EPR techniques is described. The determination of the valence electron spin density distribution by ENDOR/TRIPLE spectroscopy is demonstrated for the primary electron donor. In combination with site-directed mutagenesis, these studies show that the spin distribution in the chlorophyll donor is strongly affected by the protein environment, and a link is established with the donor's oxidation potential and function in the electron transfer process. Similarities and differences between electron donors in anoxygenic (bacterial) and oxygenic photosynthesis are briefly discussed. Application of transient and pulse EPR techniques give information also on short-lived intermediates, such as radical pairs and triplet states, and allow the determination of distances and relative orientations of these species. The extension to time-resolved pulse ENDOR spectroscopy opens the possibility to resolve the electron-nuclear hyperfine structure of these reaction intermediates, even on a very short time scale.

Conformational energetics of a partially symmetrized photosynthetic reaction centre

The partially symmetrized mutant reaction centre from the photosynthetic bacterium Rhodobacter capsulatus, named D LL, lacks the L-branch electron acceptor bacteriopheophytin (BPh L), thus rendering it of particular interest for experiments on primary excited states in photosynthesis. Here we present the results of conformational energy calculations on the D LL mutant and several BPh L-binding revertants. The calculations provide an explanation for the relative BPh L binding properties of the proteins in terms of interactions involving two residues in the binding pocket, these being a tryptophan and a methionine in the D LL protein.

Initial electron-transfer events in photosynthetic bacteria

Abstract The electron transfer from the primary donor special pair to the primary acceptor bacteriopheophytin in bacterial photosynthesis, as probed by femtosecond spectroscopy, is discussed in terms of the following four issues: unidirectionality; single-step superexchange versus the two-step sequential mechanism; the temperature dependence of the electron-transfer rate; and the improved methodology for examining the primary events in photosynthesis. New methods are still required to address the recently observed non-single exponential decay of the initial excited state of photosynthesis. Without additional information, the mechanism of the primary charge separation chemistry will remain unsettled.

Steady State of Photosynthetic Electron Transport in Cells of the Cyanophyti Synechocystis PCC 6714 Having Different Stoichiometry between PS I and PS II: Analysis of Flash-Induced Oxidation-Reduction of Cytochrome f and P700 under Steady State of Photosynthesis

Steady State of Photosynthetic Electron Transport in Cells of the Cyanophyti Synechocystis PCC 6714 Having Different Stoichiometry between PS I and PS II: Analysis of Flash-Induced Oxidation-Reduction of Cytochrome f and P700 under Steady State of Photosynthesis

The relationship between steady state phytoplankton growth and photosynthesis

The relationship between steady state phytoplankton growth and photosynthesis

Phosphorescence spectra of bacteriochlorophylls

The authors wish to report phosphorescence spectra of the lowest triplet state of a number of bacteriochlorophylls (BChls) which provide the first accurate values for the triplet state energies of these chromophores. There have been many previous attempts to estimate the triplet state energies of bacteriochlorins motivated by the possible importance of triplet states in photosynthesis, the utility of these chromophores as sensitizers and quenchers of singlet oxygen, model studies of electron transfer, and intrinsic theoretical interest in the excited state energies of extended conjugated macrocycles.

Steady State of Photosynthesis in Cyanobacterial Photosynthetic Systems Before and After Regulation of Electron Transport Composition: Overall Rate of Photosynthesis and PSI/PS II Composition

Steady State of Photosynthesis in Cyanobacterial Photosynthetic Systems Before and After Regulation of Electron Transport Composition: Overall Rate of Photosynthesis and PSI/PS II Composition

Inorganic Phosphate Concentration in the Stroma of Isolated Chloroplasts and Its Influence on Photosynthesis

The concentration of inorganic orthophosphate (Pi) was determined in the stroma of isolated chloroplasts during photosynthesis under Pi-saturated and Pi-limited conditions. Pi was determined calorimetrically or by high performance liquid chromatography of extracts of chloroplasts labelled with 32Pi. When chloroplasts were illuminated in the absence of added Pi, photosynthesis soon declined due to Pi-depletion. After 5 min in the light, photosynthesis had declined to 2% of the maximum rate. At this point, stromal Pi was estimated to be 1.4 mM by the colorimetric method and 0.2 mM by 32P chromatography. Using the colorimetric method, Pi equivalent to approximately 1 mM in the stroma was found to be associated with thylakoid membranes isolated from chloroplasts, irrespective of the Pi content of the intact chloroplasts. This was considered to be a non-metabolic pool of Pi. During steady- state photosynthesis with optimal concentrations of Pi added to the reaction medium, the stromal Pi concentration was estimated to be 2.6 mM and 1.6 mM with the colorimetric and 32P methods, respectively. Measurement of stromal 32Pi in chloroplasts illuminated with varying concentrations of 32Pi in the reaction medium suggested that photosynthesis was saturated at stromal Pi concentrations above 2.0-2.5 mM. Photophosphorylation by thylakoid membranes was saturated at Pi concentrations above 1.2-1.5 mM. It is concluded that, during photosynthesis in isolated chloroplasts in the presence of an optimal supply of Pi from the reaction medium, the stromal Pi concentration is just above that required to saturate photophosphorylation. Any decrease in the supply of Pi from the medium results in a rapid decrease in stromal Pi to the point where photophosphorylation may become Pi-limited, decreasing the rate of CO2 fixation.

Modulation of Phosphoenolpyruvate Carboxylase by 3-Phosphoglycerate: Probable Physiological Significance for C 4-photosynthesis

Summary The effect of 3-phosphoglycerate on the activity of phosphoenolpyruvate carboxylase from C 4 plants depends on pH and substrate (PEP) concentration. At low pH (7.2) a strong activation is observed due to the elimination of the positive cooperativity; therefore, it is most evident at low substrate levels. This activation may have a physiological significance for C 4 -photosynthesis, since 3-PGA is a major photosynthetic metabolite which can also serve as a stabilizer of PEP concentration during steady — state photosynthesis.

Significance of the electron trap parameters of glow curves of photosynthetic membranes

AbstractSeveral methods are available to determine the electron trap parameters such as activation energy E, preexponential frequency factor s, and the lifetime τ of the electrons in the trap states in photosynthesis. These parameters are also related by the Arrhenius expression which helps in calculating the E values without any assumptions regarding the particular order of kinetics applicable or the values of preexponential factors. Subsequent calculation then yields the values for s and τ in a straightforward manner. However, application of these methods (as well as the Arrhenius expression) to some of the experimental data has recently yielded such values that give rise to some questions regarding the meaning attributable to these parameters. The significance of these findings are discussed in view of the correlation of thermoluminescence and delayed light emission studies in photosynthetic membranes.

The very low ionization potentials of porphyrins and the possible role of rydberg states in photosynthesis

The very low ionization potentials of porphyrins lead to the prediction that in addition to bands due to (π, π *) transitions, bands due to Rydberg transitions should exist in their visible spectra. The suggestion is made that Rydberg excited states could be important in photosynthesis.

Further evidence for dissipative energy migration via triplet states in photosynthesis. The protective mechanism of carotenoids in Rhodopseudomonas spheroides chromatophores

The protective action of carotenoids against irreversible photodestruction was discovered in photosynthetic bacteria by Stanieda and coworkers. In green plant material it was found by Wolff and Witt ((1969) Z. Naturforsch. 24b, 1031–1037 and (1972) Proc. 2nd. Int. Congr. Photosynthesis Res. Stresa (Forti, G., Avron, M. and Melandri, A., eds.), Vol. 2, pp. 931–936, Dr. W. Junk, N. V. Publ. The Hague) that the formation of special carotenoid triplet states (via very rapid energy transfer from excited chlorophylls) and their fast radiationless decay in τ 1 2 ≈ 3 μ s is at least one mechanism for the protective action of carotenoids to irreversible photooxidation of the chlorophylls. Hence, it is anticipated that the same mechanism might be realized also in bacteria. The present study gives evidence for such a “triplet valve” to be established also in bacteria. This conclusion was derived from the following observations: 1. 1. The light-induced difference spectrum shows a bleaching of a carotenoid at three characteristic wavelength between 400 and 500 nm. A positive peak around 533 nm indicates the formation of a carotenoid triplet state. 2. 2. The absorption changes can be induced by red light which excites only bacteriochlorophyll. This indicates an energy transfer from bacteriochlorophyll to carotenoids. 3. 3. The light-induced carotenoid triplets decay radiationless in 3 μs in air-saturated aqueous suspensions of the chromatophores. 4. 4. The carotenoid triplet formation occurs only at actinic flash intensities where the photosynthesis becomes saturated. 5. 5. Addition of dithionite, which blocks photosynthesis, markedly increases the extent of carotenoid triplet formation. The different types of excilton migration within the photosynthetic unit are discussed, especially the routes leading to the dissipation of excess excitation energy.

Triplet states in photosynthesis

A comparison of zero field splitting (ZFS) and spin polarization of triplet spectra of bacteriochlorophyll a in vitro and in vivo provides support for the special pair model of photoreactive chlorophyll in photosynthetic bacteria. Spin polarization of the triplet spectra is a new and unique probe of primary events in the light conversion act in photosynthesis.