Events Of Photosynthesis Research Articles

Probing the design principles of photosynthetic systems through fluorescence noise measurement

Elucidating the energetic processes which govern photosynthesis, the engine of life on earth, are an essential goal both for fundamental research and for cutting-edge biotechnological applications. Fluorescent signal of photosynthetic markers has long been utilised in this endeavour. In this research we demonstrate the use of fluorescent noise analysis to reveal further layers of intricacy in photosynthetic energy transfer. While noise is a common tool analysing dynamics in physics and engineering, its application in biology has thus far been limited. Here, a distinct behaviour in photosynthetic pigments across various chemical and biological environments is measured. These changes seem to elucidate quantum effects governing the generation of oxidative radicals. Although our method offers insights, it is important to note that the interpretation should be further validated expertly to support as conclusive theory. This innovative method is simple, non-invasive, and immediate, making it a promising tool to uncover further, more complex energetic events in photosynthesis, with potential uses in environmental monitoring, agriculture, and food-tech.

Origin and Evolution of Photosystems: Lessons from Green Sulfur Bacteria

AbstractPhotosynthetic reaction centres (RCs) are the sites of primary events in photosynthesis, where photon absorption leads to charge separation. Chen et al. recently described the RC structure in a green sulfur bacterium. Together with the structure of heliobacterial RC published previously, this study is instrumental to elucidate RC evolution to modern plant photosystems and oxygen production from water.

Probing structure–function relationships in early events in photosynthesis using a chimeric photocomplex

The native core light-harvesting complex (LH1) from the thermophilic purple phototrophic bacterium Thermochromatium tepidum requires Ca2+ for its thermal stability and characteristic absorption maximum at 915 nm. To explore the role of specific amino acid residues of the LH1 polypeptides in Ca-binding behavior, we constructed a genetic system for heterologously expressing the Tch. tepidum LH1 complex in an engineered Rhodobacter sphaeroides mutant strain. This system contained a chimeric pufBALM gene cluster (pufBA from Tch. tepidum and pufLM from Rba. sphaeroides) and was subsequently deployed for introducing site-directed mutations on the LH1 polypeptides. All mutant strains were capable of phototrophic (anoxic/light) growth. The heterologously expressed Tch. tepidum wild-type LH1 complex was isolated in a reaction center (RC)-associated form and displayed the characteristic absorption properties of this thermophilic phototroph. Spheroidene (the major carotenoid in Rba. sphaeroides) was incorporated into the Tch. tepidum LH1 complex in place of its native spirilloxanthins with one carotenoid molecule present per αβ-subunit. The hybrid LH1-RC complexes expressed in Rba. sphaeroides were characterized using absorption, fluorescence excitation, and resonance Raman spectroscopy. Site-specific mutagenesis combined with spectroscopic measurements revealed that α-D49, β-L46, and a deletion at position 43 of the α-polypeptide play critical roles in Ca binding in the Tch. tepidum LH1 complex; in contrast, α-N50 does not participate in Ca2+ coordination. These findings build on recent structural data obtained from a high-resolution crystallographic structure of the membrane integrated Tch. tepidum LH1-RC complex and have unambiguously identified the location of Ca2+ within this key antenna complex.

Why chloroplasts and mitochondria retain their own genomes and genetic systems: Colocation for redox regulation of gene expression

Chloroplasts and mitochondria are subcellular bioenergetic organelles with their own genomes and genetic systems. DNA replication and transmission to daughter organelles produces cytoplasmic inheritance of characters associated with primary events in photosynthesis and respiration. The prokaryotic ancestors of chloroplasts and mitochondria were endosymbionts whose genes became copied to the genomes of their cellular hosts. These copies gave rise to nuclear chromosomal genes that encode cytosolic proteins and precursor proteins that are synthesized in the cytosol for import into the organelle into which the endosymbiont evolved. What accounts for the retention of genes for the complete synthesis within chloroplasts and mitochondria of a tiny minority of their protein subunits? One hypothesis is that expression of genes for protein subunits of energy-transducing enzymes must respond to physical environmental change by means of a direct and unconditional regulatory control—control exerted by change in the redox state of the corresponding gene product. This hypothesis proposes that, to preserve function, an entire redox regulatory system has to be retained within its original membrane-bound compartment. Colocation of gene and gene product for redox regulation of gene expression (CoRR) is a hypothesis in agreement with the results of a variety of experiments designed to test it and which seem to have no other satisfactory explanation. Here, I review evidence relating to CoRR and discuss its development, conclusions, and implications. This overview also identifies predictions concerning the results of experiments that may yet prove the hypothesis to be incorrect.

Phosphoproteomic analysis of the non-seed vascular plant model Selaginella moellendorffii

BackgroundSelaginella (Selaginella moellendorffii) is a lycophyte which diverged from other vascular plants approximately 410 million years ago. As the first reported non-seed vascular plant genome, Selaginella genome data allow comparative analysis of genetic changes that may be associated with land plant evolution. Proteomics investigations on this lycophyte model have not been extensively reported. Phosphorylation represents the most common post-translational modifications and it is a ubiquitous regulatory mechanism controlling the functional expression of proteins inside living organisms.ResultsIn this study, polyethylene glycol fractionation and immobilized metal ion affinity chromatography were employed to isolate phosphopeptides from wild-growing Selaginella. Using liquid chromatography-tandem mass spectrometry analysis, 1593 unique phosphopeptides spanning 1104 non-redundant phosphosites with confirmed localization on 716 phosphoproteins were identified. Analysis of the Selaginella dataset revealed features that are consistent with other plant phosphoproteomes, such as the relative proportions of phosphorylated Ser, Thr, and Tyr residues, the highest occurrence of phosphosites in the C-terminal regions of proteins, and the localization of phosphorylation events outside protein domains. In addition, a total of 97 highly conserved phosphosites in evolutionary conserved proteins were identified, indicating the conservation of phosphorylation-dependent regulatory mechanisms in phylogenetically distinct plant species. On the other hand, close examination of proteins involved in photosynthesis revealed phosphorylation events which may be unique to Selaginella evolution. Furthermore, phosphorylation motif analyses identified Pro-directed, acidic, and basic signatures which are recognized by typical protein kinases in plants. A group of Selaginella-specific phosphoproteins were found to be enriched in the Pro-directed motif class.ConclusionsOur work provides the first large-scale atlas of phosphoproteins in Selaginella which occupies a unique position in the evolution of terrestrial plants. Future research into the functional roles of Selaginella-specific phosphorylation events in photosynthesis and other processes may offer insight into the molecular mechanisms leading to the distinct evolution of lycophytes.

Rubisco activity is associated with photosynthetic thermotolerance in a wild rice (Oryza meridionalis)

Oryza meridionalis is a wild species of rice, endemic to tropical Australia. It shares a significant genome homology with the common domesticated rice Oryza sativa. Exploiting the fact that the two species are highly related but O. meridionalis has superior heat tolerance, experiments were undertaken to identify the impact of temperature on key events in photosynthesis. At an ambient CO(2) partial pressure of 38 Pa and irradiance of 1500 µmol quanta m(-2) s(-1), the temperature optimum of photosynthesis was 33.7 ± 0.8°C for O. meridionalis, significantly higher than the 30.6 ± 0.7°C temperature optimum of O. sativa. To understand the basis for this difference, we measured gas exchange and rubisco activation state between 20 and 42°C and modeled the response to determine the rate-limiting steps of photosynthesis. The temperature response of light respiration (R(light)) and the CO(2) compensation point in the absence of respiration (Γ(*)) were determined and found to be similar for the two species. C3 photosynthesis modeling showed that despite the difference in susceptibility to high temperature, both species had a similar temperature-dependent limitation to photosynthesis. Both rice species were limited by ribulose-1,5-bisphosphate (RuBP) regeneration at temperatures of 25 and 30°C but became RuBP carboxylation limited at 35 and 40°C. The activation state of rubisco in O. meridionalis was more stable at higher temperatures, explaining its greater heat tolerance compared with O. sativa.

From red to blue to far-red in Lhca4: How does the protein modulate the spectral properties of the pigments?

The first event of photosynthesis is the harvesting of solar energy by a large array of pigments. These pigments are coordinated to proteins that organize them to assure efficient excitation energy transfer. The protein plays an essential role in tuning the spectroscopic properties of the pigments, by determining their site energy and/or by favoring pigment–pigments interactions. Here we investigate how the protein modulates the pigment properties by using a single-point-mutation approach. We monitor changes in the low-energy absorption/emission band of Lhca4, which is well separated from the bulk absorption and thus represents an attractive model system. Moreover, it was recently shown that Lhca4 exists in at least two conformations, a dominating one emitting at 720nm and a second one emitting at 685nm (Kruger et al. PNAS 2011). Here we show that a single amino-acid substitution (from Asn to Gln, which are both chlorophyll-binding residues and only differ for one C―C bond), moves the equilibrium almost completely towards the 685-nm conformation. This indicates that small changes in the protein can have a large effect on the properties of the pigments. We show that His99, which was suggested to coordinate a red-absorbing chlorophyll (Melkozernov and Blankenship, JBC 2003), is not a chlorophyll ligand. We also show that single amino-acid substitutions nearby the chlorophylls allow to tune the emission spectrum of the pigments over a wide range of wavelengths and to modulate the excited-state lifetimes of the complex. These findings are discussed in the light of previously proposed non-photochemical quenching models.

Electron transfer in chemistry and biology — The primary events in photosynthesis

One of the most important chemical reactions is electron transfer from one atomic/molecular unit to another. This reaction, accompanied by proton and hydrogen atom transfers, occurs in a cascade in many biological processes, including photosynthesis. The key chemical steps involved in photosynthesis and the many unsolved mysteries are described in this article.

ChemInform Abstract: Fullerenes as Novel Acceptors in Photosynthetic Electron Transfer

We propose a novel strategy using fullerenes for the construction of solar energy conversion systems that mimic the primary electron transfer events in photosynthesis. Redox-active fullerenes such as C60 and C70 were covalently bound to a porphyrin and the photophysical properties of the resulting compounds were investigated. Regardless of solvent and linkage, the charge-separated state is produced efficiently in zincporphyrin–fullerene dyads, showing that fullerenes are good electron acceptors. The most intriguing characteristic of fullerenes in electron transfer is that they accelerate photoinduced charge separation as well as charge shift and slow down charge recombination, properties that are in sharp contrast with those of conventional two-dimensional aromatic acceptors such as quinones and imides. The peculiar electron transfer properties of fullerenes can be explained by the small reorganization energies, which make it possible to optimize artificial photosynthetic multistep charge separation. A combination of the two strategies, multistep electron transfer and small reorganization energy of fullerenes, has been applied to light energy conversion systems as well as the more complex molecular systems such as triads. Highly efficient photosynthetic multistep electron transfer has been realized at gold electrodes modified with self-assembled monolayers of fullerene-containing molecules. These results will provide new principles and concepts to develop artificial photosynthetic materials as well as molecular devices.

Dendritic Porphyrin–Fullerene Conjugates: Efficient Light‐Harvesting and Charge‐Transfer Events

A novel dendritic C(60)-H(2)P-(ZnP)(3) (P=porphyrin) conjugate gives rise to the successful mimicry of the primary events in photosynthesis, that is, light harvesting, unidirectional energy transfer, charge transfer, and charge-shift reactions. Owing, however, to the flexibility of the linkers that connect the C(60), H(2)P, and ZnP units, the outcome depends strongly on the rigidity/viscosity of the environment. In an agar matrix or Triton X-100, time-resolved transient absorption spectroscopic analysis and fluorescence-lifetime measurements confirm the following sequence. Initially, light harvesting is seen by the peripheral C(60)-H(2)P- *(ZnP)(3) conjugate. Once photoexcited, a unidirectional energy transfer funnels the singlet excited-state energy to H(2)P to form C(60)-*(H(2)P)-(ZnP)(3), which powers an intramolecular charge transfer that oxidizes the photoexcited H(2)P and reduces the adjacent C(60) species. In the correspondingly formed (C(60))(*-)-(H(2)P)(*+)-(ZnP)(3) conjugate, an intramolecular charge-shift reaction generates (C(60))(*-)-H(2)P-(ZnP)(3) (.+), in which the radical cation resides on one of the three ZnP moieties, and for which lifetimes of up to 460 ns are found. On the other hand, investigations in organic media (i.e., toluene, THF, and benzonitrile) reveal a short cut, that is, the peripheral ZnP unit reacts directly with C(60) to form (C(60))(*-)-H(2)P-(ZnP)(3) (*+). Substantial configurational rearrangements- placing ZnP and C(60) in proximity to each other-are, however, necessary to ensure the required through space interactions (i.e., close approach). Consequently, the lifetime of (C(60))(*-)-H(2)P-(ZnP)(3) (*+) is as short as 100 ps in benzonitrile.

Photosynthetic Antenna−Reaction Center Mimicry: Sequential Energy- and Electron Transfer in a Self-assembled Supramolecular Triad Composed of Boron Dipyrrin, Zinc Porphyrin and Fullerene

A self-assembled supramolecular triad, a model to mimic the photochemical events of photosynthetic antenna-reaction center, viz., sequential energy and electron transfer, has been newly constructed and studied. Boron dipyrrin, zinc porphyrin, and fullerene respectively constitute the energy donor, electron donor, and electron acceptor segments of the antenna-reaction center mimicry. For the construction, first, boron dipyrrin was covalently attached to a zinc porphyrin entity bearing a benzo-18-crown-6 host segment at the opposite end of the porphyrin ring. Next, an alkyl ammonium functionalized fullerene was used to self-assemble the crown ether entity via ion-dipole interactions. The newly formed supramolecular triad was fully characterized by spectroscopic, computational, and electrochemical methods. Selective excitation of the boron dipyrrin moiety in the dyad resulted in energy transfer over 97% efficiency creating singlet excited zinc porphyrin. The rate of energy transfer from the decay measurements of time-correlated singlet photon counting (TCSPC) and up-conversion techniques agreed well with that obtained by the pump-probe technique and revealed efficient photoinduced energy transfer in the dyad (time constant in the order of 10-60 ps depending upon the conformer). Upon forming the supramolecular triad by self-assembling fullerene, the excited zinc porphyrin resulted in electron transfer to the coordinated fullerene yielding a charge-separated state, thus mimicking the antenna-reaction center functionalities of photosynthesis. Nanosecond transient absorption studies yielded a lifetime of the charge-separated state to be 23 micros indicating charge stabilization in the supramolecular triad. The present supramolecular system represents a successful model to mimic the rather complex "combined antenna-reaction center" events of photosynthesis.

Coupling of laser excitation and inelastic neutron scattering: attempt to probe the dynamics of light-induced C-phycocyanin dynamics

Excitation energy transfer (EET) in light-harvesting antennae is a highly efficient key event in photosynthesis, where light-induced dynamics of the antenna pigment-protein complexes may play a functional role. So far, however, the relationship between EET and protein dynamics remains unknown. C-phycocyanin (C-PC) is the main pigment/protein complex present in the cyanobacterial antenna, called "phycobilisome". The aim of the present study was to investigate light-induced C-PC internal thermal motions (ps timescale) measured by inelastic neutron scattering. To synchronize the beginning of the laser flash (6 ns duration) with that of the neutron test pulse ( approximately 87 micros duration), we developed a novel type of "time-resolved" experimental setup on MIBEMOL time-of-flight neutron spectrometer (LLB, France). Data acquisition has been modified to get quasi-simultaneously "light" and "dark" measurements (with and without laser, respectively) and eliminate many spurious effects that could occur on the sample during the experiment. The study was carried out on concentrated C-PC ( approximately 135 g/L protein in D(2)O phosphate buffer), contained in an aluminium/sapphire sample holder (almost "transparent" for neutrons) and homogeneously illuminated inside an "integrating sphere". We observed very similar incoherent dynamical structure factors of C-PC with or without light. The vibrational density of states showed two very slightly increased vibrational modes with light, at approximately 30 and approximately 50 meV ( approximately 240 and approximately 400 cm(-1), respectively). These effects have to be verified by further experiments before probing any temporal evolution, by introducing a time delay between the laser flash and the neutron test pulse.

Time-resolved methods in biophysics. 4. Broadband pump—probe spectroscopy system with sub-20 fs temporal resolution for the study of energy transfer processes in photosynthesis

In this paper we discuss how to push the temporal resolution limits of transient absorption spectroscopy in order to detect very fast processes (energy relaxation, energy or charge transfer, vibrational coherence) taking place in molecules of biological relevance. After reviewing the main principles of femtosecond pump-probe spectroscopy, we describe an experimental setup based on two synchronized non-collinear optical parametric amplifiers (NOPAs). Each NOPA can be independently configured to generate ultra-broadband sub-10 fs visible pulses, tunable 10-15 fs visible pulses, tunable 15-40 fs near-infrared pulses (900-1500 nm). This system enables to perform pump-probe experiments over nearly two octaves of spectrum with sub-20 fs temporal resolution. We then present an application example highlighting the capability of this instrument to track excited state dynamics in biomolecules on the sub-100 fs timescale: the study of carotenoid-bacteriochlorophyll energy transfer processes in peripheral light-harvesting complexes (LH2) from purple bacteria. We show that, by comparing excited-state dynamics of the carotenoids in organic solvents and inside the LH2 complexes, it is possible to visualize in the time domain the primary events in photosynthesis.

Role of the Special Pair in the Charge‐Separating Event in Photosynthesis

We synthesized special-pair/electron-acceptor systems consisting of a complementary slipped cofacial dimer of imidazolyl-substituted zinc porphyrin, bearing pyromellitdiimide as the electron acceptor. In the case of the dimer, the first and second oxidation potentials were split into a total of four peaks in the differential pulse voltammetry measurement. Furthermore, the shift values of the first oxidation potentials obtained by changing the solvent polarity for the dimer were almost half of those observed for the monomer. These results indicate that the radical cation is delocalized over the whole pi system of the dimer. Time-resolved transient absorption measurements revealed that, relative to the corresponding monomer, the dimer accelerated the charge separation rate, but decelerated the charge recombination rate. The smaller reorganization energy of the slipped cofacial dimer relative to that of the monomeric system demonstrates the significance of the special-pair arrangement for efficient charge separation in photosynthesis.

Energy transfer followed by electron transfer in a supramolecular triad composed of boron dipyrrin, zinc porphyrin, and fullerene: a model for the photosynthetic antenna-reaction center complex.

The first example of a working model of the photosynthetic antenna-reaction center complex, constructed via self-assembled supramolecular methodology, is reported. For this, a supramolecular triad is assembled by axially coordinating imidazole-appended fulleropyrrolidine to the zinc center of a covalently linked zinc porphyrin-boron dipyrrin dyad. Selective excitation of the boron dipyrrin moiety in the boron dipyrrin-zinc porphyrin dyad resulted in efficient energy transfer (k(ENT)(singlet) = 9.2 x 10(9) s(-)(1); Phi(ENT)(singlet) = 0.83) creating singlet excited zinc porphyrin. Upon forming the supramolecular triad, the excited zinc porphyrin resulted in efficient electron transfer to the coordinated fullerenes, resulting in a charge-separated state (k(cs)(singlet) = 4.7 x 10(9) s(-)(1); Phi(CS)(singlet) = 0.9). The observed energy transfer followed by electron transfer in the present supramolecular triad mimics the events of natural photosynthesis. Here, the boron dipyrrin acts as antenna chlorophyll that absorbs light energy and transports spatially to the photosynthetic reaction center, while the electron transfer from the excited zinc porphyrin to fullerene mimics the primary events of the reaction center where conversion of the electronic excitation energy to chemical energy in the form of charge separation takes place. The important feature of the present model system is its relative "simplicity" because of the utilized supramolecular approach to mimic rather complex "combined antenna-reaction center" events of photosynthesis.

Elucidation of the Primary Events in Photosynthesis. Novel Insights Based on Enolisation and Iminium Involvement with Pheophytin in Photosystem 2

Pheophytin (Pheo) is structurally constituted as to make possible certain reactions, previously given very little attention, which appear to play crucial roles in the initial electron transfer (ET) processes. The transformations involve enolisation with subsequent formation of mono- and di-iminium cations at the Pheo core. The important impact of these ions on ET from chlorophyll (Chl) to Pheo and then to quinone are evaluated. These insights rationalise the long-standing enigmas of fast transfer, across gap ET, activation-less aspect, and essential lack of reversibility. Comparisons are made to other important areas of iminium involvement, e.g. chemistry of vision, polyaniline doping, and DNA alkylation.

Fluorescence lifetime, yield, energy transfer and spectrum in photosynthesis, 1950-1960.

The fluorescence lifetime of chlorophyll agives information about the primary photo-physical events in photosynthesis. Most of the light energy absorbed by chlorophylls is utilized for photochemistry. There are two main additional pathways competing for the absorbed light energy: fluorescence and radiationless internal conversion (heat). Only a few percent of the absorbed energy proceeds along these two pathways. This historical minireview focuses on the first direct measurements of the lifetime of chlorophyll fluorescence, the time it takes to transfer energy from phycoerythrin to chlorophyll a, and the discovery of the fluorescence band at 720 nm (F720; then attributed to a dimer of chlorophyll). These works were carried out during the the late 1950s to the early 1960s in the laboratory of Professor Eugene Rabinowitch at the University of Illinois, Urbana-Champaign [Brody (1995) Photosynth Res 43: 67-74].

Primary electron transfer in modified bacterial reaction centers: optimization of the first events in photosynthesis

Pigment exchange in bacterial reaction centers (RC) of Rb. sphaeroides allows the systematic variation of the free energies of the primary (B A) and secondary (H A) electron acceptors. The effect of energetically elevated acceptor levels on the initial electron transfer (ET) step, on the P +B A − population and on the quantum yield of charge separation is explained within the stepwise ET model. Slower and less efficient charge separation with enhanced non-exponential character is found in the modified samples indicating that the native RC is a highly optimized system where small changes of an energy level may cause a significant reduction in efficiency.

Fullerenes as Novel Acceptors in Photosynthetic Electron Transfer

We propose a novel strategy using fullerenes for the construction of solar energy conversion systems that mimic the primary electron transfer events in photosynthesis. Redox-active fullerenes such as C60 and C70 were covalently bound to a porphyrin and the photophysical properties of the resulting compounds were investigated. Regardless of solvent and linkage, the charge-separated state is produced efficiently in zincporphyrin–fullerene dyads, showing that fullerenes are good electron acceptors. The most intriguing characteristic of fullerenes in electron transfer is that they accelerate photoinduced charge separation as well as charge shift and slow down charge recombination, properties that are in sharp contrast with those of conventional two-dimensional aromatic acceptors such as quinones and imides. The peculiar electron transfer properties of fullerenes can be explained by the small reorganization energies, which make it possible to optimize artificial photosynthetic multistep charge separation. A combination of the two strategies, multistep electron transfer and small reorganization energy of fullerenes, has been applied to light energy conversion systems as well as the more complex molecular systems such as triads. Highly efficient photosynthetic multistep electron transfer has been realized at gold electrodes modified with self-assembled monolayers of fullerene-containing molecules. These results will provide new principles and concepts to develop artificial photosynthetic materials as well as molecular devices.

Fullerenes as Novel Acceptors in Photosynthetic Electron Transfer

We propose a novel strategy using fullerenes for the construction of solar energy conversion systems that mimic the primary electron transfer events in photosynthesis. Redox-active fullerenes such as C60 and C70 were covalently bound to a porphyrin and the photophysical properties of the resulting compounds were investigated. Regardless of solvent and linkage, the charge-separated state is produced efficiently in zincporphyrin–fullerene dyads, showing that fullerenes are good electron acceptors. The most intriguing characteristic of fullerenes in electron transfer is that they accelerate photoinduced charge separation as well as charge shift and slow down charge recombination, properties that are in sharp contrast with those of conventional two-dimensional aromatic acceptors such as quinones and imides. The peculiar electron transfer properties of fullerenes can be explained by the small reorganization energies, which make it possible to optimize artificial photosynthetic multistep charge separation. A combination of the two strategies, multistep electron transfer and small reorganization energy of fullerenes, has been applied to light energy conversion systems as well as the more complex molecular systems such as triads. Highly efficient photosynthetic multistep electron transfer has been realized at gold electrodes modified with self-assembled monolayers of fullerene-containing molecules. These results will provide new principles and concepts to develop artificial photosynthetic materials as well as molecular devices.