Light Harvesting in Photosystem I Supercomplexes,

Investigating the Balance between Structural Conservation and Functional Flexibility in Photosystem I

Photosynthesis, as the primary source of energy for all life forms, plays a crucial role in maintaining the global balance of energy, entropy, and enthalpy in living organisms. Among its various building blocks, photosystem I (PSI) is responsible for light-driven electron transfer, crucial for generating cellular reducing power. PSI acts as a light-driven plastocyanin-ferredoxin oxidoreductase and is situated in the thylakoid membranes of cyanobacteria and the chloroplasts of eukaryotic photosynthetic organisms. Comprehending the structure and function of the photosynthetic machinery is essential for understanding its mode of action. New insights are offered into the structure and function of PSI and its associated light-harvesting proteins, with a specific focus on the remarkable structural conservation of the core complex and high plasticity of the peripheral light-harvesting complexes.

Energy Transfer and Radical-Pair Dynamics in Photosystem I with Different Red Chlorophyll a Pigments

We establish a general kinetic scheme for the energy transfer and radical-pair dynamics in photosystem I (PSI) of Chlamydomonas reinhardtii, Synechocystis PCC6803, Thermosynechococcus elongatus and Spirulina platensis grown under white-light conditions. With the help of simultaneous target analysis of transient-absorption data sets measured with two selective excitations, we resolved the spectral and kinetic properties of the different species present in PSI. WL-PSI can be described as a Bulk Chl a in equilibrium with a higher-energy Chl a, one or two Red Chl a and a reaction-center compartment (WL-RC). Three radical pairs (RPs) have been resolved with very similar properties in the four model organisms. The charge separation is virtually irreversible with a rate of ≈900 ns-1. The second rate, of RP1 → RP2, ranges from 70-90 ns-1 and the third rate, of RP2 → RP3, is ≈30 ns-1. Since RP1 and the Red Chl a are simultaneously present, resolving the RP1 properties is challenging. In Chlamydomonas reinhardtii, the excited WL-RC and Bulk Chl a compartments equilibrate with a lifetime of ≈0.28 ps, whereas the Red and the Bulk Chl a compartments equilibrate with a lifetime of ≈2.65 ps. We present a description of the thermodynamic properties of the model organisms at room temperature.

High-Resolution Frequency-Domain Spectroscopic and Modeling Studies of Photosystem I (PSI), PSI Mutants and PSI Supercomplexes

Photosystem I (PSI) is one of the two main pigment-protein complexes where the primary steps of oxygenic photosynthesis take place. This review describes low-temperature frequency-domain experiments (absorption, emission, circular dichroism, resonant and non-resonant hole-burned spectra) and modeling efforts reported for PSI in recent years. In particular, we focus on the spectral hole-burning studies, which are not as common in photosynthesis research as the time-domain spectroscopies. Experimental and modeling data obtained for trimeric cyanobacterial Photosystem I (PSI3), PSI3 mutants, and PSI3-IsiA18 supercomplexes are analyzed to provide a more comprehensive understanding of their excitonic structure and excitation energy transfer (EET) processes. Detailed information on the excitonic structure of photosynthetic complexes is essential to determine the structure-function relationship. We will focus on the so-called "red antenna states" of cyanobacterial PSI, as these states play an important role in photochemical processes and EET pathways. The high-resolution data and modeling studies presented here provide additional information on the energetics of the lowest energy states and their chlorophyll (Chl) compositions, as well as the EET pathways and how they are altered by mutations. We present evidence that the low-energy traps observed in PSI are excitonically coupled states with significant charge-transfer (CT) character. The analysis presented for various optical spectra of PSI3 and PSI3-IsiA18 supercomplexes allowed us to make inferences about EET from the IsiA18 ring to the PSI3 core and demonstrate that the number of entry points varies between sample preparations studied by different groups. In our most recent samples, there most likely are three entry points for EET from the IsiA18 ring per the PSI core monomer, with two of these entry points likely being located next to each other. Therefore, there are nine entry points from the IsiA18 ring to the PSI3 trimer. We anticipate that the data discussed below will stimulate further research in this area, providing even more insight into the structure-based models of these important cyanobacterial photosystems.

Energy transfer and trapping in photosystem I with and without chlorophyll-f

We establish a general kinetic scheme for energy transfer and trapping in the photosystem I (PSI) of cyanobacteria grown under white light (WL) or far-red light (FRL) conditions. With the help of simultaneous target analysis of all emission and transient absorption datasets measured in five cyanobacterial strains, we resolved the spectral and kinetic properties of the different species present in PSI. WL-PSI can be described by Bulk Chl a, two Red Chl a, and a reaction center compartment (WL-RC). The FRL-PSI contains two additional Chl f compartments. The lowest excited state of the FRL-RC is downshifted by ≈ 29 nm. The rate of charge separation drops from ≈900 ns-1 in WL-RC to ≈300 ns-1 in FRL-RC. The delayed trapping in the FRL-PSI (≈130 ps) is explained by uphill energy transfer from the Chl f compartments with Gibbs free energies of ≈kBT below that of the FRL-RC.

The location of the low-energy states in Lhca1 favors excitation energy transfer to the core in the plant PSI-LHCI supercomplex

Lhca1 is one of the four pigment-protein complexes composing the outer antenna of plant Photosystem I-light-havesting I supercomplex (PSI-LHCI). It forms a functional dimer with Lhca4 but, differently from this complex, it does not contain 'red-forms,' i.e., pigments absorbing above 700 nm. Interestingly, the recent PSI-LHCI structures suggest that Lhca1 is the main point of delivering the energy harvested by the antenna to the core. To identify the excitation energy pathways in Lhca1, we developed a structure-based exciton model based on the simultaneous fit of the low-temperature absorption, linear dichroism, and fluorescence spectra of wild-type Lhca1 and two mutants, lacking chlorophylls contributing to the long-wavelength region of the absorption. The model enables us to define the locations of the lowest energy pigments in Lhca1 and estimate pathways and timescales of energy transfer within the complex and to the PSI core. We found that Lhca1 has a particular energy landscape with an unusual (compared to Lhca4, LHCII, and CP29) configuration of the low-energy states. Remarkably, these states are located near the core, facilitating direct energy transfer to it. Moreover, the low-energy states of Lhca1 are also coupled to the red-most state (red forms) of the neighboring Lhca4 antenna, providing a pathway for effective excitation energy transfer from Lhca4 to the core.

Q-Band relaxation in chlorophyll: new insights from multireference quantum dynamics

The ultrafast relaxation within the Q-bands of chlorophyll plays a crucial role in photosynthetic light-harvesting. We investigate this process via nuclear and electronic quantum dynamics on multireference potential energy surfaces.

The role of vibronic modes in formation of red antenna states of cyanobacterial PSI

Cyanobacterial photosystem I (PSI) constitutes monomeric and trimeric pigment-protein complexes whose optical properties are marked by the presence of long-wavelength absorption bands. In spite of numerous experimental studies, the nature of these bands is still under debate and requires intensive theoretical analysis. Collecting together the data of linear spectroscopy and single-molecule spectroscopy (SMS) of PSI from Arthrospira platensis, we performed quantum modeling of the optical response based on molecular exciton theory (ET) and the multimode Brownian oscillator model (MBOM). Applying MBOM, the spectra of the red antenna state were calculated considering a particular for each red state adjustment of the low-frequency vibronic modes. Within the framework of our PSI exciton model it was shown that the coupling energy between antenna chlorophylls cannot be a factor of the red states formation, thus the long-wavelength bands are calculated without attribution to so-called antenna red chlorophylls. By the fitting of Huang-Rhys factors and frequencies for the lowest vibronic modes, we were able to reproduce the effects of strong and weak electron-phonon coupling experimentally observed in SMS spectra of red antenna states. Based on our theoretical calculations and also analysis of existing crystal structures of cyanobacterial PSI, we assumed that long-wavelength Chls can be localized in the peripheral protein subunits containing one or two pigment molecules.

Thermal, electrochemical and photochemical reactions involving catalytically versatile ene reductase enzymes

Successful exploitation of biocatalytic processes employing flavoproteins requires the implementation of cost-effective solutions to circumvent the need to supply costly nicotinamide coenzymes as reducing equivalents. Chemical syntheses harnessing the power of the flavoprotein ene reductases will likely increase the range and/or optical purity of available fine chemicals and pharmaceuticals due to their ability to catalyze asymmetric bioreductions. This review will outline current progress in the design of alternative routes to ene reductase flavin activation, most notably within the Old Yellow Enzyme family. A variety of chemical, enzymatic, electrochemical and photocatalytic routes have been employed, designed to eliminate the need for nicotinamide coenzymes or provide cost-effective alternatives to efficient recycling. Photochemical approaches have also enabled novel mechanistic routes of ene reductases to become available, opening up the possibility of accessing a wider range of non-natural chemical diversity.

Biocatalysis Fueled by Light: On the Versatile Combination of Photocatalysis and Enzymes

Enzymes catalyze a plethora of highly specific transformations under mild and environmentally benign reaction conditions. Their fascinating performances attest to high synthetic potential that is often hampered by operational obstacles such as in vitro cofactor supply and regeneration. Exploiting light and combining it with biocatalysis not only helps in overcoming these drawbacks, but the fruitful liaison of these two fields of "green chemistry" also offers opportunities to unlock new synthetic reactivities. In this review we provide an overview of the wide variety of photo-biocatalysis, ranging from the photochemical delivery of electrons required in redox biocatalysis and photochemical cofactor and reagent (re)generation to direct photoactivation of enzymes enabling reactions unknown in nature. We highlight synthetically relevant transformations such as asymmetric reactions facilitated by the combination of light as energy source and enzymes' catalytic power.

Excimer Formation in Carboxylic Acid-Functionalized Perylene Diimides Attached to Silicon Dioxide Nanoparticles

The creation of artificial light-harvesting complexes involves the ordered arrangement of chromophores in space. To guarantee efficient energy-transfer processes, organic dyes must be brought into close proximity, often leading to aggregation and the formation of excimer states. In recent years, the attachment of ligand-based chromophores to nanoparticles has also generated interest in relation to improved solar harvesting and spin-dependent electronic interactions such as singlet fission and upconversion. We explore the covalent attachment of two novel perylene-diimide (PDI) carboxylic acid ligands to silicon dioxide nanoparticles. This allows us to study electronic interactions between the ligands when attached to nanoparticles because these cannot couple to the wide band gap silicon dioxide. One of the synthesized PDI ligands has sterically hindering phenols in the bay position and undergoes minimal optical changes upon attachment, but the other forms an excimer state with a red-shifted and long-lived florescence. As such, molecular structure changes offer a method to tune weak and strong interactions between ligand layers on nanocrystal surfaces.

Structural Characterization and Photochemical Properties of Mono- and Bimetallic Cu-Mabiq Complexes

We present a series of monometallic ([Cu(Mabiq)OTf] (1) and [Cu(Mabiq)] (2)) and bimetallic copper-Mabiq complexes ([Cu₂(Mabiq)(PPh₃)₂(OTf)₂] (3) and [Cu₂(Mabiq)(PPh₃)₂]PF₆ (4)). The latter compounds contain an additional Cu^I center that binds in a tetrahedral fashion to the external bipyrimidine nitrogens of the macrocyclic ligand. Compounds 3 and 4 represent the first examples of bimetallic transition metal Mabiq complexes, stable both in solution and in the solid state. The structural and electronic properties of compounds 1-4 were analyzed by means of X-ray crystallography, cyclic voltammetry, and spectroscopic methods. One-electron reduced 2 and 4 consist of a Cu^II ion coordinated by a Mabiq ligand radical, [Cu^II(Mabiq^•)]. Thus, both bimetallic compounds are mixed-valent with respect to the copper oxidation states. Complexes 2 and 4 can be generated photochemically, upon irradiation of 1 or 3 with visible light in the presence of a sacrificial electron donor.

An algal photoenzyme converts fatty acids to hydrocarbons

Although many organisms capture or respond to sunlight, few enzymes are known to be driven by light. Among these are DNA photolyases and the photosynthetic reaction centers. Here, we show that the microalga Chlorella variabilis NC64A harbors a photoenzyme that acts in lipid metabolism. This enzyme belongs to an algae-specific clade of the glucose-methanol-choline oxidoreductase family and catalyzes the decarboxylation of free fatty acids to n-alkanes or -alkenes in response to blue light. Crystal structure of the protein reveals a fatty acid-binding site in a hydrophobic tunnel leading to the light-capturing flavin adenine dinucleotide (FAD) cofactor. The decarboxylation is initiated through electron abstraction from the fatty acid by the photoexcited FAD with a quantum yield >80%. This photoenzyme, which we name fatty acid photodecarboxylase, may be useful in light-driven, bio-based production of hydrocarbons.

Trimeric Assembly of Dendritic Light-Harvesting Antenna with Two Kinds of Porphyrin Cores

A trimeric assembly of light-harvesting antennas was prepared using a copper-catalyzed Hüisgen 1,3-dipolar cycloaddition reaction between a dendrimer having a zinc diethynyldiphenylporphyrin core (ZnDEDPP) with two azide terminals and two equivalents of dendrimers having a zinc tetraphenylporphyrin core (ZnTPP) with one ethynyl terminal. The absorptions of the trimer appear in a longer-wavelength region compared to monomeric references in toluene; however, there is almost no shift in wavelength in 1,1,2,2-tetrachloroethane (TCE). Fluorescence spectra of the trimer show that the singlet energy transfer from ZnTPP to ZnDEDPP takes place more effectively in toluene than in TCE. These absorption and fluorescence studies are compatible with solvent-dependent conformation; the extended forms of the trimers are favored by solvation in polar TCE, and the folded conformation is stabilized by the attractive van der Waals and dipole-dipole interactions between the dendritic chains in nonpolar toluene.

Pivotal Role of Iron in the Regulation of Cyanobacterial Electron Transport

Iron-containing metalloproteins are the main cornerstones for efficient electron transport in biological systems. The abundance and diversity of iron-dependent proteins in cyanobacteria makes those organisms highly dependent of this micronutrient. To cope with iron imbalance, cyanobacteria have developed a survey of adaptation strategies that are strongly related to the regulation of photosynthesis, nitrogen metabolism and other central electron transfer pathways. Furthermore, either in its ferrous form or as a component of the haem group, iron plays a crucial role as regulatory signalling molecule that directly or indirectly modulates the composition and efficiency of cyanobacterial redox reactions. We present here the major mechanism used by cyanobacteria to couple iron homeostasis to the regulation of electron transport, making special emphasis in processes specific in those organisms.

Structure and energy transfer in photosystems of oxygenic photosynthesis

Oxygenic photosynthesis is the principal converter of sunlight into chemical energy on Earth. Cyanobacteria and plants provide the oxygen, food, fuel, fibers, and platform chemicals for life on Earth. The conversion of solar energy into chemical energy is catalyzed by two multisubunit membrane protein complexes, photosystem I (PSI) and photosystem II (PSII). Light is absorbed by the pigment cofactors, and excitation energy is transferred among the antennae pigments and converted into chemical energy at very high efficiency. Oxygenic photosynthesis has existed for more than three billion years, during which its molecular machinery was perfected to minimize wasteful reactions. Light excitation transfer and singlet trapping won over fluorescence, radiation-less decay, and triplet formation. Photosynthetic reaction centers operate in organisms ranging from bacteria to higher plants. They are all evolutionarily linked. The crystal structure determination of photosynthetic protein complexes sheds light on the various partial reactions and explains how they are protected against wasteful pathways and why their function is robust. This review discusses the efficiency of photosynthetic solar energy conversion.

Excitonic dynamics of Chlorophyll-a molecules in chitosan hydrogel scaffold

Non-coherent energy hopping (hopping rate 4.28 ns⁻¹) through excitonically coupled 23° aligned Chl-a molecules within chitosan hydrogel matrix, for an artificial light harvesting system.

On the question of the light-harvesting role of β-carotene in photosystem II and photosystem I core complexes

β-Carotene is the only carotenoid present in the core complexes of Photosystems I and II. Its proximity to chlorophyll a molecules enables intermolecular electronic interactions, including β-carotene to chlorophyll a electronic excitation transfers. However, it has been well documented that, compared to chlorophylls and to phycobilins, the light harvesting efficiency of β-carotenes for photosynthetic O2 evolution is poor. This is more evident in cyanobacteria than in plants and algae because they lack accessory light harvesting pigments with absorptions that overlap the β-carotene absorption. In the present work we investigated the light harvesting role of β-carotenes in the cyanobacterium Synechococcus sp. PCC 7942 using selective β-carotene excitation and selective Photosystem detection of photo-induced electron transport to and from the intersystem plastoquinones (the plastoquinone pool). We report that, although selectively excited β-carotenes transfer electronic excitation to the chlorophyll a of both photosystems, they enable only the oxidation of the plastoquinone pool by Photosystem I but not its reduction by Photosystem II. This may suggest a light harvesting role for the β-carotenes of the Photosystem I core complex but not for those of the Photosystem II core complex. According to the present investigation, performed with whole cyanobacterial cells, the lower photosynthesis yields measured with β-Car-absorbed light can be attributed to the different excitation trapping efficiencies in the reaction centers of PSI and PSII.

Evidence for reverse pathways and equilibrium in singlet energy transfers between an artificial special pair and an antenna

A dyad, 1, built on an artificial special pair (bis(meso-nonyl)zinc(II)porphyrin), [Zn2], a spacer (biphenylene), a bridge (1,4-benzene), and an antenna (di-meso-(3,5-di(t-butyl)phenyl)porphyrin free base), FB, is prepared by Suzuki coupling and is analyzed by absorption and steady state, and time-resolved emission spectroscopy at 298 and 77 K. Using bases from the Förster theory, evidence for two pathways for S 1 energy transfer, FB* → [Zn2], and [Zn2]* → FB, along with their respective rates, k ET ( S 1)1 and k ET ( S 1)-1, are extracted from the comparison of the fluorescence decays monitored at the emission maximum. At 77 K, the unquenched (1.79 ([Zn2]) and 10.6 ns (FB)) and quenched components (<100 ps; i.e. k ET ( S 1) > 10 (ns)-1), are observed, hence, demonstrating the bidirectional paths with no back energy transfer. A 298 K, only two components are detected (0.44 ([Zn2]) and 2.64 ns (FB)) and the resulting reduced τFs indicates back energy transfer, therefore cycling and equilibrium. Their global rates are 0.31 and 1.8 (ns)-1 for k ET ( S 1)1 and k ET ( S 1)-1 at 298 K. This large temperature dependence on k ET ( S 1) is fully consistent with the participation of thermal activation. Finally, DFT calculations (B3LYP) were used to illustrate a clear correlation between the relative k ET ( S 1) s and the amplitude of the MO couplings between the artificial special pair and the antenna.

Is the special pair structure a good strategy for the kinetics during the last step of the energy transfer with the nearest antenna? A chemical model approach

A cofacial bis(Mg(II)porphyrin)-C(6)H(4)-free base ([Mg(2)]-bridge-FB) dyad shows S(1) energy transfer in both directions and much slower rates than similar monoporphyrin systems are observed.

Building fluorescent DNA nanodevices on target living cell surfaces

We report 1) the anchoring of preformed fluorescent DNA nanodevices (NDs) and 2) the in situ self-assembly of fluorescent DNA NDs on target living cell surfaces. Three types of aptamer-tethered DNA NDs were built and anchored on target cell surfaces by specific target-aptamer association. The in situ nanodevice self-assembly was further demonstrated on the surfaces of target living cells in cell mixtures. These DNA NDs exhibited fluorescence emission and underwent fluorescence energy transfer on living cell surfaces.

Two-dimensional electronic spectroscopy for mapping molecular photophysics

We demonstrate the ability of two-dimensional electronic spectroscopy (2DES) to map ultrafast energy transfer and dynamics in two systems: the pigment–protein complex photosystem I (PSI) and aggregates of the conjugated polymer poly(3-hexylthiophene) (P3HT). A detailed description of our experimental set-up and data processing procedure is also given.

Two-Dimensional Electronic Spectroscopy Reveals Ultrafast Downhill Energy Transfer in Photosystem I Trimers of the Cyanobacterium Thermosynechococcus elongatus

Two-dimensional electronic spectroscopy (2DES) was used to investigate the ultrafast energy-transfer dynamics of trimeric photosystem I of the cyanobacterium Thermosynechococcus elongatus. We demonstrate the ability of 2DES to resolve dynamics in a large pigment-protein complex containing ∼300 chromophores with both high frequency and time resolution. Monitoring the waiting-time-dependent changes of the line shape of the inhomogeneously broadened Qy(0-0) transition, we directly observe downhill energy equilibration on the 50 fs time scale.

The hidden function of photosynthesis: a sensing system for environmental conditions that regulates plant acclimation responses

Plants convert light energy from the sun into chemical energy by photosynthesis. Since they are sessile, they have to deal with a wide range of conditions in their immediate environment. Many abiotic and biotic parameters exhibit considerable fluctuations which can have detrimental effects especially on the efficiency of photosynthetic light harvesting. During evolution, plants, therefore, evolved a number of acclimation processes which help them to adapt photosynthesis to such environmental changes. This includes protective mechanisms such as excess energy dissipation and processes supporting energy redistribution, e.g. state transitions or photosystem stoichiometry adjustment. Intriguingly, all these responses are triggered by photosynthesis itself via the interplay of its light reaction and the Calvin-Benson cycle with the residing environmental condition. Thus, besides its primary function in harnessing and converting light energy, photosynthesis acts as a sensing system for environmental changes that controls molecular acclimation responses which adapt the photosynthetic function to the environmental change. Important signalling parameters directly or indirectly affected by the environment are the pH gradient across the thylakoid membrane and the redox states of components of the photosynthetic electron transport chain and/or electron end acceptors coupled to it. Recent advances demonstrate that these signals control post-translational modifications of the photosynthetic protein complexes and also affect plastid and nuclear gene expression machineries as well as metabolic pathways providing a regulatory framework for an integrated response of the plant to the environment at all cellular levels.

Photoinduced charge transfer in short-distance ferrocenylsubphthalocyanine dyads

Two new ferrocenylsubphthalocyanine dyads with ferrocenylmethoxide (2) and ferrocenecarboxylate (3) substituents directly attached to the subphthalocyanine ligand via the axial position have been prepared and characterized using NMR, UV-vis, and magnetic circular dichroism (MCD) spectroscopies as well as X-ray crystallography. The redox properties of the ferrocenyl-containing dyads 2 and 3 were investigated using the cyclic voltammetry (CV) approach and compared to those of the parent subphthalocyanine 1. CV data reveal that the first reversible oxidation is ferrocene-centered, while the second oxidation and the first reduction are localized on the subphthalocyanine ligand. The electronic structures and nature of the optical bands observed in the UV-vis and MCD spectra of all target compounds were investigated by a density functional theory polarized continuum model (DFT-PCM) and time-dependent (TD)DFT-PCM approaches. It has been found that in both dyads the highest occupied molecular orbital (HOMO) to HOMO-2 are ferrocene-centered molecular orbitals, while HOMO-3 as well as lowest unoccupied molecular orbital (LUMO) and LUMO+1 are localized on the subphthalocyanine ligand. TDDFT-PCM data on complexes 1-3 are consistent with the experimental observations, which indicate the dominance of π-π* transitions in the UV-vis spectra of 1-3. The excited-state dynamics of the dyads 2 and 3 were investigated using time-correlated single photon counting, which indicates that fluorescence quenching is more efficient in dyad 3 compared to dyad 2. These fluorescence lifetime measurements were interpreted on the basis of DFT-PCM calculations.

Solution structure of a novel α-conotoxin with a distinctive loop spacing pattern

α-Pharmacological conotoxins are among the most selective ligands of nicotinic acetylcholine receptors with typical cysteine frameworks. They are characterized by the intercysteine loop and classified into various subfamilies, such as α3/5 and α4/7 conotoxins. A novel α-conotoxin, Pu14a (DCPPHPVPGMHKCVCLKTC), with a distinct loop spacing pattern between cysteines was reported recently. Pu14a belongs to the Cys framework 14 (-C-C-C-C) family containing four proline residues in the loop 1 region. Similar to another framework 14 conotoxin Lt14a (MCPPLCKPSCTNC-NH2), Pu14a has C1-C3/C2-C4 disulfide linkage, and can inhibit some subtypes of nicotinic acetylcholine receptors. In this study, the solution structure of Pu14a was investigated using 1H nuclear magnetic resonance spectroscopy to understand the structure-activity relationship of this conotoxin. 20 converged structures of this conopeptide, with RMSD value of 0.77 Å, were obtained based on distance constraints, dihedral angles and disulfide bond constraints. The three-dimensional structure of Pu14a showed remarkable difference from typical α-conotoxins because of a large intercysteine loop between C2 and C13, as well as a 3(10)-helix near the C-terminal. Furthermore, four proline residues in Pu14a adopted the trans conformation that may correlate with the large loop configuration and the biological activity of this conopeptide. The distinct structural characteristics of Pu14a will be very useful for studying the structure-activity relationship of α-conotoxins.

Close Interspecies Interactions between Prokaryotes from Sulfureous Environments

Green sulfur bacteria are obligate photolithoautotrophs that require highly reducing conditions for growth and can utilize only a very limited number of carbon substrates. These bacteria thus inhabit a very narrow ecologic niche. However, several green sulfur bacteria have overcome the limits of immobility by entering into a symbiosis with motile Betaproteobacteria in a type of multicellular association termed phototrophic consortia. One of these consortia, "Chlorochromatium aggregatum," has recently been established as the first culturable model system to elucidate the molecular basis of this symbiotic interaction. It consists of 12-20 green sulfur bacteria epibionts surrounding a central, chemoheterotrophic betaproteobacterium in a highly ordered fashion. Recent genomic, transcriptomic, and proteomic studies of "C. aggregatum" and its epibiont provide insights into the molecular basis and the origin of the stable association between the two very distantly related bacteria. While numerous genes of central metabolic pathways are upregulated during the specific symbiosis and hence involved in the interaction, only a limited number of unique putative symbiosis genes have been detected in the epibiont. Green sulfur bacteria therefore are preadapted to a symbiotic lifestyle. The metabolic coupling between the bacterial partners appears to involve amino acids and highly specific ultrastructures at the contact sites between the cells. Similarly, the interaction in the equally well studied archaeal consortia consisting of Nanoarchaeum equitans and its host Ignicoccus hospitalis is based on the transfer of amino acids while lacking the highly specialized contact sites observed in phototrophic consortia.

The role of the environment in electronic energy transfer: a molecular modeling perspective

The key role of the environment in electronic energy transfer has been underscored in recent experimental and theoretical studies. In this perspective, we provide an overview of novel quantum-mechanical methodologies aimed at describing environment effects in energy transfers. The techniques described include continuum dielectric and atomistic descriptions of the surroundings. We discuss the advantages and limitations of each technique, as well as the main insights that have emerged from their application to solvated dyads and photosynthetic pigment-protein complexes. We finally highlight the aspects that still need to be solved in order to provide a full theoretical route to the study of energy transfer phenomena in complex environments.

Isolation and characteristics of the PSI-LHCI-LHCII supercomplex under high light

We developed a novel method for the isolation of the PSI-LHCI-LHCII complex from spinach leaves. The supercomplex was resolved into a core complex (CPI), LHCII trimers, LHCI dimers and LHCII monomers using green gel electrophoresis. We then investigate changes in the fluorescence and absorption spectra of PSI-LHCI-LHCII under high light. In addition, we compared light-induced denaturation of the core protein subunits in both PSI-LHCI and PSI-LHCI-LHCII. Differences in denaturation and photochemical activity indicated that binding of LHCII increased the photosensitivity of the PSI core. Increased energy delivered to the PSI core during illumination accelerated damage to the core complex.