Light-harvesting Antenna Systems Research Articles

Synthesis of 20-iodochlorophyll derivatives and their properties including reactivity, electronic absorption, and self-aggregation

Methyl 20-iodopyropheophorbides-a possessing several substituents at the 3-position were prepared by treatment of the corresponding 20-unsubstituted chlorophyll-a derivatives with iodine and [bis(trifluoroacetoxy)iodo]benzene in chloroform and water at room temperature. The sterically demanding 20-iodination was clean, efficient, and regioselective without alteration of the 3-substituents, and also highly reproducible. The resulting 20-iodides were so reactive that they were readily transformed into 20-acetylchlorin (Pd-catalyzed cross-coupling) and 20-unsubstituted chlorin (acidic deiodination). Electronic absorption spectra in dichloromethane showed that the 20-iodination shifted the maxima to longer wavelengths in their monomeric states. Zinc complexes of 31-hydroxy-20-iodo-131-oxochlorins self-aggregated in non-polar organic solvents to give red-shifted and broadened absorption bands, similarly as in the corresponding 20-unsubstituted molecules, which are good models of the main light-harvesting antenna systems in photosynthetic green bacteria.

Self-aggregation of synthetic zinc chlorophyll derivatives possessing 3(1)-hydroxy or methoxy group and 13(1)-mono- or dicyanomethylene moiety in nonpolar organic solvents as models of chlorosomal bacteriochlorophyll-d aggregates.

Methyl 13(1)-(di)cyanomethylene-pyropheophorbides were synthesized by Knoevenagel reactions of the corresponding 13(1)-oxo-chlorins prepared from modifying chlorophyll-a with malononitrile or cyanoacetic acid. Alternatively, methyl 13(1)-cyanomethylene-pyropheophorbides were produced by Wittig reactions of 13(1)-oxo-chlorins with Ph3P=CHCN. Self-aggregation of zinc complexes of the semi-synthetic chlorophyll derivatives possessing a hydroxy or methoxy group at the 3(1)-position was examined in 1%(v/v) tetrahydrofuran or dichloromethane and hexane by electronic absorption and circular dichroism spectroscopy. Although intermolecular hydrogen-bonding between the 3(1)-hydroxy and 13(1)-oxo groups of bacteriochlorophylls-c/d/e/f was essential for their self-aggregation in natural light-harvesting antenna systems (=chlorosomes), zinc 3(1)-hydroxy-13(1)-di/monocyanomethylene-chlorins self-aggregated in the less/lesser polar organic solvents to form chlorosome-like large oligomers in spite of lacking the 13(1)-oxo moiety as the hydrogen-bonding acceptor. Zinc 3(1)-methoxy-13(1)-dicyanomethylene-chlorin gave similar self-aggregates regardless of lack of both the 3(1)-hydroxy and 13(1)-oxo groups. The present self-aggregation was ascribable to stronger coordination of the 3(1)-oxygen atom to the central zinc than the conventional systems, where the electron-withdrawing cyano group(s) increased the coordinative ability of the central zinc through the chlorin π-system.

Scrambled Self‐Assembly of Bacteriochlorophylls c and e in Aqueous Triton X‐100 Micelles

Bacteriochlorophyll (BChl) e was coassembled with BChl c in Triton X-100 micelles in aqueous solutions. The Qy absorption bands of the coaggregates were positioned between those of aggregates consisting solely of BChl c or e. The electronic absorption spectra of the coaggregates could not be reproduced by linear combinations of the spectra of the aggregates consisting solely of each pigment, but they were in line with the simulated spectra for the self-aggregates in which both BChls were randomly distributed. These suggest that BChls c and e are not spatially separated; they are homogenously distributed over the self-aggregates to give electronic spectra that are different from those of the aggregate consisting solely of each pigment. Deaggregation of the scrambled self-aggregates by excess Triton X-100 did not produce any spectral components assigned to an aggregate consisting solely of either BChl c or e. Acid-induced decomposition of the scrambled aggregates showed different kinetics from those of the aggregates consisting solely of each pigment. These also support the homogeneous distribution of BChls c and e in the scrambled self-aggregates. These results will be useful to investigate the major light-harvesting antenna systems of green photosynthetic bacteria that contain two kinds of chlorosomal BChls.

Cyclopropane-ring formation in the acyl groups of chlorosome glycolipids is crucial for acid resistance of green bacterial antenna systems

Green photosynthetic bacteria have unique light-harvesting antenna systems called chlorosomes. Chlorobaculum tepidum, a model organism of the bacteria, biosynthesized monogalactosyl- and rhamnosylgalactosyldiacylglycerides possessing a methylene-bridged palmitoleyl group characterized by a cis-substituted cyclopropane ring as the dominant glycolipids of its chlorosome surface. The formation of the cyclopropane ring was chemically inhibited by supplementation of sinefungin, an analog of S-adenosyl-l-methionine, into the bacterial cultivation. The presence of the cyclopropane ring reinforced acid resistance of the light-harvesting chlorosomes and suppressed acidic demetalation (pheophytinization) of bacteriochlorophyll-c pigments constructing the core part of chlorosomes. The ring-formation would represent direct and post-synthetic modifications of chlorosome membrane properties and was tolerant of acidic environments.

Organization and Flexibility of Cyanobacterial Thylakoid Membranes Examined by Neutron Scattering

Cyanobacteria are prokaryotes that can use photosynthesis to convert sunlight into cellular fuel. Knowledge of the organization of the membrane systems in cyanobacteria is critical to understanding the metabolic processes in these organisms. We examined the wild-type strain of Synechocystis sp. PCC 6803 and a series of mutants with altered light-harvesting phycobilisome antenna systems for changes in thylakoid membrane architecture under different conditions. Using small-angle neutron scattering, it was possible to resolve correlation distances of subcellular structures in live cells on the nanometer scale and capture dynamic light-induced changes to these distances. Measurements made from samples with varied scattering contrasts confirmed that these distances could be attributed to the thylakoid lamellar system. We found that the changes to the thylakoid system were reversible between light- and dark-adapted states, demonstrating a robust structural flexibility in the architecture of cyanobacterial cells. Chemical disruption of photosynthetic electron transfer diminished these changes, confirming the involvement of the photosynthetic apparatus. We have correlated these findings with electron microscopy data to understand the origin of the changes in the membranes and found that light induces an expansion in the center-to-center distances between the thylakoid membrane layers. These combined data lend a dynamic dimension to the intracellular organization in cyanobacterial cells.

New hybrid materials with porphyrin-ferrocene and porphyrin-pyrene covalently linked to single-walled carbon nanotubes

Novel porphyrin derivatives bearing additional pyrene or ferrocene units as light harvesting antenna systems were synthesized and fully characterized. Following a covalent functionalization approach for single-walled carbon nanotubes (SWCNTs), stable SWCNT suspensions in common organic solvents were produced. Subsequently, the resulting porphyrin-pyrene and porphyrin-ferrocene dyads were incorporated onto the nanotubes' backbone yielding donor–donor–acceptor hybrids. The resulting hybrid materials were soluble in common organic solvents and were characterized using micro-Raman, ATR-IR, UV-Vis and photoluminescence spectroscopy, transmission electron microscopy, thermogravimetric analysis and electrochemistry. Photoluminescence quenching of the porphyrin emission in both hybrid materials was detected thus suggesting the potentiality of these materials in photoelectrochemical cells.

Hierarchical Self-Assembly of a Biomimetic Light-Harvesting Antenna Based on DNA G-Quadruplexes

A new modular approach to an artificial light-harvesting antenna system is presented. The approach involves the hierarchical self-assembly of porphyrin acceptor molecules to G-quadruplexes tethered to coumarin donor moieties.

Differential mobility of pigment-protein complexes in granal and agranal thylakoid membranes of C₃ and C₄ plants.

The photosynthetic performance of plants is crucially dependent on the mobility of the molecular complexes that catalyze the conversion of sunlight to metabolic energy equivalents in the thylakoid membrane network inside chloroplasts. The role of the extensive folding of thylakoid membranes leading to structural differentiation into stacked grana regions and unstacked stroma lamellae for diffusion-based processes of the photosynthetic machinery is poorly understood. This study examines, to our knowledge for the first time, the mobility of photosynthetic pigment-protein complexes in unstacked thylakoid regions in the C₃ plant Arabidopsis (Arabidopsis thaliana) and agranal bundle sheath chloroplasts of the C₄ plants sorghum (Sorghum bicolor) and maize (Zea mays) by the fluorescence recovery after photobleaching technique. In unstacked thylakoid membranes, more than 50% of the protein complexes are mobile, whereas this number drops to about 20% in stacked grana regions. The higher molecular mobility in unstacked thylakoid regions is explained by a lower protein-packing density compared with stacked grana regions. It is postulated that thylakoid membrane stacking to form grana leads to protein crowding that impedes lateral diffusion processes but is required for efficient light harvesting of the modularly organized photosystem II and its light-harvesting antenna system. In contrast, the arrangement of the photosystem I light-harvesting complex I in separate units in unstacked thylakoid membranes does not require dense protein packing, which is advantageous for protein diffusion.

Memory-Assisted Exciton Diffusion in the Chlorosome Light-Harvesting Antenna of Green Sulfur Bacteria.

Chlorosomes are likely the largest and most efficient natural light-harvesting photosynthetic antenna systems. They are composed of large numbers of bacteriochlorophylls organized into supramolecular aggregates. We explore the microscopic origin of the fast excitation energy transfer in the chlorosome using the recently resolved structure and atomistic-detail simulations. Despite the dynamical disorder effects on the electronic transitions of the bacteriochlorophylls, our simulations show that the exciton delocalizes over the entire aggregate in about 200 fs. The memory effects associated to the dynamical disorder assist the exciton diffusion through the aggregates and enhance the diffusion coefficients as a factor of 2 as compared to the model without memory. Furthermore, exciton diffusion in the chlorosome is found to be highly anisotropic with the preferential transfer toward the baseplate, which is the next functional element in the photosynthetic system.

Supramolecular energy transfer from photoexcited chlorosomal zinc porphyrin self-aggregates to a chlorin or bacteriochlorin monomer as models of main light-harvesting antenna systems in green photosynthetic bacteria

Self-aggregates of a synthetic zinc porphyrin worked as a light absorber and photoexcited energy donor, transferred the collected energy to a small amount of 3-acetyl-(bacterio)chlorin monomer, and induced near-infrared fluorescence from the acceptors in aqueous micellar solution. These artificial supramolecular systems are novel models of the main light-harvesting antennas of green photosynthetic bacteria, chlorosomes.

Allelic Variations of a Light Harvesting Chlorophyll A/B-Binding Protein Gene (Lhcb1) Associated with Agronomic Traits in Barley

Light-harvesting chlorophyll a/b-binding protein (LHCP) is one of the most abundant chloroplast proteins in plants. Its main function is to collect and transfer light energy to photosynthetic reaction centers. However, the roles of different LHCPs in light-harvesting antenna systems remain obscure. Exploration of nucleotide variation in the genes encoding LHCP can facilitate a better understanding of the functions of LHCP. In this study, nucleotide variations in Lhcb1, a LHCP gene in barley, were investigated across 292 barley accessions collected from 35 different countries using EcoTILLING technology, a variation of the Targeting Induced Local Lesions In Genomes (TILLING). A total of 23 nucleotide variations were detected including three insert/deletions (indels) and 20 single nucleotide polymorphisms (SNPs). Among them, 17 SNPs were in the coding region with nine missense changes. Two SNPs with missense changes are predicted to be deleterious to protein function. Seventeen SNP formed 31 distinguishable haplotypes in the barley collection. The levels of nucleotide diversity in the Lhcb1 locus differed markedly with geographic origins and species of accessions. The accessions from Middle East Asia exhibited the highest nucleotide and haplotype diversity. H. spontaneum showed greater nucleotide diversity than H. vulgare. Five SNPs in Lhcb1 were significantly associated with at least one of the six agronomic traits evaluated, namely plant height, spike length, number of grains per spike, thousand grain weight, flag leaf area and leaf color, and these SNPs may be used as potential markers for improvement of these barley traits.

Artificial light-harvesting antenna systems grafted on a carbohydrate platform

An enantiopure α-D-glucopyranoside derivative has been used as a platform to prepare artificial antenna systems based on bodipy subunits. Efficient and ultrafast energy transfer (in the fs and ps time regimes) takes place in the multibodipy systems.

Photosynthetic Light-Harvesting Is Tuned by the Heterogeneous Polarizable Environment of the Protein

In photosynthesis, special antenna proteins that contain multiple light-absorbing molecules (chromophores) are able to capture sunlight and transfer the excitation energy to reaction centers with almost 100% quantum efficiencies. The critical role of the protein scaffold in holding the appropriate arrangement of the chromophores is well established and can be intuitively understood given the need to keep optimal dipole-dipole interactions between the energy-transferring chromophores, as described by Förster theory more than 60 years ago. However, the question whether the protein structure can also play an active role by tuning such dipole-dipole interactions has not been answered so far, its effect being rather crudely described by simple screening factors related to the refractive index properties of the system. Here, we present a combined quantum chemical/molecular mechanical approach to compute electronic couplings that accounts for the heterogeneous dielectric nature of the protein-solvent environment in atomic detail. We apply the method to study the effect of dielectric heterogeneity in the energy migration properties of the PE545 principal light-harvesting antenna of the cryptomonad Rhodomonas CS24. We find that dielectric heterogeneity can profoundly tune by a factor up to ∼4 the energy migration rates between chromophore sites compared to the average continuum dielectric view that has historically been assumed. Our results indicate that engineering of the local dielectric environment can potentially be used to optimize artificial light-harvesting antenna systems.

In vitro synthesis and characterization of bacteriochlorophyll-f and its absence in bacteriochlorophyll-e producing organisms

Bacteriochlorophyll(BChl)-f which has not yet been found in natural phototrophs was prepared by chemically modifying chlorophyll-b. The retention time of reverse-phase high-performance liquid chromatography of the synthetic monomeric BChl-f as well as its visible absorption and fluorescence emission spectra in a solution were identified and compared with other naturally occurring chlorophyll pigments obtained from the main light-harvesting antenna systems of green sulfur bacteria, BChls-c/d/e. Based on the above data, BChl-f was below the level of detection in three strains of green photosynthetic bacteria producing BChl-e.

The composition and structure of photosystem I-associated antenna from Cyanidioschyzon merolae

Red algae contain two types of light-harvesting antenna systems, the phycobilisomes and chlorophyll a binding polypeptides (termed Lhcr), which expand the light-harvesting capacity of the photosynthetic reaction centers. In this study, photosystem I (PSI) and its associated light-harvesting proteins were isolated from the red alga Cyanidioschyzon merolae. The structural and functional properties of the largest PSI particles observed were investigated by biochemical characterization, mass spectrometry, fluorescence emission and excitation spectroscopy, and transmission electron microscopy. Our data provide strong evidence for a stable PSI complex in red algae that possesses two distinct types of functional peripheral light-harvesting antenna complex, comprising both Lhcr and a PSI-linked phycobilisome sub-complex. We conclude that the PSI antennae system of red algae represents an evolutionary intermediate between the prokaryotic cyanobacteria and other eukaryotes, such as green algae and vascular plants.

A model of the protein–pigment baseplate complex in chlorosomes of photosynthetic green bacteria

In contrast to photosynthetic reaction centers, which share the same structural architecture, more variety is found in the light-harvesting antenna systems of phototrophic organisms. The largest antenna system described, so far, is the chlorosome found in anoxygenic green bacteria, as well as in a recently discovered aerobic phototroph. Chlorosomes are the only antenna system, in which the major light-harvesting pigments are organized in self-assembled supramolecular aggregates rather than on protein scaffolds. This unique feature is believed to explain why some green bacteria are able to carry out photosynthesis at very low light intensities. Encasing the chlorosome pigments is a protein-lipid monolayer including an additional antenna complex: the baseplate, a two-dimensional paracrystalline structure containing the chlorosome protein CsmA and bacteriochlorophyll a (BChl a). In this article, we review current knowledge of the baseplate antenna complex, which physically and functionally connects the chlorosome pigments to the reaction centers via the Fenna-Matthews-Olson protein, with special emphasis on the well-studied green sulfur bacterium Chlorobaculum tepidum (previously Chlorobium tepidum). A possible role for the baseplate in the biogenesis of chlorosomes is discussed. In the final part, we present a structural model of the baseplate through combination of a recent NMR structure of CsmA and simulation of circular dichroism and optical spectra for the CsmA-BChl a complex.

Discrete Cyclic Porphyrin Arrays as Artificial Light-Harvesting Antenna

The importance of photosynthesis has driven researchers to seek ways to mimic its fundamental features in simplified systems. The absorption of a photon by light-harvesting (antenna) complexes made up of a large number of protein-embedded pigments initiates photosynthesis. Subsequently the many pigments within the antenna system shuttle that photon via an efficient excitation energy transfer (EET) until it encounters a reaction center. Since the 1995 discovery of the circularly arranged chromophoric assemblies in the crystal structure of light-harvesting antenna complex LH2 of purple bacteria Rps. Acidophila, many designs of light-harvesting antenna systems have focused on cyclic porphyrin wheels that allow for efficient EET. In this Account, we review recent research in our laboratories in the synthesis of covalently and noncovalently linked discrete cyclic porphyrin arrays as models of the photosynthetic light-harvesting antenna complexes. On the basis of the silver(I)-promoted oxidative coupling strategy, we have prepared a series of extremely long yet discrete meso-meso-linked porphyrin arrays and covalently linked large porphyrin rings. We examined the photophysical properties of these molecules using steady-state absorption, fluorescence, fluorescence lifetime, fluorescence anisotropy decay, and transient absorption measurements. Both the pump-power dependence on the femtosecond transient absorption and the transient absorption anisotropy decay profiles are directly related to the EET processes within the porphyrin rings. Within these structures, the exciton-exciton annihilation time and the polarization anisotropy rise time are well-described in terms of the Forster-type incoherent energy hopping model. In noncoordinating solvents such as CHCl(3), meso-pyridine-appended zinc(II) porphyrins and their meso-meso-linked dimers spontaneously assemble to form tetrameric porphyrin squares and porphyrin boxes, respectively. In the latter case, we have demonstrated the rigorous homochiral self-sorting process and efficient EET along these cyclic porphyrin arrays. The meso-cinchomeronimide appended zinc(II) porphyrin forms a cyclic trimer. We have also shown that the corresponding meso-meso-linked diporphyrins undergo high-fidelity self-sorting assembling to form discrete cyclic trimer, tetramer, and pentamer with large association constants through perfect discrimination of enantiomeric and conformational differences of the meso-cinchomeronimide substituents. Collectively, these studies of covalently and noncovalently linked discrete cyclic porphyrin arrays aid in the understanding of the structural requirements for such very fast EET in natural light-harvesting complexes.

Ordered Polyelectrolyte Multilayers: Unidirectional FRET Cascade in Nanocompartmentalized Polyelectrolyte Multilayers

Multifunctional polyelectrolyte (or layer-by-layer, LbL) multilayers consisting of a set of nanocompartments separated by impermeable ultrathin barriers, whereby the thickness of the compartments is tuned in the range 1-10 nm, are synthesized. Each compartment contains a different dye, introduced by co-adsorption during multilayer deposition. Different LbL barriers are tested for impermeability towards dye diffusion while simultaneously allowing energy transfer to occur between the compartmentalized dyes. Cross-linked LbL multilayers based on poly(acrylic acid) and poly(allyl amine) are shown to provide the desired impermeability for thicknesses as small as about 2.5 nm. A proof-of-concept system is then realized involving a cascade of two FRET processes, whereby the light energy is collected in a first nanocompartment containing pyranine, sent to a second nanocompartment loaded with fluorescein, before finally being transferred to a third, Nile blue-filled compartment located at the external surface of the film. This demonstrates the possibility to fabricate complex light-harvesting antenna systems by LbL assembly while controlling the architecture of the antenna down to a few nanometers.

Energy Transfer among Light-Harvesting Macrorings Incorporated into a Bilayer Membrane

Amphiphilic bis(imidazolyl)tris(porphyrinatozinc) complexes having omega-carboxyalkyl meso substituents were prepared. The barrel-type porphyrin macroring organized through their complementary coordination was incorporated into a liposomal bilayer membrane with orientation normal to the surface. Under the conditions concentrated in the membrane, introduction of a fullerotripyridyl ligand into the cavity of the macroring quenched fluorescence not only from the host ring itself but also from other surrounding macrorings. Energy transfer among antenna macrorings in the membrane mimics the assembly of a bacterial light-harvesting antenna system.

Synthesis of Zinc 3-Hydroxymethyl-porphyrins Possessing Carbonyl Groups at the 13- and/or 15-Positions for Models of Self-Aggregative Chlorophylls in Green Photosynthetic Bacteria

Zinc 3-hydroxymethyl-protopyropheophorbide-d derivatives 1, 2, and 3 possessing carbonyl group(s) at the 13/15-, 13-, and 15-positions, respectively, and 4 lacking the C=O group were prepared from naturally occurring chlorophyll-a. Zinc porphyrins 1 and 2 possessing the 13-C=O self-aggregated in an aqueous micellar solution, similarly to the self-aggregates of BChls-c/d/e in the main light-harvesting antenna systems of green photosynthetic bacteria, whereas 3 and 4 lacking the 13-C=O failed to make such well-ordered aggregates. These results indicated that both the presence of the C=O group and its situation at the 13-position were preferable for formation of the well-ordered J-aggregates.