What We Are Learning from the Diverse Structures of the Homodimeric Type I Reaction Center-Photosystems of Anoxygenic Phototropic Bacteria.

A Type I reaction center (RC) (Fe-S type, ferredoxin reducing) is found in several phyla containing anoxygenic phototrophic bacteria. These include the heliobacteria (HB), the green sulfur bacteria (GSB), and the chloracidobacteria (CB), for which high-resolution homodimeric RC-photosystem (PS) structures have recently appeared. The 2.2-Å X-ray structure of the RC-PS of Heliomicrobium modesticaldum revealed that the core PshA apoprotein (PshA-1 and PshA-2 homodimeric pair) exhibits a structurally conserved PSI arrangement comprising five C-terminal transmembrane α-helices (TMHs) forming the RC domain and six N-terminal TMHs coordinating the light-harvesting (LH) pigments. The Hmi. modesticaldum structure lacked quinone molecules, indicating that electrons were transferred directly from the A0 (81-OH-chlorophyll (Chl) a) acceptor to the FX [4Fe-4S] component, serving as the terminal RC acceptor. A pair of additional TMHs designated as Psh X were also found that function as a low-energy antenna. The 2.5-Å resolution cryo-electron microscopy (cryo-EM) structure for the RC-PS of the green sulfur bacterium Chlorobaculum tepidum included a pair of Fenna-Matthews-Olson protein (FMO) antennae, which transfer excitations from the chlorosomes to the RC-PS (PscA-1 and PscA-2) core. A pair of cytochromes cZ (PscC) molecules was also revealed, acting as electron donors to the RC bacteriochlorophyll (BChl) a' special pair, as well as PscB, housing the [4Fe-4S] cluster FA and FB, and the associated PscD protein. While the FMO components were missing from the 2.6-Å cryo-EM structure of the Zn- (BChl) a' special pair containing RC-PS of Chloracidobacterium thermophilum, a unique architecture was revealed that besides the (PscA)2 core, consisted of seven additional subunits including PscZ in place of PscD, the PscX and PscY cytochrome c serial electron donors and four low mol. wt. subunits of unknown function. Overall, these diverse structures have revealed that (i) the HB RC-PS is the simplest light-energy transducing complex yet isolated and represents the closest known homolog to a common homodimeric RC-PS ancestor; (ii) the symmetrically localized Ca2+-binding sites found in each of the Type I homodimeric RC-PS structures likely gave rise to the analogously positioned Mn4CaO5 cluster of the PSII RC and the TyrZ RC donor site; (iii) a close relationship between the GSB RC-PS and the PSII Chl proteins (CP)43 and CP47 was demonstrated by their strongly conserved LH-(B)Chl localizations; (iv) LH-BChls of the GSB-RC-PS are also localized in the conserved RC-associated positions of the PSII ChlZ-D1 and ChlZ-D2 sites; (v) glycosylated carotenoids of the GSB RC-PS are located in the homologous carotenoid-containing positions of PSII, reflecting an O2-tolerance mechanism capable of sustaining early stages in the evolution of oxygenic photosynthesis. In addition to the close relationships found between the homodimeric RC-PS and PSII, duplication of the gene encoding the ancestral Type I RC apoprotein, followed by genetic divergence, may well account for the appearance of the heterodimeric Type I and Type II RCs of the extant oxygenic phototrophs. Accordingly, the long-held view that PSII arose from the anoxygenic Type II RC is now found to be contrary to the new evidence provided by Type I RC-PS homodimer structures, indicating that the evolutionary origins of anoxygenic Type II RCs, along with their distinct antenna rings are likely to have been preceded by the events that gave rise to their oxygenic counterparts.

Experimental and Theoretical Mutation of Exciton States on the Smallest Type-I Photosynthetic Reaction Center Complex of a Green Sulfur Bacterium Chlorobaclum tepidum.

The exciton states on the smallest type-I photosynthetic reaction center complex of a green sulfur bacterium Chlorobaculum tepidum (GsbRC) consisting of 26 bacteriochlorophylls a (BChl a) and four chlorophylls a (Chl a) located on the homodimer of two PscA reaction center polypeptides were investigated. This analysis involved the study of exciton states through a combination of theoretical modeling and the genetic removal of BChl a pigments at eight sites. (1) A theoretical model of the pigment assembly exciton state on GsbRC was constructed using Poisson TrESP (P-TrESP) and charge density coupling (CDC) methods based on structural information. The model reproduced the experimentally obtained absorption spectrum, circular dichroism spectrum, and excitation transfer dynamics, as well as explained the effects of mutation. (2) Eight BChl a molecules at different locations on the GsbRC were selectively removed by genetic exchange of the His residue, which ligates the central Mg atom of BChl a, with the Leu residue on either one or two PscAs in the RC. His locations are conserved among all type-I RC plant polypeptide, cyanobacteria, and bacteria amino acid sequences. (3) Purified mutant-GsbRCs demonstrated distinct absorption and fluorescence spectra at 77 K, which were different from each other, suggesting successful pigment removal. (4) The same mutations were applied to the constructed theoretical model to analyze the outcomes of these mutations. (5) The combination of theoretical predictions and experimental mutations based on structural information is a new tool for studying the function and evolution of photosynthetic reaction centers.

Late acquisition of the rTCA carbon fixation pathway by Chlorobi.

The reverse tricarboxylic acid (rTCA) cycle is touted as a primordial mode of carbon fixation due to its autocatalytic propensity and oxygen intolerance. Despite this inferred antiquity, however, the earliest rock record affords scant supporting evidence. In fact, based on the chimeric inheritance of rTCA cycle steps within the Chlorobiaceae, even the use of the chemical fossil record of this group is now subject to question. While the 1.64-billion-year-old Barney Creek Formation contains chemical fossils of the earliest known putative Chlorobiaceae-derived carotenoids, interferences from the accompanying hydrocarbon matrix have hitherto precluded the carbon isotope measurements necessary to establish the physiology of the organisms that produced them. Overcoming this obstacle, here we report a suite of compound-specific carbon isotope measurements identifying a cyanobacterially dominated ecosystem featuring heterotrophic bacteria. We demonstrate chlorobactane is 13C-depleted when compared to contemporary equivalents, showing only slight 13C-enrichment over co-existing cyanobacterial carotenoids. The absence of this diagnostic isotopic fingerprint, in turn, confirms phylogenomic hypotheses that call for the late assembly of the rTCA cycle and, thus, the delayed acquisition of autotrophy within the Chlorobiaceae. We suggest that progressive oxygenation of the Earth System caused an increase in the marine sulfate inventory thereby providing the selective pressure to fuel the Neoproterozoic shift towards energy-efficient photoautotrophy within the Chlorobiaceae.

Cryo-electron microscopy structure of the intact photosynthetic light-harvesting antenna-reaction center complex from a green sulfur bacterium.

The photosynthetic reaction center complex (RCC) of green sulfur bacteria (GSB) consists of the membrane-imbedded RC core and the peripheric energy transmitting proteins called Fenna-Matthews-Olson (FMO). Functionally, FMO transfers the absorbed energy from a huge peripheral light-harvesting antenna named chlorosome to the RC core where charge separation occurs. In vivo, one RC was found to bind two FMOs, however, the intact structure of RCC as well as the energy transfer mechanism within RCC remain to be clarified. Here we report a structure of intact RCC which contains a RC core and two FMO trimers from a thermophilic green sulfur bacterium Chlorobaculum tepidum at 2.9 Å resolution by cryo-electron microscopy. The second FMO trimer is attached at the cytoplasmic side asymmetrically relative to the first FMO trimer reported previously. We also observed two new subunits (PscE and PscF) and the N-terminal transmembrane domain of a cytochrome-containing subunit (PscC) in the structure. These two novel subunits possibly function to facilitate the binding of FMOs to RC core and to stabilize the whole complex. A new bacteriochlorophyll (numbered as 816) was identified at the interspace between PscF and PscA-1, causing an asymmetrical energy transfer from the two FMO trimers to RC core. Based on the structure, we propose an energy transfer network within this photosynthetic apparatus.

Testing quantum speedups in exciton transport through a photosynthetic complex using quantum stochastic walks.

Photosynthesis is a highly efficient process, nearly 100 percent of the red photons falling on the surface of leaves reach the reaction center and get transformed into energy. Most theoretical studies on photosynthetic complexes focus mainly on the Fenna-Matthews-Olson complex obtained from green-sulfur bacteria. Quantum coherence was speculated to play a significant role in this very efficient transport process. However, recent reports indicate quantum coherence via exciton transport may not be as relevant as coherence originating via vibronic processes to photosynthesis. Regardless of the origin, there has been a debate on whether quantum coherence results in any speedup of the exciton transport process. To address this we model exciton transport in FMO using a quantum stochastic walk (QSW) with only incoherence, pure dephasing and with both dephasing and incoherence. We find that the QSW model with pure dephasing leads to a substantial speedup in exciton transport as compared to a QSW model which includes both dephasing and incoherence and one which includes only incoherence, both of which experience slowdowns.

Anoxic chlorophyll maximum enhances local organic matter remineralization and nitrogen loss in Lake Tanganyika.

In marine and freshwater oxygen-deficient zones, the remineralization of sinking organic matter from the photic zone is central to driving nitrogen loss. Deep blooms of photosynthetic bacteria, which form the suboxic/anoxic chlorophyll maximum (ACM), widespread in aquatic ecosystems, may also contribute to the local input of organic matter. Yet, the influence of the ACM on nitrogen and carbon cycling remains poorly understood. Using a suite of stable isotope tracer experiments, we examined the transformation of nitrogen and carbon under an ACM (comprising of Chlorobiaceae and Synechococcales) and a non-ACM scenario in the anoxic zone of Lake Tanganyika. We find that the ACM hosts a tight coupling of photo/litho-autotrophic and heterotrophic processes. In particular, the ACM was a hotspot of organic matter remineralization that controlled an important supply of ammonium driving a nitrification-anammox coupling, and thereby played a key role in regulating nitrogen loss in the oxygen-deficient zone.

Rubredoxin from the green sulfur bacterium Chlorobaculum tepidum donates a redox equivalent to the flavodiiron protein in an NAD(P)H dependent manner via ferredoxin-NAD(P)+ oxidoreductase.

The green sulfur bacterium, Chlorobaculum tepidum, is an anaerobic photoautotroph that performs anoxygenic photosynthesis. Although genes encoding rubredoxin (Rd) and a putative flavodiiron protein (FDP) were reported in the genome, a gene encoding putative NADH-Rd oxidoreductase is not identified. In this work, we expressed and purified the recombinant Rd and FDP and confirmed dioxygen reductase activity in the presence of ferredoxin-NAD(P)+ oxidoreductase (FNR). FNR from C. tepidum and Bacillus subtilis catalyzed the reduction of Rd at rates comparable to those reported for NADH-Rd oxidoreductases. Also, we observed substrate inhibition at high concentrations of NADPH similar to that observed with ferredoxins. In the presence of NADPH, B. subtilis FNR and Rd, FDP promoted dioxygen reduction at rates comparable to those reported for other bacterial FDPs. Taken together, our results suggest that Rd and FDP participate in the reduction of dioxygen in C. tepidum and that FNR can promote the reduction of Rd in this bacterium.

Benchmark and performance of long-range corrected time-dependent density functional tight binding (LC-TD-DFTB) on rhodopsins and light-harvesting complexes.

The chromophores of rhodopsins (Rh) and light-harvesting (LH) complexes still represent a major challenge for a quantum chemical description due to their size and complex electronic structure. Since gradient corrected and hybrid density functional approaches have been shown to fail for these systems, only range-separated functionals seem to be a promising alternative to the more time consuming post-Hartree-Fock approaches. For extended sampling of optical properties, however, even more approximate approaches are required. Recently, a long-range corrected (LC) functional has been implemented into the efficient density functional tight binding (DFTB) method, allowing to sample the excited states properties of chromophores embedded into proteins using quantum mechanical/molecular mechanical (QM/MM) with the time-dependent (TD) DFTB approach. In the present study, we assess the accuracy of LC-TD-DFT and LC-TD-DFTB for rhodopsins (bacteriorhodopsin (bR) and pharaonis phoborhodopsin (ppR)) and LH complexes (light-harvesting complex II (LH2) and Fenna-Matthews-Olson (FMO) complex). This benchmark study shows the improved description of the color tuning parameters compared to standard DFT functionals. In general, LC-TD-DFTB can exhibit a similar performance as the corresponding LC functionals, allowing a reliable description of excited states properties at significantly reduced cost. The two chromophores investigated here pose complementary challenges: while huge sensitivity to external field perturbation (color tuning) and charge transfer excitations are characteristic for the retinal chromophore, the multi-chromophoric character of the LH complexes emphasizes a correct description of inter-chromophore couplings, giving less importance to color tuning. None of the investigated functionals masters both systems simultaneously with satisfactory accuracy. LC-TD-DFTB, at the current stage, although showing a systematic improvement compared to TD-DFTB cannot be recommended for studying color tuning in retinal proteins, similar to some of the LC-DFT functionals, because the response to external fields is still too weak. For sampling of LH-spectra, however, LC-TD-DFTB is a viable tool, allowing to efficiently sample absorption energies, as shown for three different LH complexes. As the calculations indicate, geometry optimization may overestimate the importance of local minima, which may be averaged over when using trajectories. Fast quantum chemical approaches therefore may allow for a direct sampling of spectra in the near future.

Prosthecochloris marina sp. nov., a new green sulfur bacterium from the coastal zone of the South China Sea.

A Gram-negative, anaerobic photoautotroph, nonmotile, oval bacterium possessing gas vesicles and having no prosthecae, designated as V1, was isolated from the South China Sea coastal zone. It had chlorosomes as photosynthetic structures, and bacteriochlorophyll c as the major photosynthetic pigment. The strain was found to grow at 20-35 °C, pH 6.3-8.0 (optimum, pH 7.1) and with 0.7-5.8% (w/v) NaCl (optimum, 1-1.8%). In the presence of sulfide and bicarbonate, acetate, and fructose promoted growth. The DNA G+C content was 47 mol%. While the new isolate belonged to the Chlorobiaceae genus Prosthecochloris, it exhibited low similarity of the 16S rRNA gene sequences (96.21-96.78%) to other members of this genus. Comparison of the genome nucleotide sequences of strain V1 revealed that the new isolate was remote from the Chlorobiaceae type strains both in dDDH (16.8-18.9%) and in ANI (75.2-77.8%). We propose to assign the isolate to a new species, Prosthecochloris marina sp. nov., with the type strain V1T ( = VKM-3301T = KCTC 15824T).

Neutron and X-ray analysis of the Fenna-Matthews-Olson photosynthetic antenna complex from Prosthecochloris aestuarii.

The Fenna-Matthews-Olson protein from Prosthecochloris aestuarii (PaFMO) has been crystallized in a new form that is amenable to high-resolution X-ray and neutron analysis. The crystals belonged to space group H3, with unit-cell parameters a = b = 83.64, c = 294.78 Å, and diffracted X-rays to ∼1.7 Šresolution at room temperature. Large PaFMO crystals grown to volumes of 0.3-0.5 mm3 diffracted neutrons to 2.2 Šresolution on the MaNDi neutron diffractometer at the Spallation Neutron Source. The resolution of the neutron data will allow direct determination of the positions of H atoms in the structure, which are believed to be fundamentally important in tuning the individual excitation energies of bacteriochlorophylls in this archetypal photosynthetic antenna complex. This is one of the largest unit-cell systems yet studied using neutron diffraction, and will allow the first high-resolution neutron analysis of a photosynthetic antenna complex.

The influence of quaternary structure on the stability of Fenna-Matthews-Olson (FMO) antenna complexes.

The trimeric nature of the Fenna-Matthews-Olson (FMO) protein antenna complex from green sulfur phototrophic bacteria was investigated. Mutations were introduced into the protein at positions 142 and 198, which were chosen to destabilize the intra-trimer salt bridges between adjacent monomers. Strains bearing the mutations R142L, R198L, or their combination, exhibited altered optical absorption spectra of purified membranes and fluoresced more intensely than the wild type. In particular, the introduction of the R142L mutation resulted in slower culture growth rates, as well as an FMO complex that was not able to be isolated in appreciable quantities, while the R198L mutation yielded an FMO complex with increased sensitivity to sodium thiocyanate and Triton X-100 treatments. Native and denaturing PAGE experiments suggest that much of the FMO complexes in the mutant strains pool with the insoluble material upon membrane solubilization with n-dodecyl ß-D-maltoside, a mild nonionic detergent. Taken together, our results suggest that the quaternary structure of the FMO complex, the homotrimer, is an important factor in the maintenance of the complex's tertiary structure.

A paralog of a bacteriochlorophyll biosynthesis enzyme catalyzes the formation of 1,2-dihydrocarotenoids in green sulfur bacteria.

Chlorobaculum tepidum, a green sulfur bacterium, utilizes chlorobactene as its major carotenoid, and this organism also accumulates a reduced form of this monocyclic pigment, 1',2'-dihydrochlorobactene. The protein catalyzing this reduction is the last unidentified enzyme in the biosynthetic pathways for all of the green sulfur bacterial pigments used for photosynthesis. The genome of C. tepidum contains two paralogous genes encoding members of the FixC family of flavoproteins: bchP, which has been shown to encode an enzyme of bacteriochlorophyll biosynthesis; and bchO, for which a function has not been assigned. Here we demonstrate that a bchO mutant is unable to synthesize 1',2'-dihydrochlorobactene, and when bchO is heterologously expressed in a neurosporene-producing mutant of the purple bacterium, Rhodobacter sphaeroides, the encoded protein is able to catalyze the formation of 1,2-dihydroneurosporene, the major carotenoid of the only other organism reported to synthesize 1,2-dihydrocarotenoids, Blastochloris viridis Identification of this enzyme completes the pathways for the synthesis of photosynthetic pigments in Chlorobiaceae, and accordingly and consistent with its role in carotenoid biosynthesis, we propose to rename the gene cruI Notably, the absence of cruI in B. viridis indicates that a second 1,2-carotenoid reductase, which is structurally unrelated to CruI (BchO), must exist in nature. The evolution of this carotenoid reductase in green sulfur bacteria is discussed herein.

Evaluation of Electron-Phonon Coupling and Spectral Densities of Pigment-Protein Complexes by Line-Narrowed Optical Spectroscopy.

Spectral hole burning (SHB) and difference fluorescence line narrowing (ΔFLN) are routinely used for investigations of electron-phonon coupling in photosynthetic pigment-protein complexes as well as in other amorphous systems at cryogenic temperatures. Nevertheless, the Huang-Rhys factors S, an integral measure of electron-phonon coupling strength, and the phonon spectral densities obtained by SHB and ΔFLN over the past years have differed significantly in the case of certain photosynthetic pigment-protein complexes. In this work, the specific properties of both types of line-narrowing spectroscopic techniques that may lead to these discrepancies are critically analyzed by a combined experimental and computational approach, using the CP29 antenna complex of green plants as a suitable model system. We confirm that only ΔFLN at low fluence, by providing access to the homogeneously broadened spectrum, is able to deliver correct S values, while SHB may significantly under- or overestimate them, depending on the burn fluence. We also discuss possible other sources of discrepancies in the literature data, e.g., in the case of LHC II aggregates and correct numerical errors found in some previous records.

The taphonomic fate of isorenieratene in Lower Jurassic shales-controlled by iron?

Fossil derivatives of isorenieratene, an accessory pigment in brown-colored green sulfur bacteria, are often used as tracers for photic zone anoxia through Earth's history, but their diagenetic behavior is still incompletely understood. Here, we assess the preservation of isorenieratene derivatives in organic-rich shales (1.5-8.4 wt.% TOC) from two Lower Jurassic anoxic systems (Bächental oil shale, Tyrol, Austria; Posidonia Shale, Baden-Württemberg, Germany). Bitumens and kerogens were investigated using catalytic hydropyrolysis (HyPy), closed-system hydrous pyrolysis (in gold capsules), gas chromatography-mass spectrometry (GC-MS) and gas chromatography combustion isotope ratio-mass spectrometry (GC-C-IRMS). Petrography and biomarkers indicate a syngenetic relationship between bitumens and kerogens. All bitumens contain abundant isorenieratane, diverse complex aromatized isorenieratene derivatives, and a pseudohomologous series of 2,3,6-trimethyl aryl isoprenoids. In contrast, HyPy and mild closed-system hydrous pyrolysis of the kerogens yielded only minor amounts of these compounds. Given the overall low maturity of the organic matter (below oil window), it appears that isorenieratene and its abundant derivatives from the bitumen had not been incorporated into the kerogens. Accordingly, sulfur cross-linking, the key mechanism for sequestration of functionalized lipids into kerogens in anoxic systems, was not effective in the Jurassic environments studied. We explain this by (i) early cyclization/aromatization and (ii) hydrogenation reactions that have prevented effective sulfurization. In addition, (iii) sulfide was locally removed via anoxygenic photosynthesis and efficiently trapped by the reaction with sedimentary iron, as further indicated by elevated iron contents (4.0-8.7 wt.%) and the presence of abundant pyrite aggregates in the rock matrix. Although the combined processes have hampered the kerogen incorporation of isorenieratene and its derivatives, they may have promoted the long-term preservation of these biomarkers in the bitumen fraction via early defunctionalization. This particular taphonomy of aromatic carotenoids has to be considered in studies of anoxic iron-rich environments (e.g., the Proterozoic ocean).

Theory of Anisotropic Circular Dichroism of Excitonically Coupled Systems: Application to the Baseplate of Green Sulfur Bacteria.

A simple exciton theory for the description of anisotropic circular dichroism (ACD) spectra of multichromophoric systems is presented that is expected to be of general use for the analysis of structure-function relationships of molecular aggregates such as photosynthetic light-harvesting antennae. The theory is applied to the baseplate of green sulfur bacteria. It is demonstrated that only the combined analysis of ACD and circular dichroism (CD) spectra for the present baseplate bacteriochlorophyll (BChl) a dimer allows for an unambiguous determination of the parameters of the exciton Hamiltonian from experimental data. The analysis of experimental absorption and linear dichroism spectra suggests that either the NMR structure has to be refined or in addition to the dimers seen in the NMR structure and in the CD and ACD spectra, BChl a monomers are present in the baseplate carotenosome sample. A refined dimer structure is presented, explaining all four optical spectra.

Ultrafast Spectroscopic Investigation of Energy Transfer in Site-Directed Mutants of the Fenna-Matthews-Olson (FMO) Antenna Complex from Chlorobaculum tepidum.

Ultrafast transient absorption (TA) and time-resolved fluorescence (TRF) spectroscopic studies were performed on several mutants of the bacteriochlorophyll (BChl) a-containing Fenna-Matthews-Olson (FMO) complex from the green sulfur bacterium Chlorobaculum tepidum. These mutants were generated to perturb a particular BChl a site and determine its effects on the optical spectroscopic properties of the pigment-protein complex. Measurements conducted at 77 K under both oxidizing and reducing conditions revealed changes in the dynamics of the various spectral components as compared to the data set from wild-type FMO. TRF results show that under reducing conditions all FMO samples decay with a similar lifetime in the ∼2 ns range. The oxidized samples revealed varying fluorescence lifetimes of the terminal BChl a emitter, considerably shorter than those recorded for the reduced samples, indicating that the quenching mechanism in wild-type FMO is still present in the mutants. Global fitting of TA data yielded similar overall results, and in addition, the lifetimes of early decaying components were determined. Target analyses of TA data for select FMO samples generated kinetic models that better simulate the TA data. A comparison of the lifetime of excitonic components for all samples reveals that the mutations affect mainly the early kinetic components, but not that of the lowest energy exciton, which reflects the flexibility of energy transfer in FMO.

Hyperfine Sublevel Correlation Spectroscopy Studies of Iron-Sulfur Cluster in Rieske Protein from Green Sulfur Bacterium Chlorobaculum tepidum.

The magnetic properties of the Rieske protein purified from Chlorobaculum tepidum were investigated using electron paramagnetic resonance and hyperfine sublevel correlation spectroscopy (HYSCORE). The g-values of the Fe2S2 center were gx = 1.81, gy = 1.90, and gz = 2.03. Four classes of nitrogen signals were obtained by HYSCORE. Nitrogens 1 and 2 had relatively strong magnetic hyperfine couplings and were assigned as the nitrogen directly ligated to Fe. Nitrogens 3 and 4 had relatively weak magnetic hyperfine couplings and were assigned as the other nitrogen of the His ligands and peptide nitrogen connected to the sulfur atom via hydrogen bonding, respectively. The anisotropy of nitrogen 3 reflects the different spin density distributions on the His ligands, which influences the electron transfer to quinone.

Origin of bimodal fluorescence enhancement factors of Chlorobaculum tepidum reaction centers on silver island films.

We focus on the spectral dependence of plasmon-induced enhancement of fluorescence of Chlorobaculum tepidum reaction centers. When deposited on silver island film, they exhibit up to a 60-fold increase in fluorescence. The dependence of enhancement factors on the excitation wavelength is not correlated with the absorption spectrum of the plasmonic structure. In particular, the presence of one (or multiple) trimers of the Fenna-Matthews-Olson (FMO) protein reveals itself in bimodal distribution of enhancement factors for the excitation at 589 nm, the wavelength corresponding to bacteriochlorophyll absorption of FMO and the core of the RC. We conclude that the structure of multichromophoric complexes can substantially affect the impact of plasmonic excitations, which is important in the context of assembling functional biohybrid systems.

Mutation-induced perturbation of the special pair P840 in the homodimeric reaction center in green sulfur bacteria.

Homodimeric photosynthetic reaction centers (RCs) in green sulfur bacteria and heliobacteria are functional homologs of Photosystem (PS) I in oxygenic phototrophs. They show unique features in their electron transfer reactions; however, detailed structural information has not been available so far. We mutated PscA-Leu688 and PscA-Val689 to cysteine residues in the green sulfur bacterium Chlorobaculum tepidum; these residues were predicted to interact with the special pair P840, based on sequence comparison with PS I. Spectroelectrochemical measurements showed that the L688C and V689C mutations altered a near-infrared difference spectrum upon P840 oxidation, as well as the redox potential of P840. Light-induced Fourier transform infrared difference measurements showed that the L688C mutation induced a differential signal of the S-H stretching vibration in the P840(+)/P840 spectrum, as reported in P800(+)/P800 difference spectrum in a heliobacterial RC. Spectral changes in the 13(1)-keto C=O region, caused by both mutations, revealed corresponding changes in the electronic structure of P840 and in the hydrogen-bonding interaction at the 13(1)-keto C=O group. These results suggest that there is a common spatial configuration around the special pair sites among type 1 RCs. The data also provided evidence that P840 has a symmetric electronic structure, as expected from a homodimeric RC.

Native FMO-reaction center supercomplex in green sulfur bacteria: an electron microscopy study.

Chlorobaculum tepidum is a representative of green sulfur bacteria, a group of anoxygenic photoautotrophs that employ chlorosomes as the main light-harvesting structures. Chlorosomes are coupled to a ferredoxin-reducing reaction center by means of the Fenna-Matthews-Olson (FMO) protein. While the biochemical properties and physical functioning of all the individual components of this photosynthetic machinery are quite well understood, the native architecture of the photosynthetic supercomplexes is not. Here we report observations of membrane-bound FMO and the analysis of the respective FMO-reaction center complex. We propose the existence of a supercomplex formed by two reaction centers and four FMO trimers based on the single-particle analysis of the complexes attached to native membrane. Moreover, the structure of the photosynthetic unit comprising the chlorosome with the associated pool of RC-FMO supercomplexes is proposed.