Characterization of the FMO protein from the aerobic chlorophototroph, Candidatus Chloracidobacterium thermophilum

Candidatus Chloracidobacterium (Cab.) thermophilum is a recently discovered aerobic chlorophototroph belonging to the phylum Acidobacteria. From analyses of genomic sequence data, this organism was inferred to have type-1 homodimeric reaction centers, chlorosomes, and the bacteriochlorophyll (BChl) a-binding Fenna–Matthews–Olson protein (FMO). Here, we report the purification and characterization of Cab. thermophilum FMO. Absorption, fluorescence emission, and CD spectra of the FMO protein were measured at room temperature and at 77 K. The spectroscopic features of this FMO protein were different from those of the FMO protein of green sulfur bacteria (GSB) and suggested that exciton coupling of the BChls in the FMO protein is weaker than in FMO of GSB especially at room temperature. HPLC analysis of the pigments extracted from the FMO protein only revealed the presence of BChl a esterified with phytol. Despite the distinctive spectroscopic properties, the residues known to bind BChl a molecules in the FMO of GSB are well conserved in the primary structure of the Cab. thermophilum FMO protein. This suggests that the FMO of Cab. thermophilum probably also binds seven or possibly eight BChl a(P) molecules. The results imply that, without changing pigment composition or structure dramatically, the FMO protein has acquired properties that allow it to perform light harvesting efficiently under aerobic conditions.     

A comparative study of the optical characteristics of intact cells of photosynthetic green sulfur bacteria containing bacteriochlorophyll c,d or e

Energy transfer and pigment arrangement in intact cells of the green sulfur bacteria Prosthecochloris aestuarii, Chlorobium vibrioforme and chlorobium phaeovibrioides, containing bacteriochlorophyll (BChl) c, d or e as main light harvesting pigment, respectively, were studied by means of absorption, fluorescence, circular dichroism and linear dichroism spectroscopy at low temperature. The results indicate a very similar composition of the antenna in the three species and a very similar structure of main light harvesting components, the chlorosome and the membrane-bound BChl a protein. In all three species the Q_y transition dipoles of BChl c, d or e are oriented approximately parallel to the long axis of the chlorosome. Absorption and fluorescence excitation spectra demonstrate the presence of at least two BChl c-e pools in the chlorosomes of all three species, long-wavelength absorbing BChls being closest to the membrane. In C. phaeovibrioides, energy from BChl e is transferred with an efficiency of 25% to the chlorosomal BChl a at 6 K, whereas the efficiency of transfer from BChl e to the BChl a protein is 10%. These numbers are compatible with the hypothesis that the chlorosomal BChl a is an intermediary in the energy transfer from the chlorosome to the membrane.Abbreviations BChl bacteriochlorophyll - Chl chlorophyll - CD circular dichroism - LD linear dichroism     

Antenna organization and energy transfer in membranes of Heliobacterium chlorum

Absorption, fluorescence emission and fluorescence excitation spectra of membranes of the recently discovered photosynthetic bacterium Heliobacterium chlorum (Gest, H. and Favinger, J.L. (1983) Arch. Microbiol. 136, 11–16) showed that at 4 K at least three spectroscopically different forms of bacteriochlorophyll g (BChl g 778, BChl g 793 and BChl g 808) can be discerned in the antenna system. Efficient energy transfer occurs from the short-wave-absorbing bacteriochlorophylls to BChl g 808. Energy transfer to bacteriochlorophyll, albeit with lower efficiency (70%), also occurred from the main carotenoid, neurosporene, and from a pigment absorbing at 670 nm. The complex structure of the antenna system is also reflected by fluorescence polarization and linear and circular dichroism spectra. Significant circular dichroism was only observed for BChl g 793, and different orientations were observed for the various Q_y transition dipoles, the one of BChl g 808 making a smaller angle with the plane of the membrane than those of the other bacteriochlorophylls.     

Excitation transfer in chlorosomes of green photosynthetic bacteria

The formation of excited states and energy transfer in chlorosomes of the green photosynthetic bacteria Chlorobium limicola and Chloroflexus aurantiacus were studied by measurements of flash-induced absorbance changes and fluorescence. Upon excitation with 35 ps, 532 nm flashes, large absorbance decreases around 750 nm were observed that were due to the disappearance of ground state absorption of the main pigment, bacteriochlorophyll (BChl) c. The absorbance changes decayed after the flash with a time constant of approx. 1 ns, together with faster components. Absorbance changes that could be ascribed to formation of excited BChl a were much smaller than those of BChl c. The yields of BChl c and BChl a fluorescence were measured as a function of the energy density of the exciting flash. At high energy a strong quenching occurred caused by annihilation of singlet excited states. An analysis of the results shows that energy transfer between BChl c molecules is very efficient and that in C. limicola excitations can probably move freely through the entire chlorosome (which contains about 10 000 BChls c). The chlorosome thus serves as a common antenna for several reaction centres. The small amounts of BChl a present in the chlorosomes of both species form clusters of only a few molecules. Upon cooling to 4 K the sizes of the domains of BChl c for energy transfer decreased considerably. The results are discussed in relation to recently suggested models for the pigment organization within chlorosomes.     

Pigment organization and energy transfer in the green photosynthetic bacterium Chloroflexus aurantiacus. III. Energy transfer in whole cells

The transfer of excitation energy in intact cells of the thermophilic green photosynthetic bacterium Chloroflexus aurantiacus was studied both at low temperature and under more physiological conditions. Analysis of excitation spectra measured at 4K indicates that the minor fraction of bacteriochlorophyll a present in the chlorosome functions as an intermediate in energy transfer between the main light-harvesting pigment BChl c and the membrane-bound B808-866 antenna complex. This supports the hypothesis that BChl a is associated with the base plate which connects the chlorosome with the membrane. The overall efficiency for energy transfer from the chlorosome to the membrane is only 15% at 4K. High efficiencies of close to 100% are observed above 40°C near the temperature where the cultures are grown. Cooling to 20°C resulted in a sudden drop of the transfer efficiency which appeared to originate in the chlorosome. This decrease may be related to a lipid phase transition. Further cooling mainly affected the efficiency of transfer between the chlorosome and the membrane. This effect can only partially be explained by a decreased Förster overlap between the chlorosomal BChl a and BChl a 808 associated with the membrane-bound antenna system. The temperature dependence of the fluorescence yield of BChl a 866 also appeared to be affected by lipid phase transitions, suggesting that this fluorescence can be used as a native probe of the physical state of the membrane.     

Pigment organization and energy transfer in the green photosynthetic bacterium Chloroflexus aurantiacus

We have studied the pigment arrangement in purified cytoplasmic membranes of the thermophilic green bacterium Chloroflexus aurantiacus. The membranes contain 30–35 antenna bacteriochlorophyll a molecules per reaction center; these are organized in the B808–866 light-harvesting complex, together with carotenoids in a 2:1 molar ratio. Measurements of linear dichroism in a pressed polyacrylamide gel permitted the accurate determination of the orientation of the optical transition dipole moments with respect to the membrane plane. Combination of linear dichroism and low temperature fluorescence polarization data shows that the Q_y transitions of the BChl 866 molecules all lie almost perfectly parallel to the membrane plane, but have no preferred orientation within the plane. The BChl 808 Q_y transitions make an average angle of about 44° with this plane. This demonstrates that there are clear structural differences between the B808–866 complex of C. aurantiacus and the B800–850 complex of purple bacteria. Excitation energy transfer from carotenoid to BChl a proceeds with about 40% efficiency, while the efficiency of energy transfer from BChl 808 to BChl 866 approaches 100%. From the minimal energy transfer rate between the two spectral forms of BChl a, obtained by analysis of low temperature fluorescence emission spectra, a maximal distance between BChl 808 and BChl 866 of 23 was derived.Abbreviations BChl bacteriochlorophyll - BPheo bacteriopheophytin - CD circular dichroism - LD linear dichroism - Tris Tris(hydroxymethyl)aminomethane     

Prompt and delayed fluorescence in pigment-protein complexes of a green photosynthetic bacterium

Excitation spectra were measured at 4 K of bacteriochlorophyll a fluorescence in reaction center containing pigment-protein complexes obtained from the green photosynthetic bacterium Prosthecochloris aestuarii. Excitation spectra for the longest-wave emission (838 nm) showed bands of bacteriochlorophyll a, carotenoid, and of a pigment with absorption bands at 670, 438 and possibly near 420 nm, which is probably identical to an unidentified porphyrin described in the preceding paper (Swarthoff, T., Kramer, H.J.M. and Amesz, J. (1982) Biochim. Biophys. Acta 681, 354–358). At room temperature the longest-wave emission is stimulated by a magnetic field, which indicates that at least part of the emission is delayed fluorescence brought about by a reversal of the primary charge separation. Below about 150 K no stimulation was observed. The excitation spectra for short-wave emission (828 nm) were very similar to the absorption spectrum of the isolated antenna bacteriochlorophyll a-protein complex, and showed bands of bacteriochlorophyll a only. This indicates that two forms of the antenna protein exist that are spectroscopically similar: a soluble form that is released by treatment with guanidine hydrochloride and a bound form that remains attached to the reaction center complex. The bands of the antenna complexes were weak in the excitation spectra of the 838 nm fluorescence, which indicates that the efficiency of energy transfer to the reaction center complex is low.     

Chlorosome antenna complexes from green photosynthetic bacteria

Chlorosomes are the distinguishing light-harvesting antenna complexes that are found in green photosynthetic bacteria. They contain bacteriochlorophyll (BChl) c, d, e in natural organisms, and recently through mutation, BChl f, as their principal light-harvesting pigments. In chlorosomes, these pigments self-assemble into large supramolecular structures that are enclosed inside a lipid monolayer to form an ellipsoid. The pigment assembly is dictated mostly by pigment–pigment interactions as opposed to protein–pigment interactions. On the bottom face of the chlorosome, the CsmA protein aggregates into a paracrystalline baseplate with BChl a, and serves as the interface to the next energy acceptor in the system. The exceptional light-harvesting ability at very low light conditions of chlorosomes has made them an attractive subject of study for both basic and applied science. This review, incorporating recent advancements, considers several important aspects of chlorosomes: pigment biosynthesis, organization of pigments and proteins, spectroscopic properties, and applications to bio-hybrid and bio-inspired devices.     

Low-temperature optical properties and pigment organization of the B875 light-harvesting bacteriochlorophyll-protein complex of purple photosynthetic bacteria

Optical and structural properties of the B875 light-harvesting complex of purple bacteria were examined by measurements of low-temperature circular dichroism (CD) and excitation spectra of fluorescence polarization. In the B875 complex isolated from wild-type Rhodopseudomonas sphaeroides, fluorescence polarization increased steeply across the long-wavelength Q_y bacteriochlorophyll a (BChl) absorption band at both 4 and approx. 300 K. With the native complex in the photosynthetic membranes of Rhodospirillum rubrum and Rps. sphaeroides wild-type and R26-carotenoidless strains, this significant increase in polarization from 0.12 to 0.40 was only observed at low temperature. A polarization of ?0.2 was observed upon excitation in the Q_x BChl band. The results indicate that about 15% of the BChl molecules in the complex absorb at wavelengths about 12 nm longer than the other BChls. All BChls have approximately the same orientation with their Q_y transition dipoles essentially parallel and their Q_x transitions perpendicular to the plane of the membrane. At low temperature, energy transfer to the long-wavelength BChls is irreversible, yielding a high degree of polarization upon direct excitation, whereas at room temperature a partial depolarization of fluorescence by energy transfer between different subunits occurs in the membrane, but not in the isolated complex. CD spectra appear to reflect the two spectral forms of B875 BChl in Rps. sphaeroides membranes. They also reveal structural differences between the complexes of Rps. sphaeroides and Rhs. rubrum, in both BChl and carotenoid regions. The CD spectrum of isolated B875 indicates that the interactions between the BChls but not the carotenoids are altered upon isolation.     

Optical and structural properties of chlorosomes of the photosynthetic green sulfur bacterium Chlorobium limicola

Isolated chlorosomes of the photosynthetic green sulfur bacterium Chorobium limicola upon cooling to 4 K showed, in addition to the near-infrared absorption band at 753 nm due to bacteriochlorophyll c, a weak band near 800 nm that could be attributed to bacteriochlorophyll a. The emission spectrum showed bands of bacteriochlorophyll c and a at 788 and 828 nm, respectively. The fluorescence excitation spectrum indicated a high efficiency of energy transfer from bacteriochlorophyll c to bacteriochlorophyll a. When all bacteriochlorophyll c absorption had been lost upon storage, no appreciable change in the optical properties of the bacteriochlorophyll a contained in these ‘depleted chlorosomes’ was observed. The fluorescence and absorption spectra of the chlorosomal bacteriochlorophyll a were clearly different from those of the soluble bacteriochlorophyll a protein present in these bacteria. The results provide strong evidence that bacteriochlorophyll a, although present in a small amount, is an integral constituent of the chlorosome. It presumably functions in the transfer of energy from the chlorosome to the photosynthetic membrane; its spectral properties and the orientation of its near-infrared optical transitions as determined by linear dichroism are such as to favor this energy transfer.     

Size and structure of antenna complexes of photosynthetic bacteria as studied by singlet-singlet quenching of the bacteriochlorophyll fluorescence yield

We have measured the singlet-singlet quenching of the bacteriochlorophyll (BChl) fluorescence yield as a function of excitation intensity in a number of antenna complexes isolated from photosynthetic bacteria. Our results show that the lithium dodecyl sulfate (LDS)-B875, LDS-B800 – 850 and lauryldimethylamine N-oxide complexes of Rhodopseudomonas sphaeroides contain 8, greater than 25 and greater than 600 BChl a molecules, respectively. The size of the Rhodopspirillum rubrum B880 complex is greater than 70 BChl a and that of the water-soluble BChl a complex from Prosthecochloris aestuarii about 20–25 BChl a. These results are discussed in relation to current models of the arrangement of antenna complexes within the photosynthetic membranes.     

Native electrospray mass spectrometry reveals the nature and stoichiometry of pigments in the FMO photosynthetic antenna protein

The nature and stoichiometry of pigments in the Fenna-Matthews-Olson (FMO) photosynthetic antenna protein complex were determined by native electrospray mass spectrometry. The FMO antenna complex was the first chlorophyll-containing protein that was crystallized. Previous results indicate that the FMO protein forms a trimer with seven bacteriochlorophyll a in each monomer. This model has long been a working basis to understand the molecular mechanism of energy transfer through pigment/pigment and pigment/protein coupling. Recent results have suggested, however, that an eighth bacteriochlorophyll is present in some subunits. In this report, a direct mass spectrometry measurement of the molecular weight of the intact FMO protein complex clearly indicates the existence of an eighth pigment, which is assigned as a bacteriochlorophyll a by mass analysis of the complex and HPLC analysis of the pigment. The eighth pigment is found to be easily lost during purification, which results in its partial occupancy in the mass spectra of the intact complex prepared by different procedures. The results are consistent with the recent X-ray structural models. The existence of the eighth bacteriochlorophyll a in this model antenna protein gives new insights into the functional role of the FMO protein and motivates the need for new theoretical and spectroscopic assignments of spectral features of the FMO protein.     

Pigment organization of the B800–850 antenna complex of Rhodopseudomonas sphaeroides

The B800–850 antenna complex of Rhodopseudomonas sphaeroides was studied by comparing the spectral properties of several different types of complexes, isolated from chromatophores by means of the detergents lithium dodecyl sulfate (LDS) or lauryl dimethylamine N-oxide (LDAO). Fluorescence polarization spectra of the BChl 800 emission at 4 K indicated that rapid energy transfer between at least two BChl 800 molecules occurs with a rate constant of energy transfer k_ET > 3 · 10¹² s^?1. The maximal dipole-dipole distance between the two BChl 800 molecules was calculated to be 18–19 Å. The porphyrin rings of the BChl 800 molecules are oriented parallel to each other, while their Q_y transition moments are mutually perpendicular. The energy-transfer efficiency from carotenoid to bacteriochlorophyll measured in different complexes showed that two functionally different carotenoids are present associated with, respectively, BChl 800 and BChl 850. Fluorescence polarization and linear dichroism spectra revealed that these carotenoids have different absorption spectra and a different orientation with respect to the membrane. The carotenoid associated with BChl 800 absorbs some nanometers more to the red and its orientation is approximately parallel to the membrane, while the carotenoid associated with BChl 850 is oriented more or less perpendicular to the membrane. The fluorescence polarization of BChl 850 was the same for the different complexes. This indicates that the observed polarization of the fluorescence is determined by the smallest complex obtained which contains 8–10 BChl 850 molecules. The B800–850 complex isolated with LDAO thus must consist of a highly ordered array of smaller structures. On basis of these results a minimal model is proposed for the basic unit consisting of four BChl 850 and two BChl 800 and three carotenoid molecules.     

Excitation energy transfer in the green photosynthetic bacterium Chloroflexus aurantiacus: A specific effect of 1-hexanol on the optical properties of baseplate and energy transfer processes

The effect of 1-hexanol on spectral properties and the processes of energy transfer of the green gliding photosynthetic bacterium Chloroflexus aurantiacus was investigated with reference to the baseplate region. On addition of 1-hexanol to a cell suspension in a concentration of one-fourth saturation, a specific change in the baseplate region was induced: that is, a bleach of the 793-nm component, and an increase in absorption of the 813-nm component. This result was also confirmed by fluorescence spectra of whole cells and isolated chlorosomes. The processes of energy transfer were affected in the overall transfer efficiency but not kinetically, indicating that 1-hexanol suppressed the flux of energy flow from the baseplate to the B806-866 complexes in the cytoplasmic membranes. The fluorescence excitation spectrum suggests a specific site of interaction between bacteriochlorophyll (BChl) c with a maximum at 771 nm in the rod elements and BChl a with a maximum at 793 nm in the baseplate, which is a funnel for a fast transfer of energy to the B806-866 complexes in the membranes. The absorption spectrum of chlorosomes was resolved to components consistently on the basis, including circular dichroism and magnetic circular dichroism spectra; besides two major BChl c forms, bands corresponding to tetramer, dimer, and monomer were also discernible, which are supposed to be intermediary components for a higher order structure. A tentative model for the antenna system of C. aurantiacus is proposed.Abbreviations A670 a component whose absorption maximum is located at 670 nm - (B)Chl (bacterio)chlorophyll - CD circular dichroism - F675 a component whose emission maximum is located at 675 nm - FMO protein Fenna-Mathews-Olson protein - LD linear dichroism - LH light-harvesting - McD magnetic circular dichroism - PS photosystem - RC reaction center     

Pigment composition of the photosynthetic membrane and reaction center of the green bacterium Prosthecochloris aestuarii

We have performed a quantitative analysis of the pigment composition of different pigment-protein complexes present in the membrane of the green sulfur bacterium Prosthecochloris aestuarii, using the resolving power of reversed-phase high-performance liquid chromatography. The most purified photochemically active complexes contained only carotenoids (OH-chlorobactene and rhodopin), bacteriochlorophyll a and a chlorophyllous pigment with absorption maxima at 663 and 433 nm, like bacteriochlorophyll c. However, the lipophilicity of this pigment, labeled BChl 663, is quite high and indicates that it contains 5–6 additional methylene groups compared to the BChl c homologue known as most lipophilic. Comparison of the BChl 663 content of various pigment-protein complexes indicates that BChl 663 is present in an amount of 10–15 molecules per reaction center. BChl 663 absorbs at 670 nm in vivo, with a specific extinction coefficient of 85 (±10) mM⁻¹ · cm⁻¹. In view of the evidence that the primary electron acceptor in P. aestuarii is a pigment with absorption maximum at 670 nm (Nuijs, A.M., Vasmel, H., Joppe, H.L.P., Duysens, L.N.M. and Amesz, J. (1985) Biochim. Biophys. Acta 807, 24–34) a direct consequence of these experiments is the fact that only BChl 663 can be a likely candidate for the role of primary electron acceptor as no other pigments absorbing around 670 nm (e.g., bacteriopheophytin c) are present in a photochemically active pigment-protein complex derived from the membrane of this green bacterium.     

Variable fluorescence in green sulfur bacteria

Green sulfur bacteria possess a complex photosynthetic machinery. The dominant light harvesting systems are chlorosomes, which consist of bacteriochlorophyll c, d or e oligomers with small amounts of protein. The chlorosomes are energetically coupled to the membrane-embedded iron sulfur-type reaction center via a bacteriochlorophyll a-containing baseplate protein and the Fenna-Matthews-Olson (FMO) antenna protein. The fluorescence yield and spectral properties of these photosynthetic complexes were investigated in intact cells of several species of green sulfur bacteria under physiological, anaerobic conditions. Surprisingly, green sulfur bacteria show a complex modulation of fluorescence yield upon illumination that is very similar to that observed in oxygenic phototrophs. Within a few seconds of illumination, the fluorescence reaches a maximum, which decreases within a minute of illumination to a lower steady state. Fluorescence spectroscopy reveals that the fluorescence yield during both processes is primarily modulated on the FMO-protein level, while the emission from chlorosomes remains mostly unchanged. The two most likely candidates that modulate bacteriochlorophyll fluorescence are (1) direct excitation quenching at the FMO-protein level and (2) indirect modulation of FMO-protein fluorescence by the reduction state of electron carriers that are part of the reaction center.     

Versatile design of biohybrid light-harvesting architectures to tune location,density, and spectral coverage of attached synthetic chromophores for enhanced energy capture

Biohybrid antennas built upon chromophore–polypeptide conjugates show promise for the design of efficient light-capturing modules for specific purposes. Three new designs, each of which employs analogs of the β-polypeptide from Rhodobacter sphaeroides, have been investigated. In the first design, amino acids at seven different positions on the polypeptide were individually substituted with cysteine, to which a synthetic chromophore (bacteriochlorin or Oregon Green) was covalently attached. The polypeptide positions are at –2, –6, –10, –14, –17, –21, and –34 relative to the 0-position of the histidine that coordinates bacteriochlorophyll a (BChl a). All chromophore–polypeptides readily formed LH1-type complexes upon combination with the α-polypeptide and BChl a. Efficient energy transfer occurs from the attached chromophore to the circular array of 875 nm absorbing BChl a molecules (denoted B875). In the second design, use of two attachment sites (positions –10 and –21) on the polypeptide affords (1) double the density of chromophores per polypeptide and (2) a highly efficient energy-transfer relay from the chromophore at –21 to that at –10 and on to B875. In the third design, three spectrally distinct bacteriochlorin–polypeptides were prepared (each attached to cysteine at the –14 position) and combined in an ~1:1:1 mixture to form a heterogeneous mixture of LH1-type complexes with increased solar coverage and nearly quantitative energy transfer from each bacteriochlorin to B875. Collectively, the results illustrate the great latitude of the biohybrid approach for the design of diverse light-harvesting systems.     

Pigment Composition in the Reaction Center Complex from the Thermophilic Green Sulfur Bacterium, Chlorobium tepidum: Carotenoid Glucoside Esters, Menaquinone and Chlorophylls

Distribution of pigments in the reaction center (RC) complex,chlorosomes and chlorosome-free membranes prepared from thegreen sulfur bacterium, Chlorobium tepidum, was analyzed. TheRC complex contained approximately 40 molecules of bacteriochlorophyll(BChl) a per P840, half of which are estimated to be in theFenna-Matthews-Olson (FMO) protein. Carotenes (2 molecules perP840) occupied only one third of the total carotenoids. Theremaining carotenoids (4 to 5 molecules per P840) were OH-chlorobacteneglucoside ester and OH-     

Energy dissipation efficiency in the CP43 assembly intermediate complex of photosystem II

Photosystem II in oxygenic organisms is a large membrane bound rapidly turning over pigment protein complex. During its biogenesis, multiple assembly intermediates are formed, including the CP43-preassembly complex (pCP43). To understand the energy transfer dynamics in pCP43, we first engineered a His-tagged version of the CP43 in a CP47-less strain of the cyanobacterium Synechocystis 6803. Isolated pCP43 from this engineered strain was subjected to advanced spectroscopic analysis to evaluate its excitation energy dissipation characteristics. These included measurements of steady-state absorption and fluorescence emission spectra for which correlation was tested with Stepanov relation. Comparison of fluorescence excitation and absorptance spectra determined that efficiency of energy transfer from β-carotene to chlorophyll a is 39 %. Time-resolved fluorescence images of pCP43-bound Chl a were recorded on streak camera, and fluorescence decay dynamics were evaluated with global fitting. These demonstrated that the decay kinetics strongly depends on temperature and buffer used to disperse the protein sample and fluorescence decay lifetime was estimated in 3.2–5.7 ns time range, depending on conditions. The pCP43 complex was also investigated with femtosecond and nanosecond time-resolved absorption spectroscopy upon excitation of Chl a and β-carotene to reveal pathways of singlet excitation relaxation/decay, Chl a triplet dynamics and Chl a → β-carotene triplet state sensitization process. The latter demonstrated that Chl a triplet in the pCP43 complex is not efficiently quenched by carotenoids. Finally, detailed kinetic analysis of the rise of the population of β-carotene triplets determined that the time constant of the carotenoid triplet sensitization is 40 ns.     

The three-dimensional structure of the FMO protein from <Emphasis Type="Italic">Pelodictyon phaeum</Emphasis> and the implications for energy transfer

The Fenna–Matthews–Olson (FMO) antenna protein from the green bacterium Pelodictyon phaeum mediates the transfer of energy from the peripheral chlorosome antenna complex to the membrane-bound reaction center. The three-dimensional structure of this protein has been solved using protein crystallography to a resolution limit of 2.0 Å, with R _work and R _free values of 16.6 and 19.9%, respectively. The structure is a trimer of three identical subunits related by a threefold symmetry axis. Each subunit has two beta sheets that surround 8 bacteriochlorophylls. The bacteriochlorophylls are all five-coordinated, with the axial ligand being a histidine, serine, backbone carbonyl, or bound water molecule. The arrangement of the bacteriochlorophylls is generally well conserved in comparison to other FMO structures, but differences are apparent in the interactions with the surrounding protein. In this structure the position and orientation of the eighth bacteriochlorophyll is well defined and shows differences in its location and the coordination of the central Mg compared to previous models. The implications of this structure on the ability of the FMO protein to perform energy transfer are discussed in terms of the experimental optical measurements.