Monomeric BChl Research Articles

Energy transfer and primary charge separation upon selective femtosecond excitation at 810 nm in the reaction center complex from Heliobacterium modesticaldum

The kinetics of excitation energy transfer and charge separation process in the photosynthetic reaction center of Heliobacterium modesticaldum were investigated with femtosecond transient absorption spectroscopy at room temperature. This is the first report using the highly purified reaction center isolated from heliobacteria. The bacteriochlorophyll (BChl) g pigment pool was selectively excited at its Qy band of 810 nm. The transient difference spectra were structured enough to be assigned to major pigment pools (Bmd787, Bmd800, and Bmd812), despite the broad absorption spectrum of the Qy region at room temperature. The spectral equilibration among three different spectral pools completed in less than 0.2 ps, indicating a very rapid energy transfer process because of the strong excitonic interaction between BChl gs. The decay-associated difference spectra (DADS) obtained by a global fit analysis from 450 nm to 900 nm revealed a very rapid energy transfer process and a subsequent trapping, with a time constant of 20 ps, to form the initial charge separation state of P800+A0–. The 20 ps component showed the bleaching Qy band at 816 nm and would derive from the excited red-BChl gs or might be due to the exciton coupling between P800 and a monomeric accessory BChl g. Furthermore, the non-decaying component spectrum exhibited a slightly broader peak centered at 792 nm, ascribed to the overlapping of two bleaching bands derived from both the P800 itself and an accessory BChl g excitonically coupled with the primary acceptor 81−OH-Chl aF. This was supported by the appearance of another bleaching band at 573 nm, which was detected in the Qx region of BChl gs.

Mutation H(M202)L does not lead to the formation of a heterodimer of the primary electron donor in reaction centers of Rhodobacter sphaeroides when combined with mutation I(M206)H

In photosynthetic reaction centers (RCs) of purple bacteria, conserved histidine residues [His L173 and His M202 in Rhodobacter (Rba.) sphaeroides] are known to serve as fifth axial ligands to the central Mg atom of the bacteriochlorophyll (BChl) molecules (PA and PB, respectively) that constitute the homodimer (BChl/BChl) primary electron donor P. In a number of previous studies, it has been found that replacing these residues with leucine, which cannot serve as a ligand to the Mg ion of BChl, leads to the assembly of heterodimer RCs with P represented by the BChl/BPheo pair. Here, we show that a homodimer P is assembled in Rba. sphaeroides RCs if the mutation H(M202)L is combined with the mutation of isoleucine to histidine at position M206 located in the immediate vicinity of PB. The resulting mutant H(M202)L/I(M206)H RCs are characterized using pigment analysis, redox titration, and a number of spectroscopic methods. It is shown that, compared to wild-type RCs, the double mutation causes significant changes in the absorption spectrum of the P homodimer and the electronic structure of the radical cation P+, but has only minor effect on the pigment composition, the P/P+ midpoint potential, and the initial electron-transfer reaction. The results are discussed in terms of the nature of the axial ligand to the Mg of PB in mutant H(M202)L/I(M206)H RCs and the possibility of His M202 participation in the previously proposed through-bond route for electron transfer from the excited state P* to the monomeric BChl BA in wild-type RCs.

Bacteriochlorophyll interaction with singlet oxygen in membranes of purple photosynthetic bacteria: does the protective function of carotenoids exist?

The direct action of singlet oxygen on the bacteriochlorophyll (BChl) of light-harvesting complexes in the membranes of four types of purple non-sulfur and sulfur photosynthesizing bacteria with and without carotenoids has been studied. It has been found that BChl in carotenoid-less samples is generally more resistant to the action of singlet oxygen compared to the control. It is assumed that carotenoids are not needed to protect BChl of bacterial light-harvesting complexes from singlet oxygen, and in the classic work of Griffith et al. [1] the apoptosis process in carotenoid-less mutant cells, which involves the destruction of complexes, the appearance of monomeric BChl and generation of singlet oxygencaused by BChl, followed by BChl oxidation, was mistakenly attributed to the protective function of carotenoids.

Bacteriochlorophyll Interaction with Singlet Oxygen in Membranes of Purple Photosynthetic Bacteria: Does the Protective Function of Carotenoids Exist?

The direct action of singlet oxygen on the bacteriochlorophyll (BChl) of light-harvesting complexes in the membranes of four species of purple non-sulfur and sulfur photosynthesizing bacteria with and without carotenoids was studied. It was found that BChl in carotenoidless samples is generally more resistant to the action of singlet oxygen compared to the control. It is assumed that carotenoids are not required to protect BChl of bacterial light-harvesting complexes from singlet oxygen, and in the classic work by Griffith et al. [1] the apoptosis process in carotenoidless mutant cells, which involves the destruction of complexes, the appearance of monomeric BChl, and the generation of singlet oxygen caused by BChl, followed by BChl oxidation, was mistakenly attributed to the protective function of carotenoids.

Supramolecular self-assembly of bacteriochlorophyll c molecules in aerosolized droplets to synthesize biomimetic chlorosomes

The unique properties of chlorosomes, arising out of the self-assembled bateriochlorophyll (BChl) c structure, have made them attractive for use in solar cells. In this work, we have demonstrated the self-assembly of BChl c in aerosolized droplets to mimic naturally occurring chlorosomes. We compare two different methods for self-assembly of BChl c, one using a single-solvent and the other using two-solvents, and demonstrate the superiority of the two-solvent method. Results show that the self-assembled BChl c sprayed at different concentrations resulted in a varying red shift of 69–75 nm in absorption spectrum compared to the solution, which has peak at 668 nm corresponding to the monomeric BChl c. The sample fluoresces at 780 nm indicating a quality of self-assembly comparable to that observed in naturally occurring chlorosomes. In order to mimic chlorosomes, solution containing BChl c, BChl a, lipids and carotenes in same proportion as in chlorosomes is sprayed. The resulting self-assembly has an absorption peak at 750 nm, shifted by 82 nm compared to that of monomers and the fluorescence peak at 790 nm. Thus in presence of lipids and carotenes, both the absorption and fluorescence peaks are red shifted. Further, using grazing incidence small angle X-ray scattering (GISAXS), we characterized the deposited films, and the 2D X-ray scattering patterns of sample clearly indicate the distinct lamellar structure as present in chlorosomes. The results of this work provide new insights into self-assembly in aerosolized droplets, which can be used for assembling a wide range of molecules.

Formation of 55-kDa fragments under impaired coordination bonds and hydrophobic interactions in peripheral light-harvesting complexes isolated from photosynthetic purple bacteria

Size exclusion chromatography was used to assess the relative size of intact and diphenylamine-treated (DPA, with suppressed carotenoid synthesis) peripheral light-harvesting complexes (LH2 complexes) of the sulfurbacterium Allochromatium minutissimum. Both LH2 complexes were nonamers and had the same elution volume V(e), coinciding with that for the LH2 complex of Rhodoblastus acidophilus (strain 10050). Their molecular mass was 150 kDa. Bot pheophytinization of bacteriochlorophyll (BChl) at low pH and treatment with the detergent LDAO, affecting the hydrophobic interactions between the neighboring protomers, result in the fragmentation of the ring of the isolated LH2 complexes and formation of 55-kDa fragments with molecular masses corresponding to one-third of the initial value. Fragmentation caused by both pheophytinization and detergent treatment was much more rapid in DPA-treated LH2 complexes than in the intact ones. The 55-kDa fragments formed at low pH values contained monomeric bacteriopheophytin, while the fragments of a similar molecular mass formed at pH 8.0 in the presence of the detergent contained monomeric BChl. The observed fragmentation was hypothesized to reflect the inherent C3 symmetry of the LH2 complexes, with the preliminarily assembled trimers used as building blocks.

Role of bacteriochlorophyll in stabilization of the structure of the core and peripheral light-harvesting complexes from purple photosynthetic bacteria

Pheophytinization of bacteriochlorophyll (BChl) at low pH was investigated in the core (LH1) and peripheral (LH2) light-harvesting complexes, as well as in the ensemble of the reaction center (RC) with the LH1 complex. The stages in disintegration of the native BChl forms in the LH1 complex and in its ensemble with RC were revealed. They were observed as emergence of the absorption band of monomeric BChl and an increase in its intensity, followed by its transformation into the band of monomeric bacteriopheophytin (BPh) and then into the band of aggregated BPh. Unlike the LH1 complex, in the case of the LH2 complex monomeric BChl was never detected as an intermediate product. While the spectra revealed formation of monomeric BPh, its accumulation did not occur, since its aggregation is very rapid compared to that in the LH1 complex and in the RC-LH1 ensemble. PAG electrophoresis revealed that pheophytinization of BChl in the LH2 complex was accompanied by disruption of the stable cylindrical structure of this complex with emergence of characteristic fragments consisting of α and β peptides and bearing monomeric BPh, as well as of the α peptide aggregates bearing BPh aggregates. Unlike the LH2 complex, BChl pheophytinization in the LH1 complex did not result in its fragmentation. This is an indication of different types of structural stabilization in the LH1 and LH2 complexes. In the LH2 complex, coordination of bacteriochlorophyll Mg2+ by conservative histidine residues of the α and β polypeptides is the main factor responsible for the maintenance of its cylindrical structure. Stability of the LH1 complex is probably based primarily on the highly specific hydrophobic interactions between the surfaces of individual polypeptide chains, since the presence of hydrogen bonds results in autonomy of each αβ3BChl2 subunit, rather than in stabilization of the LH1 complex as a whole.

Temporal and spectral characterization of the photosynthetic reaction center from Heliobacterium modesticaldum

A time-resolved spectroscopic study of the isolated photosynthetic reaction center (RC) from Heliobacterium modesticaldum reveals that thermal equilibration of light excitation among the antenna pigments followed by trapping of excitation and the formation of the charge-separated state P800 (+)A0 (-) occurs within~25ps. This time scale is similar to that reported for plant and cyanobacterial photosystem I (PS I) complexes. Subsequent electron transfer from the primary electron acceptor A0 occurs with a lifetime of~600ps, suggesting that the RC of H. modesticaldum is functionally similar to that of Heliobacillus mobilis and Heliobacterium chlorum. The (A0 (-)-A0) and (P800 (+)-P800) absorption difference spectra imply that an 8(1)-OH-Chl a F molecule serves as the primary electron acceptor and occupies the position analogous to ec3 (A0) in PS I, while a monomeric BChl g pigment occupies the position analogous to ec2 (accessory Chl). The presence of an intense photobleaching band at 790nm in the (A0 (-)-A0) spectrum suggests that the excitonic coupling between the monomeric accessory BChl g and the 8(1)-OH-Chl a F in the heliobacterial RC is significantly stronger than the excitonic coupling between the equivalent pigments in PS I.

Protein Influence on Charge-Asymmetry of the Primary Donor in Photosynthetic Bacterial Reaction Centers Containing a Heterodimer: Effects on Photophysical Properties and Electron Transfer

The substantial electronic distinctions between bacteriochlorophyll (BChl) and its Mg-free analogue bacteriopheophytin (BPh) are exploited in two sets of Rhodobacter capsulatus reaction center (RC) mutants that contain a heterodimeric BChl-BPh primary electron donor (D). The BPh component of the M-heterodimer (Mhd) or L-heterodimer (Lhd) obtains from substituting a Leu for His M200 or for His L173, respectively. Lhd-β and Mhd-β RCs serve as the initial templates in the two mutant sets, where β denotes that the L-side BPh acceptor (HL) has been replaced by a BChl (due to substituting His for Leu M212). Three variants each of Lhd-β and Mhd-β mutants were constructed: (1) a swap (denoted YF) of the native Phe (L181) and Tyr (M208) residues, which flank D and the nearby M- and L-side monomeric BChl cofactors, respectively, giving Tyr (L181) and Phe (M208); (2) addition of a hydrogen bond (denoted L131LH) to the ring V keto group of the L-macrocycle of D, via replacing the native Leu at L131 with His; (3) the combination of 1 and 2. A low yield of electron transfer (ET) to the M-side BPh (HM) is observed in all four Lhd-containing RCs. Comparison with the yield of ET to β on the L-side shows that electron density on the L-macrocycle of D* favors ET to the M-side cofactors and vice versa. Increasing or decreasing the electronic asymmetry of D* via the YF, L131LH mutations or the combination results in consistent trends in the characteristics of the long-wavelength ground state absorption band of D, the rate constant of internal conversion of D* to the ground state, and the rate constants for ET to both the L- and M-side cofactors. A surprising correlation is that an increase in the charge asymmetry in D* not only increases the D* internal-conversion rate constant, but also the rate constants for ET to both the L- and M-side cofactors, spanning time scales of tens of picoseconds to several nanoseconds. The YF swap has a previously unrecognized effect on the electronic asymmetry of D*, resulting in increased charge asymmetry for the Mhd and decreased charge asymmetry for the Lhd. This result indicates that the native Tyr (M208) and Phe (L181) in the wild-type RC promote an electron distribution in P* that is the reverse of that favorable for ET to the photoactive L-branch. This conclusion reinforces the view that the native configuration of these residues promotes ET to the L branch primarily by poising the free energies of the charge-separated states. Overall, this work addresses the extent to which electronic couplings complement energetics in underpinning the directionality of ET in the bacterial RC.

Spectroscopic insights into the decreased efficiency of chlorosomes containing bacteriochlorophyll f

Chlorosomes are light-harvesting antenna complexes that occur in green photosynthetic bacteria which have only been shown naturally to contain bacteriochlorophyll (BChl) c, d, or e as the principal light-harvesting pigments. BChl f has long been thought to be an obvious fourth member of the so-called Chlorobium chlorophylls, because it possesses a C-7 formyl group like BChl e and lacks a methyl group at C-20 like BChl d. In organisms that synthesize BChl c or e, the bchU gene product encodes the enzyme that methylates the C-20 position of these molecules. A bchU null mutant of the green sulfur bacterium Chlorobaculum limnaeum strain 1677T, which normally synthesizes BChl e, has recently been generated via insertional inactivation, and it produces chlorosomes containing BChl f [Vogl et al., 2012]. In this study, chlorosomes containing BChl f and monomeric BChl f in pyridine were characterized using a variety of spectroscopic techniques, including fluorescence emission and excitation spectroscopy, fluorescence lifetime and quantum yield determinations, and circular dichroism. These spectroscopic measurements, as well as Gaussian simulation of the data, show that chlorosomes containing BChl f are less efficient in energy transfer than those with BChl e. This can primarily be attributed to the decreased spectral overlap between the oligomeric BChl f (energy donor) fluorescence emission and the BChl a (energy acceptor) absorption in the chlorosome baseplate. This study allows us to hypothesize that, if they exist in nature, BChl f-containing organisms most likely live in rare high-light, anoxic conditions devoid of Chl a, d, or BChl e filtering. Abstract referenceK. Vogl, M. Tank, G.S. Orf, R.E. Blankenship, D.A. Bryant, Bacteriochlorophyll f: properties of chlorosomes containing the “forbidden chlorophyll,” Front. Microbiol. 3 (2012) 298.

The site-directed mutation I(L177)H in Rhodobacter sphaeroides reaction center affects coordination of PA and BB bacteriochlorophylls

To explore the influence of the I(L177)H single mutation on the properties of the nearest bacteriochlorophylls (BChls), three reaction centers (RCs) bearing double mutations were constructed in the photosynthetic purple bacterium Rhodobacter sphaeroides, and their properties and pigment content were compared with those of the correspondent single mutant RCs. Each pair of the mutations comprised the amino acid substitution I(L177)H and another mutation altering histidine ligand of BChl PA or BChl BB. Contrary to expectations, the double mutation I(L177)H+H(L173)L does not bring about a heterodimer RC but causes a 46nm blue shift of the long-wavelength P absorbance band. The histidine L177 or a water molecule were suggested as putative ligands for PA in the RC I(L177)H+H(L173)L although this would imply a reorientation of the His backbone and additional rearrangements in the primary donor environment or even a repositioning of the BChl dimer. The crystal structure of the mutant I(L177)H reaction center determined to a resolution of 2.9Å shows changes at the interface region between the BChl PA and the monomeric BChl BB. Spectral and pigment analysis provided evidence for β-coordination of the BChl BB in the double mutant RC I(L177)H+H(M182)L and for its hexacoordination in the mutant reaction center I(L177)H. Computer modeling suggests involvement of two water molecules in the β-coordination of the BChl BB. Possible structural consequences of the L177 mutation affecting the coordination of the two BChls PA and BB are discussed. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

The rate of Qx→Qy relaxation in bacteriochlorophylls of reaction centers from Rhodobacter sphaeroides determined by kinetics of the ultrafast carotenoid bandshift

Transient absorption changes induced by excitation of isolated reaction centers (RCs) from Rhodobacter sphaeroides with 600nm laser pulses of 20fs (full width at half maximum) were monitored in the wavelength region of 420–560nm. The spectral features of the spectrum obtained are characteristic for an electrochromic band shift of the single carotenoid (Car) molecule spheroidene, which is an integral constituent of these RCs. This effect is assigned to an electrochromic bandshift of Car due to the local electric field of the dipole moment formed by electronic excitation of bacteriochlorophyll (BChl) molecule(s) in the neighborhood of Car. Based on the known distances between the pigments, the monomeric BChl (BB) in the inactive B-branch is inferred to dominate this effect. The excitation of BB at 600nm leads to a transition into the S2 state (Qx band), which is followed by rapid internal conversion to the S1 state (Qy band), thus leading to a change of strength and orientation of the dipole moment, i.e., of the electric field acting on the Car molecule. Therefore, the time course of the electrochromic bandshift reflects the rate of the internal conversion from S2 to S1 of BB. The evaluation of the kinetics leads to a value of 30fs for this relaxation process. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

Effect of low pH values on chromatophores, core complexes, and peripheral light harvesting complexes isolated from the sulfur bacterium Allochromatium minutissimum

Spectral methods have been used to trace pheophytinization of bacteriochlorophyll (BChl) in the membranes of chromatophores isolated from normal and carotenoidless cells of the purple bacterium Allochromatium minutissimum as well as in the core complexes and peripheral light harvesting complexes in the media with different detergents at low pH values. The well-marked staging of damage of native BChl forms with the absorption band of 885 nm has been revealed: (1) the formation and increase of the absorption band of monomeric BChl (785 nm); (2) pheophytinization of resultant monomeric BChl, and (3) aggregation of bacteriopheophytin (BPheo). Compared to the initial carotenoid complexes, carotenoidless pigment protein complexes were less resistant to the effect of low pH values, especially at the stages of BChl monomerization and pheophytinization. However, BPheo aggregation in them was slower. The electrophoresis in PAAG has shown that BChl pheophytinization in peripheral light harvesting complexes is accompanied by disruption of the ring-shaped structures of the complexes, with appearance of typical fragments consisting of α- and β- peptides and carrying monomeric BPheo, and by formation of α-peptide aggregates carrying BPheo aggregates.

Examination of stability of mutant photosynthetic reaction center of Rhodobacter sphaeroides I(L177)H and determination of location of bacteriochlorophyll covalently bound to the protein

We demonstrated earlier that as a result of the I(L177)H mutation in the photosynthetic reaction center (RC) of the bacterium Rhodobacter sphaeroides, one of the bacteriochlorophylls (BChl) binds with the L-subunit, simultaneously raising coordination stability of the central magnesium atom of the bacteriochlorophyll associated with the protein. In this study, spectral properties of wild type RC and I(L177)H in the presence of urea and SDS as well as at 48 degrees C were examined. It is shown that the I(L177)H mutation decreases the RC stability. Under denaturing conditions, some changes indicating breakdown of oligomeric structure of the complex and loss of interaction between pigments and their protein environment are observed in I(L177)H RC spectra. In addition, pheophytinization of bacteriochlorophylls occurs in both types of RC in the presence of SDS. However, an 811-nm band is observed in the spectrum of the mutant RC under these conditions, which indicates retention of one of the BChl molecules in the protein binding site and stable coordination of its central magnesium atom. It is shown that in both types of RC, monomeric BChl B(B) can be modified by sodium borohydride treatment and then extracted by acetone-methanol mixture. Spectral properties of the BChl covalently bound with the protein in I(L177)H RC do not change. The results demonstrate that BChl P(A) is the molecule of BChl tightly bound with the L-subunit in mutant RC as it was supposed earlier.

Excitation energy pathways in the photosynthetic units of reaction center LM- and H-subunit deletion mutants of Rhodospirillum rubrum

Light-induced reaction dynamics of isolated photosynthetic membranes obtained from wild-type (WT) and reaction center (RC)-subunit deletion strains SPUHK1 (an H-subunit deletion mutant) and SK Delta LM (an (L+M) deletion mutant) of the purple non-sulphur bacterium Rhodospirillum rubrum have been investigated by femtosecond transient absorption spectroscopy. Upon excitation of the spirilloxanthin (Spx) S(2) state at 546 nm, of the bacteriochlorophyll Soret band at 388 nm and probing spectral regions, which are characteristic for carotenoids, similar dynamics in the SPUHK1, SK Delta LM and WT strains could be observed. The excitation of Spx S(2) is followed by the simultaneous population of the lower singlet excited states S(1) and S* which decay with lifetimes of 1.4 and 5 ps, respectively for the mutants, and 1.4 and 4 ps, respectively, for the wild-type. The excitation of the BChl Soret band is followed by relaxation into BChl lower excited states which compete with excitation energy transfer BChl-to-Spx. The deexcitation pathway BChl(Soret) --> Spx(S(2)) --> Spx(S(1)) occurs with the same transition rate for all investigated samples (WT, SPUHK1 and SK Delta LM). The kinetic traces measured for the Spx S(1) --> S(N) transition display similar behaviour for all samples showing a positive signal which increases within the first 400 fs (i.e. the time needed for the excitation energy to reach the Spx S(1) excited state) and decays with a lifetime of about 1.5 ps. This suggests that the Spx excited state dynamics in the investigated complexes do not differ significantly. Moreover, a longer excited state lifetime of BChl for SPUHK1 in comparison to WT was observed, consistent with a photochemical quenching channel present in the presence of RC. For long delay times, photobleaching of the RC special pair and an electrochromic blue shift of the monomeric BChl a can be observed only for the WT but not for the mutants. The close similarity of the excited state decay processes of all strains indicates that the pigment geometry of the LH1 complex in native membranes is unaffected by the presence of an RC and allows us to draw a model representation of the WT, SK Delta LM and SPUHK1 PSU complexes.

Identification of the Intermediate Charge-Separated State P +βL− in a Leucine M214 to Histidine Mutant of the Rhodobacter sphaeroides Reaction Center Using Femtosecond Midinfrared Spectroscopy

Energy and electron transfer in a Leu M214 to His (LM214H) mutant of the Rhodobacter sphaeroides reaction center (RC) were investigated by applying time-resolved visible pump/midinfrared probe spectroscopy at room temperature. This mutant replacement of the Leu at position M214 resulted in the incorporation of a bacteriochlorophyll (BChl) in place of the native bacteriopheophytin in the L-branch of cofactors (denoted β L). Purified LM214H RCs were excited at 600 nm (unselective excitation), at 800 nm (direct excitation of the monomeric BChl cofactors B L and B M), and at 860 nm (direct excitation of the primary donor (P) BChl pair (P L/P M)). Absorption changes associated with carbonyl (C=O) stretch vibrational modes (9-keto, 10a-ester, and 2a-acetyl) of the cofactors and of the protein were recorded in the region between 1600 cm −1 and 1770 cm −1, and the data were subjected to both a sequential analysis and a simultaneous target analysis. After photoexcitation of the LM214H RC, P ∗ decayed on a timescale of ∼6.3 ps to P +B L −. The decay of P +B L − occurred with a lifetime of ∼2 ps, ∼3 times slower than that observed in wild-type and R-26 RCs (∼0.7 ps). Further electron transfer to the β L BChl resulted in formation of the P + β L − state, and its infrared absorbance difference spectrum is reported for the first time, to our knowledge. The fs midinfrared spectra of P +B L − and P + β L − showed clear differences related to the different environments of the two BChls in the mutant RC.

Optimal coupling of subantennas as a strategy for efficient functioning of the light-harvesting antennas in photosynthesizing organisms: Model computations

This work continues the cycle of studies into the strategy of efficient functioning of the natural light-harvesting molecular antennas within the framework of the previously proposed concept of stringent optimization of the structure of the photosynthesizing machinery according to the functional criterion [1]. The presence of inhomogeneous antennas composed of different subantennas poses the problem of their coupling [2]. One of the structural optimizing factors is the relative orientation of Q y transition dipoles of the light-harvesting pigments in subantennas [3]. This paper briefs the mathematical modeling of the dynamics of excitation energy transfer in the superantenna of the photosynthesizing green bacteria Chloroflexus ( Cf. ) aurantiacus. This allowed us to compute the optimal relative orientation of the Q y transition dipoles in subantenna molecules using the formalism of probability matrices and to demonstrate the significance of their optimal coupling for an efficient and stable functioning of the superantenna. The superantenna of Cf. aurantiacus is localized to three subcellular structures: chlorosomes, containing the oligomeric BChl c subantenna B740; the baseplate, containing the monomeric BChl a subantenna B798; and cytoplasmic membrane, containing the BChl a subantenna B808‐866, analogous to the subantenna B800‐ 850 of purple bacteria [4]. The excitation energy transfer B740 → B798 → B808 → B866 proceeds according to the thermodynamic potential [5]. The Q y transition dipoles in BChl c clusters of subantenna B740 are mutually parallel and oriented along the long axis of the chlorosome [6], whereas the Q y transition dipoles in B808 and B866 subantenna molecules make angles of ~43° and ~8° with the plane of the membrane, respectively [7]. The orientation of Q y transition dipoles for the intermediate subantenna B798 in BChl ‡ is unknown. In this work, we modeled the energy transfer B740 → B798 → B808 to analyze the optimization of this process by optimizing the orientation of Q y transition dipoles in BChl ‡ subantenna B798. An infinite three-dimensional antenna displaying

Selective Chemical Shift Assignment of B800 and B850 Bacteriochlorophylls in Uniformly [13C,15N]-Labeled Light-Harvesting Complexes by Solid-State NMR Spectroscopy at Ultra-High Magnetic Field

The electronic ground states of the bacteriochlorophyll a type B800 and type B850 in the light-harvesting 2 complex of Rhodopseudomonas acidophila strain 10050 have been characterized by magic angle spinning (MAS) dipolar (13)C-(13)C correlation NMR spectroscopy. Uniformly [(13)C,(15)N] enriched light-harvesting 2 (LH2) complexes were prepared biosynthetically, while [(13)C,(15)N]-B800 LH2 complexes were obtained after reconstitution of apoprotein with uniformly [(13)C,(15)N]-enriched bacteriochlorophyll cofactors. Extensive sets of isotropic (13)C NMR chemical shifts were obtained for each bacteriochlorin ring species in the LH2 protein. (13)C isotropic shifts in the protein have been compared to the corresponding shifts of monomeric BChl a dissolved in acetone-d(6). Density functional theory calculations were performed to estimate ring current effects induced by adjacent cofactors. By correction for the ring current shifts, the (13)C shift effects due to the interactions with the protein matrix were resolved. The chemical shift changes provide a clear evidence for a global electronic effect on the B800 and B850 macrocycles, which is attributed to the dielectrics of the protein environment, in contrast with local effects due to interaction with specific amino acid residues. Considerable shifts of -6.2 < Deltasigma < +5.8 ppm are detected for (13)C nuclei in both the B800 and the B850 bacteriochlorin rings. Because the shift effects for the B800 and B850 are similar, the polarization of the electronic ground states induced by the protein environment is comparable for both cofactors and corresponds with a red shift of approximately 30 nm relative to the monomeric BChl dissolved in acetone-d(6). The electronic coupling between the B850 cofactors due to macrocycle overlap is the predominant mechanism behind the additional red shift in the B850.

Comparison between chlorosomes containing bacteriochlorophyll- c and chlorosomes containing bacteriochlorophyll- d isolated from two substrains of green sulfur photosynthetic bacterium Chlorobium vibrioforme NCIB 8327

Chlorosomes containing bacteriochlorophyll(BChl)- c and those containing BChl- d were isolated from two substrains of Chlorobium vibrioforme f. sp. thiosulfatophilum NCIB 8327, respectively. The two types of chlorosomes were investigated from the following aspect, what kinds of effects the molecular structure of chlorosomal BChls had on structural and spectroscopic properties of in vivo self-aggregates in chlorosomes without alteration of the other components such as chlorosomal proteins and lipids; both chlorosomes were expected to have the same components except for light-harvesting BChls. In their visible absorption spectra, the differences of Soret and Q y peak positions between BChl- c containing and BChl- d containing chlorosomes were similar to the differences between monomeric BChl- c and d. An inverse S-shaped CD signal in the Q y region of BChl- d containing chlorosomes was 1.4 times larger than that of BChl- c containing chlorosomes, when the Q y absorbance of the two chlorosomes was almost the same. This implies that the excitonic interaction of BChl- d is larger than that of BChl- c in natural chlorosomes. Resonance Raman spectroscopy showed that BChl self-assemblies in both chlorosomes were essentially formed by the same local structural interaction among 3 1-hydroxy group, 13-keto group, and central magnesium. BChl- d self-aggregates in chlorosomes were more tolerant of 1-hexanol than in vivo BChl- c aggregates, suggesting that the molecular structure of BChl- d provided more stable self-assemblies than BChl- c in natural chlorosomes.

Light control over the size of an antenna unit building block as an efficient strategy for light harvesting in photosynthesis

It was shown that an increase in the bacteriochlorophyll (BChl) c antenna size observed upon lowering growth light intensities led to enhancement of the hyperchromism of the BChl c Q y absorption band of the green photosynthetic bacterium Chloroflexus aurantiacus. With femtosecond difference absorption spectroscopy, it was shown that the amplitude of bleaching of the oligomeric BChl c Q y band (as compared to that for monomeric BChl a) increased with increasing BChl c content in chlorosomes. This BChl c bleaching amplitude was about doubled as the chlorosomal antenna size was about trebled. Both sets of findings clearly show that a unit BChl c aggregate in the chlorosomal antenna is variable in size and governed by the grow light intensity, thus ensuring the high efficiency of energy transfer within the BChl c antenna regardless of its size.