Exciton delocalization in the B808-866 antenna of the green bacterium Chloroflexus aurantiacus as revealed by ultrafast pump-probe spectroscopy.

A model of pigment organization in the B808-866 bacteriochlorophyll a antenna of the green photosynthetic bacterium Chloroflexus aurantiacus based on femtosecond pump-probe studies is proposed. The building block of the antenna was assumed to be structurally similar to that of the B800-850 light-harvesting 2 (LH2) antenna of purple bacteria and to have the form of two concentric rings of N strongly coupled BChl866 pigments and of N/2 weakly coupled BChl808 monomers, where N = 24 or 32. We have shown that the Qy transition dipoles of BChl808 and BChl866 molecules form the angles 43 degrees +/- 3 degrees and 8 degrees +/- 4 degrees, respectively, with the plane of the corresponding rings. Using the exciton model, we have obtained a quantitative fit of the pump-probe spectra of the B866 and B808 bands. The anomalously high bleaching value of the B866 band with respect to the B808 monomeric band provided the direct evidence for a high degree of exciton delocalization in the BChl866 ring antenna. The coherence length of the steady-state exciton wave packet corresponds to five or six BChl866 molecules at room temperature.     

Exciton dynamics in the chlorosomal antenna of the green bacterium Chloroflexus aurantiacus: experimental and theoretical studies of femtosecond pump-probe spectra

Femtosecond absorption difference spectra were measured for chlorosomes isolated from the green bacterium Chloroflexus aurantiacus at room temperature. Using the relative difference absorption of the oligomeric BChl c and monomeric BChl a bands, the size of a unit BChl c aggregate as well as the exciton coherence size were estimated for the chlorosomal BChl c antenna under study. A quantitative fit of the data was obtained within the framework of the exciton model proposed before [Fetisova et al. (1996) Biophys J 71: 995–1010]. The size of the antenna unit was found to be 24 exciton-coupled BChl c molecules. The anomalously high bleaching value of the oligomeric B740 band with respect to the monomeric B795 band provided the direct evidence for a high degree of exciton delocalization in the chlorosomal B740 BChl c antenna. The effective delocalization size of individual exciton wavefunctions (the thermally averaged inverse participation ratio) in the chlorosomal BChl c antenna is 9.5, whereas the steady-state wavepacket corresponds to the coherence size (the inverse participation ratio of the density matrix) of 7.4 at room temperature.This revised version was published online in October 2005 with corrections to the Cover Date.     

Purification, characterization and crystallization of the core complex from thermophilic purple sulfur bacterium Thermochromatium tepidum

A light-harvesting-reaction center (LH1-RC) core complex has been highly purified from a thermophilic purple sulfur bacterium, Thermochromatium tepidum. The bacteriochlorophyll (BChl) a molecules in the LH1 exhibit a Q(y) transition at 914 nm, more than 25 nm red-shift from those of its mesophilic counterparts. The LH1-RC complex was isolated in a monomeric form as confirmed by sucrose density gradient centrifugation, blue native PAGE and size-exclusion chromatography. Four subunits (L, M, H and a tetraheme cytochrome) in RC and two polypeptides (alpha and beta) in LH1 were identified. Spirilloxanthin was determined to be the predominant carotenoid in the core complex. The purified core complex was highly stable, no significant change in the LH1 Q(y) transition was observed over 10 days of incubation at room temperature in dark. Circular dichroism spectrum of the LH1 complex was characterized by low intensity and nonconservative spectral shape, implying a high symmetry of the large LH1 ring and interaction between the BChl a and carotenoid molecules. A dimeric feature of the BChl a molecules in LH1 was revealed by magnetic circular dichroism spectrum. Crystals of the core complex were obtained which diffracted X-rays to about 10 A.     

Dynamics of energy transfer and trapping in the light-harvesting antenna of Rhodopseudomonas viridis.

By low intensity picosecond absorption spectroscopy it is shown that the exciton lifetime in the light-harvesting antenna of Rhodopseudomonas (Rps.) viridis membranes with photochemically active reaction centers at room temperature is 60 +/- 10 ps. This lifetime reflects the overall trapping rate of the excitation energy by the reaction center. With photochemically inactive reaction centers, in the presence of P+, the exciton lifetime increases to 150 +/- 15 ps. Prereducing the secondary electron acceptor QA does not prevent primary charge separation, but slows it down from 60 to 90 +/- 10 ps. Picosecond kinetics measured at 77 K with inactive reaction centers indicates that the light-harvesting antenna is spectrally homogeneous. Picosecond absorption anisotropy measurements show that energy transfer between identical Bchlb molecules occurs on the subpicosecond time scale. Using these experimental results as input to a random-walk model, results in strict requirements for the antenna-RC coupling. The model analysis prescribes fast trapping (approximately 1 ps) and an approximately 0.5 escape probability from the reaction center, which requires a more tightly coupled RC and antenna, as compared with the Bchla-containing bacteria Rhodospirillum (R.) rubrum and Rhodobacter (Rb.) sphaeroides.     

High pressure studies of energy transfer and strongly coupled bacteriochlorophyll dimers in photosynthetic protein complexes

High pressure is used with hole burning and absorption spectroscopies at low temperatures to study the pressure dependence of the B800B850 energy transfer rate in the LH2 complex of Rhodobacter sphaeroides and to assess the extent to which pressure can be used to identify and characterize states associated with strongly coupled chlorophyll molecules. Pressure tuning of the B800–B850 gap from 750 cm^\s-1 at 0.1 MPa to 900 cm^-1 at 680 MPa has no measurable effect on the 2 ps energy transfer rate of the B800–850 complex at 4.2 K. An explanation for this resilience against pressure, which is supported by earlier hole burning studies, is provided. It is based on weak coupling nonadiabatic transfer theory and takes into account the inhomogeneous width of the B800–B850 energy gap, the large homogeneous width of the B850 band from exciton level structure and the Franck-Condon factors of acceptor protein phonons and intramolecular BChl a modes. The model yields reasonable agreement with the 4.2 K energy transfer rate and is consistent with its weak temperature dependence. It is assumed that it is the C₉-ring exciton levels which lie within the B850 band that are the key acceptor levels, meaning that BChl a modes are essential to the energy transfer process. These ring exciton levels derive from the strongly allowed lowest energy component of the basic B850 dimer. However, the analysis of B850s linear pressure shift suggests that another Förster pathway may also be important. It is one that involves the ring exciton levels derived from the weakly allowed upper component of the B850 dimer which we estimate to be quasi-degenerate with B800. In the second part of the paper, which is concerned with strong BChl monomer-monomer interactions of dimers, we report that the pressure shifts of B875 (LH2), the primary donor absorption bands of bacterial RC (P870 of Rb. sphaeroides and P960 of Rhodopseudomonas viridis) and B1015 (LH complex of Rps. viridis) are equal and large in value (-0.4 cm⁰¹/MPa at 4.2 K) relative to those of isolated monomers in polymers and proteins (< -0.1 cm⁰¹/MPa). The shift rate for B850 at 4.2 K is-0.28 cm^–1/MPa. A model is presented which appears to be capable of providing a unified explanation for the pressure shifts.Abbreviations B800 BChl antenna band absorbing (at room temperature) at 800 nm (B850, B875, B1015 are defined similarly) - CD circular dichroism - FC factor Franck-Condon factor - FMO comple Fenna-Matthews-Olson complex - L-S theory Laird-Skinner theory - LH1 core light-harvesting complex of the BChl antenna complexes - LH2 peripheral light-harvesting complex of the BChl antenna complexes - NPHB non-photochemical hole burning - P960 absorption band of special pair of Rhodopseudomonas viridis absorbing at 960 nm (room temperature). P870 of Rhodobacter sphaeroides is defined similarly - QM/MM results quantum mechanical/molecular mechanical results - RC reaction center - ZPH zero phonon hole     

Analysis of absorption spectra of purple bacterial reaction centers in the near infrared region by higher order derivative spectroscopy

Reaction centers (RCs) of purple bacteria are uniquely suited objects to study the mechanisms of the photosynthetic conversion of light energy into chemical energy. A recently introduced method of higher order derivative spectroscopy [I.K. Mikhailyuk, H. Lokstein, A.P. Razjivin, A method of spectral subband decomposition by simultaneous fitting the initial spectrum and a set of its derivatives, J. Biochem. Biophys. Methods 63 (2005) 10-23] was used to analyze the NIR absorption spectra of RC preparations from Rhodobacter (R.) sphaeroides strain 2R and Blastochloris (B.) viridis strain KH, containing bacteriochlorophyll (BChl) a and b, respectively. Q(y) bands of individual RC porphyrin components (BChls and bacteriopheophytins, BPheo) were identified. The results indicate that the upper exciton level P(y+) of the photo-active BChl dimer in RCs of R. sphaeroides has an absorption maximum of 810nm. The blue shift of a complex integral band at approximately 800nm upon oxidation of the RC is caused primarily by bleaching of P(y+), rather than by an electrochromic shift of the absorption band(s) of the monomeric BChls. Likewise, the disappearance of a band peaking at 842nm upon oxidation of RCs from B. viridis indicates that this band has to be assigned to P(y+). A blue shift of an absorption band at approximately 830nm upon oxidation of RCs of B. viridis is also essentially caused by the disappearance of P(y+), rather than by an electrochromic shift of the absorption bands of monomeric BChls. Absorption maxima of the monomeric BChls, B(B) and B(A) are at 802 and 797nm, respectively, in RCs of R. sphaeroides at room temperature. BPheo co-factors H(B) and H(A) peak at 748 and 758nm, respectively, at room temperature. For B. viridis RCs the spectral positions of H(B) and H(A) were found to be 796 and 816nm, respectively, at room temperature.     

Purification, characterization and crystallization of the core complex from thermophilic purple sulfur bacterium Thermochromatium tepidum

A light-harvesting-reaction center (LH1-RC) core complex has been highly purified from a thermophilic purple sulfur bacterium, Thermochromatium tepidum. The bacteriochlorophyll (BChl) a molecules in the LH1 exhibit a Q_y transition at 914 nm, more than 25 nm red-shift from those of its mesophilic counterparts. The LH1-RC complex was isolated in a monomeric form as confirmed by sucrose density gradient centrifugation, blue native PAGE and size-exclusion chromatography. Four subunits (L, M, H and a tetraheme cytochrome) in RC and two polypeptides (α and β) in LH1 were identified. Spirilloxanthin was determined to be the predominant carotenoid in the core complex. The purified core complex was highly stable, no significant change in the LH1 Q_y transition was observed over 10 days of incubation at room temperature in dark. Circular dichroism spectrum of the LH1 complex was characterized by low intensity and nonconservative spectral shape, implying a high symmetry of the large LH1 ring and interaction between the BChl a and carotenoid molecules. A dimeric feature of the BChl a molecules in LH1 was revealed by magnetic circular dichroism spectrum. Crystals of the core complex were obtained which diffracted X-rays to about 10 Å.     

Low Light Adaptation: Energy Transfer Processes in Different Types of Light Harvesting Complexes from Rhodopseudomonas palustris

Energy transfer processes in photosynthetic light harvesting 2 (LH2) complexes isolated from purple bacterium Rhodopseudomonas palustris grown at different light intensities were studied by ground state and transient absorption spectroscopy. The decomposition of ground state absorption spectra shows contributions from B800 and B850 bacteriochlorophyll (BChl) a rings, the latter component splitting into a low energy and a high energy band in samples grown under low light (LL) conditions. A spectral analysis reveals strong inhomogeneity of the B850 excitons in the LL samples that is well reproduced by an exponential-type distribution. Transient spectra show a bleach of both the low energy and high energy bands, together with the respective blue-shifted exciton-to-biexciton transitions. The different spectral evolutions were analyzed by a global fitting procedure. Energy transfer from B800 to B850 occurs in a mono-exponential process and the rate of this process is only slightly reduced in LL compared to high light samples. In LL samples, spectral relaxation of the B850 exciton follows strongly nonexponential kinetics that can be described by a reduction of the bleach of the high energy excitonic component and a red-shift of the low energetic one. We explain these spectral changes by picosecond exciton relaxation caused by a small coupling parameter of the excitonic splitting of the BChl a molecules to the surrounding bath. The splitting of exciton energy into two excitonic bands in LL complex is most probably caused by heterogenous composition of LH2 apoproteins that gives some of the BChls in the B850 ring B820-like site energies, and causes a disorder in LH2 structure.     

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.     

Energy transfer and charge separation in the purple non-sulfur bacterium Roseospirillum parvum

The antenna reaction centre system of the recently described purple non-sulfur bacterium Roseospirillum parvum strain 930I was studied with various spectroscopic techniques. The bacterium contains bacteriochlorophyll (BChl) a, 20% of which was esterified with tetrahydrogeranylgeraniol. In the near-infrared, the antenna showed absorption bands at 805 and 909 nm (929 nm at 6 K). Fluorescence bands were located at 925 and 954 nm, at 300 and 6 K, respectively. Fluorescence excitation spectra and time resolved picosecond absorbance difference spectroscopy showed a nearly 100% efficient energy transfer from BChl 805 to BChl 909, with a time constant of only 2.6 ps. This and other evidence indicate that both types of BChl belong to a single LH1 complex. Flash induced difference spectra show that the primary electron donor absorbs at 886 nm, i.e. at 285 cm(-1) higher energy than the long wavelength antenna band. Nevertheless, the time constant for trapping in the reaction centre was the same as for almost all other purple bacteria: 55+/-5 ps. The shape as well as the amplitude of the absorbance difference spectrum of the excited antenna indicated exciton interaction and delocalisation of the excited state over the BChl 909 ring, whereas BChl 805 appeared to have a monomeric nature.     

Solvation effect of bacteriochlorophyll excitons in light-harvesting complex LH2

We have characterized the influence of the protein environment on the spectral properties of the bacteriochlorophyll (Bchl) molecules of the peripheral light-harvesting (or LH2) complex from Rhodobacter sphaeroides. The spectral density functions of the pigments responsible for the 800 and 850 nm electronic transitions were determined from the temperature dependence of the Bchl absorption spectra in different environments (detergent micelles and native membranes). The spectral density function is virtually independent of the hydrophobic support that the protein experiences. The reorganization energy for the B850 Bchls is 220 cm(-1), which is almost twice that of the B800 Bchls, and its Huang-Rhys factor reaches 8.4. Around the transition point temperature, and at higher temperatures, both the static spectral inhomogeneity and the resonance interactions become temperature-dependent. The inhomogeneous distribution function of the transitions exhibits less temperature dependence when LH2 is embedded in membranes, suggesting that the lipid phase protects the protein. However, the temperature dependence of the fluorescence spectra of LH2 cannot be fitted using the same parameters determined from the analysis of the absorption spectra. Correct fitting requires the lowest exciton states to be additionally shifted to the red, suggesting the reorganization of the exciton spectrum.     

Excitation energy transfer in chlorosomes of green bacteria: theoretical and experimental studies.

A theory of excitation energy transfer within the chlorosomal antennae of green bacteria has been developed for an exciton model of aggregation of bacteriochlorophyll (BChl) c (d or e). This model of six exciton-coupled BChl chains with low packing density, approximating that in vivo, and interchain distances of approximately 2 nm was generated to yield the key spectral features found in natural antennae, i.e., the exciton level structure revealed by spectral hole burning experiments and polarization of all the levels parallel to the long axis of the chlorosome. With picosecond fluorescence spectroscopy it was demonstrated that the theory explains the antenna-size-dependent kinetics of fluorescence decay in chlorosomal antenna, measured for intact cells of different cultures of the green bacterium C. aurantiacus, with different chlorosomal antenna size determined by electron microscopic examination of the ultrathin sections of the cells. The data suggest a possible mechanism of excitation energy transfer within the chlorosome that implies the formation of a cylindrical exciton, delocalized over a tubular aggregate of BChl c chains, and Forster-type transfer of such a cylindrical exciton between the nearest tubular BChl c aggregates as well as to BChl a of the baseplate.     

Two-photon excitation spectrum of fluorescence of the light-harvesting complex B800–850 from Allochromatium minutissimum within 1200–1500 (600–750) nm spectral range is not carotenoid mediated

Two types of peripheral light-harvesting complexes LH2 (B800–850) from photosynthetic purple bacterium Allochromatium minutissimum were studied. First type containing carotenoids was prepared from wild type cells. The other one was obtained from carotenoid depleted cells grown with diphenylamine. We have shown that under laser femtosecond excitation within absorption 1200–1500 nm wavelength range the two-photon excitation of LH2 complexes takes place. This can be observed as fluorescence of bacteriochlorophyll (BChl) spectral form B850 (BChl molecules of circular aggregate with strong exciton interaction in 850 nm spectral domain). LH2 fluorescence excitation spectra under two-photon excitation are the same for carotenoid-containing and carotenoidless preparations. In both cases the broad band with peak near 1350 (675) nm (FWHM ～ 240 (120) nm) was found. It is concluded that the broad band with peak near 1350 (675) nm in two-photon excitation spectra of LH2 complexes from Allochromatium minutissimum cannot be interpreted as two-photon excitation band of the optically forbidden S₀ → S₁ transition of carotenoids (rhodopin). Possible nature of this band is discussed.     

Certain species of the Proteobacteria possess unusual bacteriochlorophyll a environments in their light-harvesting proteins

In this work, we have examined, using Fourier-transform Raman (FT-R) spectroscopy, the bacteriochlorophyll a (BChl a) binding sites in light-harvesting (LH) antennae from different species of the Proteobacteria that exhibit unusal absorption properties. While the LH1 complexes from Erythromicrobium (E.) ramosum (RC-B871) and Rhodospirillum centenum (B875) present classic FT-R spectra in the carbonyl high-frequency region, we show that in the blue-shifted LH1 complex, absorbing at 856 nm, from Roseococcus thiosulfatophilus, as well as in the B798-832 LH2 from E. ramosum, or in the B830 complex from the obligate phototrophic bacterium Chromatium purpuratum, some H-bonds between the acetyl carbonyl of the BChl a and the surrounding protein are missing. The molecular mechanisms responsible for the unusual absorption of these complexes are thus similar to those responsible for tuning of the absorption of the LH2 complexes between 850 and 820 nm. Furthermore, our results suggest that the binding pocket of the monomeric BChl in the LH2 from E. ramosum is different from that of Rps. acidphila or Rb. sphaeroides. The FT-R spectra of Chromatium purpuratum indicate that, in contrast with every LH2 complex previously studied by FT-R spectroscopy, no free-from-interaction keto groupings exist in this complex.     

Fast energy transfer between BChl d and BChl c in chlorosomes of the green sulfur bacterium Chlorobium limicola

We have studied energy transfer in chlorosomes of Chlorobium limicola UdG6040 containing a mixture of about 50% bacteriochlorophyll (BChl) c and BChl d each. BChl d-depleted chlorosomes were obtained by acid treatment. The energy transfer between the different pigment pools was studied using both steady-state and time-resolved fluorescence spectroscopy at room temperature and low temperature. The steady-state emission of the intact chlorosome originated mainly from BChl c, as judged by comparison of fluorescence emission spectra of intact and BChl d-depleted chlorosomes. This indicated that efficient energy transfer from BChl d to BChl c takes place. At room temperature BChl c/d to BChl a excitation energy transfer (EET) was characterized by two components of 27 and 74 ps. At low temperature we could also observe EET from BChl d to BChl c with a time constant of approximately 4 ps. Kinetic modeling of the low temperature data indicated heterogeneous fluorescence kinetics and suggested the presence of an additional BChl c pool, E790, which is more or less decoupled from the baseplate BChl a. This E790 pool is either a low-lying exciton state of BChl c which acts as a trap at low temperature or alternatively represents the red edge of a broad inhomogeneous absorption band of BChl c. We present a refined model for the organization of the spatially separated pigment pools in chlorosomes of Cb. limicola UdG6040 in which BChl d is situated distal and BChl c proximal with respect to the baseplate.     

Probing the structure of the core light-harvesting complex (LH1) of Rhodopseudomonas viridis by dissociation and reconstitution methodology

A subunit complex was formed from the core light-harvesting complex (LH1) of bacteriochlorophyll(BChl)-b-containing Rhodopseudomonas viridis. The addition of octyl glucoside to a carotenoid-depleted Rps. viridis membrane preparation resulted in a subunit complex absorbing at 895 nm, which could be quantitatively dissociated to free BChl b and then reassociated to the subunit. When carotenoid was added back, the subunit could be reassociated to LH1 with a 25% yield. Additionally, the Rps. viridis - and -polypeptides were isolated, purified, and then reconstituted with BChl b. They formed a subunit absorbing near 895 nm, similar to the subunit formed by titration of the carotenoid depleted membrane, but did not form an LH1-type complex at 1015 nm. The same results were obtained with the -polypeptide alone and BChl b. Isolated polypeptides were also tested for their interaction with BChl a. They formed subunit and LH1-type complexes similar to those formed using polypeptides isolated from BChl-a-containing bacteria but displayed 6–10 nm smaller red shifts in their long-wavelength absorption maxima. Thus, the larger red shift of BChl-b-containing Rps. viridis is not attributable solely to the protein structure. The -polypeptide of Rps. viridis differed from the other -polypeptides tested in that it could form an LH1-type complex with BChl a in the absence of the - and -polypeptides. It apparently contains the necessary information required to assemble into an LH1-type complex. When the -polypeptide was tested in reconstitution with BChl a and BChl b with the - and -polypeptides, it had no effect; its role remains undetermined.Abbreviations B820 the subunit form of the core light-harvesting complex in BChl-a-containing bacteria which has an absorption maximum at or near 820 nm - B875 the core light-harvesting complex of Rhodobacter sphaeroides which has an absorption maximum at 875 nm - B881 the core light-harvesting complex of wild-type Rhodospirillum rubrum which has an absorption maximum at 881 nm - B895 the subunit form of the core light-harvesting complex in Rps. viridis which has an absorption maximum near 888–895 nm - B1015 the core light-harvesting complex of Rps. viridis which has an absorption maximum at 1015 nm - CD circular dichroism - LH1 the core light-harvesting complex - OG n-octyl -d-glucopyranoside     

Characterization of the different peripheral light-harvesting complexes from high- and low-light grown cells from Rhodopseudomonas palustris

In this paper we demonstrate that the spectroscopically different peripheral light-harvesting complexes from Rhodopseudomonas palustris, strain 2.6.1, isolated from high- and low-light grown cells have widely differing bacteriochlorophyll a (BChl a) resonance Raman spectra in the high-frequency carbonyl region (1550-1750 cm-1). Complexes synthesized in low-light grown cells exhibit Raman spectra characteristic of B800-850 and B800-820 complexes, depending on the excitation conditions. The in vivo strategy for low-light adaptation in this bacterium is thus somewhat different from that generally encountered in the Rhodospirillaceae. In these bacteria, as typified by Rps. acidophila and Rps. cryptolactis, low-light conditions induce the synthesis of B800-820 only complexes in which the hydrogen bonds between the acetyl carbonyl and the B850 binding pocket are broken, inducing changes in the absorption properties of the monomeric bacteriochlorophylls. In the case of Rps. palustris, additional spectral effects occur due to the coupling of the electronic levels of the differently interacting dimers. The extensive use of differential alpha/beta-polypeptide expression [Tadros et al. (1993) Eur. J. Biochem. 217, 867-875] thus allows Rps. palustris to alter its BChl a binding site environments causing the observed spread of BChl a Qy transitions, ranging from 801 to 856 nm.     

Spectral diffusion of the lowest exciton component in the core complex from Rhodopseudomonas palustris studied by single-molecule spectroscopy

We have recorded fluorescence-excitation spectra from individual RC–LH1 complexes from Rhodopseudomonas palustris. The spectra feature a few broad bands accompanied by a sharp line at the low-energy side of the spectrum which is ascribed to the lowest exciton state of the BChl a assembly. Recording several fluorescence-excitation spectra from the same individual complex in rapid succession reveals that the linewidth of the lowest exciton transition is determined by spectral diffusion which increases for higher excitation energies.     

Spectroscopy on the B850 band of individual light-harvesting 2 complexes of Rhodopseudomonas acidophila. I. Experiments and Monte Carlo simulations

The electronic structure of the circular aggregate of 18 bacteriochlorophyll a (BChl a) molecules responsible for the B850 absorption band of the light-harvesting 2 (LH2) complex of the photosynthetic purple bacterium Rhodopseudomonas acidophila has been studied by measuring fluorescence-excitation spectra of individual complexes at 1.2 K. The spectra reveal several well-resolved bands that are obscured in the single, broad B850 band observed in conventional absorption measurements on bulk samples. They are interpreted consistently in terms of the exciton model for the circular aggregate of BChl a molecules. From the energy separation between the different exciton transitions a reliable value of the intermolecular interaction is obtained. The spectra of the individual complexes allow for a distinction between the intra- and the intercomplex disorder. In addition to the random disorder, a regular modulation of the interaction has to be assumed to account for all the features of the observed spectra. This modulation has a C(2) symmetry, which strongly suggests a structural deformation of the ring into an ellipse.