Comparison of the LH2 Antenna Complexes of Rhodopseudomonas acidophila (Strain 10050) and Rhodobacter sphaeroides by High-Pressure Absorption, High-Pressure Hole Burning, and Temperature-Dependent Absorption Spectroscopies

Effects of Detergents on the Spectral Features of B820 Bacteriochlorophyll a in Light-Harvesting Complex 3

Excitonic coupling of bacteriochlorophyll (BChl) a in light-harvesting (LH) proteins of purple photosynthetic bacteria is key for efficient photon capture and energy transfer. Environmental factors can affect the spectral features of these BChl a pigments and investigating these effects can provide insight into the molecular mechanisms underlying the photosynthetic spectral tuning. The present study analyzes the spectral alterations of the Q_y band of B820 BChl a within the LH3 protein in relation to the type and concentration of detergents in the buffer. Changing the detergent from lauryl dimethylamine N-oxide (LDAO) to n-dodecyl-β-d-maltoside (DDM) caused a red shift in the B820 Q_y band accompanied by hyperchromism; these spectral alterations were completely reversed by exchanging back from DDM to LDAO. These results reflect the different effects of harsh vs mild detergents on the perturbation of LH3. The B820 Q_y band did not change when LDAO or NaCl concentration was altered, suggesting that electrostatic effects by external components have little influence on the spectral features of B820 BChl a in LH3.

Comparison of tryptophan fluorescence lifetimes in cyanobacterial photosystem I frozen in the light and in the dark

The dependence on temperature of tryptophan fluorescence lifetime in trimeric photosystem I (PSI) complexes from cyanobacteria Synechocystis sp. PCC 6803 during the heating of pre-frozen to - 180 °C in the dark or in the light-activated preparations has been studied. Fluorescence lifetime in samples frozen in the light was longer than in samples frozen in the dark. For samples in 65% glycerol at λ_reg = 335 nm and at 20 °C, the lifetime of components were as follows: τ₁ ≈ 1.2 ns, τ₂ ≈ 4.9 ns, and τ₃ ≈ 20 ns. The contribution of the first component was negligible. To analyze the contribution of components 2 and 3 derived from frozen-thawed samples, two temperature ranges from - 180 to - 90 °C and above - 90 °C are considered. In doing so, the contributions of these components appear antiphase course to each other. The dependence on temperature of these contributions is explained by the influence of the microconformational protein dynamics on the tryptophan fluorescence lifetime. In the present work, a comparative analysis of temperature-dependent conformational dynamics and electron transfer in cyanobacterial PSI (Schlodder et al., in Biochemistry 37:9466-9476, 1998) and Rhodobacter sphaeroides reaction center complexes (Knox et al., in J Photochem Photobiol B 180:140-148, 2018) was also carried out.

Selective Removal of B800 Bacteriochlorophyll a from Light-Harvesting Complex 2 of the Purple Photosynthetic Bacterium Phaeospirillum molischianum

The selective removal of B800 bacteriochlorophyll (BChl) a from light-harvesting complex 2 (LH2) in purple photosynthetic bacteria is a clue about elucidation of the mechanism for the transfer of energy from these pigments to B850 BChl a and their roles in the LH2 protein structure. We demonstrated that the kinetics of the removal of B800 BChl a from two representative LH2 proteins derived from Phaeospirillum molischianum and Rhodoblastus acidophilus differed significantly, in contrast to the calculated binding enthalpy. These results may be interpreted as changes in the local structure near B800 BChl a with respect to the geometries of the original crystal structures upon removal of B800 BChl a. Despite the difficulty of removing B800 BChl a from molischianum-LH2, we prepared the molischianum-LH2 protein lacking B800 BChl a by combination of two detergents, n-dodecyl β-d-maltoside and n-octyl β-d-glucoside, under acidic conditions. Spectral and atomic force microscopy analyses indicated that the absence of B800 BChl a had little effect on the local structure in the vicinity of B850 BChl a and the circular arrangement in this protein. These results suggest that the hydrophobic domain near B850 BChl a is rigid and plays a major role in the structural formation of molischianum-LH2.

The effect of light and temperature on the dynamic state of Rhodobacter sphaeroides reaction centers proteins determined from changes in tryptophan fluorescence lifetime and P+QA- recombination kinetics

The temperature dependencies of the rate of dark recombination of separated charges between the photoactive bacteriochlorophyll and the primary quinone acceptor (Q_A) in photosynthetic reaction centers (RCs) of the purple bacteria Rhodobacter sphaeroides (Rb. sphaeroides) were investigated. Measurements were performed in water-glycerol and trehalose environments after freezing to -180 °C in the dark and under actinic light with subsequent heating. Simultaneously, the RC tryptophanyl fluorescence lifetime in the spectral range between 323 and 348 nm was measured under these conditions. A correlation was found between the temperature dependencies of the functional and dynamic parameters of RCs in different solvent mixtures. For the first time, differences in the average fluorescence lifetime of tryptophanyl residues were measured between RCs frozen in the dark and in the actinic light. The obtained results can be explained by the RC transitions between different conformational states and the dynamic processes in the structure of the hydrogen bonds of RCs. We assumed that RCs exist in two main microconformations - "fast" and "slow", which are characterized by different rates of P⁺ and Q_A^- recombination reactions. The "fast" conformation is induced in frozen RCs in the dark, while the "slow" conformation of RC occurs when the RC preparation is frozen under actinic light. An explanation of the temperature dependencies of tryptophan fluorescence lifetimes in RC proteins was made under the assumption that temperature changes affect mainly the electron transfer from the indole ring of the tryptophan molecule to the nearest amide or carboxyl groups.

Excitation energy transfer in phycobiliproteins of the cyanobacterium Acaryochloris marina investigated by spectral hole burning

The cyanobacterium Acaryochloris marina developed two types of antenna complexes, which contain chlorophyll-d (Chl d) and phycocyanobilin (PCB) as light-harvesting pigment molecules, respectively. The latter membrane-extrinsic complexes are denoted as phycobiliproteins (PBPs). Spectral hole burning was employed to study excitation energy transfer and electron-phonon coupling in PBPs. The data reveal a rich spectral substructure with a total of four low-energy electronic states whose absorption bands peak at 633, 644, 654, and at about 673 nm. The electronic states at ~633 and 644 nm can be tentatively attributed to phycocyanin (PC) and allophycocyanin (APC), respectively. The remaining low-energy electronic states including the terminal emitter at 673 nm may be associated with different isoforms of PC, APC, or the linker protein. Furthermore, the hole burning data reveal a large number of excited state vibrational frequencies, which are characteristic for the chromophore PCB. In summary, the results are in good agreement with the low-energy level structure of PBPs and electron-phonon coupling parameters reported by Gryliuk et al. (BBA 1837:1490-1499, 2014) based on difference fluorescence line-narrowing experiments.

Polaronic effects at finite temperatures in the B850 ring of the LH2 complex

Energy transfer and relaxation dynamics in the B850 ring of LH2 molecular aggregates are described, taking into account the polaronic effects, by a stochastic time-dependent variational approach.

Subtle spectral effects accompanying the assembly of bacteriochlorophylls into cyclic light harvesting complexes revealed by high-resolution fluorescence spectroscopy

We have observed that an assembly of the bacteriochloropyll a molecules into B850 and B875 groups of cyclic bacterial light-harvesting complexes LH2 and LH1, respectively, results an almost total loss of the intra-molecular vibronic structure in the fluorescence spectrum, and simultaneously, an essential enhancement of its phonon sideband due to electron-phonon coupling. While the suppression of the vibronic coupling in delocalized (excitonic) molecular systems is predictable, as also confirmed by our model calculations, a boost of the electron-phonon coupling is rather unexpected. The latter phenomenon is explained by exciton self-trapping, promoted by mixing the molecular exciton states with charge transfer states between the adjacent chromophores in the tightly packed B850 and B875 arrangements. Similar, although less dramatic trends were noted for the light-harvesting complexes containing chlorophyll pigments.

Protein dynamics tunes excited state positions in light-harvesting complex II

Light harvesting and excitation energy transfer in photosynthesis are relatively well understood at cryogenic temperatures up to ∼100 K, where crystal structures of several photosynthetic complexes including the major antenna complex of green plants (LHC II) are available at nearly atomic resolution. The situation is much more complex at higher or even physiological temperatures, because the spectroscopic properties of antenna complexes typically undergo drastic changes above ∼100 K. We have addressed this problem using a combination of quasielastic neutron scattering (QENS) and optical spectroscopy on native LHC II and mutant samples lacking the Chl 2/Chl a 612 pigment molecule. Absorption difference spectra of the Chl 2/Chl a 612 mutant of LHC II reveal pronounced changes of spectral position and their widths above temperatures as low as ∼80 K. The complementary QENS data indicate an onset of conformational protein motions at about the same temperature. This finding suggests that excited state positions in LHC II are affected by protein dynamics on the picosecond time scale. In more detail, this means that at cryogenic temperatures the antenna complex is trapped in certain protein conformations. At higher temperature, however, a variety of conformational substates with different spectral position may be thermally accessible. At the same time, an analysis of the widths of the absorption difference spectra of Chl 2/Chl a 612 reveals three different reorganization energies or Huang-Rhys factors in different temperature ranges, respectively. These findings imply that (dynamic) pigment-protein interactions fine-tune electronic energy levels and electron-phonon coupling of LHC II for efficient excitation energy transfer at physiological temperatures.

Excimer formation in a confined space: photophysics of ladderphanes with tetraarylethylene linkers

Communication between chromophores is vital for both natural and non-natural photophysical processes. Spatial confinements offer unique conditions to scrutinize such interactions. Polynorbornene- and polycyclobutene-based ladderphanes are ideal model compounds in which all tetraarylethylene (TAE) linkers are aligned coherently. The spans for each of the monomeric units in these ladderphanes are 4.5-5.5 Å. Monomers do not exhibit emission, because bond rotation in TAE can quench the excited-state energy. However, polymers emit at 493 nm (Φ=0.015) with large Stokes shift under ambient conditions and exhibit dual emission at 450 and 493 nm at 150 K. When the temperature is lowered, the emission intensity at 450 nm increases, whereas that at 493 nm decreases. At 100 K, both monomers and polymers emit only at 450 nm. This shorter-wavelength emission arises from the intrinsic emission of TAE chromophore, and the emission at 493 nm could be attributed to the excimer emission in the confined space of ladderphanes. The fast kinetics suggest diffusion-controlled formation of the excimer.

Single-molecule spectroscopy unmasks the lowest exciton state of the B850 assembly in LH2 from Rps. acidophila

We have recorded fluorescence-excitation and emission spectra from single LH2 complexes from Rhodopseudomonas (Rps.) acidophila. Both types of spectra show strong temporal spectral fluctuations that can be visualized as spectral diffusion plots. Comparison of the excitation and emission spectra reveals that for most of the complexes the lowest exciton transition is not observable in the excitation spectra due to the cutoff of the detection filter characteristics. However, from the spectral diffusion plots we have the full spectral and temporal information at hand and can select those complexes for which the excitation spectra are complete. Correlating the red most spectral feature of the excitation spectrum with the blue most spectral feature of the emission spectrum allows an unambiguous assignment of the lowest exciton state. Hence, application of fluorescence-excitation and emission spectroscopy on the same individual LH2 complex allows us to decipher spectral subtleties that are usually hidden in traditional ensemble spectroscopy.

Excitation energy transfer and electron-vibrational coupling in phycobiliproteins of the cyanobacterium Acaryochloris marina investigated by site-selective spectroscopy

In adaption to its specific environmental conditions, the cyanobacterium Acaryochloris marina developed two different types of light-harvesting complexes: chlorophyll-d-containing membrane-intrinsic complexes and phycocyanobilin (PCB) - containing phycobiliprotein (PBP) complexes. The latter complexes are believed to form a rod-shaped structure comprising three homo-hexamers of phycocyanin (PC), one hetero-hexamer of phycocyanin and allophycocyanin (APC) and probably a linker protein connecting the PBPs to the reaction centre. Excitation energy transfer and electron-vibrational coupling in PBPs have been investigated by selectively excited fluorescence spectra. The data reveal a rich spectral substructure with a total of five low-energy electronic states with fluorescence bands at 635nm, 645nm, 654nm, 659nm and a terminal emitter at about 673 nm. The electronic states at ~635 and 645 nm are tentatively attributed to PC and APC, respectively, while an apparent heterogeneity among PC subunits may also play a role. The other fluorescence bands may be associated with three different isoforms of the linker protein. Furthermore, a large number of vibrational features can be identified for each electronic state with intense phonon sidebands peaking at about 31 to 37cm⁻¹, which are among the highest phonon frequencies observed for photosynthetic antenna complexes. The corresponding Huang-Rhys factors S fall in the range between 0.98 (terminal emitter), 1.15 (APC), and 1.42 (PC). Two characteristic vibronic lines at about 1580 and 1634cm⁻¹ appear to reflect CNH⁺ and CC stretching modes of the PCB chromophore, respectively. The exact phonon and vibrational frequencies vary with electronic state implying that the respective PCB chromophores are bound to different protein environments. This article is part of a special issue entitled: photosynthesis research for sustainability: keys to produce clean energy.

Structural implications of hydrogen-bond energetics in membrane proteins revealed by high-pressure spectroscopy

The light-harvesting 1 (LH1) integral membrane complex of Rhodobacter sphaeroides provides a convenient model system in which to examine the poorly understood role of hydrogen bonds (H-bonds) as stabilizing factors in membrane protein complexes. We used noncovalently bound arrays of bacteriochlorophyll chromophores within native and genetically modified variants of LH1 complexes to monitor local changes in the chromophore binding sites induced by externally applied hydrostatic pressure. Whereas membrane-bound complexes demonstrated very high resilience to pressures reaching 2.1 GPa, characteristic discontinuous shifts and broadenings of the absorption spectra were observed around 1 GPa for detergent-solubilized proteins, in similarity to those observed when specific (α or β) H-bonds between the chromophores and the surrounding protein were selectively removed by mutagenesis. These pressure effects, which were reversible upon decompression, allowed us to estimate the rupture energies of H-bonds to the chromophores in LH1 complexes. A quasi-independent, additive role of H-bonds in the α- and β-sublattices in reinforcing the wild-type LH1 complex was established. A comparison of a reaction-center-deficient LH1 complex with complexes containing reaction centers also demonstrated a stabilizing effect of the reaction center. This study thus provides important insights into the design principles of natural photosynthetic complexes.

A comparative spectroscopic and kinetic study of photoexcitations in detergent-isolated and membrane-embedded LH2 light-harvesting complexes

Integral membrane proteins constitute more than third of the total number of proteins present in organisms. Solubilization with mild detergents is a common technique to study the structure, dynamics, and catalytic activity of these proteins in purified form. However beneficial the use of detergents may be for protein extraction, the membrane proteins are often denatured by detergent solubilization as a result of native lipid membrane interactions having been modified. Versatile investigations of the properties of membrane-embedded and detergent-isolated proteins are, therefore, required to evaluate the consequences of the solubilization procedure. Herein, the spectroscopic and kinetic fingerprints have been established that distinguish excitons in individual detergent-solubilized LH2 light-harvesting pigment-protein complexes from them in the membrane-embedded complexes of purple photosynthetic bacteria Rhodobacter sphaeroides. A wide arsenal of spectroscopic techniques in visible optical range that include conventional broadband absorption-fluorescence, fluorescence anisotropy excitation, spectrally selective hole burning and fluorescence line-narrowing, and transient absorption-fluorescence have been applied over broad temperature range between physiological and liquid He temperatures. Significant changes in energetics and dynamics of the antenna excitons upon self-assembly of the proteins into intracytoplasmic membranes are observed, analyzed, and discussed. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

Davydov splitting of excitons in cyclic bacteriochlorophyll a nanoaggregates of bacterial light-harvesting complexes between 4.5 and 263 K

The nature of electronic excitations created by photon absorption in the cyclic B850 aggregates of 18 bacteriochlorophyll molecules of LH2 antenna complexes of photosynthetic bacteria is studied over a broad temperature range using absorption, fluorescence, and fluorescence anisotropy spectra. The latter technique has been proved to be suitable for revealing the hidden structure of excitons in inhomogeneously broadened spectra of cyclic aggregates. A theoretical model that accounts for differences of absorbing excitons in undeformed and emitting exciton polarons in deformed antenna lattices is also developed. Only a slight decrease of the exciton bandwidth and exciton coupling energy with temperature is observed. Survival of excitons in the whole temperature span from cryogenic to nearly ambient temperatures strongly suggests that collective, coherent electronic excitations might play a role in the functional light-harvesting process taking place at physiological temperatures.

Low-temperature protein dynamics of the B800 molecules in the LH2 light-harvesting complex: spectral hole burning study and comparison with single photosynthetic complex spectroscopy

Previously published and new spectral hole burning (SHB) data on the B800 band of LH2 light-harvesting antenna complex of Rps. acidophila are analyzed in light of recent single photosynthetic complex spectroscopy (SPCS) results (for a review, see Berlin et al. Phys. Life Rev. 2007, 4, 64.). It is demonstrated that, in general, SHB-related phenomena observed for the B800 band are in qualitative agreement with the SPCS data and the protein models involving multiwell multitier protein energy landscapes. Regarding the quantitative agreement, we argue that the single-molecule behavior associated with the fastest spectral diffusion (smallest barrier) tier of the protein energy landscape is inconsistent with the SHB data. The latter discrepancy can be attributed to SPCS probing not only the dynamics of of the protein complex per se, but also that of the surrounding amorphous host and/or of the host-protein interface. It is argued that SHB (once improved models are developed) should also be able to provide the average magnitudes and probability distributions of light-induced spectral shifts and could be used to determine whether SPCS probes a set of protein complexes that are both intact and statistically relevant. SHB results are consistent with the B800 --> B850 energy-transfer models including consideration of the whole B850 density of states.

Spectral diffusion and electron-phonon coupling of the B800 BChl a molecules in LH2 complexes from three different species of purple bacteria

We have investigated the spectral diffusion and the electron-phonon coupling of B800 bacteriochlorophyll a molecules in the peripheral light-harvesting complex LH2 for three different species of purple bacteria, Rhodobacter sphaeroides, Rhodospirillum molischianum, and Rhodopseudomonas acidophila. We come to the conclusion that B800 binding pockets for Rhodobacter sphaeroides and Rhodopseudomonas acidophila are rather similar with respect to the polarity of the protein environment but that the packaging of the alphabeta-polypeptides seems to be less tight in Rb. sphaeroides with respect to the other two species.

Spectral hole burning: examples from photosynthesis

The optical spectra of photosynthetic pigment-protein complexes usually show broad absorption bands, often consisting of a number of overlapping, "hidden" bands belonging to different species. Spectral hole burning is an ideal technique to unravel the optical and dynamic properties of such hidden species. Here, the principles of spectral hole burning (HB) and the experimental set-up used in its continuous wave (CW) and time-resolved versions are described. Examples from photosynthesis studied with hole burning, obtained in our laboratory, are then presented. These examples have been classified into three groups according to the parameters that were measured: (1) hole widths as a function of temperature, (2) hole widths as a function of delay time and (3) hole depths as a function of wavelength. Two examples from light-harvesting (LH) 2 complexes of purple bacteria are given within the first group: (a) the determination of energy-transfer times from the chromophores in the B800 ring to the B850 ring, and (b) optical dephasing in the B850 absorption band. One example from photosystem II (PSII) sub-core complexes of higher plants is given within the second group: it shows that the size of the complex determines the amount of spectral diffusion measured. Within the third group, two examples from (green) plants and purple bacteria have been chosen for: (a) the identification of "traps" for energy transfer in PSII sub-core complexes of green plants, and (b) the uncovering of the lowest k = 0 exciton-state distribution within the B850 band of LH2 complexes of purple bacteria. The results prove the potential of spectral hole burning measurements for getting quantitative insight into dynamic processes in photosynthetic systems at low temperature, in particular, when individual bands are hidden within broad absorption bands. Because of its high-resolution wavelength selectivity, HB is a technique that is complementary to ultrafast pump-probe methods. In this review, we have provided an extensive bibliography for the benefit of scientists who plan to make use of this valuable technique in their future research.

Excitation energy transfer between closely spaced multichromophoric systems: effects of band mixing and intraband relaxation

We theoretically analyze the excitation energy transfer between two closely spaced linear molecular J-aggregates, whose excited states are Frenkel excitons. The aggregate with the higher (lower) exciton band edge energy is considered as the donor (acceptor). The celebrated theory of Förster resonance energy transfer (FRET), which relates the transfer rate to the overlap integral of optical spectra, fails in this situation. We point out that, in addition to the well-known fact that the point-dipole approximation breaks down (enabling energy transfer between optically forbidden states), also the perturbative treatment of the electronic interactions between donor and acceptor system, which underlies the Förster approach, in general loses its validity due to overlap of the exciton bands. We therefore propose a nonperturbative method, in which donor and acceptor bands are mixed and the energy transfer is described in terms of a phonon-assisted energy relaxation process between the two new (renormalized) bands. The validity of the conventional perturbative approach is investigated by comparing to the nonperturbative one; in general, this validity improves for lower temperature and larger distances (weaker interactions) between the aggregates. We also demonstrate that the interference between intraband relaxation and energy transfer renders the proper definition of the transfer rate and its evaluation from experiment a complicated issue that involves the initial excitation condition. Our results suggest that the best way of determining this transfer rate between two J-aggregates is to measure the fluorescence kinetics of the acceptor J-band after resonant excitation of the donor J-band.

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.

Multichromophoric Förster Resonance Energy Transfer from B800 to B850 in the Light Harvesting Complex 2: Evidence for Subtle Energetic Optimization by Purple Bacteria

This work provides a detailed account of the application of our multichromophoric Förster resonance energy transfer (MC-FRET) theory (Phys. Rev. Lett. 2004, 92, 218301) for the calculation of the energy transfer rate from the B800 unit to the B850 unit in the light harvesting complex 2 (LH2) of purple bacteria. The model Hamiltonian consists of the B800 unit represented by a single bacteriochlorophyll (BChl), the B850 unit represented by its entire set of BChls, the electronic coupling between the two units, and the bath terms representing all environmental degrees of freedom. The model parameters are determined, independent of the rate calculation, from the literature data and by a fitting to an ensemble line shape. Comparing our theoretical rate and a low-temperature experimental rate, we estimate the magnitude of the BChl-Qy transition dipole to be in the range of 6.5-7.5 D, assuming that the optical dielectric constant of the medium is in the range of 1.5-2. We examine how the bias of the average excitation energy of the B800-BChl relative to that of the B850-BChl affects the energy transfer time by calculating the transfer rates based on both our MC-FRET theory and the original FRET theory, varying the value of the bias. Within our model, we find that the value of bias 260 cm-1, which we determine from the fitting to an ensemble line shape, is very close to the value at which the ratio between MC-FRET and FRET rates is a maximum. This provides evidence that the bacterial system utilizes the quantum mechanical coherence among the multiple chromophores within the B850 in a constructive way so as to achieve efficient energy transfer from B800 to B850.

Optical spectroscopic studies of light-harvesting by pigment-reconstituted peridinin-chlorophyll-proteins at cryogenic temperatures

Low temperature, steady-state, optical spectroscopic methods were used to study the spectral features of peridinin-chlorophyll-protein (PCP) complexes in which recombinant apoprotein has been refolded in the presence of peridinin and either chlorophyll a (Chl a), chlorophyll b (Chl b), chlorophyll d (Chl d), 3-acetyl-chlorophyll a (3-acetyl-Chl a) or bacteriochlorophyll a (BChl a). Absorption spectra taken at 10 K provide better resolution of the spectroscopic bands than seen at room temperature and reveal specific pigment-protein interactions responsible for the positions of the Qy bands of the chlorophylls. The study reveals that the functional groups attached to Ring I of the two protein-bound chlorophylls modulate the Qy and Soret transition energies. Fluorescence excitation spectra were used to compute energy transfer efficiencies of the various complexes at room temperature and these were correlated with previously reported ultrafast, time-resolved optical spectroscopic dynamics data. The results illustrate the robust nature and value of the PCP complex, which maintains a high efficiency of antenna function even in the presence of non-native chlorophyll species, as an effective tool for elucidating the molecular details of photosynthetic light-harvesting.

Theoretical prediction of spectral and optical properties of bacteriochlorophylls in thermally disordered LH2 antenna complexes

A general approach for calculating spectral and optical properties of pigment-protein complexes of known atomic structure is presented. The method, that combines molecular dynamics simulations, quantum chemistry calculations, and statistical mechanical modeling, is demonstrated by calculating the absorption and circular dichroism spectra of the B800-B850 bacteriochlorophylls of the LH2 antenna complex from Rs. molischianum at room temperature. The calculated spectra are found to be in good agreement with the available experimental results. The calculations reveal that the broadening of the B800 band is mainly caused by the interactions with the polar protein environment, while the broadening of the B850 band is due to the excitonic interactions. Since it contains no fitting parameters, in principle, the proposed method can be used to predict optical spectra of arbitrary pigment-protein complexes of known structure.

Energy transfer in photosynthesis: experimental insights and quantitative models

We overview experimental and theoretical studies of energy transfer in the photosynthetic light-harvesting complexes LH1, LH2, and LHCII performed during the past decade since the discovery of high-resolution structure of these complexes. Experimental findings obtained with various spectroscopic techniques makes possible a modelling of the excitation dynamics at a quantitative level. The modified Redfield theory allows a precise assignment of the energy transfer pathways together with a direct visualization of the whole excitation dynamics where various regimes from a coherent motion of delocalized exciton to a hopping of localized excitations are superimposed. In a single complex it is possible to observe the switching between these regimes driven by slow conformational motion (as we demonstrate for LH2). Excitation dynamics under quenched conditions in higher-plant complexes is discussed.

Temperature broadening of LH2 absorption in glycerol solution

In order to determine the relationship between the pigment-protein and the pigment-pigment interactions, the measurements of absorption spectra of the peripheral light-harvesting complex LH2 from the purple bacteria Rhodobacter sphaeroides solvated in glycerol/buffer solution were carried out in a wide temperature range, from 4 to 250 K. The SDFs used for simulating the temperature dependence of B800 and B850 bands were determined in a parametric form. To fit experimental spectra the overall exciton-phonon coupling had to be assumed to be weak for B850 (lambda/2V approximately 0.3, where lambda is the reorganization energy and V is the nearest-neighbor dipole-dipole coupling for bacteriochlorophylls). At physiological temperatures the intermediate nuclear bath dynamics compares with the magnitude of energy gap fluctuations. Slower dynamics with kappa approximately 0.39, where kappa is the ratio of the nuclear relaxation rate and the line width parameter, determines the spectral shape of B850 whilst faster modulations characterize B800 (kappa approximately 2.39). The static disorder for the B800 band is relatively high with the characteristic value of the inhomogeneous bandwidth Gamma(inh) approximately 120 cm-1, while for the B850 band this value is almost equal to the dipole-dipole coupling strength (Gamma(inh) approximately 360 cm-1). It has been found that the LH2 absorption spectrum is likely to be influenced by the temperature dependence of the dielectric constant of the solution in the high temperature range, when the glycerol/buffer solution is in the liquid state.

The structure and thermal motion of the B800-850 LH2 complex from Rps.acidophila at 2.0A resolution and 100K: new structural features and functionally relevant motions

The structure at 100K of integral membrane light-harvesting complex II (LH2) from Rhodopseudomonas acidophila strain 10050 has been refined to 2.0A resolution. The electron density has been significantly improved, compared to the 2.5A resolution map, by high resolution data, cryo-cooling and translation, libration, screw (TLS) refinement. The electron density reveals a second carotenoid molecule, the last five C-terminal residues of the alpha-chain and a carboxy modified alpha-Met1 which forms the ligand of the B800 bacteriochlorophyll. TLS refinement has enabled the characterisation of displacements between molecules in the complex. B850 bacteriochlorophyll molecules are arranged in a ring of 18 pigments composed of nine approximate dimers. These pigments are strongly coupled and at their equilibrium positions the excited state dipole interaction energies, within and between dimers, are approximately 370cm(-1) and 280cm(-1), respectively. This difference in coupling energy is similar in magnitude to changes in interaction energies arising from the pigment displacements described by TLS tensors. The displacements appear to be non-random in nature and appear to be designed to optimise the modulation of pigment energy interactions. This is the first time that LH2 pigment displacements have been quantified experimentally. The calculated energy changes indicate that there may be significant contributions to inter-pigment energy interactions from molecular displacements and these may be of importance to photosynthetic energy transfer.

Spectroscopy on individual light-harvesting 1 complexes of Rhodopseudomonas acidophila

In this paper the fluorescence-excitation spectra of individual LH1-RC complexes (Rhodopseudomonas acidophila) at 1.2 K are presented. All spectra show a limited number of broad bands with a characteristic polarization behavior, indicating that the excitations are delocalized over a large number of pigments. A significant variation in the number of bands, their bandwidths, and polarization behavior is observed. Only 30% of the spectra carry a clear signature of delocalized excited states of a circular structure of the pigments. The large spectral variety suggests that besides site heterogeneity also structural heterogeneity determines the optical spectrum of the individual LH1-RC complexes. Further research should reveal if such heterogeneity is a native property of the complex or induced during the experimental procedures.

Bacterial photosynthesis begins with quantum-mechanical coherence

In the antenna system of photosynthetic bacteria, pigments form circular aggregates whose excitations are excitons with quantum-mechanical coherence extending over many pigments. These excitons play crucial roles in light harvesting, storage, and excitation-energy transfer (EET). EET takes place rapidly to and/or from optically forbidden exciton states, without total transition dipole, within the antenna system and to the reaction center. Such EETs cannot be rationalized by Förster's formula, the traditional theory on EET, because it allows EET only between optically allowed states. The coherence in the excitons seems to prohibit rapid EET on this formula. The bacteria overcome this difficulty by circumventing the coherence, using the effects of the physical size of an aggregate that is larger than the shortest distance between pigments in the donor and pigments in the acceptor. The shortest-distance pair therein cannot detect whether the aggregate has a nonvanishing total transition dipole or not, since the pair see effectively only the transition dipole on the other pigment in themselves. The transition dipole facilitates rapid EET even to and/or from optically forbidden exciton states. Such EETs have enabled us to develop a general formula for the rate constant of EET. This is a formula in the weak-interaction limit, and so is Förster's formula, but it correctly takes into account the above size effect.

Absorption and CD spectroscopy and modeling of various LH2 complexes from purple bacteria

The absorption (OD) and circular dichroism (CD) spectra of LH2 complexes from various purple bacteria have been measured and modeled. Based on the lineshapes of the spectra we can sort the LH2 complexes into two distinguishable groups: "acidophila"-like (type 1) and "molischianum"-like (type 2). Starting from the known geometric structures of Rhodopseudomonas (Rps.) acidophila and Rhodospirillum (Rsp.) molischianum we can model the OD and CD spectra of all species by just slightly varying some key parameters: the interaction strength, the energy difference of alpha- and beta-bound B850 bacteriochlorophylls (BChls), the orientation of the B800 and B850 BChls, and the (in)homogeneous broadening. Although the ring size can vary, the data are consistent with all the LH2 complexes having basically very similar structures.