Fluorescence spectroscopy of conformational changes of single LH2 complexes

High-Resolution Frequency-Domain Spectroscopic and Modeling Studies of Photosystem I (PSI), PSI Mutants and PSI Supercomplexes

Photosystem I (PSI) is one of the two main pigment-protein complexes where the primary steps of oxygenic photosynthesis take place. This review describes low-temperature frequency-domain experiments (absorption, emission, circular dichroism, resonant and non-resonant hole-burned spectra) and modeling efforts reported for PSI in recent years. In particular, we focus on the spectral hole-burning studies, which are not as common in photosynthesis research as the time-domain spectroscopies. Experimental and modeling data obtained for trimeric cyanobacterial Photosystem I (PSI3), PSI3 mutants, and PSI3-IsiA18 supercomplexes are analyzed to provide a more comprehensive understanding of their excitonic structure and excitation energy transfer (EET) processes. Detailed information on the excitonic structure of photosynthetic complexes is essential to determine the structure-function relationship. We will focus on the so-called "red antenna states" of cyanobacterial PSI, as these states play an important role in photochemical processes and EET pathways. The high-resolution data and modeling studies presented here provide additional information on the energetics of the lowest energy states and their chlorophyll (Chl) compositions, as well as the EET pathways and how they are altered by mutations. We present evidence that the low-energy traps observed in PSI are excitonically coupled states with significant charge-transfer (CT) character. The analysis presented for various optical spectra of PSI3 and PSI3-IsiA18 supercomplexes allowed us to make inferences about EET from the IsiA18 ring to the PSI3 core and demonstrate that the number of entry points varies between sample preparations studied by different groups. In our most recent samples, there most likely are three entry points for EET from the IsiA18 ring per the PSI core monomer, with two of these entry points likely being located next to each other. Therefore, there are nine entry points from the IsiA18 ring to the PSI3 trimer. We anticipate that the data discussed below will stimulate further research in this area, providing even more insight into the structure-based models of these important cyanobacterial photosystems.

A Brief Introduction to Single-Molecule Fluorescence Methods

One of the most popular single-molecule approaches in biological science is single-molecule fluorescence microscopy, which will be the subject of the following section of this volume. Fluorescence methods provide the sensitivity required to study biology on the single-molecule level, but they also allow access to useful measurable parameters on time and length scales relevant for the biomolecular world. Before several detailed experimental approaches will be addressed, we will first give a general overview of single-molecule fluorescence microscopy. We start with discussing the phenomenon of fluorescence in general and the history of single-molecule fluorescence microscopy. Next, we will review fluorescent probes in more detail and the equipment required to visualize them on the single-molecule level. We will end with a description of parameters measurable with such approaches, ranging from protein counting and tracking, single-molecule localization super-resolution microscopy, to distance measurements with Förster resonance energy transfer and orientation measurements with fluorescence polarization.

Room-Temperature Excitation-Emission Spectra of Single LH2 Complexes Show Remarkably Little Variation

Excitation spectroscopy gives direct insight into the excited state manifold, energy transfer, transient intermediates, vibrations, and so on. Unfortunately, excitation spectroscopy of single molecules under ambient conditions has remained challenging. Here we present excitation spectra alongside emission spectra of the same individual light-harvesting complex LH2 of the purple bacteria Rps. acidophila. The acquisition of both the excited and ground state spectra allows us to quantify disorder and interband correlations, which are key variables for the interpretation of observed long-lasting coherences. We have overcome the low photostability and small fluorescence quantum yield that are inherent to many biologically relevant systems by combining single-molecule Fourier transform spectroscopy, low excitation intensities, and effective data analysis. We find that LH2 complexes show little spectral variation (130-170 cm^-1), that their two absorption bands (B800-B850) are uncorrelated, and that the Stokes shift is not constant. The low amount of spectral disorder underlines the protective role of the protein scaffold, benefiting the efficient energy transport throughout the light-harvesting membrane.

Role of missing carotenoid in reducing the fluorescence of single monomeric photosystem II core complexes

The fluorescence of monomeric photosystem II core complexes (mPSIIcc) of the cyanobacterium Thermosynechococcus elongatus, originating from redissolved crystals, is investigated by using single-molecule spectroscopy (SMS) at 1.6 K. The emission spectra of individual mPSIIcc are dominated by sharp zero-phonon lines, showing the existence of different emitters compatible with the F685, F689, and F695 bands reported formerly. The intensity of F695 is reduced in single mPSIIcc as compared to single PSIIcc-dimers (dPSIIcc). Crystal structures show that one of the β-carotene (β-Car) cofactors located at the monomer-monomer interface in dPSIIcc is missing in mPSIIcc. This β-Car in dPSIIcc is in van der Waals distance to chlorophyll (Chl) 17 in the CP47 subunit. We suggest that this Chl contributes to the F695 emitter. A loss of β-Car cofactors in mPSIIcc preparations will lead to an increased lifetime of the triplet state of Chl 17, which can explain the reduced singlet emission of F695 as observed in SMS.

Robust light harvesting by a noisy antenna

Photosynthetic light harvesting can be very efficient in solar energy conversion while taking place in a highly disordered and noisy physiological environment. This efficiency is achieved by the ultrafast speed of the primary photosynthetic processes, which is enabled by a delicate interplay of quantum effects, thermodynamics and environmental noise. The primary processes take place in light-harvesting antennas built from pigments bound to a fluctuating protein scaffold. Here, we employ ultrafast single-molecule spectroscopy to follow fluctuations of the femtosecond energy transfer times in individual LH2 antenna complexes of purple bacteria. By combining single molecule results with ensemble spectroscopy through a unified theoretical description of both, we show how the protein fluctuations alter the excitation energy transfer dynamics. We find that from the thirteen orders of magnitude of possible timescales from picoseconds to minutes, the relevant fluctuations occur predominantly on a biological timescale of seconds, i.e. in the domain of slow protein motion. The measured spectra and dynamics can be explained by the protein modulating pigment excitation energies only. Moreover, we find that the small spread of pigment mean energies allows for excitation delocalization between the coupled pigments to survive. These unique features provide fast energy transport even in the presence of disorder. We conclude that this is the mechanism that enables LH2 to operate as a robust light-harvester, in spite of its intrinsically noisy biological environment.

A Brief Introduction to Single-Molecule Fluorescence Methods

One of the more popular single-molecule approaches in biological science is single-molecule fluorescence microscopy, which will be the subject of the following section of this volume. Fluorescence methods provide the sensitivity required to study biology on the single-molecule level, but they also allow access to useful measurable parameters on time and length scales relevant for the biomolecular world. Before several detailed experimental approaches will be addressed, we will first give a general overview of single-molecule fluorescence microscopy. We start with discussing the phenomenon of fluorescence in general and the history of single-molecule fluorescence microscopy. Next, we will review fluorescent probes in more detail and the equipment required to visualize them on the single-molecule level. We will end with a description of parameters measurable with such approaches, ranging from protein counting and tracking, single-molecule localization super-resolution microscopy, to distance measurements with Förster Resonance Energy Transfer and orientation measurements with fluorescence polarization.

Apoprotein heterogeneity increases spectral disorder and a step-wise modification of the B850 fluorescence peak position

It has already been established that the quaternary structure of the main light-harvesting complex (LH2) from the photosynthetic bacterium Rhodopseudomonas palustris is a nonameric 'ring' of PucAB heterodimers and under low-light culturing conditions an increased diversity of PucB synthesis occurs. In this work, single molecule fluorescence emission studies show that different classes of LH2 'rings' are present in "low-light" adapted cells and that an unknown chaperon process creates multiple sub-types of 'rings' with more conformational sub-states and configurations. This increase in spectral disorder significantly augments the cross-section for photon absorption and subsequent energy flow to the reaction centre trap when photon availability is a limiting factor. This work highlights yet another variant used by phototrophs to gather energy for cellular development.

New Insights from Sum Frequency Generation Vibrational Spectroscopy into the Interactions of Islet Amyloid Polypeptides with Lipid Membranes

Studies of amyloid polypeptides on membrane surfaces have gained increasing attention in recent years. Several studies have revealed that membranes can catalyze protein aggregation and that the early products of amyloid aggregation can disrupt membrane integrity, increasing water permeability and inducing ion cytotoxicity. Nonetheless, probing aggregation of amyloid proteins on membrane surfaces is challenging. Surface-specific methods are required to discriminate contributions of aggregates at the membrane interface from those in the bulk phase and to characterize protein secondary structures in situ and in real time without the use of perturbing spectroscopic labels. Here, we review the most recent applications of sum frequency generation (SFG) vibrational spectroscopy applied in conjunction with computational modeling techniques, a joint experimental and computational methodology that has provided valuable insights into the aggregation of islet amyloid polypeptide (IAPP) on membrane surfaces. These applications show that SFG can provide detailed information about structures, kinetics, and orientation of IAPP during interfacial aggregation, relevant to the molecular mechanisms of type II diabetes. These recent advances demonstrate the promise of SFG as a new approach for studying amyloid diseases at the molecular level and for the rational drug design targeting early aggregation products on membrane surfaces.

Variation of exciton-vibrational coupling in photosystem II core complexes from Thermosynechococcus elongatus as revealed by single-molecule spectroscopy

The spectral properties and dynamics of the fluorescence emission of photosystem II core complexes are investigated by single-molecule spectroscopy at 1.6 K. The emission spectra are dominated by sharp zero-phonon lines (ZPLs). The sharp ZPLs are the result of weak to intermediate exciton-vibrational coupling and slow spectral diffusion. For several data sets, it is possible to surpass the effect of spectral diffusion by applying a shifting algorithm. The increased signal-to-noise ratio enables us to determine the exciton-vibrational coupling strength (Huang-Rhys factor) with high precision. The Huang-Rhys factors vary between 0.03 and 0.8. The values of the Huang-Rhys factors show no obvious correlation between coupling strength and wavelength position. From this result, we conclude that electrostatic rather than exchange or dispersive interactions are the main contributors to the exciton-vibrational coupling in this system.

Effect of TMAO and betaine on the energy landscape of photosystem I

The accumulation of organic co-solvents in cells is a basic strategy for organisms from various species to increase stress tolerance in extreme environments. Widespread representatives of this class of co-solvents are trimethylamine-N-oxide (TMAO) and betaine; these small molecules are able to stabilize the native conformation of proteins and prevent their aggregation. Despite their importance, detailed experimental studies on the impact of these co-solvents on the energy landscape of proteins have not yet been carried out. We use single-molecule spectroscopy at cryogenic temperatures to examine the influence of these physiological relevant co-solvents on photosystem I (PSI) from Thermosynechococcus elongatus. In contrast to PSI ensemble spectra, which are almost unaffected by the addition of TMAO and betaine, statistical analysis of the fluorescence emission from individual PSI trimers yields insight into the interaction of the co-solvents with PSI. The results show an increased homogeneity upon addition of TMAO or betaine. The number of detectable zero-phonon lines (ZPLs) is reduced, indicating spectral diffusion processes with faster rates. In the framework of energy landscape model these findings indicate that co-solvents lead to reduced barrier heights between energy valleys, and thus efficient screening of protein conformations can take place.

Combined topographic, spectroscopic, and model analyses of inhomogeneous energetic coupling of linear light harvesting complex II aggregates in native photosynthetic membranes

Light harvesting by LH1 and LH2 antenna proteins in the photosynthetic membranes of purple bacteria has been extensively studied in recent years for the fundamental understanding of the energy transfer dynamics and mechanism. Here we report the inhomogeneous structural organization of the LH2 complexes in photosynthetic membranes, giving evidence for the existence of energetically coupled linear LH2 aggregates in the native photosynthetic membranes of purple bacteria. Focusing on systematic model analyses, we combined AFM imaging and spectroscopic analysis with energetic coupling model analysis to characterize the inhomogeneous linear aggregation of LH2. Our AFM imaging results reveal that the LH2 complexes form linear aggregates with the monomer number varying from one to eight and each monomer tilted along the aggregated structure in photosynthetic membranes. The spectroscopic results support the attribution of aggregated LH2 complexes in the photosynthetic membranes, and the model calculation values for the absorption, emission and lifetime are consistent with the experimentally determined spectroscopic values, further proving a molecular-level understanding of the energetic coupling and energy transfer among the LH2 complexes in the photosynthetic membranes.

Förster energy transfer theory as reflected in the structures of photosynthetic light-harvesting systems

Förster’s theory of resonant energy transfer underlies a fundamental process in nature, namely the harvesting of sunlight by photosynthetic life forms. The theoretical framework developed by Förster and others describes how electronic excitation migrates in the photosynthetic apparatus of plants, algae, and bacteria from light absorbing pigments to reaction centers where light energy is utilized for the eventual conversion into chemical energy. The demand for highest possible efficiency of light harvesting appears to have shaped the evolution of photosynthetic species from bacteria to plants which, despite a great variation in architecture, display common structural themes founded on the quantum physics of energy transfer as described first by Förster. Herein, Förster’s theory of excitation transfer is summarized, including recent extensions, and the relevance of the theory to photosynthetic systems as evolved in purple bacteria, cyanobacteria, and plants is demonstrated. Förster’s energy transfer formula, as used widely today in many fields of science, is also derived.

Conformational switching explains the intrinsic multifunctionality of plant light-harvesting complexes

The light-harvesting complexes of photosystem I and II (Lhcas and Lhcbs) of plants display a high structural homology and similar pigment content and organization. Yet, the spectroscopic properties of these complexes, and accordingly their functionality, differ substantially. This difference is primarily due to the charge-transfer (CT) character of a chlorophyll dimer in all Lhcas, which mixes with the excitonic states of these complexes, whereas this CT character is generally absent in Lhcbs. By means of single-molecule spectroscopy near room temperature, we demonstrate that the presence or absence of such a CT state in Lhcas and Lhcbs can occasionally be reversed; i.e., these complexes are able to interconvert conformationally to quasi-stable spectral states that resemble the Lhcs of the other photosystem. The high structural similarity of all the Lhca and Lhcb proteins suggests that the stable conformational states that give rise to the mixed CT-excitonic state are similar for all these proteins, and similarly for the conformations that involve no CT state. This indicates that the specific functions related to Lhca and Lhcb complexes are realized by different stable conformations of a single generic protein structure. We propose that this functionality is modulated and controlled by the protein environment.

Parameters of the protein energy landscapes of several light-harvesting complexes probed via spectral hole growth kinetics measurements

The parameters of barrier distributions on the protein energy landscape in the excited electronic state of the pigment/protein system have been determined by means of spectral hole burning for the lowest-energy pigments of CP43 core antenna complex and CP29 minor antenna complex of spinach Photosystem II (PS II) as well as of trimeric and monomeric LHCII complexes transiently associated with the pea Photosystem I (PS I) pool. All of these complexes exhibit sixty to several hundred times lower spectral hole burning yields as compared with molecular glassy solids previously probed by means of the hole growth kinetics measurements. Therefore, the entities (groups of atoms), which participate in conformational changes in protein, appear to be significantly larger and heavier than those in molecular glasses. No evidence of a small (∼1 cm(-1)) spectral shift tier of the spectral diffusion dynamics has been observed. Therefore, our data most likely reflect the true barrier distributions of the intact protein and not those related to the interface or surrounding host. Possible applications of the barrier distributions as well as the assignments of low-energy states of CP29 and LHCII are discussed in light of the above results.

Time-dependent atomistic view on the electronic relaxation in light-harvesting system II

Aiming at a better understanding of the molecular details in light absorption during photosynthesis, spatial and temporal correlation functions as well as spectral densities have been determined. At the focus of the present study are the light-harvesting II complexes of the purple bacterium Rhodospirillum molischianum. The calculations are based on a time-dependent combination of molecular dynamics simulations and quantum chemistry methods. Using a 12 ps long trajectory, different quantum chemical methods have been compared to each other. Furthermore, several approaches to determine the couplings between the individual chromophores have been tested. Correlations between energy gap fluctuations of different individual pigments are analyzed but found to be negligible. From the energy gap fluctuations, spectral densities are extracted which serve as input for calculations of optical properties and exciton dynamics. To this end, the spectral densities are tested by determining the linear absorption of the complete two-ring system. One important difference from earlier studies is given by the severely extended length of the trajectory along which the quantum chemical calculations have been performed. Due to this extension, more accurate and reliable data have been obtained in the low frequency regime which is important in the dynamics of electronic relaxation.

A brief introduction to single-molecule fluorescence methods

One of the more popular single-molecule approaches in biological science is single-molecule fluorescence microscopy, which is the subject of the following section of this volume. Fluorescence methods provide the sensitivity required to study biology on the single-molecule level, but they also allow access to useful measurable parameters on time and length scales relevant for the biomolecular world. Before several detailed experimental approaches are addressed, we first give a general overview of single-molecule fluorescence microscopy. We start with discussing the phenomenon of fluorescence in general and the history of single-molecule fluorescence microscopy. Next, we review fluorescent probes in more detail and the equipment required to visualize them on the single-molecule level. We end with a description of parameters measurable with such approaches, ranging from protein counting and tracking, to distance measurements with Förster Resonance Energy Transfer and orientation measurements with fluorescence polarization.

Fluorescence spectral dynamics of single LHCII trimers

Single-molecule spectroscopy was employed to elucidate the fluorescence spectral heterogeneity and dynamics of individual, immobilized trimeric complexes of the main light-harvesting complex of plants in solution near room temperature. Rapid reversible spectral shifts between various emitting states, each of which was quasi-stable for seconds to tens of seconds, were observed for a fraction of the complexes. Most deviating states were characterized by the appearance of an additional, red-shifted emission band. Reversible shifts of up to 75 nm were detected. By combining modified Redfield theory with a disordered exciton model, fluorescence spectra with peaks between 670 nm and 705 nm could be explained by changes in the realization of the static disorder of the pigment-site energies. Spectral bands beyond this wavelength window suggest the presence of special protein conformations. We attribute the large red shifts to the mixing of an excitonic state with a charge-transfer state in two or more strongly coupled chlorophylls. Spectral bluing is explained by the formation of an energy trap before excitation energy equilibration is completed.

Energy Transfer Dynamics in an RC-LH1-PufX Tubular Photosynthetic Membrane

Light absorption and the subsequent transfer of excitation energy are the first two steps of the photosynthetic process, carried out by protein-bound pigments, mainly bacteriochlorophylls (BChls), in photosynthetic bacteria. BChls are anchored in light-harvesting (LH) complexes, such as light-harvesting complex I (LH1), which directly associates with the reaction center (RC), forming the RC-LH1 core complex. In Rhodobacter sphaeroides, RC-LH1 core complexes contain an additional protein, PufX, and assemble into dimeric RC-LH1-PufX core complexes. In the absence of light-harvesting complexes II, the former complexes can aggregate into a helically ordered tubular photosynthetic membrane. We examined the excitation transfer dynamics in a single RC-LH1-PufX core complex dimer using the hierarchical equations of motion for dissipative quantum dynamics that accurately, yet computationally costly, treat the coupling between BChls and their protein environment. A widely employed description, generalized Förster theory, was also used to calculate the transfer rates of the same excitonic system in order to verify the accuracy of this computationally cheap method. Additionally, in light of the structural uncertainties in the Rhodobacter sphaeroides RC-LH1-PufX core complex, geometrical alterations were introduced in the BChl organization. It is shown that the energy transfer dynamics is not affected by the considered changes in the BChl organization, and that generalized Förster theory provides accurate transfer rates. An all-atom model for a tubular photosynthetic membrane is then constructed on the basis of electron microscopy data, and the overall energy transfer properties of this membrane are computed.

In situ misfolding of human islet amyloid polypeptide at interfaces probed by vibrational sum frequency generation

Kinetic analysis of conformational changes of proteins at interfaces is crucial for understanding many biological processes at membrane surfaces. In this study, we demonstrate that surface-selective sum frequency generation (SFG) spectroscopy can be used to investigate kinetics of conformational changes of proteins at interfaces. We focus on an intrinsically disordered protein, human islet amyloid polypeptide (hIAPP) that is known to misfold into the beta-sheet structure upon interaction with membranes. Using the ssp polarization setting (s-polarized SFG, s-polarized visible, and p-polarized infrared), we observe changes in the amide I spectra of hIAPP at the air/water interface after addition of dipalmitoylphosphoglycerol (DPPG) that correspond to the lipid-induced changes in secondary structures. We also used the chiral-sensitive psp polarization setting to obtain amide I spectra and observed a gradual buildup of the chiral structures that display the vibrational characteristics of parallel beta-sheets. We speculate that the second-order chiral-optical response at the antisymmetric stretch frequency of parallel beta-sheet at 1622 cm(-1) could be a highly characteristic optical property of the beta-sheet aggregates not only for hIAPP, but possibly also for other amyloid proteins. Analyzing the achiral and chiral amide I spectra, we conclude that DPPG induces the misfolding of hIAPP from alpha-helical and random-coil structures to the parallel beta-sheet structure at the air/water interface. We propose that SFG could complement existing techniques in obtaining kinetic and structural information for probing structures and functions of proteins at interfaces.

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.

Physical origins and models of energy transfer in photosynthetic light-harvesting

We perform a quantitative comparison of different energy transfer theories, i.e. modified Redfield, standard and generalized Förster theories, as well as combined Redfield-Förster approach. Physical limitations of these approaches are illustrated and critical values of the key parameters indicating their validity are found. We model at a quantitative level the spectra and dynamics in two photosynthetic antenna complexes: in phycoerythrin 545 from cryptophyte algae and in trimeric LHCII complex from higher plants. These two examples show how the structural organization determines a directed energy transfer and how equilibration within antenna subunits and migration between subunits are superimposed.

Long-range energy propagation in nanometer arrays of light harvesting antenna complexes

Here we report the first observation of long-range transport of excitation energy within a biomimetic molecular nanoarray constructed from LH2 antenna complexes from Rhodobacter sphaeroides. Fluorescence microscopy of the emission of light after local excitation with a diffraction-limited light beam reveals long-range transport of excitation energy over micrometer distances, which is much larger than required in the parent bacterial system. The transport was established from the influence of active energy-guiding layers on the observed fluorescence emission. We speculate that such an extent of energy migration occurs as a result of efficient coupling between many hundreds of LH2 molecules. These results demonstrate the potential for long-range energy propagation in hybrid systems composed of natural light harvesting antenna molecules from photosynthetic organisms.

Strategies for extracting structural information from 2D IR spectroscopy of amyloid: application to islet amyloid polypeptide

The 37-residue human islet amyloid polypeptide (hIAPP or amylin) self-assembles into fibers, the assembly of which has been associated with the disease mechanism of type II diabetes. Infrared spectroscopy in conjunction with isotope labeling is proving to be a powerful tool for studying the aggregation process of hIAPP and other amyloid forming proteins with residue specific structure and kinetic information, but the relationship between the spectroscopic observables and the structure is not fully established. We report a detailed analysis of the linear and 2D IR spectra of hIAPP fibers isotope labeled at seven different residue positions. The features of the 2D IR spectra, including the frequencies, linewidths, intensities, and polarization dependence of the diagonal and cross-peaks, rely heavily on the position of the isotope labeled residue. In order to understand how these measured parameters depend on fiber secondary and tertiary structure, we have simulated 1D and 2D IR spectra utilizing idealized structural models in addition to a recently published solid-state NMR based model of the amyloid fibril. The analysis provides a more rigorous foundation for interpreting the infrared spectra of amyloids. In addition, we demonstrate that 2D IR spectra can be employed to distinguish between residues in beta-sheets versus those in turn regions, and that transitional residues between secondary structures can be identified by the suppression of their cross-peaks in 2D IR spectra. This latter approach is not limited to amyloid fibrils and will be generally useful in identifying regions of secondary structure in proteins using 2D IR spectroscopy and isotope labeling.

Single-molecule spectroscopy reveals that individual low-light LH2 complexes from Rhodopseudomonas palustris 2.1.6. have a heterogeneous polypeptide composition

Rhodopseudomonas palustris belongs to the group of purple bacteria that have the ability to produce LH2 complexes with unusual absorption spectra when they are grown at low-light intensity. This ability is often related to the presence of multiple genes encoding the antenna apoproteins. Here we report, for the first time to our knowledge, direct evidence that individual low-light LH2 complexes have a heterogeneous alphabeta-apoprotein composition that modulates the site energies of Bchl a molecules, producing absorption bands at 800, 820, and 850 nm. The arrangement of the Bchl a molecules in the "tightly coupled ring" can be modeled by nine alphabeta-Bchls dimers, such that the Bchls bound to six alphabeta-pairs have B820-like site energies and the remaining Bchl a molecules have B850-like site energies. Furthermore, the experimental data can only be satisfactorily modeled when these six alphabeta-pairs with B820 Bchl a molecules are distributed such that the symmetry of the assembly is reduced to C(3). It is also clear from the measured single-molecule spectra that the energies of the electronically excited states in the mixed B820/850 ring are mainly influenced by diagonal disorder.

Low-temperature single-molecule spectroscopy on photosynthetic pigment-protein complexes from purple bacteria

The primary reactions of purple bacterial photosynthesis take place within two well characterized pigment-protein complexes, the core Reaction Center-Light Harvesting 1 (RC-LH1) complex and the more peripheral Light Harvesting 2 (LH2) complex. These antenna complexes serve to absorb incident solar radiation and to transfer it to the reaction-centers, where it is used to 'power' the photosynthetic redox reaction. This review provides an overview of how the character of the electronically excited states of these pigment-protein complexes are determined by quantum mechanics and how the respective spectral signatures can be observed by single-molecule spectroscopy.

Protein dynamics-induced variation of excitation energy transfer pathways

Strong anticorrelation between the fluorescence emission of different emitters is observed by employing single-molecule fluorescence spectroscopy on photosystem I at cryogenic temperatures. This anticorrelation demonstrates a time-dependent interaction between pigments participating in the exciton transfer chain, implying that uniquely defined energy transfer pathways within the complex do not exist. Fluctuations of the chromophores themselves or their immediate protein surroundings induce changes in their site energy, and, as a consequence, these fluctuations change the coupling within the excitation transfer pathways. The time scales of the site energy fluctuations of the individual emitters do not meet the time scales of the observed correlated emission behavior. Therefore, the emitters must be fed individually by energetically higher lying states, causing the observed intensity variations. This phenomenon is shown for photosystem I pigment-protein complexes from 2 different cyanobacteria (Thermosynechococcus elongatus and Synechocystis sp. PCC 6803) with strongly different spectral properties underlining the general character of the findings. The variability of energy transfer pathways might play a key role in the extreme robustness of light-harvesting systems in general.

Structural model and excitonic properties of the dimeric RC-LH1-PufX complex from Rhodobacter sphaeroides

The light-harvesting apparatus of the purple bacterial photosynthetic unit consists of a pool of peripheral light-harvesting complexes that transfer excitation energy to a reaction center (RC) via the surrounding pigment-protein complex LH1. Recent electron microscopy and atomic force microscopy studies have revealed that RC-LH1 units of Rhodobacter sphaeroides form membrane-bending dimeric complexes together with the polypeptide PufX. We present a structural model for these RC-LH1-PufX dimeric complexes constructed using the molecular dynamics flexible fitting method based on an EM density map. The arrangement of the LH1 BChls displays a distortion near the proposed location of the PufX polypeptide. The resulting atomic model for BChl arrays is used to compute the excitonic properties of the dimeric RC-LH1 complex. A comparison is presented between the structural and excitonic features of the S-shaped dimeric BChl array of Rhodobacter sphaeroides and the circular BChl arrangement found in other purple bacteria.

Spectral dynamics of individual bacterial light-harvesting complexes: alternative disorder model

The bacterial (Rhodopseudomonas acidophila) photosynthetic peripheral light-harvesting complex of type 2 (LH2) exhibits rich fluorescence spectral dynamics at room temperature. The fluorescence spectrum of individual LH2 shifts either to the blue or to the red during the experimental observation time of a few minutes. These spectral changes are often reversible and occur between levels of a distinctly different peak wavelength. Furthermore, they are accompanied by a change of the spectral line shape. To interpret the dynamics of spectral changes, an energetic disorder model associated with easily explainable structural changes of the protein is proposed. This model assumes that each pigment in the tightly coupled ring of bacteriochlorophylls can be in two states of electronic transition energy due to the protein-pigment interaction. The transition between these structural, and hence spectroscopic, states occurs through the thermally induced conformational potential energy barrier crossing. Although simplified, the model allows us to reproduce the bulk fluorescence spectrum, the distribution of the single-molecule spectral peak wavelength and its changes, and the statistics of the duration of the spectral states. It also provides an intuitively clear picture of possible protein dynamics in LH2. At the same time, it requires additional sophistication since it essentially does not reproduce the red occurrences of single LH2 spectra.

Atomic-level structural and functional model of a bacterial photosynthetic membrane vesicle

The photosynthetic unit (PSU) of purple photosynthetic bacteria consists of a network of bacteriochlorophyll-protein complexes that absorb solar energy for eventual conversion to ATP. Because of its remarkable simplicity, the PSU can serve as a prototype for studies of cellular organelles. In the purple bacterium Rhodobacter sphaeroides the PSU forms spherical invaginations of the inner membrane, approximately 70 nm in diameter, composed mostly of light-harvesting complexes, LH1 and LH2, and reaction centers (RCs). Atomic force microscopy studies of the intracytoplasmic membrane have revealed the overall spatial organization of the PSU. In the present study these atomic force microscopy data were used to construct three-dimensional models of an entire membrane vesicle at the atomic level by using the known structure of the LH2 complex and a structural model of the dimeric RC-LH1 complex. Two models depict vesicles consisting of 9 or 18 dimeric RC-LH1 complexes and 144 or 101 LH2 complexes, representing a total of 3,879 or 4,464 bacteriochlorophylls, respectively. The in silico reconstructions permit a detailed description of light absorption and electronic excitation migration, including computation of a 50-ps excitation lifetime and a 95% quantum efficiency for one of the model membranes, and demonstration of excitation sharing within the closely packed RC-LH1 dimer arrays.