Exciton states of the antenna and energy trapping by the reaction center

Modeling of excitation dynamics in large-size molecular systems: Hierarchical equations with compartmentalization

We describe the new method that can be useful for calculation of the excitation dynamics in large molecular arrays that can be split into compartments with weak exciton coupling between them. In this method, the dynamics within each compartment is evaluated nonperturbatively using hierarchical equations of motion (HEOM), whereas transfers between the exciton states belonging to different compartments are treated by the generalized Förster (gF) theory. In a combined HEOM-gF approach, the number of equations increases linearly when adding new compartments as opposed to pure HEOM, where a depth of hierarchy exhibits strong non-linear grows when scaling the total number of molecules. Comparing the combined HEOM-gF method with an exact HEOM solution enabled us to estimate the parameters corresponding to a validity range of the proposed theory. The possibility of using the method for modeling of energy transfers in photosynthetic antenna supercomplexes is discussed.

Unique features of the 'photo-energetics' of purple bacteria: a critical survey by the late Aleksandr Yuryevich Borisov (1930-2019)

We provide here an edited version of the "Farewell discussion" by the late Aleksandr (Alex) Yuryevich (Yu) Borisov (1930-2019) on several aspects related to the excitation energy transfer in photosynthetic bacteria. It is preceded by a prolog giving the events that led to our decision to publish it. Further, we include here a few photographs to give a personal glimpse of this unique biophysicist of our time. In addition, we provide here a reminiscence, by Andrei B. Rubin, on the scientific beginnings of Borisov. This article follows a Tribute to Borisov by Semenov et al. (2019, Photosynthesis Research, this issue).

The origin of the "dark" absorption band near 675 nm in the purple bacterial core light-harvesting complex LH1: two-photon measurements of LH1 and its subunit B820

A comparative two-photon excitation spectroscopic study of the exciton structure of the core antenna complex (LH1) and its subunit B820 was carried out. LH1 and its subunit B820 were isolated from cells of the carotenoid-less mutant G9 of Rhodospirillum rubrum. The measurements were performed by two-photon pump-probe spectroscopy. Samples were excited by 70 fs pulses at 1390 nm at a frequency of 1 kHz. Photoinduced absorption changes were recorded in the spectral range from 780 to 1020 nm for time delays of the probe pulse relative to the pump pulse in the - 1.5 to 11 ps range. All measurements were performed at room temperature. Two-photon excitation caused bleaching of exciton bands (k = 0, k = ± 1) of the circular bacteriochlorophyll aggregate of LH1. In the case of the B820 subunit, two-photon excitation did not cause absorption changes in this spectral range. It is proposed that in LH1 upper exciton branch states are mixed with charge-transfer (CT) states. In B820 such mixing is absent, precluding two-photon excitation in this spectral region. Usually, CT states are optically "dark", i.e., one photon-excitation forbidden. Thus, their investigation is rather complicated by conventional spectroscopic methods. Thus, our study provides a novel approach to investigate CT states and their interaction(s) with other excited states in photosynthetic light-harvesting complexes and other molecular aggregates.

Light harvesting in phototrophic bacteria: structure and function

This review serves as an introduction to the variety of light-harvesting (LH) structures present in phototrophic prokaryotes. It provides an overview of the LH complexes of purple bacteria, green sulfur bacteria (GSB), acidobacteria, filamentous anoxygenic phototrophs (FAP), and cyanobacteria. Bacteria have adapted their LH systems for efficient operation under a multitude of different habitats and light qualities, performing both oxygenic (oxygen-evolving) and anoxygenic (non-oxygen-evolving) photosynthesis. For each LH system, emphasis is placed on the overall architecture of the pigment–protein complex, as well as any relevant information on energy transfer rates and pathways. This review addresses also some of the more recent findings in the field, such as the structure of the CsmA chlorosome baseplate and the whole-cell kinetics of energy transfer in GSB, while also pointing out some areas in need of further investigation.

Direct Measurement of Energy Migration in Supramolecular Carbocyanine Dye Nanotubes

Exciton transport lengths in double-walled and bundled cylindrical 3,3'-bis- (2-sulfopropyl)-5,5',6,6'-tetrachloro-1,1'-dioctylbenzimida-carbocyanine (C8S3) J-aggregates were measured using direct imaging of fluorescence from individual aggregates deposited on solid substrates. Regions identified in confocal images were excited with a focused laser spot, and the resulting fluorescence emission was imaged onto an electron multiplying charged coupled device camera. A two-dimensional Gaussian fitting scheme was used to quantitatively compare the excitation beam profile to the broadened aggregate emission profiles. The double-walled tubes exhibit average exciton transport lengths of 140 nm, while exciton transport in the bundled nanotubes was found to be remarkably long, with distances reaching many hundreds of nanometers. A steady-state one-dimensional diffusion model for the broadening of the emission profiles yields diffusion coefficients of 120 nm(2) ps(-1) for the nanotubes and 7000 nm(2) ps(-1) for the aggregate bundles. The level of structural hierarchy dramatically affects the exciton transport capabilities in these artificial light-harvesting systems, and energy migration is not limited to a single dimension in J-aggregate bundles.

From localized excited States to excitons: changing of conceptions of primary photosynthetic processes in the twentieth century

A short description of two theories of the primary photosynthetic processes is given. Generally accepted in 1950s-1990s, the localized excited states theory has been changed to the modern exciton theory. Appearance of the new experimental data and the light-harvesting complex crystal structure are reasons why the exciton theory has become important. The bulk of data for the old theory and outstanding experiments that have been the driving force for a new theory are discussed in detail.

Efficiency of photochemical stages of photosynthesis in purple bacteria (a critical survey)

Based on currently available data, the energy transfer efficiency in the successive photophysical and photochemical stages has been analyzed for purple bacteria. This analysis covers the stages starting from migration of the light-induced electronic excitations from the bulk antenna pigments to the reaction centers up to irreversible stage of the electron transport along the transmembrane chain of cofactors-carriers. Some natural factors are revealed that significantly increase the rates of efficient processes in these stages. The influence on their efficiency by the "bottleneck" in the energy migration chain is established. The overall quantum yield of photosynthesis in these stages is determined.

Excitation energy transfer and trapping dynamics in the core complex of the filamentous photosynthetic bacterium Roseiflexus castenholzii

The light-harvesting core complex of the thermophilic filamentous anoxygenic phototrophic bacterium Roseiflexus castenholzii is intrinsic to the cytoplasmic membrane and intimately bound to the reaction center (RC). Using ultrafast transient absorption and time-resolved fluorescence spectroscopy with selective excitation, energy transfer, and trapping dynamics in the core complex have been investigated at room temperature in both open and closed RCs. Results presented in this report revealed that the excited energy transfer from the BChl 800 to the BChl 880 band of the antenna takes about 2 ps independent of the trapping by the RC. The time constants for excitation quenching in the core antenna BChl 880 by open and closed RCs were found to be 60 and 210 ps, respectively. Assuming that the light harvesting complex is generally similar to LH1 of purple bacteria, the possible structural and functional aspects of this unique antenna complex are discussed. The results show that the core complex of Roseiflexus castenholzii contains characteristics of both purple bacteria and Chloroflexus aurantiacus.

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.

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.

Circular symmetry of the light-harvesting 1 complex from Rhodospirillum rubrum is not perturbed by interaction with the reaction center

The effect of the interaction of the reaction center (RC) upon the geometrical arrangement of the bacteriochlorophyll a (BChla) pigments in the light-harvesting 1 complex (LH1) from Rhodospirillum rubrum has been examined using single molecule spectroscopy. Fluorescence excitation spectra at 1.8 K obtained from single detergent-solubilized as well as single membrane-reconstituted LH1-RC complexes showed predominantly (>70%) a single broad absorption maximum at 880-900 nm corresponding to the Q(y) transition of the LH1 complex. This absorption band was independent of the polarization direction of the excitation light. The remaining complexes showed two mutually orthogonal absorption bands in the same wavelength region with moderate splittings in the range of DeltaE = 30-85 cm(-1). Our observations are in agreement with simulated spectra of an array of 32 strongly coupled BChla dipoles arranged in perfect circular symmetry possessing only a diagonal disorder of <or=150 cm(-1). However, in contrast to LH1 complexes alone, excitation spectra that consist of a single absorption band were observed more frequently in the presence of the reaction center. Our results show that the interaction of the RC with the LH1 complex stabilizes the circular symmetric arrangement of the bacteriochlorophyll pigments and are in contradiction to recent studies by other groups using single molecule spectroscopy as well as cryoelectronmicroscopy and atomic force microscopy indicating that the RC induces an elliptical distortion of the LH1 complex. Possible reasons for this discrepancy are discussed.

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.

Dynamics of excitation energy transfer in the LH1 and LH2 light-harvesting complexes of photosynthetic bacteria

Photosynthetic light harvesting is a unique life process that occurs with amazing efficiency. Since the discovery of the structure of the bacterial peripheral light-harvesting complex (LH2), this process has been studied using a variety of advanced laser spectroscopic methods. We are now in a position to discuss the physical origins of excitation energy transfer and trapping in the LH2 and LH1 antennae of photosynthetic purple bacteria. We demonstrate that the time evolution of the state created by the light is determined by the combined action of excitonic pigment-pitment interactions, energetic disorder, and coupling to nuclear motion in a pigment-protein complex. A quantitative fit of experimental data using Redfield theory allowed us to determine the pathways and time scales of exciton and vibrational relaxation and analyze separately different contributions to the measured transient absorption dynamics. Furthermore, these dynamics were observed to be strongly dependent on the excitation wavelength. A numerical fit of this dependence turns out to be extremely critical to a variation of the structure and disorder parameters and, therefore, can be used as a test for different antenna models (disordered ring, elliptical deformations, correlated disorder, etc.). The calculated equilibration dynamics in the exciton basis allow a visualization of the exciton motion using a density matrix picture in real space.

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.

Site inhomogeneity and exciton delocalization in the photosynthetic antenna

The influence of energy disorder on exciton states of molecular aggregates (the dimer and the circular aggregate) was analyzed. The dipole strength and inhomogeneous line shapes of exciton states were calculated by means of numerical diagonalization of Hamiltonian with diagonal energy disorder without intersite correlation. The disorder degree corresponding to destruction of coherent exciton states was estimated. The circular aggregates were treated as a model of light-harvesting antenna structures of photosynthetic bacteria. It was concluded that the site inhomogeneity typical for LH1 and LH2 complexes of purple bacteria cannot significantly influence the exciton delocalization over the whole antenna.

Exciton delocalization in the antenna of purple bacteria: exciton spectrum calculations using Z-ray data and experimental site inhomogeneity

Electron absorption and circular dichroism spectra of the peripheral light-harvesting complex (LH2) of photosynthetic purple bacteria were calculated taking into account the real-life spatial arrangement and experimental inhomogeneous broadening of bacteriochlorophyll molecules. It was shown that strong excitonic interactions between 18 bacteriochlorophyll molecules (BCh1850) within the circular aggregate of the LH2 complex result in an exciton delocalization over all these pigment molecules. The site inhomogeneity (spectral disorder) practically has no influence on exciton delocalization. The splitting between two lowest exciton levels corresponds to experimentally revealed splitting by hole-burning studies of the LH2 complex.

Energy trapping and detrapping by wild type and mutant reaction centers of purple non-sulfur bacteria

Time-correlated single photon counting was used to study energy trapping and detrapping kinetics at 295 K in Rhodobacter sphaeroides chromatophore membranes containing mutant reaction centers. The mutant reaction centers were expressed in a background strain of Rb. sphaeroides which contained only B880 antenna complexes and no B800-850 antenna complexes. The excited state decay times in the isolated reaction centers from these strains were previously shown to vary by roughly 15-fold, from 3.4 to 52 ps, due to differences in the charge separation rates in the different mutants (Allen and Williams (1995) J Bioenerg Biomembr 27: 275-283). In this study, measurements were also performed on wild type Rhodospirillum rubrum and Rb. sphaeroides B880 antenna-only mutant chromatophores for comparison. The emission kinetics in membranes containing mutant reaction centers was complex. The experimental data were analyzed in terms of a kinetic model that involved fast excitation migration between antenna complexes followed by reversible energy transfer to the reaction center and charge separation. Three emission time constants were identified by fitting the data to a sum of exponential decay components. They were assigned to trapping/quenching of antenna excitations by the reaction center, recombination of the P(+)H(-) charge-separated state of the reaction center reforming an emitting state, and emission from uncoupled antenna pigment-protein complexes. The first varied from 60 to 160 ps, depending on the reaction center mutation; the second was 200-300 ps, and the third was about 700 ps. The observed weak linear dependence of the trapping time on the primary charge separation time, together with the known sub-picosecond exciton migration time within the antenna, supports the concept that it is energy transfer from the antenna to the reaction center, rather than charge separation, that limits the overall energy trapping time in wild type chromatophores. The component due to charge recombination reforming the excited state is minor in wild type membranes, but increases substantially in mutants due to the decreasing free energy gap between the states P(*) and P(+)H(-).

Excitation delocalization over the whole core antenna of photosynthetic purple bacteria evidenced by non-linear pump-probe spectroscopy

Anomalously high values of photoinduced absorption changes were revealed in the antenna of photosynthetic purple bacteria. They were found to be 4-16 times greater at the bleaching peak of the antenna than at the bleaching peak of the BChl dimer of the reaction center. This is direct proof of excitation delocalization over many pigment molecules. Calculations according to the model of exciton delocalization over all core antenna BChls allow one to explain the observed phenomenon.