Excitons in core LH1 antenna complexes of photosynthetic bacteria: Evidence for strong resonant coupling and off-diagonal disorder

Tuning by Hydrogen Bonding in Photosynthesis

Hydrogen bonding plays a crucial role in stabilizing proteins throughout their folding process. In photosynthetic light-harvesting chromoproteins, enriched with pigment chromophores, hydrogen bonds also fine-tune optical absorption to align with the solar irradiation spectrum. Despite its significance for photosynthesis, the precise mechanism of spectral tuning through hydrogen bonding remains inadequately understood. This study investigates wild-type and genetically engineered LH2 and LH1 light-harvesting complexes from Rhodobacter sphaeroides using a unique set of advanced spectroscopic techniques combined with simple exciton modeling. Our findings reveal an intricate interplay between exciton and site energy shift mechanisms, challenging the prevailing belief that spectral changes observed in these complexes upon the modification of tertiary structure hydrogen bonds almost directly follow shifting site energies. These deeper insights into natural adaptation processes hold great promise for advancing sustainable solar energy conversion technologies.

Enhancing solar spectrum utilization in photosynthesis: exploring exciton and site energy shifts as key mechanisms

Photosynthesis is a critical process that harnesses solar energy to sustain life across Earth's intricate ecosystems. Central to this phenomenon is nuanced adaptation to a spectrum spanning approximately from 300 nm to nearly 1100 nm of solar irradiation, a trait enabling plants, algae, and phototrophic bacteria to flourish in their respective ecological niches. While the Sun's thermal radiance and the Earth's atmospheric translucence naturally constrain the ultraviolet extent of this range, a comprehension of how to optimize the utilization of near-infrared light has remained an enduring pursuit. This study unveils the remarkable capacity of the bacteriochlorophyll b-containing purple photosynthetic bacterium Blastochloris viridis to harness solar energy at extreme long wavelengths, a property attributed to a synergistic interplay of exciton and site energy shift mechanisms. Understanding the unique native adaptation mechanisms offers promising prospects for advancing sustainable energy technologies of solar energy conversion.

Dominant role of excitons in photosynthetic color-tuning and light-harvesting

Photosynthesis is a vital process that converts sunlight into energy for the Earth's ecosystems. Color adaptation is crucial for different photosynthetic organisms to thrive in their ecological niches. Although the presence of collective excitons in light-harvesting complexes is well known, the role of delocalized excited states in color tuning and excitation energy transfer remains unclear. This study evaluates the characteristics of photosynthetic excitons in sulfur and non-sulfur purple bacteria using advanced optical spectroscopic techniques at reduced temperatures. The exciton effects in these bacteriochlorophyll a-containing species are generally much stronger than in plant systems that rely on chlorophylls. Their exciton bandwidth varies based on multiple factors such as chromoprotein structure, surroundings of the pigments, carotenoid content, hydrogen bonding, and metal ion inclusion. The study nevertheless establishes a linear relationship between the exciton bandwidth and Qy singlet exciton absorption peak, which in case of LH1 core complexes from different species covers almost 130 nm. These findings provide important insights into bacterial color tuning and light-harvesting, which can inspire sustainable energy strategies and devices.

Exciton Origin of Color-Tuning in Ca2+-Binding Photosynthetic Bacteria

Flexible color adaptation to available ecological niches is vital for the photosynthetic organisms to thrive. Hence, most purple bacteria living in the shade of green plants and algae apply bacteriochlorophyll a pigments to harvest near infra-red light around 850–875 nm. Exceptions are some Ca²⁺-containing species fit to utilize much redder quanta. The physical basis of such anomalous absorbance shift equivalent to ~5.5 kT at ambient temperature remains unsettled so far. Here, by applying several sophisticated spectroscopic techniques, we show that the Ca²⁺ ions bound to the structure of LH1 core light-harvesting pigment–protein complex significantly increase the couplings between the bacteriochlorophyll pigments. We thus establish the Ca-facilitated enhancement of exciton couplings as the main mechanism of the record spectral red-shift. The changes in specific interactions such as pigment–protein hydrogen bonding, although present, turned out to be secondary in this regard. Apart from solving the two-decade-old conundrum, these results complement the list of physical principles applicable for efficient spectral tuning of photo-sensitive molecular nano-systems, native or synthetic.

Quantum Redirection of Antenna Absorption to Photosynthetic Reaction Centers

The early steps of photosynthesis involve the photoexcitation of reaction centers (RCs) and light-harvesting (LH) units. Here, we show that the historically overlooked excitonic delocalization across RC and LH pigments results in a redistribution of absorption amplitudes that benefits the absorption cross section of the optical bands associated with the RC of several species. While we prove that this redistribution is robust to the microscopic details of the dephasing between these units in the purple bacterium Rhodospirillum rubrum, we are able to show that the redistribution witnesses a more fragile, but persistent, coherent population dynamics which directs excitations from the LH toward the RC units under incoherent illumination and physiological conditions. Even though the redirection does not seem to affect importantly the overall efficiency in photosynthesis, stochastic optimization allows us to delineate clear guidelines and develop simple analytic expressions in order to amplify the coherent redirection in artificial nanostructures.

Spectrally Selective Spectroscopy of Native Ca-Containing and Ba-Substituted LH1-RC Core Complexes from Thermochromatium tepidum

The LH1-RC core complex from the thermophilic photosynthetic purple sulfur bacterium Thermochromatium tepidum has recently attracted interest of many researchers because of its several unique properties, such as increased robustness against environmental hardships and the much red-shifted near-infrared absorption spectrum of the LH1 antenna exciton polarons. The known near-atomic-resolution crystal structure of the complex well supported this attention. Yet several mechanistic aspects of the complex prominence remained to be understood. In this work, samples of the native, Ca²⁺-containing core complexes were investigated along with those destabilized by Ba²⁺ substitution, using various spectrally selective steady-state and picosecond time-resolved spectroscopic techniques at physiological and cryogenic temperatures. As a result, the current interpretation of exciton spectra of the complex was significantly clarified. Specifically, by evaluating the homogeneous and inhomogeneous compositions of the spectra, we showed that there is little to no effect of cation substitution on the dynamic or kinetic properties of antenna excitons. Reasons of the extra red shift of absorption/fluorescence spectra observed in the Ca-LH1-RC and not in the Ba-LH1-RC complex should thus be searched in subtle structural differences following the inclusion of different cations into the core complex scaffold.

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.

Optical Signatures of Quantum Delocalization over Extended Domains in Photosynthetic Membranes

The prospect of coherent dynamics and excitonic delocalization across several light-harvesting structures in photosynthetic membranes is of considerable interest, but challenging to explore experimentally. Here we demonstrate theoretically that the excitonic delocalization across extended domains involving several light-harvesting complexes can lead to unambiguous signatures in the optical response, specifically, linear absorption spectra. We characterize, under experimentally established conditions of molecular assembly and protein-induced inhomogeneities, the optical absorption in these arrays from polarized and unpolarized excitation, and demonstrate that it can be used as a diagnostic tool to determine the resonance coupling between iso-energetic light-harvesting structures. The knowledge of these couplings would then provide further insight into the dynamical properties of transfer, such as facilitating the accurate determination of Förster rates.

Unified analysis of ensemble and single-complex optical spectral data from light-harvesting complex-2 chromoproteins for gaining deeper insight into bacterial photosynthesis

Bacterial light-harvesting pigment-protein complexes are very efficient at converting photons into excitons and transferring them to reaction centers, where the energy is stored in a chemical form. Optical properties of the complexes are known to change significantly in time and also vary from one complex to another; therefore, a detailed understanding of the variations on the level of single complexes and how they accumulate into effects that can be seen on the macroscopic scale is required. While experimental and theoretical methods exist to study the spectral properties of light-harvesting complexes on both individual complex and bulk ensemble levels, they have been developed largely independently of each other. To fill this gap, we simultaneously analyze experimental low-temperature single-complex and bulk ensemble optical spectra of the light-harvesting complex-2 (LH2) chromoproteins from the photosynthetic bacterium Rhodopseudomonas acidophila in order to find a unique theoretical model consistent with both experimental situations. The model, which satisfies most of the observations, combines strong exciton-phonon coupling with significant disorder, characteristic of the proteins. We establish a detailed disorder model that, in addition to containing a C_{2}-symmetrical modulation of the site energies, distinguishes between static intercomplex and slow conformational intracomplex disorders. The model evaluations also verify that, despite best efforts, the single-LH2-complex measurements performed so far may be biased toward complexes with higher Huang-Rhys factors.

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.

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.

Excitons in intact cells of photosynthetic bacteria

Live cells and regular crystals seem fundamentally incompatible. Still, effects characteristic to ideal crystals, such as coherent sharing of excitation, have been recently used in many studies to explain the behavior of several photosynthetic complexes, especially the inner workings of the light-harvesting apparatus of the oldest known photosynthetic organisms, the purple bacteria. To this date, there has been no concrete evidence that the same effects are instrumental in real living cells, leaving a possibility that this is an artifact of unnatural study conditions, not a real effect relevant to the biological operation of bacteria. Hereby, we demonstrate survival of collective coherent excitations (excitons) in intact cells of photosynthetic purple bacteria. This is done by using excitation anisotropy spectroscopy for tracking the temperature-dependent evolution of exciton bands in light-harvesting systems of increasing structural complexity. The temperature was gradually raised from 4.5 K to ambient temperature, and the complexity of the systems ranged from detergent-isolated complexes to complete bacterial cells. The results provide conclusive evidence that excitons are indeed one of the key elements contributing to the energetic and dynamic properties of photosynthetic organisms.

Open Quantum Dynamics Calculations with the Hierarchy Equations of Motion on Parallel Computers

Calculating the evolution of an open quantum system, i.e., a system in contact with a thermal environment, has presented a theoretical and computational challenge for many years. With the advent of supercomputers containing large amounts of memory and many processors, the computational challenge posed by the previously intractable theoretical models can now be addressed. The hierarchy equations of motion present one such model and offer a powerful method that remained under-utilized so far due to its considerable computational expense. By exploiting concurrent processing on parallel computers the hierarchy equations of motion can be applied to biological-scale systems. Herein we introduce the quantum dynamics software PHI, that solves the hierarchical equations of motion. We describe the integrator employed by PHI and demonstrate PHI's scaling and efficiency running on large parallel computers by applying the software to the calculation of inter-complex excitation transfer between the light harvesting complexes 1 and 2 of purple photosynthetic bacteria, a 50 pigment system.

How Quantum Coherence Assists Photosynthetic Light Harvesting

This perspective examines how hundreds of pigment molecules in purple bacteria cooperate through quantum coherence to achieve remarkable light harvesting efficiency. Quantum coherent sharing of excitation, which modifies excited state energy levels and combines transition dipole moments, enables rapid transfer of excitation over large distances. Purple bacteria exploit the resulting excitation transfer to engage many antenna proteins in light harvesting, thereby increasing the rate of photon absorption and energy conversion. We highlight here how quantum coherence comes about and plays a key role in the photosynthetic apparatus of purple bacteria.

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.

Direct visualization of exciton reequilibration in the LH1 and LH2 complexes of Rhodobacter sphaeroides by multipulse spectroscopy

The dynamics of the excited states of the light-harvesting complexes LH1 and LH2 of Rhodobacter sphaeroides are governed, mainly, by the excitonic nature of these ring-systems. In a pump-dump-probe experiment, the first pulse promotes LH1 or LH2 to its excited state and the second pulse dumps a portion of the excited state. By selective dumping, we can disentangle the dynamics normally hidden in the excited-state manifold. We find that by using this multiple-excitation technique we can visualize a 400-fs reequilibration reflecting relaxation between the two lowest exciton states that cannot be directly explored by conventional pump-probe. An oscillatory feature is observed within the exciton reequilibration, which is attributed to a coherent motion of a vibrational wavepacket with a period of ∼150 fs. Our disordered exciton model allows a quantitative interpretation of the observed reequilibration processes occurring in these antennas.

The effect of correlated bath fluctuations on exciton transfer

Excitation dynamics of various light harvesting systems have been investigated with many theoretical methods including various non-Markovian descriptions of dissipative quantum dynamics. It is typically assumed that each excited state is coupled to an independent thermal environment, i.e., that fluctuations in different environments are uncorrelated. Here the assumption is dropped and the effect of correlated bath fluctuations on excitation transfer is investigated. Using the hierarchy equations of motion for dissipative quantum dynamics it is shown for models of the B850 bacteriochlorophylls of LH2 that correlated bath fluctuations have a significant effect on the LH2→LH2 excitation transfer rate. It is also demonstrated that inclusion of static disorder is crucial for an accurate description of transfer dynamics.

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.

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.