Photosynthetic Light-Harvesting: Reconciling Dynamics and Structure of Purple Bacterial LH2 Reveals Function of Photosynthetic Unit

Exciton Transfer Simulations in a Light-Harvesting 2 Complex Reveal the Transient Delocalization Mechanism

The striking efficiency of exciton transfer in light-harvesting (LH) complexes has remained a topic of debate since the revision of the long-held role of electronic coherences. To address this issue, we have developed a neural network for the pigments in the LH2 complex of Rhodospirillum molischianum that allows nonadiabatic molecular dynamic (NAMD) simulations of exciton transfer in a coupled quantum mechanical/molecular mechanics (QM/MM) embedding. The calculated exciton occupations are averaged over hundreds of trajectories, each lasting several picoseconds. We have obtained transitions within the B800 and B850 rings that agree well with the experimental results, indicating an incoherent hopping process in the B800 ring and a more delocalized transfer in the B850 subsystem. The reorganization energies and excitonic couplings are comparable to each other, indicating that the "transient delocalization" transport model is the underlying cause of the highly efficient exciton transport in the B850 ring. This phenomenon can be attributed to a localized exciton that shows occasional large delocalization events. Our results indicate that the reason for the striking efficiency is the unusual electronic property of bacteriochlorophyll, manifested in minimal inner and outer sphere reorganization energies.

Modulating the Charge Transfer Coupling in Boron-Dipyrromethene Homodimers by π-Bridge Units

To mimic the excitation energy conversion mechanisms observed in natural light-harvesting systems, we have extensively investigated photoinduced symmetry-breaking charge separations (SBCSs) in various multichromophoric model systems have been extensively investigated. However, designing multichromophoric model systems capable of simultaneously achieving ultrafast and complete SBCS remains a significant challenge. In this study, we employed benzene, thiophene, and furan as π-bridges to develop a series of boron dipyrromethene (BODIPY) homodimers. Spectral analysis, together with an estimation of the π-bridge-dependent charge transfer (CT) coupling using the fragment charge difference method, reveals that π-bridge units with different electron-donating abilities can effectively modulate the CT coupling between chromophores. Notably, the furan-based π-bridge, exhibiting the most pronounced electron-donating character, facilitates symmetry-breaking charge transfer (SBCT), i.e., excimer formation with a time constant of about 12 ps in weak polar toluene. Furthermore, a dramatic increase in the SBCS rate constant was observed in highly polar acetonitrile, improving from 60.4 ps for the benzene-bridged homodimer to 2.9 ps for the furan-bridged counterpart. These findings underscore the potential of π-bridge units in tuning the photophysical properties of covalent molecular aggregates by optimizing such systems for specific applications such as organic photovoltaics and photocatalysis.

Theory of photosynthetic membrane influence on B800-B850 energy transfer in the LH2 complex

Photosynthetic organisms rely on a network of light-harvesting protein-pigment complexes to efficiently absorb sunlight and transfer excitation energy to reaction center proteins where charge separation occurs. In photosynthetic purple bacteria, these complexes are embedded within the cell membrane, with lipid composition affecting complex clustering, thereby impacting inter-complex energy transfer. However, the impact of the lipid bilayer on intra-complex excitation dynamics is less understood. Recent experiments have addressed this question by comparing photo-excitation dynamics in detergent-isolated light-harvesting complex 2 (LH2) to LH2 complexes embedded in membrane discs mimicking the biological environment, revealing differences in spectra and energy-transfer rates. In this paper, we use available quantum chemical and spectroscopy data to develop a complementary theoretical study on the excitonic structure and intra-complex energy-transfer kinetics of the LH2 of photosynthetic purple bacteria Rhodoblastus (Rbl.) acidophilus (formerly Rhodopseudomonas acidophila) in two different conditions: the LH2 in a membrane environment and detergent-isolated LH2. We find that dark excitonic states, crucial for B800-B850 energy transfer within LH2, are more delocalized in the membrane model. Using nonperturbative and generalized Förster calculations, we show that such increased quantum delocalization results in a 30% faster B800 to B850 transfer rate in the membrane model, in agreement with experimental results. We identify the dominant energy-transfer pathways in each environment and demonstrate how differences in the B800 to B850 transfer rate arise from changes in LH2's electronic properties when embedded in the membrane. Furthermore, by accounting for the quasi-static variations of electronic excitation energies in the LH2, we show that the broadening of the distribution of the B800-B850 transfer rates is affected by the lipid composition. We argue that such variation in broadening could be a signature of a speed-accuracy trade-off, commonly seen in biological process.

Photosynthetic Energy Transfer: Missing in Action (Detected Spectroscopy)?

In recent years, action-detected ultrafast spectroscopies have gained popularity offering distinct advantages over their coherently detected counterparts, such as spatially resolved and operando measurements with high sensitivity. However, there are also fundamental limitations connected to the process of signal generation in action-detected experiments. Here we perform fluorescence-detected two-dimensional electronic spectroscopy (F-2DES) of the light-harvesting II (LH2) complex from purple bacteria. We demonstrate that the B800-B850 energy transfer process in LH2 is weak but observable in F-2DES, unlike in coherently detected 2DES where the energy transfer is visible with 100% contrast. We explain the weak signatures using a disordered excitonic model that accounts for experimental conditions. We further derive a general formula for the presence of excited-state signals in multichromophoric aggregates, dependent on the aggregate geometry, size, excitonic coupling and disorder. We find that the prominence of excited-state dynamics in action-detected spectroscopy offers a unique probe of excitonic delocalization in multichromophoric systems.

Coherence in Chemistry: Foundations and Frontiers

Coherence refers to correlations in waves. Because matter has a wave-particle nature, it is unsurprising that coherence has deep connections with the most contemporary issues in chemistry research (e.g., energy harvesting, femtosecond spectroscopy, molecular qubits and more). But what does the word "coherence" really mean in the context of molecules and other quantum systems? We provide a review of key concepts, definitions, and methodologies, surrounding coherence phenomena in chemistry, and we describe how the terms "coherence" and "quantum coherence" refer to many different phenomena in chemistry. Moreover, we show how these notions are related to the concept of an interference pattern. Coherence phenomena are indeed complex, and ambiguous definitions may spawn confusion. By describing the many definitions and contexts for coherence in the molecular sciences, we aim to enhance understanding and communication in this broad and active area of chemistry.

Spectral modulation of B850 bacteriochlorophyll a in light-harvesting complex 2 from purple photosynthetic bacterium Thermochromatium tepidum by detergents and calcium ions

Spectral variations of light-harvesting (LH) proteins of purple photosynthetic bacteria provide insight into the molecular mechanisms underlying spectral tuning of circular bacteriochlorophyll (BChl) arrays, which play crucial roles in photoenergy conversion in these organisms. Here we investigate spectral changes of the Qy band of B850 BChl a in LH2 protein from purple sulfur bacterium Thermochromatium tepidum (tepidum-LH2) by detergents and Ca2+. The tepidum-LH2 solubilized with lauryl dimethylamine N-oxide and n-octyl-β-D-glucoside (LH2LDAO and LH2OG, respectively) exhibited blue-shift of the B850 Qy band with hypochromism compared with the tepidum-LH2 solubilized with n-dodecyl-β-D-maltoside (LH2DDM), resulting in the LH3-like spectral features. Resonance Raman spectroscopy indicated that this blue-shift was ascribable to the loss of hydrogen-bonding between the C3-acetyl group in B850 BChl a and the LH2 polypeptides. Ca2+ produced red-shift of the B850 Qy band in LH2LDAO by forming hydrogen-bond for the C3-acetyl group in B850 BChl a, probably due to a change in the microenvironmental structure around B850. Ca2+-induced red-shift was also observed in LH2OG although the B850 acetyl group is still free from hydrogen-bonding. Therefore, the Ca2+-induced B850 red-shift in LH2OG would originate from an electrostatic effect of Ca2+. The current results suggest that the B850 Qy band in tepidum-LH2 is primarily tuned by two mechanisms, namely the hydrogen-bonding of the B850 acetyl group and the electrostatic effect.

Prominent Role of Charge Transfer in the Spectral Tuning of Photosynthetic Light-Harvesting I Complex

Purple bacteria possess two ring-shaped protein complexes, light-harvesting 1 (LH1) and 2 (LH2), both of which function as antennas for solar energy utilization for photosynthesis but exhibit distinct absorption properties. The two antennas have differing amounts of bacteriochlorophyll (BChl) a; however, their significance in spectral tuning remains elusive. Here, we report a high-precision evaluation of the physicochemical factors contributing to the variation in absorption maxima between LH1 and LH2, namely, BChl a structural distortion, protein electrostatic interaction, excitonic coupling, and charge transfer (CT) effects, as derived from detailed spectral calculations using an extended version of the exciton model, in the model purple bacterium Rhodospirillum rubrum. Spectral analysis confirmed that the electronic structure of the excited state in LH1 extended to the BChl a 16-mer. Further analysis revealed that the LH1-specific redshift (∼61% in energy) is predominantly accounted for by the CT effect resulting from the closer inter-BChl distance in LH1 than in LH2. Our analysis explains how LH1 and LH2, both with chemically identical BChl a chromophores, use distinct physicochemical effects to achieve a progressive redshift from LH2 to LH1, ensuring efficient energy transfer to the reaction center special pair.

Light harvesting in purple bacteria does not rely on resonance fine-tuning in peripheral antenna complexes

The ring-like peripheral light-harvesting complex 2 (LH2) expressed by many phototrophic purple bacteria is a popular model system in biological light-harvesting research due to its robustness, small size, and known crystal structure. Furthermore, the availability of structural variants with distinct electronic structures and optical properties has made this group of light harvesters an attractive testing ground for studies of structure-function relationships in biological systems. LH2 is one of several pigment-protein complexes for which a link between functionality and effects such as excitonic coherence and vibronic coupling has been proposed. While a direct connection has not yet been demonstrated, many such interactions are highly sensitive to resonance conditions, and a dependence of intra-complex dynamics on detailed electronic structure might be expected. To gauge the sensitivity of energy-level structure and relaxation dynamics to naturally occurring structural changes, we compare the photo-induced dynamics in two structurally distinct LH2 variants. Using polarization-controlled 2D electronic spectroscopy at cryogenic temperatures, we directly access information on dynamic and static disorder in the complexes. The simultaneous optimal spectral and temporal resolution of these experiments further allows us to characterize the ultrafast energy relaxation, including exciton transport within the complexes. Despite the variations in PPC molecular structure manifesting as clear differences in electronic structure and disorder, the energy-transport and-relaxation dynamics remain remarkably similar. This indicates that the light-harvesting functionality of purple bacteria within a single LH2 complex is highly robust to structural perturbations and likely does not rely on finely tuned electronic- or electron-vibrational resonance conditions.

The influence of cationic antiseptics on the processes of light energy conversion in various photosynthetic pigment-protein complexes

The widespread use of disinfectants and antiseptics, and consequently their release into the environment, determines the relevance of studying their potential impact on the main producers of organic matter on the planet-photosynthetic organisms. The review examines the effects of some biguanides and quaternary ammonium compounds, octenidine, miramistin, chlorhexidine, and picloxidine, on the functioning of the photosynthetic apparatus of various organisms (Strakhovskaya et al. in Photosynth Res 147:197-209, 2021; Knox et al. in Photosynth Res 153:103, 2022; Paschenko et al. in Photosynth Res 155:93-105, 2023a, Photosynth Res 2023b). A common feature of these antiseptics is the combination of hydrophobic and hydrophilic regions in the molecules, the latter carrying a positive charge(s). The comparison of the results obtained with intact bacterial membrane vesicles (chromatophores) and purified pigment-protein complexes (photosystem II and I) of oxygenic organisms allows us to draw conclusions about the mechanisms of the cationic antiseptic action on the functional properties of the components of the photosynthetic apparatus.

Coarse-Grained Approach to Simulate Signatures of Excitation Energy Transfer in Two-Dimensional Electronic Spectroscopy of Large Molecular Systems

Two-dimensional electronic spectroscopy (2DES) has proven to be a highly effective technique in studying the properties of excited states and the process of excitation energy transfer in complex molecular assemblies, particularly in biological light-harvesting systems. However, the accurate simulation of 2DES for large systems still poses a challenge because of the heavy computational demands it entails. In an effort to overcome this limitation, we devised a coarse-grained 2DES method. This method encompasses the treatment of the entire system by dividing it into distinct weakly coupled segments, which are assumed to communicate predominantly through incoherent exciton transfer. We first demonstrate the efficiency of this method through simulation on a model dimer system, which demonstrates a marked improvement in calculation efficiency, with results that exhibit good concordance with reference spectra calculated with less approximate methods. Additionally, the application of this method to the light-harvesting antenna 2 (LH2) complex of purple bacteria showcases its advantages, accuracy, and limitations. Furthermore, simulating the anisotropy decay in LH2 induced by energy transfer and its comparison with experiments confirm that the method is capable of accurately describing dynamical processes in a biologically relevant system. This method presented lends itself to an extension that accounts for the effect of intrasegment relaxation processes on the 2DES spectra, which for computational efficiency are ignored in the implementation reported here. It is envisioned that the method will be employed in the future to accurately and efficiently calculate 2D spectra of more extensive systems, such as photosynthetic supercomplexes.

Spectral changes of light-harvesting complex 2 lacking B800 bacteriochlorophyll a under neutral pH conditions

Exchange of B800 bacteriochlorophyll (BChl) a in light-harvesting complex 2 (LH2) is promising for a better understanding of the mechanism on intracomplex excitation energy transfer of this protein. Structural and spectroscopic properties of LH2 lacking B800 BChl a (B800-depleted LH2), which is an important intermediate protein in the B800 exchange, will be useful to tackle the energy transfer mechanism in LH2 by the B800 exchange strategy. In this study, we report a unique spectral change of B800-depleted LH2, in which the Qy absorption band of B800 BChl a is automatically recovered under neutral pH conditions. This spectral change was facilitated by factors for destabilization of LH2, namely, a detergent, lauryl dimethylamine N-oxide, and an increase in temperature. Spectral analyses in the preparation of an LH2 variant denoted as B800-recovered LH2 indicated that most BChl a that was released by decomposition of part of B800-depleted LH2 was a source of the production of B800-recovered LH2. Characterization of purified B800-recovered LH2 demonstrated that its spectroscopic and structural features was quite similar to those of native LH2. The current results indicate that the recovery of the B800 Qy band of B800-depleted LH2 originates from the combination of decomposition of part of B800-depleted LH2 and in situ reconstitution of BChl a into the B800 binding pockets of residual B800-depleted LH2, resulting in the formation of stable B800-recovered LH2.

Molecular and Supramolecular Materials: From Light-Harvesting to Quantum Information Science and Technology

The past two decades have witnessed immense advances in quantum information technology (QIT), benefited by advances in physics, chemistry, biology, and materials science and engineering. It is intriguing to consider whether these diverse molecular and supramolecular structures and materials, partially inspired by quantum effects as observed in sophisticated biological systems such as light-harvesting complexes in photosynthesis and the magnetic compass of migratory birds, might play a role in future QIT. If so, how? Herein, we review materials and specify the relationship between structures and quantum properties, and we identify the challenges and limitations that have restricted the intersection of QIT and chemical materials. Examples are broken down into two categories: materials for quantum sensing where nonclassical function is observed on the molecular scale and systems where nonclassical phenomena are present due to intermolecular interactions. We discuss challenges for materials chemistry and make comparisons to related systems found in nature. We conclude that if chemical materials become relevant for QIT, they will enable quite new kinds of properties and functions.

Controlling Symmetry-Breaking Charge Separation in Pyrene Bichromophores

So far, symmetry-breaking charge separation (SB-CS) has been observed with a limited number of chromophores and is usually inhibited by the formation of an excimer. , We show here that thanks to of fine-tuning of the interchromophore coupling via structural control, SB-CS can be operative with pyrene, despite its high propensity to form an excimer. This is realized with a bichromophoric system consisting of two pyrenes attached to a crown ether macrocycle, which can bind cations of different sizes. By combining stationary and time-resolved spectroscopy together with molecular dynamics simulations, we demonstrate that the excited-state dynamics can be totally changed depending on the binding cation. Whereas strong coupling leads to rapid excimer formation, too weak coupling results in noninteracting chromophores. However, intermediate coupling, achieved upon binding of Mg2+, allows for SB-CS to be operative.

Efficient 2D to 0D Energy Transfer in HgTe Nanoplatelet-Quantum Dot Heterostructures through High-Speed Exciton Diffusion

Large area absorbers with localized defect emission are of interest for energy concentration via the antenna effect. Transfer between 2D and 0D quantum-confined structures is advantageous as it affords maximal lateral area antennas with continuously tunable emission. We report the quantum efficiency of energy transfer in in situ grown HgTe nanoplatelet (NPL)/quantum dot (QD) heterostructures to be near unity (>85%), while energy transfer in separately synthesized and well separated solutions of HgTe NPLs to QDs only reaches 47 ± 11% at considerably higher QD concentrations. Using Kinetic Monte Carlo simulations, we estimate an exciton diffusion constant of 1-10 cm2/s in HgTe NPLs, the same magnitude as that of 2D semiconductors. We also simulate in-solution energy transfer between NPLs and QDs, recovering an R-4 dependence consistent with 2D-0D near-field energy transfer even in randomly distributed NPL/QD mixtures. This highlights the advantage of NPLs 2D morphology and the efficiency of NPL/QD heterostructures and mixtures for energy harvesting.

Waveguided energy transfer in pseudo-two-dimensional systems

Resonance energy transfer (RET) is an important and ubiquitous process whereby energy is transferred from a donor chromophore to an acceptor chromophore without contact via Coulombic coupling. There have been a number of recent advances exploiting the quantum electrodynamics (QED) framework for RET. Here, we extend the QED RET theory to investigate whether real photon exchange can allow for excitation transfer over very long distances if the exchanged photon is waveguided. To study this problem, we consider RET in two spatial dimensions. We derive the RET matrix element using QED in two dimensions, consider an even greater confinement by deriving the RET matrix element for a two-dimensional waveguide using ray theory, and compare the resulting RET elements in 3D and 2D and for the 2D waveguide. We see greatly enhanced RET rates over long distances for both the 2D and 2D waveguide systems and see a great preference for transverse photon mediated transfer in the 2D waveguide system.

Coherence Maps and Flow of Excitation Energy in the Bacterial Light Harvesting Complex 2

We present and analyze coherence maps [ J. Phys. Chem. B 2022, 126, 9361-9375] to investigate the quantum coherences that are created, sustained, and damped by vibrational modes during the transfer of excitation energy from the B800 (outer) to the B850 (inner) ring of the light harvesting complex 2 (LH2) of purple bacteria with a variety of initial conditions. The reduced density matrix of the 24-pigment complex, where the ground and excited electronic states of each bacteriochlorophyll are explicitly coupled to 50 intramolecular vibrations at room temperature, is obtained from fully quantum-mechanical small matrix path integral (SMatPI) calculations. The coherence maps show a very rapid localization within the outer ring, accompanied by the formation of inter-ring quantum superpositions that evolve to a partial quantum delocalization at equilibrium, and quantify in state-to-state detail the flow of energy within the complex.

Atomic force microscopic analysis of the light-harvesting complex 2 from purple photosynthetic bacterium Thermochromatium tepidum

Structural information on the circular arrangements of repeating pigment-polypeptide subunits in antenna proteins of purple photosynthetic bacteria is a clue to a better understanding of molecular mechanisms for the ring-structure formation and efficient light harvesting of such antennas. Here, we have analyzed the ring structure of light-harvesting complex 2 (LH2) from the thermophilic purple bacterium Thermochromatium tepidum (tepidum-LH2) by atomic force microscopy. The circular arrangement of the tepidum-LH2 subunits was successfully visualized in a lipid bilayer. The average top-to-top distance of the ring structure, which is correlated with the ring size, was 4.8 ± 0.3 nm. This value was close to the top-to-top distance of the octameric LH2 from Phaeospirillum molischianum (molischianum-LH2) by the previous analysis. Gaussian distribution of the angles of the segments consisting of neighboring subunits in the ring structures of tepidum-LH2 yielded a median of 44°, which corresponds to the angle for the octameric circular arrangement (45°). These results indicate that tepidum-LH2 has a ring structure consisting of eight repeating subunits. The coincidence of an octameric ring structure of tepidum-LH2 with that of molischianum-LH2 is consistent with the homology of amino acid sequences of the polypeptides between tepidum-LH2 and molischianum-LH2.

Conversion of B800 Bacteriochlorophyll a to 3-Acetyl Chlorophyll a in the Light-Harvesting Complex 3 by In Situ Oxidation

The spectral features of energy donors and acceptors and the relationship between them in photosynthetic light-harvesting proteins are crucial for photofunctions of these proteins. Engineering energy donors and acceptors in light-harvesting proteins affords the means to increase our understanding of their photofunctional mechanisms. Herein, we demonstrate the conversion of energy-donating B800 bacteriochlorophyll (BChl) a to 3-acetyl chlorophyll (AcChl) a in light-harvesting complex 3 (LH3) from Rhodoblastus acidophilus by in situ oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone. AcChl a in the B800 site exhibited a Q_y band that was 111 nm blue-shifted with respect to BChl a in oxidized LH3. The structure of LH3 was barely influenced by the oxidation process, based on circular dichroism spectroscopy and size-exclusion chromatography evidence. In oxidized LH3, AcChl a transferred excitation energy to B820 BChl a, but the rate of excitation energy transfer (EET) was lower than in native LH3. The intracomplex EET in oxidized LH3 was slightly faster than in oxidized light-harvesting complex 2 (LH2). This difference is rationalized by an increase in overlap of the luminescence band of AcChl a with the long tail of the B820 absorption band in oxidized LH3 compared with that of the B850 band in oxidized LH2.

An efficient protocol for excited states of large biochromophores

Efficient energy transport in photosynthetic antenna is a long-standing source of inspiration for artificial light harvesting materials. However, characterizing the excited states of the constituent chromophores poses a considerable challenge to mainstream quantum chemical and semiempirical excited state methods due to their size and complexity and the accuracy required to describe small but functionally important changes in their properties. In this paper, we explore an alternative approach to calculating the excited states of large biochromophores, exemplified by a specific method for calculating the Q_y transition of bacteriochlorophyll a, which we name Chl-xTB. Using a diagonally dominant approximation to the Casida equation and a bespoke parameterization scheme, Chl-xTB can match time-dependent density functional theory's accuracy and semiempirical speed for calculating the potential energy surfaces and absorption spectra of chlorophylls. We demonstrate that Chl-xTB (and other prospective realizations of our protocol) can be integrated into multiscale models, including concurrent excitonic and point-charge embedding frameworks, enabling the analysis of biochromophore networks in a native environment. We exploit this capability to probe the low-frequency spectral densities of excitonic energies and interchromophore interactions in the light harvesting antenna protein LH2 (light harvesting complex 2). The impact of low-frequency protein motion on interchromophore coupling and exciton transport has routinely been ignored due to the prohibitive costs of including it in simulations. Our results provide a more rigorous basis for continued use of this approximation by demonstrating that exciton transition energies are unaffected by low-frequency vibrational coupling to exciton interaction energies.

Optical cavity-mediated exciton dynamics in photosynthetic light harvesting 2 complexes

Strong light-matter interaction leads to the formation of hybrid polariton states and alters the photophysical dynamics of organic materials and biological systems without modifying their chemical structure. Here, we experimentally investigated a well-known photosynthetic protein, light harvesting 2 complexes (LH2) from purple bacteria under strong coupling with the light mode of a Fabry-Perot optical microcavity. Using femtosecond pump probe spectroscopy, we analyzed the polariton dynamics of the strongly coupled system and observed a significant prolongation of the excited state lifetime compared with the bare exciton, which can be explained in terms of the exciton reservoir model. Our findings indicate the potential of tuning the dynamic of the whole photosynthetic unit, which contains several light harvesting complexes and reaction centers, with the help of strong exciton-photon coupling, and opening the discussion about possible design strategies of artificial photosynthetic devices.

Carotenoid responds to excess energy dissipation in the LH2 complex from Rhodoblastus acidophilus

The functions of both (bacterio) chlorophylls and carotenoids in light-harvesting complexes have been extensively studied during the past decade, yet, the involvement of BChl a high-energy Soret band in the cascade of light-harvesting processes still remains a relatively unexplored topic. Here, we present transient absorption data recorded after excitation of the Soret band in the LH2 complex from Rhodoblastus acidophilus. Comparison of obtained data to those recorded after excitation of rhodopin glucoside and B800 BChl a suggests that no Soret-to-Car energy transfer pathway is active in LH2 complex. Furthermore, a spectrally rich pattern observed in the spectral region of rhodopin glucoside ground state bleaching (420-550 nm) has been assigned to an electrochromic shift. The results of global fitting analysis demonstrate two more features. A 6 ps component obtained exclusively after excitation of the Soret band has been assigned to the response of rhodopin glucoside to excess energy dissipation in LH2. Another time component, ~ 450 ps, appearing independently of the excitation wavelength was assigned to BChl a-to-Car triplet-triplet transfer. Presented data demonstrate several new features of LH2 complex and its behavior following the excitation of the Soret band.

Energy transfer dynamics and the mechanism of biohybrid photosynthetic antenna complexes chemically linked with artificial chromophores

A light-harvesting strategy is crucial for the utilisation of solar energy. In this study, we addressed the expanding light-harvesting (LH) wavelength of photosynthetic LH complex 2 (LH2, from Rhodoblastus acidophilus strain 10050) through covalent conjugation with extrinsic chromophores. To further understand the conjugation architecture and mechanism of excitation energy transfer (EET), we examined the effects of the linker length and spectral overlap integral between the emission and absorption spectra of the energy donor and acceptor pigments. In the former case, contrary to the intuition based on the Förster resonance energy transfer (FRET) theory, the observed energy transfer rate was similar regardless of the linker length, and the energy transfer efficiency increased with longer linkers. In the latter case, despite the energy transfer rate increases at higher spectral overlaps, it was quantitatively inconsistent with the FRET theory. The mechanism of EET beyond the FRET theory was discussed in terms of the higher-lying exciton state of B850, which mediates efficient EET despite the small spectral overlap. This systematic investigation provides insights for the development of efficient artificial photosynthetic systems.

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.

Observation of anomalous carotenoid and blind chlorophyll activations in photosystem I under synthetic membrane confinements

The role of natural thylakoid membrane confinements in architecting the robust structural and electrochemical properties of PSI is not fully understood. Most PSI studies till date extract the proteins from their natural confinements that can lead to non-native conformations. Recently our group had successfully reconstituted PSI in synthetic lipid membranes using detergent-mediated liposome solubilizations. In this study, we investigate the alterations in chlorophylls and carotenoids interactions and reorganization in PSI based on spectral property changes induced by its confinement in anionic DPhPG and zwitterionic DPhPC phospholipid membranes. To this end, we employ a combination of absorption, fluorescence, and circular dichroism (CD) spectroscopic measurements. Our results indicate unique activation and alteration of photoresponses from the PSI carotenoid (Car) bands in PSI-DPhPG proteoliposomes that can tune the Excitation Energy Transfer (EET), otherwise absent in PSI at non-native environments. Specifically, we observe broadband light harvesting via enhanced absorption in the otherwise non-absorptive green region (500-580 nm) of the Chlorophylls (Chl) along with ~64% increase in the full-width half maximum of the Qy band (650-720 nm). The CD results indicate enhanced Chl-Chl and Chl-Car interactions along with conformational changes in protein secondary structures. Such distinct changes in the Car and Chl bands are not observed in PSI confined in DPhPC. The fundamental insights into membrane microenvironments tailoring PSI subunits reorganization and interactions provide novel strategies for tuning photoexcitation processes and rational designing of biotic-abiotic interfaces in PSI-based photoelectrochemical energy conversion systems.

B800-to-B850 relaxation of excitation energy in bacterial light harvesting: All-state, all-mode path integral simulations

We report fully quantum mechanical simulations of excitation energy transfer within the peripheral light harvesting complex (LH2) of Rhodopseudomonas molischianum at room temperature. The exciton–vibration Hamiltonian comprises the 16 singly excited bacteriochlorophyll states of the B850 (inner) ring and the 8 states of the B800 (outer) ring with all available electronic couplings. The electronic states of each chromophore couple to 50 intramolecular vibrational modes with spectroscopically determined Huang–Rhys factors and to a weakly dissipative bath that models the biomolecular environment. Simulations of the excitation energy transfer following photoexcitation of various electronic eigenstates are performed using the numerically exact small matrix decomposition of the quasiadiabatic propagator path integral. We find that the energy relaxation process in the 24-state system is highly nontrivial. When the photoexcited state comprises primarily B800 pigments, a rapid intra-band redistribution of the energy sharply transitions to a significantly slower relaxation component that transfers 90% of the excitation energy to the B850 ring. The mixed character B850* state lacks the slow component and equilibrates very rapidly, providing an alternative energy transfer channel. This (and also another partially mixed) state has an anomalously large equilibrium population, suggesting a shift to lower energy by virtue of exciton–vibration coupling. The spread of the vibrationally dressed states is smaller than that of the eigenstates of the bare electronic Hamiltonian. The total population of the B800 band is found to decay exponentially with a 1/ e time of 0.5 ps, which is in good agreement with experimental results.

Efficient nano-photonic antennas based on dark states in quantum emitter rings

Nanoscopic arrays of quantum emitters can feature highly sub-radiant collective excitations with a lifetime exponentially growing with emitter number. Adding an absorptive impurity as an energy dump in the center of a ring shaped polygon allows to exploit this feature to create highly efficient single photon antennas. Here among regular polygons with an identical center absorbing emitter, a nonagon exhibits a distinct optimum of the absorption efficiency. This special enhancement originates from the unique emergence of a subradiant eigenstate with dominant center occupation. Only for nine emitters the sum of coupling strengths of each emitter to all others matches the center to the ring coupling. Analogous to a parabolic mirror the antenna ring then concentrates incoming radiation at its center without being significantly excited itself. Similar large efficiency enhancements, which even prevail for broadband excitation, can also be engineered for other antenna sizes by tailoring the frequency and magnitude of the central absorber. Interestingly, for very small structures a quantum treatment predicts an even stronger enhancement for the single photon absorption enhancement than a classical dipole model. As natural light harvesting structures are often based on ring shaped structures, the underlying principle might be exploited there as well.

Superradiance and Directional Exciton Migration in Metal-Organic Frameworks

Crystalline metal-organic frameworks (MOFs) are promising synthetic analogues of photosynthetic light-harvesting complexes (LHCs). The precise assembly of linkers (organic chromophores) around the topology-defined pores offers the evolution of unique photophysical behaviors that are reminiscence of LHCs. These include MOF excited states with photoabsorbed energy that is spatially dispersed over multiple linkers defining the molecular excitons. The multilinker molecular excitons display superradiance─a hallmark of coupled oscillators seen in LHCs─with radiative rate constant (k_rad) exceeding that of a single linker. Our theoretical model and experimental results on three zirconium MOFs, namely, PCN-222(Zn), NU-1000, and SIU-100, with similar topology but varying linkers suggest that the size of such molecular excitons depends on the electronic symmetry of the linker. This multilinker exciton model effectively predicts the energy transfer rate constant; corresponding single-step exciton hopping time, ranging from a few picoseconds in SIU-100 and NU-1000 to a few hundreds of picoseconds in PCN-222(Zn), matches well with the experimental data. The model also predicts the anisotropy of exciton displacement with preferential migration along the crystallographic c-axis. Overall, these findings establish various missing links defining the exciton size and dynamics in MOF-assembled linkers. The understandings will provide design principles, especially, positioning the catalysts or electrode relative to the linker orientation for low-density solar energy conversion systems.

Effect of Dipyridamole on Membrane Energization and Energy Transfer in Chromatophores of Rba. sphaeroides

Effect of dipyridamole (DIP) at concentrations up to 1 mM on fluorescent characteristics of light-harvesting complexes LH2 and LH1, as well as on conditions of photosynthetic electron transport chain in the bacterial chromatophores of Rba. sphaeroides was investigated. DIP was found to affect efficiency of energy transfer from the light-harvesting complex LH2 to the LH1-reaction center core complex and to produce the long-wavelength ("red") shift of the absorption band of light-harvesting bacteriochlorophyll molecules in the IR spectral region at 840-900 nm. This shift is associated with the membrane transition to the energized state. It was shown that DIP is able to reduce the photooxidized bacteriochlorophyll of the reaction center, which accelerated electron flow along the electron transport chain, thereby stimulating generation of the transmembrane potential on the chromatophore membrane. The results are important for clarifying possible mechanisms of DIP influence on the activity of membrane-bound functional proteins. In particular, they might be significant for interpreting numerous therapeutic effects of DIP.

Bridging Carotenoid-to-Bacteriochlorophyll Energy Transfer of Purple Bacteria LH2 With Temperature Variations: Insights From Conformational Changes

Photosynthesis is a key process for converting light energy into chemical energy and providing food for lives on Earth. Understanding the mechanism for the energy transfers could provide insights into regulating energy transfers in photosynthesis and designing artificial photosynthesis systems. Many efforts have been devoted to exploring the mechanism of temperature variations affecting the excitonic properties of LH2. In this study, we performed all-atom molecular dynamics (MD) simulations and quantum mechanics calculations for LH2 complex from purple bacteria along with its membrane environment under three typical temperatures: 270, 300, and 330 K. The structural analysis from validated MD simulations showed that the higher temperature impaired interactions at N-terminus of both α and β polypeptide helices and led to the dissociation of this hetero polypeptide dimer. Rhodopin-β-D-glucosides (RG1) moved centripetally with α polypeptide helices when temperature increased and enlarged their distances with bacteriochlorophylls molecules that have the absorption peak at 850 nm (B850), which resulted in reducing the coupling strengths between RG1 and B850 molecules. The present study reported a cascading mechanism for temperature regulating the energy transfers in LH2 of purple bacteria.

Scaling relations of exciton diffusion in linear aggregates with static and dynamic disorder

Exciton diffusion plays an important role in many opto-electronic processes and phenomena. Understanding the interplay of intermolecular coupling, static energetic disorder, and dephasing caused by environmental fluctuations (dynamic disorder) is crucial to optimize exciton diffusion under various physical conditions. We report on a systematic analysis of the exciton diffusion constant in linear aggregates using the Haken-Strobl-Reineker model to describe this interplay. We numerically investigate the static-disorder scaling of (i) the diffusion constant in the limit of small dephasing rate, (ii) the dephasing rate at which the diffusion is optimized, and (iii) the value of the diffusion constant at the optimal dephasing rate. Three scaling regimes are found, associated with, respectively, fully delocalized exciton states (finite-size effects), weakly localized states, and strongly localized states. The scaling powers agree well with analytically estimated ones. In particular, in the weakly localized regime, the numerical results corroborate the so-called quantum Goldilocks principle to find the optimal dephasing rate and maximum diffusion constant as a function of static disorder, while in the strong-localization regime, these quantities can be derived fully analytically.

Estimation of the relative contributions to the electronic energy transfer rates based on Förster theory: The case of C-phycocyanin chromophores

In the present study, we provide a reformulation of the theory originally proposed by Förster which allows for simple and convenient formulas useful to estimate the relative contributions of transition dipole moments of a donor and acceptor (chemical factors), their orientation factors (intermolecular structural factors), intermolecular center-to-center distances (intermolecular structural factors), spectral overlaps of absorption and emission spectra (photophysical factors), and refractive index (material factor) to the excitation energy transfer (EET) rate constant. To benchmark their validity, we focused on the EET occurring in C-phycocyanin (C-PC) chromophores. To this aim, we resorted to quantum chemistry calculations to get optimized molecular structures of the C-PC chromophores within the density functional theory (DFT) framework. The absorption and emission spectra, as well as transition dipole moments, were computed by using the time-dependent DFT (TDDFT). Our method was applied to several types of C-PCs showing that the EET rates are determined by an interplay of their specific physical, chemical, and geometrical features. These results show that our formulas can become a useful tool for a reliable estimation of the relative contributions of the factors regulating the EET transfer rate.

The Relationship between the Spatial Arrangement of Pigments and Exciton Transition Moments in Photosynthetic Light-Harvesting Complexes

Considering bacteriochlorophyll molecules embedded in the protein matrix of the light-harvesting complexes of purple bacteria (known as LH2 and LH1-RC) as examples of systems of interacting pigment molecules, we investigated the relationship between the spatial arrangement of the pigments and their exciton transition moments. Based on the recently reported crystal structures of LH2 and LH1-RC and the outcomes of previous theoretical studies, as well as adopting the Frenkel exciton Hamiltonian for two-level molecules, we performed visualizations of the LH2 and LH1 exciton transition moments. To make the electron transition moments in the exciton representation invariant with respect to the position of the system in space, a system of pigments must be translated to the center of mass before starting the calculations. As a result, the visualization of the transition moments for LH2 provided the following pattern: two strong transitions were outside of LH2 and the other two were perpendicular and at the center of LH2. The antenna of LH1-RC was characterized as having the same location of the strongest moments in the center of the complex, exactly as in the B850 ring, which actually coincides with the RC. Considering LH2 and LH1 as supermolecules, each of which has excitation energies and corresponding transition moments, we propose that the outer transitions of LH2 can be important for inter-complex energy exchange, while the inner transitions keep the energy in the complex; moreover, in the case of LH1, the inner transitions increased the rate of antenna-to-RC energy transfer.

Hydrostatic High-Pressure-Induced Denaturation of LH2 Membrane Proteins

The denaturation of globular proteins by high pressure is frequently associated with the release of internal voids and/or the exposure of the hydrophobic protein interior to a polar aqueous solvent. Similar evidence with respect to membrane proteins is not available. Here, we investigate the impact of hydrostatic pressures reaching 12 kbar on light-harvesting 2 integral membrane complexes of purple photosynthetic bacteria using two types of innate chromophores in separate strategic locations: bacteriochlorophyll-a in the hydrophobic interior and tryptophan at both protein-solvent interfacial gateways to internal voids. The complexes from mutant Rhodobacter sphaeroides with low resilience against pressure were considered in parallel with the naturally robust complexes of Thermochromatium tepidum. In the former case, a firm correlation was established between the abrupt blue shift of the bacteriochlorophyll-a exciton absorption, a known indicator of the breakage of tertiary structure pigment-protein hydrogen bonds, and the quenching of tryptophan fluorescence, a supposed result of further protein solvation. No such effects were observed in the reference complex. While these data may be naively taken as supporting evidence of the governing role of hydration, the analysis of atomistic model structures of the complexes confirmed the critical part of the structure in the pressure-induced denaturation of the membrane proteins studied.

Vibration-mediated energy transport in bacterial reaction center: Simulation study

Exciton energy relaxation in a bacterial Reaction Center (bRC) pigment-protein aggregate presumably involves emission of high energy vibrational quanta to cover wide energy gaps between excitons. Here, we assess this hypothesis utilizing vibronic two-particle theory in modeling of the excitation relaxation process in bRC. Specific high frequency molecular vibrational modes are included explicitly one at a time in order to check which high frequency vibrations are involved in the excitation relaxation process. The low frequency bath modes are treated perturbatively within Redfield relaxation theory. The analysis of the population relaxation rate data indicates energy flow pathways in bRC and suggests that specific vibrations may be responsible for the excitation relaxation process.

Spatial Patterns of Light-Harvesting Antenna Complex Arrangements Tune the Transfer-to-Trap Efficiency of Excitons in Purple Bacteria

In photosynthesis, the efficiency with which a photogenerated exciton reaches the reaction center is dictated by chromophore energies and the arrangement of chromophores in the supercomplex. Here, we explore the interplay between the arrangement of light-harvesting antennae and the efficiency of exciton transport in purple bacterial photosynthesis. Using a Miller-Abrahams-based exciton hopping model, we compare different arrangements of light-harvesting proteins on the intracytoplasmic membrane. We find that arrangements with aggregated LH1s have a higher efficiency than arrangements with randomly distributed LH1s in a wide range of physiological light fluences. This effect is robust to the introduction of defects on the intracytoplasmic membrane. Our result explains the absence of species with aggregated LH1 arrangements in low-light niches and the large increase seen in the expression of LH1 dimer complexes in high fluences. We suggest that the effect seen in our study is an adaptive strategy toward solar light fluence across different purple bacterial species.

Energy Transfer Dynamics in Light-Harvesting Complex 2 Variants Containing Oxidized B800 Bacteriochlorophyll a

Excitation energy transfer (EET) in light-harvesting proteins is vital for photosynthetic activities. The pigment compositions and their organizations in these proteins are responsible for the EET functions. Thus, changing the pigment compositions in light-harvesting proteins contributes to a better understanding of EET mechanisms. In this study, we investigated the EET dynamics of two light-harvesting complex 2 (LH2) variants, in which nine B800 bacteriochlorophyll (BChl) a pigments were entirely or half converted to 3-acetyl chlorophyll (AcChl) a. The AcChl a pigments showed a Q_y band, which was blue-shifted by 107 nm from B800 BChl a in the two variants. EET from AcChl a to B850 BChl a was observed in both fully oxidized and half-oxidized LH2 variants, but the EET rates were lower than that from B800 to B850 BChl a. EET from AcChl a to the co-present B800 was barely detected in the half-oxidized LH2. The preferential EET from AcChl a to B850 instead of B800 was rationalized by little spectral overlap of AcChl a with B800 BChl a and the pigment geometry in the protein. The EET rate from B800 to B850 BChl a in the half-oxidized LH2 was analogous to that in native LH2, indicating that partial oxidation of B800 did not disturb the EET channel from the residual B800 to B850.

Semiclassical Modified Redfield and Generalized Förster Theories of Exciton Relaxation/Transfer in Light-Harvesting Complexes: The Quest for the Principle of Detailed Balance

A conceptual problem of transfer theories that use a semiclassical description of the electron-vibrational coupling is the neglect of the correlation between momenta and coordinates of nuclei. In the Redfield theory of exciton relaxation, this neglect leads to a violation of the principle of detailed balance; equal “uphill” and “downhill” transfer rate constants are obtained. Here, we investigate how this result depends on nuclear reorganization effects, neglected in Redfield but taken into account in the modified Redfield theory. These reorganization effects, resulting from a partial localization of excited states, are found to promote a preferential “downhill” relaxation of excitation energy. However, for realistic spectral densities of light-harvesting antennae in photosynthesis, the reorganization effects are too small to compensate for the missing coordinate–momentum uncertainty. For weaker excitonic couplings as they occur between domains of strongly coupled pigments, we find the principle of detailed balance to be fulfilled in a semiclassical variant of the generalized Förster theory. A qualitatively correct description of the transfer is obtained with this theory at a significantly lower computational cost as with the quantum generalized Förster theory. Larger deviations between the two theories are expected for large energy gaps as they occur in complexes with chemically different pigments.

Fully Quantum Modeling of Exciton Diffusion in Mesoscale Light Harvesting Systems

It has long been a challenge to accurately and efficiently simulate exciton-phonon dynamics in mesoscale photosynthetic systems with a fully quantum mechanical treatment due to extensive computational resources required. In this work, we tackle this seemingly intractable problem by combining the Dirac-Frenkel time-dependent variational method with Davydov trial states and implementing the algorithm in graphic processing units. The phonons are treated on the same footing as the exciton. Tested with toy models, which are nanoarrays of the B850 pigments from the light harvesting 2 complexes of purple bacteria, the methodology is adopted to describe exciton diffusion in huge systems containing more than 1600 molecules. The superradiance enhancement factor extracted from the simulations indicates an exciton delocalization over two to three pigments, in agreement with measurements of fluorescence quantum yield and lifetime in B850 systems. With fractal analysis of the exciton dynamics, it is found that exciton transfer in B850 nanoarrays exhibits a superdiffusion component for about 500 fs. Treating the B850 ring as an aggregate and modeling the inter-ring exciton transfer as incoherent hopping, we also apply the method of classical master equations to estimate exciton diffusion properties in one-dimensional (1D) and two-dimensional (2D) B850 nanoarrays using derived analytical expressions of time-dependent excitation probabilities. For both coherent and incoherent propagation, faster energy transfer is uncovered in 2D nanoarrays than 1D chains, owing to availability of more numerous propagating channels in the 2D arrangement.

Bioinspired Applications of Porphyrin Derivatives

Porphyrin derivatives are ubiquitous in nature and have important biological roles, such as in light harvesting, oxygen transport, and catalysis. Owing to their intrinsic π-conjugated structure, porphyrin derivatives exhibit characteristic photophysical and electrochemical properties. In biological systems, porphyrin derivatives are associated with various protein molecules through noncovalent interactions. For example, hemoglobin, which is responsible for oxygen transport in most vertebrates, consists of four subunits of a globular protein with an iron porphyrin derivative prosthetic group. Furthermore, noncovalently arranged porphyrin derivatives are the fundamental chromophores in light-harvesting systems for photosynthesis in plants and algae. These biologically important roles originate from the functional versatility of porphyrin derivatives. Specifically, porphyrins are excellent host compounds, forming coordination complexes with various metal ions that adds functionality to the porphyrin unit, such as redox activity and additional ligand binding at the central metal ion. In addition, porphyrins are useful building blocks for functional supramolecular assemblies because of their flat and symmetrical molecular architectures, and their excellent photophysical properties are typically utilized for the fabrication of bioactive functional materials. In this Account, we summarize our endeavors over the past decade to develop functional materials based on porphyrin derivatives using bioinspired approaches. In the first section, we discuss several synthetic receptors that act as artificial allosteric host systems and can be used for the selective detection of various chemicals, such as cyanide, chloride, and amino acids. In the second section, we introduce multiporphyrin arrays as mimics of natural light-harvesting complexes. The active control of energy transfer processes by additional guest binding and the fabrication of organic photovoltaic devices using porphyrin derivatives are also introduced. In the third section, we introduce several types of porphyrin-based supramolecular assemblies. Through noncovalent interactions such as metal-ligand interaction, hydrogen bonding, and π-π interaction, porphyrin derivatives were constructed as supramolecular polymers with formation of fiber or toroidal assembly. In the last section, the application of porphyrin derivatives for biomedical nanodevice fabrication is introduced. Even though porphyrins were good candidates as photosensitizers for photodynamic therapy, they have limitations for biomedical application owing to aggregation in aqueous media. We suggested ionic dendrimer porphyrins and they showed excellent photodynamic therapy (PDT) efficacy.

Physical properties of porphyrin-based crystalline metal‒organic frameworks

Metal‒organic frameworks (MOFs) are widely studied molecular assemblies that have demonstrated promise for a range of potential applications. Given the unique and well-established photophysical and electrochemical properties of porphyrins, porphyrin-based MOFs are emerging as promising candidates for energy harvesting and conversion applications. Here we discuss the physical properties of porphyrin-based MOFs, highlighting the evolution of various optical and electronic features as a function of their modular framework structures and compositional variations.

Excitation Energy Transfer from Bacteriochlorophyll b in the B800 Site to B850 Bacteriochlorophyll a in Light-Harvesting Complex 2

Control of the spectral overlap between energy donors and acceptors provides insight into excitation energy transfer (EET) mechanisms in photosynthetic light-harvesting proteins. Substitution of energy-donating B800 bacteriochlorophyll (BChl) a with other pigments in the light-harvesting complex 2 (LH2) of purple photosynthetic bacteria has been extensively performed; however, most studies on the B800 substitution have focused on the decrease in the spectral overlap integral with energy-accepting B850 BChl a by reconstitution of chlorophylls into the B800 site. Here, we reconstitute BChl b into the B800 site of the LH2 protein from Rhodoblastus acidophilus to increase the spectral overlap with B850 BChl a. BChl b in the B800 site had essentially the same hydrogen-bonding pattern as B800 BChl a, whereas it showed a red-shifted Q_y absorption band at 831 nm. The EET rate from BChl b to B850 BChl a in the reconstituted LH2 was similar to that of native LH2 despite the red shift of the Q_y band of the energy donor. These results demonstrate the importance of the contribution of the density of excitation states of the B850 circular assembly, which incorporates higher lying optically forbidden states, to intracomplex EET in LH2.

The effect of some antiseptic drugs on the energy transfer in chromatophore photosynthetic membranes of purple non-sulfur bacteria Rhodobacter sphaeroides

Chromatophores of purple non-sulfur bacteria (PNSB) are invaginations of the cytoplasmic membrane that contain a relatively simple system of light-harvesting protein-pigment complexes, a photosynthetic reaction center (RC), a cytochrome complex, and ATP synthase, which transform light energy into the energy of synthesized ATP. The high content of negatively charged phosphatidylglycerol (PG) and cardiolipin (CL) in PNSB chromatophore membranes makes these structures potential targets that bind cationic antiseptics. We used the methods of stationary and kinetic fluorescence spectroscopy to study the effect of some cationic antiseptics (chlorhexidine, picloxydine, miramistin, and octenidine at concentrations up to 100 μM) on the spectral and kinetic characteristics of the components of the photosynthetic apparatus of Rhodobacter sphaeroides chromatophores. Here we present the experimental data on the reduced efficiency of light energy conversion in the chromatophore membranes isolated from the photosynthetic bacterium Rb. sphaeroides in the presence of cationic antiseptics. The addition of antiseptics did not affect the energy transfer between the light-harvesting LH1 complex and reaction center (RC). However, it significantly reduced the efficiency of the interaction between the LH2 and LH1 complexes. The effect was maximal with 100 μM octenidine. It has been proved that molecules of cationic antiseptics, which apparently bind to the heads of negatively charged cardiolipin molecules located in the rings of light-harvesting pigments on the cytoplasmic surface of the chromatophores, can disturb the optimal conditions for efficient energy migration in chromatophore membranes.

Energy transduction through FRET in self-assembled soft nanostructures based on surfactants/polymers: current scenario and prospects

The self-assembled systems of surfactants/polymers, which are capable of supporting energy funneling between fluorophores, have recently gained significant attraction. Surfactant and polymeric micelles form nanoscale structures spanning a radius of 2-10 nm are generally suitable for the transduction of energy among fluorophores. These systems have shown great potential in Förster resonance energy transfer (FRET) due to their unique characteristics of being aqueous based, tendency to remain self-assembled, spontaneous formation, tunable nature, and responsiveness to different external stimuli. This review presents current developments in the field of energy transfer, particularly the multi-step FRET processes in the self-assembled nanostructures of surfactants/polymers. The part one of this review presents a background and brief overview of soft systems and discusses certain aspects of the self-assemblies of surfactants/polymers and their co-solubilization property to bring fluorophores to close proximity to transduce energy. The second part of this review deals with single-step and multi-step FRET in the self-assemblies of surfactants/polymers and links FRET systems with advanced smart technologies including multicolor formation, data encryption, and artificial antenna systems. This review also discusses the diverse examples in the literature to present the emerging applications of FRET. Finally, the prospects regarding further improvement of FRET in self-assembled soft systems are outlined.

In situ formation of photoactive B-ring reduced chlorophyll isomer in photosynthetic protein LH2

Natural chlorophylls have a D-ring reduced chlorin π-system; however, no naturally occurring photosynthetically active B-ring reduced chlorins have been reported. Here we report a B-ring reduced chlorin, 17,18-didehydro-bacteriochlorophyll (BChl) a, produced by in situ oxidation of B800 bacteriochlorophyll (BChl) a in a light-harvesting protein LH2 from a purple photosynthetic bacterium Phaeospirillum molischianum. The regioselective oxidation of the B-ring of B800 BChl a is rationalized by its molecular orientation in the protein matrix. The formation of 17,18-didehydro-BChl a produced no change in the local structures and circular arrangement of the LH2 protein. The B-ring reduced 17,18-didehydro-BChl a functions as an energy donor in the LH2 protein. The photoactive B-ring reduced Chl isomer in LH2 will be helpful for understanding the photofunction and evolution of photosynthetic cyclic tetrapyrrole pigments.

Real-Time Path Integral Simulation of Exciton-Vibration Dynamics in Light-Harvesting Bacteriochlorophyll Aggregates

The mechanism of excitation energy transfer in photoexcited bacteriochlorophyll (BChl) aggregates poses intriguing questions, which have important implications for the observed efficiency of photosynthesis. We investigate this process through fully quantum mechanical calculations of exciton-vibration dynamics in chains and rings of BChl a molecules, with parameters characterizing the B850 ring of the LH2 complex of photosynthetic bacteria. The calculations are performed using the modular path integral methodology, which allows the exact treatment of 50 intramolecular vibrations on each pigment using parameters obtained from spectroscopic Huang-Rhys factors with computational effort that scales linearly with aggregate length. Our results indicate that the interplay between electronic and vibrational time scales leads to the rapid suppression but not the overdamping of electronic coherence, which facilitates the spreading of excitation energy throughout the aggregate.

Membrane Dynamics in Phototrophic Bacteria

Photosynthetic membranes are typically densely packed with proteins, and this is crucial for their function in efficient trapping of light energy. Despite being crowded with protein, the membranes are fluid systems in which proteins and smaller molecules can diffuse. Fluidity is also crucial for photosynthetic function, as it is essential for biogenesis, electron transport, and protein redistribution for functional regulation. All photosynthetic membranes seem to maintain a delicate balance between crowding, order, and fluidity. How does this work in phototrophic bacteria? In this review, we focus on two types of intensively studied bacterial photosynthetic membranes: the chromatophore membranes of purple bacteria and the thylakoid membranes of cyanobacteria. Both systems are distinct from the plasma membrane, and both have a distinctive protein composition that reflects their specialized roles. Chromatophores are formed from plasma membrane invaginations, while thylakoid membranes appear to be an independent intracellular membrane system. We discuss the techniques that can be applied to study the organization and dynamics of these membrane systems, including electron microscopy techniques, atomic force microscopy, and many variants of fluorescence microscopy. We go on to discuss the insights that havebeen acquired from these techniques, and the role of membrane dynamics in the physiology of photosynthetic membranes. Membrane dynamics on multiple timescales are crucial for membrane function, from electron transport on timescales of microseconds to milliseconds to regulation and biogenesis on timescales of minutes to hours. We emphasize the open questions that remain in the field.