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.

Choosing an Optimal Solvent Is Crucial for Obtaining Cell-Penetrating Peptide Nanoparticles with Desired Properties and High Activity in Nucleic Acid Delivery.

Cell-penetrating peptides (CPPs) are highly promising transfection agents that can deliver various compounds into living cells, including nucleic acids (NAs). Positively charged CPPs can form non-covalent complexes with negatively charged NAs, enabling simple and time-efficient nanoparticle preparation. However, as CPPs have substantially different chemical and physical properties, their complexation with the cargo and characteristics of the resulting nanoparticles largely depends on the properties of the surrounding environment, i.e., solution. Here, we show that the solvent used for the initial dissolving of a CPP determines the properties of the resulting CPP particles formed in an aqueous solution, including the activity and toxicity of the CPP-NA complexes. Using different biophysical methods such as dynamic light scattering (DLS), atomic force microscopy (AFM), transmission and scanning electron microscopy (TEM and SEM), we show that PepFect14 (PF14), a cationic amphipathic CPP, forms spherical particles of uniform size when dissolved in organic solvents, such as ethanol and DMSO. Water-dissolved PF14, however, tends to form micelles and non-uniform aggregates. When dissolved in organic solvents, PF14 retains its α-helical conformation and biological activity in cell culture conditions without any increase in cytotoxicity. Altogether, our results indicate that by using a solvent that matches the chemical nature of the CPP, the properties of the peptide-cargo particles can be tuned in the desired way. This can be of critical importance for in vivo applications, where CPP particles that are too large, non-uniform, or prone to aggregation may induce severe consequences.

Mitochondrial Irc3 helicase of the thermotolerant yeast Ogataea polymorpha displays dual DNA- and RNA-stimulated ATPase activity.

Irc3 is one of the six mitochondrial helicases described in Saccharomyces cerevisiae. Physiological functions of Irc3 are not completely understood as both DNA metabolic processes and mRNA translation have been suggested to be direct targets of the helicase. In vitro analysis of Irc3 has been hampered by the modest thermostability of the S. cerevisiae protein. Here, we purified a homologous helicase (Irc3op) of the thermotolerant yeast Ogataea polymorpha that retains structural integrity and catalytic activity at temperatures above 40 °C. Irc3op can complement the respiratory deficiency phenotype of a S. cerevisiae irc3Δ mutant, indicating conservation of biochemical functions. The ATPase activity of Irc3op is best stimulated by branched and double- stranded DNA cofactors. Single-stranded DNA binds Irc3op tightly but is a weak activator of the ATPase activity. We could also detect a lower level stimulation with RNA, especially with molecules possessing a compact three-dimensional structure. These results support the idea that that Irc3 might have dual specificity and remodel both DNA and RNA molecules in vivo. Furthermore, our analysis of kinetic parameters predicts that Irc3 could have a regulatory function via sensing changes of the mitochondrial ATP pool or respond to the accumulation of single-stranded DNA.

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.

Asymmetry in the Q y Fluorescence and Absorption Spectra of Chlorophyll a Pertaining to Exciton Dynamics.

Significant asymmetry found between the high-resolution Q y emission and absorption spectra of chlorophyll-a is herein explained, providing basic information needed to understand photosynthetic exciton transport and photochemical reactions. The Q y spectral asymmetry in chlorophyll has previously been masked by interference in absorption from the nearby Q x transition, but this effect has recently been removed using extensive quantum spectral simulations or else by analytical inversion of absorption and magnetic circular dichroism data, allowing high-resolution absorption information to be accurately determined from fluorescence-excitation spectra. To compliment this, here, we measure and thoroughly analyze the high-resolution differential fluorescence line narrowing spectra of chlorophyll-a in trimethylamine and in 1-propanol. The results show that vibrational frequencies often change little between absorption and emission, yet large changes in line intensities are found, this effect also being strongly solvent dependent. Among other effects, the analysis in terms of four basic patterns of Duschinsky-rotation matrix elements, obtained using CAM-B3LYP calculations, predicts that a chlorophyll-a molecule excited into a specific vibrational level, may, without phase loss or energy relaxation, reemit the light over a spectral bandwidth exceeding 1,000 cm-1 (0.13 eV) to influence exciton-transport dynamics.

The two light-harvesting membrane chromoproteins of Thermochromatium tepidum expose distinct robustness against temperature and pressure.

An increased robustness against high temperature and the much red-shifted near-infrared absorption spectrum of excitons in the LH1-RC core pigment-protein complex from the thermophilic photosynthetic purple sulfur bacterium Thermochromatium tepidum has recently attracted much interest. In the present work, thermal and hydrostatic pressure stability of the peripheral LH2 and core LH1-RC complexes from this bacterium were in parallel investigated by various optical spectroscopy techniques applied over a wide spectral range from far-ultraviolet to near-infrared. In contrast to expectations, very distinct robustness of the complexes was established, while the sturdiness of LH2 surpassed that of LH1-RC both with respect to temperatures between 288 and 360 K, and pressures between 1 bar and 14 kbar. Subtle structural variances related to the hydrogen bond network are likely responsible for the extra stability of LH2.

Absorption-emission symmetry breaking and the different origins of vibrational structures of the 1Qy and 1Qx electronic transitions of pheophytin a.

The vibrational structure of the optical absorption and fluorescence spectra of the two lowest-energy singlet electronic states (Qy and Qx) of pheophytin a were carefully studied by combining low-resolution and high-resolution spectroscopy with quantum chemical analysis and spectral modeling. Large asymmetry was revealed between the vibrational structures of the Qy absorption and fluorescence spectra, integrally characterized by the total Huang-Rhys factor and reorganization energy in absorption of Svib A = 0.43 ± 0.06, λA = 395 cm-1 and in emission of Svib E = 0.35 ± 0.06, λE = 317 cm-1. Time-dependent density-functional theory using the CAM-B3LYP, ωB97XD, and MN15 functionals could predict and interpret this asymmetry, with the exception of one vibrational mode per model, which was badly misrepresented in predicted absorption spectra; for CAM-B3LYP and ωB97XD, this mode was a Kekulé-type mode depicting aromaticity. Other computational methods were also considered but performed very poorly. The Qx absorption spectrum is broad and could not be interpreted in terms of a single set of Huang-Rhys factors depicting Franck-Condon allowed absorption, with Herzberg-Teller contributions to the intensity being critical. For it, CAM-B3LYP calculations predict that Svib A (for modes >100 cm-1) = 0.87 and λA = 780 cm-1, with effective x and y polarized Herzberg-Teller reorganization energies of 460 cm-1 and 210 cm-1, respectively, delivering 15% y-polarized intensity. However, no method was found to quantitatively determine the observed y-polarized contribution, with contributions of up to 50% being feasible.

Higher Order Vibronic Sidebands of Chlorophyll a and Bacteriochlorophyll a for Enhanced Excitation Energy Transfer and Light Harvesting.

Optical absorption and fluorescence spectra of molecules in condensed phases often show extensive sidebands. Originating from electron-vibrational and electron-phonon couplings, these spectral tails bear important information on the dynamics of electronic states and processes the molecules are involved in. The vibronic sidebands observed in conjugate Qy absorption and fluorescence spectra of chlorophyll a and bacteriochlorophyll a are relatively weak, characterized by the total Huang-Rhys factor which is less than one. Therefore, it is widely considered that only fundamental intramolecular modes are responsible for their formation. Here, we provide evidence for extra-long and structured fluorescence tails of chlorophyll a and bacteriochlorophyll a as far as 4000 cm-1 from respective spectral origins, far beyond the frequency range of fundamental modes. According to quantum chemical simulations, these sidebands extending to ∼960 nm in chlorophyll a and ∼1140 nm in bacteriochlorophyll a into the infrared part of the optical spectrum are mainly contributed to by vibrational overtones of the fundamental modes. Because energy transfer and relaxation processes generally depend on vibronic overlap integrals, these findings potentially contribute to better understanding of many vital photo-induced phenomena, including photosynthetic light harvesting.

Controlling Photosynthetic Excitons by Selective Pigment Photooxidation.

As a basis of photosynthesis, photoinduced oxidation of (bacterio)chlorophyll molecules in the special reaction center complexes has been a subject of extensive research. In contrast, the generally harmful photooxidation of antenna chromoproteins has received much less attention. Here, we have established the permanent structural changes in the LH2 antenna bacteriochlorophyll-protein complex from a sulfur photosynthetic purple bacterium Ectothiorhodospira haloalkaliphila taking place at physiological conditions upon intense optical irradiation. To this end, a crystal structure of the LH2 complex from E. haloalkaliphila was first resolved by X-ray diffraction to 3.7 Å, verifying a great similarity with the earlier structure from Phaesporillum molischianum. Analysis of the various steady-state and picosecond time-resolved optical spectroscopy data and related model simulations then confirmed that the major spectral effects observed-bleaching and blue-shifting of the B850 exciton band and correlated emergence of a higher-energy C700 exciton band-are associated with photooxidation of increasing numbers of B850 bacteriochlorophylls into 3-acetyl-chlorophylls, with no noticeable damage to the pigment-binding protein scaffold. A prospective noninvasive method for an in situ optical control of excitons by selective photooxidation of pigment chromophores was thus revealed and demonstrated in a structurally well-defined native system.

Evaluation of Electron-Phonon Coupling and Spectral Densities of Pigment-Protein Complexes by Line-Narrowed Optical Spectroscopy.

Spectral hole burning (SHB) and difference fluorescence line narrowing (ΔFLN) are routinely used for investigations of electron-phonon coupling in photosynthetic pigment-protein complexes as well as in other amorphous systems at cryogenic temperatures. Nevertheless, the Huang-Rhys factors S, an integral measure of electron-phonon coupling strength, and the phonon spectral densities obtained by SHB and ΔFLN over the past years have differed significantly in the case of certain photosynthetic pigment-protein complexes. In this work, the specific properties of both types of line-narrowing spectroscopic techniques that may lead to these discrepancies are critically analyzed by a combined experimental and computational approach, using the CP29 antenna complex of green plants as a suitable model system. We confirm that only ΔFLN at low fluence, by providing access to the homogeneously broadened spectrum, is able to deliver correct S values, while SHB may significantly under- or overestimate them, depending on the burn fluence. We also discuss possible other sources of discrepancies in the literature data, e.g., in the case of LHC II aggregates and correct numerical errors found in some previous records.

High temperature limit of photosynthetic excitons.

Excitons in light-harvesting complexes are known to significantly improve solar-energy harnessing. Here we demonstrate photosynthetic excitons at super-physiological temperatures reaching 60-80 °C in different species of mesophilic photosynthetic bacteria. It is shown that the survival of light-harvesting excitons in the peripheral LH2 antennae is restricted by thermal decomposition of the pigment-protein complex rather than by any intrinsic property of excitons. The regular spatial organization of the bacteriochlorophyll a pigments supporting excitons in this complex is lost upon the temperature-induced breakdown of its tertiary structure. Secondary structures of the complexes survive even higher temperatures. The discovered pivotal role of the protein scaffold in the stabilization of excitons comprises an important aspect of structure-function relationship in biology. These results also intimately entangle the fundamental issues of quantum mechanical concepts in biology and in the folding of proteins.

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.

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

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

In situ high-resolution structure of the baseplate antenna complex in Chlorobaculum tepidum.

Photosynthetic antenna systems enable organisms harvesting light and transfer the energy to the photosynthetic reaction centre, where the conversion to chemical energy takes place. One of the most complex antenna systems, the chlorosome, found in the photosynthetic green sulfur bacterium Chlorobaculum (Cba.) tepidum contains a baseplate, which is a scaffolding super-structure, formed by the protein CsmA and bacteriochlorophyll a. Here we present the first high-resolution structure of the CsmA baseplate using intact fully functional, light-harvesting organelles from Cba. tepidum, following a hybrid approach combining five complementary methods: solid-state NMR spectroscopy, cryo-electron microscopy, isotropic and anisotropic circular dichroism and linear dichroism. The structure calculation was facilitated through development of new software, GASyCS for efficient geometry optimization of highly symmetric oligomeric structures. We show that the baseplate is composed of rods of repeated dimers of the strongly amphipathic CsmA with pigments sandwiched within the dimer at the hydrophobic side of the helix.

Spectral and kinetic effects accompanying the assembly of core complexes of Rhodobacter sphaeroides.

In the present work, spectral and kinetic changes accompanying the assembly of the light-harvesting 1 (LH1) complex with the reaction center (RC) complex into monomeric RC-LH1 and dimeric RC-LH1-PufX core complexes of the photosynthetic purple bacterium Rhodobacter sphaeroides are systematically studied over the temperature range of 4.5-300K. The samples were interrogated with a combination of optical absorption, hole burning, fluorescence excitation, steady state and picosecond time resolved fluorescence spectroscopy. Fair additivity of the LH1 and RC absorption spectra suggests rather weak electronic coupling between them. A low-energy tail revealed at cryogenic temperatures in the absorption spectra of both monomeric and dimeric core complexes is proved to be due to the special pair of the RC. At selected excitation intensity and temperature, the fluorescence decay time of core complexes is shown to be a function of multiple factors, most importantly of the presence/absence of RCs, the supramolecular architecture (monomeric or dimeric) of the complexes, and whether the complexes were studied in a native membrane environment or in a detergent - purified state.

Challenges facing an understanding of the nature of low-energy excited states in photosynthesis.

While the majority of the photochemical states and pathways related to the biological capture of solar energy are now well understood and provide paradigms for artificial device design, additional low-energy states have been discovered in many systems with obscure origins and significance. However, as low-energy states are naively expected to be critical to function, these observations pose important challenges. A review of known properties of low energy states covering eight photochemical systems, and options for their interpretation, are presented. A concerted experimental and theoretical research strategy is suggested and outlined, this being aimed at providing a fully comprehensive understanding.

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.

A conserved π-cation and an electrostatic bridge are essential for 11R-lipoxygenase catalysis and structural stability.

Lipoxygenases (LOXs) are lipid-peroxidizing enzymes that consist of a regulatory calcium- and membrane-binding PLAT (polycystin-1, lipoxygenase, α-toxin) domain and a catalytic domain. In a previous study, the crystal structure of an 11R-LOX revealed a conserved π-cation bridge connecting these two domains which could mediate the regulatory effect of the PLAT domain to the active site. Here we analyzed the role of residues Trp107 and Lys172 that constitute the π-cation bridge in 11R-LOX along with Arg106 and Asp173-a potential salt bridge, which could also contribute to the inter-domain communication. According to our kinetic assays and protein unfolding experiments conducted using differential scanning fluorimetry and circular dichroism spectroscopy, mutants with a disrupted link display diminished catalytic activity alongside reduced stability of the protein fold. The results demonstrate that both these bridges contribute to the two-domain interface, and are important for proper enzyme activation.

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

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