Fluorescence of the various red antenna states in photosystem I complexes from cyanobacteria is affected differently by the redox state of P700

Presence of low-energy chlorophylls d in photosystem I trimer and monomer cores isolated from Acaryochloris sp. NBRC 102871

Acaryochloris species belong to a special category of cyanobacteria possessing chlorophyll (Chl) d. One of the photosynthetic characteristics of Acaryochloris marina MBIC11017 is that the absorption spectra of photosystem I (PSI) showed almost no bands and shoulders of low-energy Chls d over 740 nm. In contrast, the absorption spectra of other Acaryochloris species showed a shoulder around 740 nm, suggesting that low-energy Chls d within PSI are diversified among Acaryochloris species. In this study, we purified PSI trimer and monomer cores from Acaryochloris sp. NBRC 102871 and examined their protein and pigment compositions and spectral properties. The protein bands and pigment compositions of the PSI trimer and monomer of NBRC102871 were virtually identical to those of MBIC11017. The absorption spectra of the NBRC102871 PSIs exhibited a shoulder around 740 nm, whereas the fluorescence spectra of PSI trimer and monomer displayed maximum peaks at 754 and 767 nm, respectively. These spectral properties were different from those of MBIC11017, indicating the presence of low-energy Chls d within the NBRC102871 PSIs. Moreover, we analyzed the NBRC102871 genome to identify amino acid sequences of PSI proteins and compared them with those of the A. marina MBIC11017 and MBIC10699 strains whose genomes are available. The results showed that some of the sequences in NBRC102871 were distinct from those in MBIC11017 and MBIC10699. These findings provide insights into the variety of low-energy Chls d with respect to the protein environments of PSI cores among the three Acaryochloris strains.

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

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

Isolation and characterization of trimeric and monomeric PSI cores from Acaryochloris marina MBIC11017

Photosystem I (PSI) catalyzes light-induced electron-transfer reactions and has been observed to exhibit various oligomeric states and different energy levels of chlorophylls (Chls) in response to oligomerization. However, the biochemical and spectroscopic properties of a PSI monomer containing Chls d are not well understood. In this study, we successfully isolated and characterized PSI monomers from the cyanobacterium Acaryochloris marina MBIC11017, and compared their properties with those of the A. marina PSI trimer. The PSI trimers and monomers were prepared using trehalose density gradient centrifugation after anion-exchange and hydrophobic interaction chromatography. The polypeptide composition of the PSI monomer was found to be consistent with that of the PSI trimer. The absorption spectrum of the PSI monomer showed the Qy band of Chl d at 704 nm, which was blue-shifted from the peak at 707 nm observed in the PSI-trimer spectrum. The fluorescence-emission spectrum of the PSI monomer measured at 77 K exhibited a peak at 730 nm without a broad shoulder in the range of 745-780 nm, which was clearly observed in the PSI-trimer spectrum. These spectroscopic properties of the A. marina PSI trimer and monomer suggest different formations of low-energy Chls d between the two types of PSI cores. Based on these findings, we discuss the location of low-energy Chls d in A. marina PSIs.

Mass spectrometry and spectroscopic characterization of a tetrameric photosystem I supercomplex from Leptolyngbya ohadii, a desiccation-tolerant cyanobacterium

Cyanobacteria inhabiting desert biological soil crusts face the harsh conditions of the desert. They evolved a suite of strategies toward desiccation-hydration cycles mixed with high light irradiations, etc. In this study we purified and characterized the structure and function of Photosystem I (PSI) from Leptolyngbya ohadii, a desiccation-tolerant desert cyanobacterium. We discovered that PSI forms tetrameric (PSI-Tet) aggregate. We investigated it by using sucrose density gradient centrifugation, clear native PAGE, high performance liquid chromatography, mass spectrometry (MS), time-resolved fluorescence (TRF) and time-resolved transient absorption (TA) spectroscopy. MS analysis identified the presence of two PsaB and two PsaL proteins in PSI-Tet and uniquely revealed that PsaLs are N-terminally acetylated in contrast to non-modified PsaL in the trimeric PSI from Synechocystis sp. PCC 6803. Chlorophyll (Chl) a fluorescence decay profiles of the PSI-Tet performed at 77 K revealed two emission bands at ∼690 nm and 725 nm with the former appearing only at early delay time. The main fluorescence emission peak, associated with emission from the low energy Chls a, decays within a few nanoseconds. TA studies demonstrated that the 725 nm emission band is associated with low energy Chls a with absorption band clearly resolved at ∼710 nm at 77 K. In summary, our work suggests that the heterogenous composition of PsaBs and PsaL in PSI-Tet is related with the adaptation mechanisms needed to cope with stressful conditions under which this bacterium naturally grows.

Structural basis for the absence of low-energy chlorophylls in a photosystem I trimer from Gloeobacter violaceus

Photosystem I (PSI) is a multi-subunit pigment-protein complex that functions in light-harvesting and photochemical charge-separation reactions, followed by reduction of NADP to NADPH required for CO₂ fixation in photosynthetic organisms. PSI from different photosynthetic organisms has a variety of chlorophylls (Chls), some of which are at lower-energy levels than its reaction center P700, a special pair of Chls, and are called low-energy Chls. However, the sites of low-energy Chls are still under debate. Here, we solved a 2.04-Å resolution structure of a PSI trimer by cryo-electron microscopy from a primordial cyanobacterium Gloeobacter violaceus PCC 7421, which has no low-energy Chls. The structure shows the absence of some subunits commonly found in other cyanobacteria, confirming the primordial nature of this cyanobacterium. Comparison with the known structures of PSI from other cyanobacteria and eukaryotic organisms reveals that one dimeric and one trimeric Chls are lacking in the Gloeobacter PSI. The dimeric and trimeric Chls are named Low1 and Low2, respectively. Low2 is missing in some cyanobacterial and eukaryotic PSIs, whereas Low1 is absent only in Gloeobacter. These findings provide insights into not only the identity of low-energy Chls in PSI, but also the evolutionary changes of low-energy Chls in oxyphototrophs.

Recent advances in single-molecule spectroscopy studies on light-harvesting processes in oxygenic photosynthesis

Photosynthetic light-harvesting complexes (LHCs) play a crucial role in concentrating the photon energy from the sun that otherwise excites a typical pigment molecule, such as chlorophyll-a, only several times a second. Densely packed pigments in the complexes ensure efficient energy transfer to the reaction center. At the same time, LHCs have the ability to switch to an energy-quenching state and thus play a photoprotective role under excessive light conditions. Photoprotection is especially important for oxygenic photosynthetic organisms because toxic reactive oxygen species can be generated through photochemistry under aerobic conditions. Because of the extreme complexity of the systems in which various types of pigment molecules strongly interact with each other and with the surrounding protein matrixes, there has been long-standing difficulty in understanding the molecular mechanisms underlying the flexible switching between the light-harvesting and quenching states. Single-molecule spectroscopy studies are suitable to reveal the conformational dynamics of LHCs reflected in the fluorescence properties that are obscured in ordinary ensemble measurements. Recent advanced single-molecule spectroscopy studies have revealed the dynamical fluctuations of LHCs in their fluorescence peak position, intensity, and lifetime. The observed dynamics seem relevant to the conformational plasticity required for the flexible activations of photoprotective energy quenching. In this review, we survey recent advances in the single-molecule spectroscopy study of the light-harvesting systems of oxygenic photosynthesis.

The structure of photosystem I from a high-light-tolerant cyanobacteria

Photosynthetic organisms have adapted to survive a myriad of extreme environments from the earth's deserts to its poles, yet the proteins that carry out the light reactions of photosynthesis are highly conserved from the cyanobacteria to modern day crops. To investigate adaptations of the photosynthetic machinery in cyanobacteria to excessive light stress, we isolated a new strain of cyanobacteria, Cyanobacterium aponinum 0216, from the extreme light environment of the Sonoran Desert. Here we report the biochemical characterization and the 2.7 Å resolution structure of trimeric photosystem I from this high-light-tolerant cyanobacterium. The structure shows a new conformation of the PsaL C-terminus that supports trimer formation of cyanobacterial photosystem I. The spectroscopic analysis of this photosystem I revealed a decrease in far-red absorption, which is attributed to a decrease in the number of long- wavelength chlorophylls. Using these findings, we constructed two chimeric PSIs in Synechocystis sp. PCC 6803 demonstrating how unique structural features in photosynthetic complexes can change spectroscopic properties, allowing organisms to thrive under different environmental stresses.

The role of vibronic modes in formation of red antenna states of cyanobacterial PSI

Cyanobacterial photosystem I (PSI) constitutes monomeric and trimeric pigment-protein complexes whose optical properties are marked by the presence of long-wavelength absorption bands. In spite of numerous experimental studies, the nature of these bands is still under debate and requires intensive theoretical analysis. Collecting together the data of linear spectroscopy and single-molecule spectroscopy (SMS) of PSI from Arthrospira platensis, we performed quantum modeling of the optical response based on molecular exciton theory (ET) and the multimode Brownian oscillator model (MBOM). Applying MBOM, the spectra of the red antenna state were calculated considering a particular for each red state adjustment of the low-frequency vibronic modes. Within the framework of our PSI exciton model it was shown that the coupling energy between antenna chlorophylls cannot be a factor of the red states formation, thus the long-wavelength bands are calculated without attribution to so-called antenna red chlorophylls. By the fitting of Huang-Rhys factors and frequencies for the lowest vibronic modes, we were able to reproduce the effects of strong and weak electron-phonon coupling experimentally observed in SMS spectra of red antenna states. Based on our theoretical calculations and also analysis of existing crystal structures of cyanobacterial PSI, we assumed that long-wavelength Chls can be localized in the peripheral protein subunits containing one or two pigment molecules.

The structure of a red-shifted photosystem I reveals a red site in the core antenna

Photosystem I coordinates more than 90 chlorophylls in its core antenna while achieving near perfect quantum efficiency. Low energy chlorophylls (also known as red chlorophylls) residing in the antenna are important for energy transfer dynamics and yield, however, their precise location remained elusive. Here, we construct a chimeric Photosystem I complex in Synechocystis PCC 6803 that shows enhanced absorption in the red spectral region. We combine Cryo-EM and spectroscopy to determine the structure⁻function relationship in this red-shifted Photosystem I complex. Determining the structure of this complex reveals the precise architecture of the low energy site as well as large scale structural heterogeneity which is probably universal to all trimeric Photosystem I complexes. Identifying the structural elements that constitute red sites can expand the absorption spectrum of oxygenic photosynthetic and potentially modulate light harvesting efficiency.

Cyanobacterial photosystem I has a highly conserved core antenna consisting of eleven subunits and more than 90 chlorophylls. Here via CryoEM and spectroscopy, the authors determine the location of a red-shifted low-energy chlorophyll that allows harvesting of longer wavelengths of light.

Evidence that chlorophyll f functions solely as an antenna pigment in far-red-light photosystem I from Fischerella thermalis PCC 7521

The Photosystem I (PSI) reaction center in cyanobacteria is comprised of ~96 chlorophyll (Chl) molecules, including six specialized Chl molecules denoted Chl1A/Chl1B (P₇₀₀), Chl2A/Chl2B, and Chl3A/Chl3B that are arranged in two branches and function in primary charge separation. It has recently been proposed that PSI from Chroococcidiopsis thermalis (Nürnberg et al. (2018) Science 360, 1210-1213) and Fischerella thermalis PCC 7521 (Hastings et al. (2019) Biochim. Biophys. Acta 1860, 452-460) contain Chl f in the positions Chl2A/Chl2B. We tested this proposal by exciting RCs from white-light grown (WL-PSI) and far-red light grown (FRL-PSI) F. thermalis PCC 7521 with femtosecond pulses and analyzing the optical dynamics. If Chl f were in the position Chl2A/Chl2B in FRL-PSI, excitation at 740 nm should have produced the charge-separated state P₇₀₀⁺A₀^- followed by electron transfer to A₁ with a τ of ≤25 ps. Instead, it takes ~230 ps for the charge-separated state to develop because the excitation migrates uphill from Chl f in the antenna to the trapping center. Further, we observe a strong electrochromic shift at 685 nm in the final P₇₀₀⁺A₁^- spectrum that can only be explained if Chl a is in the positions Chl2A/Chl2B. Similar arguments rule out the presence of Chl f in the positions Chl3A/Chl3B; hence, Chl f is likely to function solely as an antenna pigment in FRL-PSI. We additionally report the presence of an excitonically coupled homo- or heterodimer of Chl f absorbing around 790 nm that is kinetically independent of the Chl f population that absorbs around 740 nm.

Formation of a PSI-PSII megacomplex containing LHCSR and PsbS in the moss Physcomitrella patens

Mosses are one of the earliest land plants that diverged from fresh-water green algae. They are considered to have acquired a higher capacity for thermal energy dissipation to cope with dynamically changing solar irradiance by utilizing both the "algal-type" light-harvesting complex stress-related (LHCSR)-dependent and the "plant-type" PsbS-dependent mechanisms. It is hypothesized that the formation of photosystem (PS) I and II megacomplex is another mechanism to protect photosynthetic machinery from strong irradiance. Herein, we describe the analysis of the PSI-PSII megacomplex from the model moss, Physcomitrella patens, which was resolved using large-pore clear-native polyacrylamide gel electrophoresis (lpCN-PAGE). The similarity in the migration distance of the Physcomitrella PSI-PSII megacomplex to the Arabidopsis megacomplex shown during lpCN-PAGE suggested that the Physcomitrella PSI-PSII and Arabidopsis megacomplexes have similar structures. Time-resolved chlorophyll fluorescence measurements show that excitation energy was rapidly and efficiently transferred from PSII to PSI, providing evidence of an ordered association of the two photosystems. We also found that LHCSR and PsbS co-migrated with the Physcomitrella PSI-PSII megacomplex. The megacomplex showed pH-dependent chlorophyll fluorescence quenching, which may have been induced by LHCSR and/or PsbS proteins with the collaboration of zeaxanthin. We discuss the mechanism that regulates the energy distribution balance between two photosystems in Physcomitrella.

The PSI-PSII Megacomplex in Green Plants

Energy dissipation is crucial for land and shallow-water plants exposed to direct sunlight. Almost all green plants dissipate excess excitation energy to protect the photosystem reaction centers, photosystem II (PSII) and photosystem I (PSI), and continue to grow under strong light. In our previous work, we reported that about half of the photosystem reaction centers form a PSI-PSII megacomplex in Arabidopsis thaliana, and that the excess energy was transferred from PSII to PSI fast. However, the physiological function and structure of the megacomplex remained unclear. Here, we suggest that high-light adaptable sun-plants accumulate the PSI-PSII megacomplex more than shade-plants. In addition, PSI of sun-plants has a deep trap to receive excitation energy, which is low-energy chlorophylls showing fluorescence maxima longer than 730 nm. This deep trap may increase the high-light tolerance of PSI by improving excitation energy dissipation. Electron micrographs suggest that one PSII dimer is directly sandwiched between two PSIs with 2-fold rotational symmetry in the basic form of the PSI-PSII megacomplex in green plants. This structure should enable fast energy transfer from PSII to PSI and allow energy in PSII to be dissipated via the deep trap in PSI.

Adaptation of light-harvesting functions of unicellular green algae to different light qualities

Oxygenic photosynthetic organisms perform photosynthesis efficiently by distributing captured light energy to photosystems (PSs) at an appropriate balance. Maintaining photosynthetic efficiency under changing light conditions requires modification of light-harvesting and energy-transfer processes. In the current study, we examined how green algae regulate their light-harvesting functions in response to different light qualities. We measured low-temperature time-resolved fluorescence spectra of unicellular green algae Chlamydomonas reinhardtii and Chlorella variabilis cells grown under different light qualities. By observing the delayed fluorescence spectra, we demonstrated that both types of green algae primarily modified the associations between light-harvesting chlorophyll protein complexes (LHCs) and PSs (PSII and PSI). Under blue light, Chlamydomonas transferred more energy from LHC to chlorophyll (Chl) located far from the PSII reaction center, while energy was transferred from LHC to PSI via different energy-transfer pathways in Chlorella. Under green light, both green algae exhibited enhanced energy transfer from LHCs to both PSs. Red light induced fluorescence quenching within PSs in Chlamydomonas and LHCs in Chlorella. In Chlorella, energy transfer from PSII to PSI appears to play an important role in balancing excitation between PSII and PSI.

Single Molecule Fluorescence Spectroscopy of PSI Trimers from Arthrospira platensis: A Computational Approach

Based on single molecule spectroscopy analysis and our preliminary theoretical studies, the linear and fluorescence spectra of the PSI trimer from Arthrospira platensis with different realizations of the static disorder were modeled at cryogenic temperature. Considering the previously calculated spectral density of chlorophyll, an exciton model for the PSI monomer and trimer including the red antenna states was developed taking into account the supposed similarity of PSI antenna structures from Thermosynechococcus e., Synechocystis sp. PCC6803, and Arthrospira platensis. The red Chls in the PSI monomer were assumed to be in the nearest proximity of the reaction center. The PSI trimer model allowed the simulation of experimentally measured zero phonon line distribution of the red states considering a weak electron-phonon coupling for the antenna exciton states. However, the broad absorption and fluorescence spectra of an individual emitter at 760 nm were calculated by adjusting the Huang-Rhys factors of the chlorophyll lower phonon modes assuming strong electron-phonon coupling.

Efficiency of excitation energy trapping in the green photosynthetic bacterium Chlorobaculum tepidum

During the millions of years of evolution, photosynthetic organisms have adapted to almost all terrestrial and aquatic habitats, although some environments are obviously more suitable for photosynthesis than others. Photosynthetic organisms living in low-light conditions require on the one hand a large light-harvesting apparatus to absorb as many photons as possible. On the other hand, the excitation trapping time scales with the size of the light-harvesting system, and the longer the distance over which the formed excitations have to be transferred, the larger the probability to lose excitations. Therefore a compromise between photon capture efficiency and excitation trapping efficiency needs to be found. Here we report results on the whole cells of the green sulfur bacterium Chlorobaculum tepidum. Its efficiency of excitation energy transfer and charge separation enables the organism to live in environments with very low illumination. Using fluorescence measurements with picosecond resolution, we estimate that despite a rather large size and complex composition of its light-harvesting apparatus, the quantum efficiency of its photochemistry is around ~87% at 20 °C, ~83% at 45 °C, and about ~81% at 77 K when part of the excitation energy is trapped by low-energy bacteriochlorophyll a molecules. The data are evaluated using target analysis, which provides further insight into the functional organization of the low-light adapted photosynthetic apparatus.

Redox-state dependent blinking of single photosystem I trimers at around liquid-nitrogen temperature

Efficient light harvesting in a photosynthetic antenna system is disturbed by a ragged and fluctuating energy landscape of the antenna pigments in response to the conformation dynamics of the protein. This situation is especially pronounced in Photosystem I (PSI) containing red shifted chlorophylls (red Chls) with the excitation energy much lower than the primary donor. The present study was conducted to clarify light-harvesting dynamics of PSI isolated from Synechocystis sp. PCC6803 by using single-molecule spectroscopy at liquid‑nitrogen temperatures. Fluorescence emission at around 720 nm from the red Chls in single PSI trimers was monitored at 80-100 K. Intermittent variations in the emission intensities, so-called blinking, were frequently observed. Its time scale lay in several tens of seconds. The blinking amplitude depended on the redox state of the phylloquinone (A₁). Electrochromic shifts of Chls induced by the negative charge on A₁ were calculated based on the X-ray crystallographic structure. A Chl molecule, Chl-A839 (numbering according to PDB 5OY0), bound near A₁ was found to have a large electrochromic shift. This Chl has strong exciton coupling with neighboring Chl (A838) whose site energy was predicted to be determined by interaction with an arginine residue (ArgF84) [Adolphs et al., 2010]. A possible scenario of the blinking was proposed. Conformational fluctuations of ArgF84 seesaw the excitation-energy of Chl-A838, which perturbs the branching ratio of excitation-energy between the red Chl and the cationic form of P700 as a quencher. The electrochromic shift of Chl-A839 enhances the effect of the conformation dynamics of ArgF84.

Deficiency of the Stroma-Lamellar Protein LIL8/PSB33 Affects Energy Transfer Around PSI in Arabidopsis

Light-harvesting-like (LIL) proteins are a group of proteins that share a consensus amino acid sequence with light-harvesting Chl-binding (LHC) proteins. We hypothesized that they might be involved in photosynthesis-related processes. In order to gain a better understanding of a potential role in photosynthesis-related processes, we examined the most recently identified LIL protein, LIL8/PSB33. Recently, it was suggested that this protein is an auxiliary PSII core protein which is involved in organization of the PSII supercomplex. However, we found that the majority of LIL8/PSB33 was localized in stroma lamellae, where PSI is predominant. Moreover, the PSI antenna sizes measured under visible light were slightly increased in the lil8 mutants which lack LIL8/PSB33 protein. Analysis of fluorescence decay kinetics and fluorescence decay-associated spectra indicated that energy transfer to quenching sites within PSI was partially hampered in these mutants. On the other hand, analysis of the steady-state fluorescence spectra in these mutants indicates that a population of LHCII is energetically disconnected from PSII. Taken together, we suggest that LIL8/PSB33 is involved in the fine-tuning of light harvesting and/or energy transfer around both photosystems.

Remembering Navasard V. Karapetyan (1936-2015)

Navasard Vaganovich Karapetyan (September 6, 1936-March 6, 2015) began his scientific career at the Bach Institute of Biochemistry of the Russian Academy of Sciences, Moscow, and was associated with this institute for over 56 years. He worked in the area of biochemistry and biophysics of photosynthesis and was especially known for his studies on chlorophyll a fluorescence in higher plants and cyanobacteria, molecular organization of Photosystem I, photoprotective energy dissipation, and dynamics of energy migration in the two photosystems. We present here a brief biography and comments on the work of Navasard Karapetyan. We remember him as an enthusiastic person who had an unflagging curiosity, energy and profound sincere interest in many aspects of photosynthesis research.

Temperature dependence of metal-enhanced fluorescence of photosystem I from Thermosynechococcus elongatus

We report the temperature dependence of metal-enhanced fluorescence (MEF) of individual photosystem I (PSI) complexes from Thermosynechococcus elongatus (T. elongatus) coupled to gold nanoparticles (AuNPs). A strong temperature dependence of shape and intensity of the emission spectra is observed when PSI is coupled to AuNPs. For each temperature, the enhancement factor (EF) is calculated by comparing the intensity of individual AuNP-coupled PSI to the mean intensity of 'uncoupled' PSI. At cryogenic temperature (1.6 K) the average EF was 4.3-fold. Upon increasing the temperature to 250 K the EF increases to 84-fold. Single complexes show even higher EFs up to 441.0-fold. At increasing temperatures the different spectral pools of PSI from T. elongatus become distinguishable. These pools are affected differently by the plasmonic interactions and show different enhancements. The remarkable increase of the EFs is explained by a rate model including the temperature dependence of the fluorescence yield of PSI and the spectral overlap between absorption and emission spectra of AuNPs and PSI, respectively.

Femtosecond Visible Transient Absorption Spectroscopy of Chlorophyll f-Containing Photosystem I

Photosystem I (PSI) from Chroococcidiopsis thermalis PCC 7203 grown under far-red light (FRL; >725 nm) contains both chlorophyll a and a small proportion of chlorophyll f. Here, we investigated excitation energy transfer and charge separation using this FRL-grown form of PSI (FRL-PSI). We compared femtosecond transient visible absorption changes of normal, white-light (WL)-grown PSI (WL-PSI) with those of FRL-PSI using excitation at 670 nm, 700 nm, and (in the case of FRL-PSI) 740 nm. The possibility that chlorophyll f participates in energy transfer or charge separation is discussed on the basis of spectral assignments. With selective pumping of chlorophyll f at 740 nm, we observe a final ∼150 ps decay assigned to trapping by charge separation, and the amplitude of the resulting P700^+•A₁^-• charge-separated state indicates that the yield is directly comparable to that of WL-PSI. The kinetics shows a rapid 2 ps time constant for almost complete transfer to chlorophyll f if chlorophyll a is pumped with a wavelength of 670 nm or 700 nm. Although the physical role of chlorophyll f is best supported as a low-energy radiative trap, the physical location should be close to or potentially within the charge-separating pigments to allow efficient transfer for charge separation on the 150 ps timescale. Target models can be developed that include a branching in the formation of the charge separation for either WL-PSI or FRL-PSI.

In vitro kinetics of P700+ reduction of Thermosynechococcus elongatus trimeric Photosystem I complexes by recombinant cytochrome c 6 using a Joliot-type LED spectrophotometer

The reduction rate of photo-oxidized Photosystem I (PSI) with various natural and artificial electron donors have been well studied by transient absorption spectroscopy. The electron transfer rate from various donors to P₇₀₀⁺ has been measured for a wide range of photosynthetic organisms encompassing cyanobacteria, algae, and plants. PSI can be a limiting component due to tedious extraction and purification methods required for this membrane protein. In this report, we have determined the in vivo, intracellular cytochrome c ₆ (cyt c ₆)/PSI ratio in Thermosynechococcus elongatus (T.e.) using quantitative Western blot analysis. This information permitted the determination of P₇₀₀⁺ reduction kinetics via recombinant cyt c ₆ in a physiologically relevant ratio (cyt c ₆: PSI) with a Joliot-type, LED-driven, pump-probe spectrophotometer. Dilute PSI samples were tested under varying cyt c ₆ concentration, temperature, pH, and ionic strength, each of which shows similar trends to the reported literature utilizing much higher PSI concentrations with laser-based spectrophotometer. Our results do however indicate kinetic differences between actinic light sources (laser vs. LED), and we have attempted to resolve these effects by varying our LED light intensity and duration. The standardized configuration of this spectrophotometer will also allow a more uniform kinetic analysis of samples in different laboratories. We can conclude that our findings from the LED-based system display an added total protein concentration effect due to multiple turnover events of P₇₀₀⁺ reduction by cyt c ₆ during the longer illumination regime.

Revealing the radiative and non-radiative relaxation rates of the fluorescent dye Atto488 in a λ/2 Fabry-Pérot-resonator by spectral and time resolved measurements

Using a Fabry-Pérot-microresonator with controllable cavity lengths in the λ/2-regime, we show the controlled modification of the vibronic relaxation dynamics of a fluorescent dye molecule in the spectral and time domain. By altering the photonic mode density around the fluorophores we are able to shape the fluorescence spectrum and enhance specifically the probability of the radiative transitions from the electronic excited state to distinct vibronic excited states of the electronic ground state. Analysis and correlation of the spectral and time resolved measurements by a theoretical model and a global fitting procedure allows us to reveal quantitatively the spectrally distributed radiative and non-radiative relaxation dynamics of the respective dye molecule under ambient conditions at the ensemble level.

Long-wavelength chlorophylls in photosystem I of cyanobacteria: origin, localization, and functions

The structural organization of photosystem I (PSI) complexes in cyanobacteria and the origin of the PSI antenna long-wavelength chlorophylls and their role in energy migration, charge separation, and dissipation of excess absorbed energy are discussed. The PSI complex in cyanobacterial membranes is organized preferentially as a trimer with the core antenna enriched with long-wavelength chlorophylls. The contents of long-wavelength chlorophylls and their spectral characteristics in PSI trimers and monomers are species-specific. Chlorophyll aggregates in PSI antenna are potential candidates for the role of the long-wavelength chlorophylls. The red-most chlorophylls in PSI trimers of the cyanobacteria Arthrospira platensis and Thermosynechococcus elongatus can be formed as a result of interaction of pigments peripherally localized on different monomeric complexes within the PSI trimers. Long-wavelength chlorophylls affect weakly energy equilibration within the heterogeneous PSI antenna, but they significantly delay energy trapping by P700. When the reaction center is open, energy absorbed by long-wavelength chlorophylls migrates to P700 at physiological temperatures, causing its oxidation. When the PSI reaction center is closed, the P700 cation radical or P700 triplet state (depending on the P700 redox state and the PSI acceptor side cofactors) efficiently quench the fluorescence of the long-wavelength chlorophylls of PSI and thus protect the complex against photodestruction.

Red chlorophyll excitation dynamics in Arthrospira platensis photosystem I trimeric complexes as studied by femtosecond transient absorption spectroscopy

Femtosecond absorption spectroscopy was applied to study for the first time excitation dynamics in isolated photosystem I trimers from Arthrospira platensis, which display extremely long-wavelength absorption peaks. Pump-probe spectra observed at 77K in the timescale of dozens of picoseconds upon 70-fs excitation revealed two maxima near 710 and 730 nm, which correspond to red chlorophyll forms. Bleaching at 680 nm developed in ∼ 200 fs, whereas the bleaching kinetics at 710 and 730 nm exhibited two components with time constants of 1 and 5.5 ps. Comparison of the kinetics of bleaching development at 710 nm and 730 nm with that of bleaching decay at 680 nm indicated that both long-wavelength forms of trimers are populated mainly via direct energy transfer from bulk chlorophyll.

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

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

Confocal sample-scanning microscope for single-molecule spectroscopy and microscopy with fast sample exchange at cryogenic temperatures

The construction of a microscope with fast sample transfer system for single-molecule spectroscopy and microscopy at low temperatures using 2D/3D sample-scanning is reported. The presented construction enables the insertion of a sample from the outside (room temperature) into the cooled (4.2 K) cryostat within seconds. We describe the mechanical and optical design and present data from individual Photosystem I complexes. With the described setup numerous samples can be investigated within one cooling cycle. It opens the possibility to investigate biological samples (i) without artifacts introduced by prolonged cooling procedures and (ii) samples that require preparation steps like plunge-freezing or specific illumination procedures prior to the insertion into the cryostat.

Red antenna states of Photosystem I trimers from Arthrospira platensis revealed by single-molecule spectroscopy

Single-molecule fluorescence spectroscopy at 1.4K was used to investigate the spectral properties of red (long-wavelength) chlorophylls in trimeric Photosystem I (PSI) complexes from the cyanobacterium Arthrospira platensis. Three distinct red antenna states could be identified in the fluorescence spectra of single PSI trimers from A. platensis in the presence of oxidized P700. Two of them are responsible for broad emission bands centered at 726 and 760nm. These bands are similar to those found in bulk fluorescence spectra measured at cryogenic temperatures. The broad fluorescence bands at ≅726 and ≅760nm belong to individual emitters that are broadened by strong electron-phonon coupling giving rise to a large Stokes-shift of about 20nm and rapid spectral diffusion. An almost perpendicular orientation of the transition dipole moments of F726 and F760 has to be assumed because direct excitation energy transfer does not occur between F726 and F760. For the first time a third red state assigned to the pool absorbing around 708nm could be detected by its zero-phonon lines. The center of the zero-phonon line distribution is found at ≅714nm. The spectral properties of the three red antenna states show a high similarity to the red antenna states found in trimeric PSI of Thermosynechoccocus elongatus. Based on these findings a similar organization of the red antenna states in PSI of these two cyanobacteria is discussed.