Triplet states of bacteriochlorophyll and carotenoids in chromatophores of photosynthetic bacteria

Triplet Energy Migration in Cytoskeletal Polymers

Dexter energy transfer (DET) of triplet electronic states is used to direct energy in photovoltaics, quench reactive singlet oxygen species in biological systems, and generate them in photodynamic therapy. However, the extent to which repeated DET between aromatic residues can lead to triplet energy migration in proteins has not been investigated. Here, we computationally describe DET rates in microtubules, actin filaments and the intermediate filament, vimentin. We discover instances where interaromatic residue Dexter couplings within individual protein subunits of these polymers are similar those of small molecules used for organic electronics. However, interaromatic residue coupling is mostly weak (<10-3 eV), limiting triplet energy diffusion lengths to 6.1, 0.5 and 1.0 Å in microtubules, actin filaments and vimentin, respectively. On the other hand, repeated förster resonance energy transfer (FRET) between aromatic residues leads to singlet energy diffusion lengths of 12.4 Å for actin filaments and about 8.6 Å for both microtubules and vimentin filaments. Our work shows that singlet energy migration dominates over triplet energy migration in cytoskeletal polymers.

Ultrafast spectroscopy of the hydrophilic carotenoid crocin at various pH

This study delves into the pH-dependent effects on the excited-state behavior of crocin, a hydrophilic carotenoid with diverse functions in biological systems. Steady-state spectroscopy demonstrates notable changes in absorption and fluorescence spectra, characterized by a pH-dependent blue shift and altered resolution of vibrational bands. Transient absorption spectra further elucidate these effects, highlighting a significant blue shift in the S1-Sn peak with increasing pH. Detailed kinetic analysis shows the pH-dependent dynamics of crocin's excited states. At pH 11, a shortening of effective conjugation is observed, resulting in a prolonged S1/ICT lifetime. Conversely, at pH 9, our data suggest a more complex scenario, suggesting the presence of two distinct crocin species with different relaxation patterns. This implies structural alterations within the crocin molecule, potentially linked to the deprotonation of hydroxyl groups in crocin and/or saponification at high pH.

Phasor Analysis Reveals Multicomponent Fluorescence Kinetics in the LH2 Complex from Marichromatium purpuratum

We investigated the fluorescence kinetics of LH2 complexes from Marichromatium purpuratum, the cryo-EM structure of which has been recently elucidated with 2.4 Å resolution. The experiments have been carried out as a function of the excitation density by varying both the excitation fluence and the repetition rate of the laser excitation. Instead of the usual multiexponential fitting procedure, we applied the less common phasor formalism for evaluating the transients because this allows for a model-free analysis of the data without a priori knowledge about the number of processes that contribute to a particular decay. For the various excitation conditions, this analysis reproduces consistently three lifetime components with decay times below 100 ps, 500 ps, and 730 ps, which were associated with the quenched state, singlet-triplet annihilation, and fluorescence decay, respectively. Moreover, it reveals that the number of decay components that contribute to the transients depends on whether the excitation wavelength is in resonance with the B800 BChl a molecules or with the carotenoids. Based on the mutual arrangement of the chromophores in their binding pockets, this leads us to conclude that the energy transfer pathways within the LH2 complex of this species differ significantly from each other for exciting either the B800 BChl molecules or the carotenoids. Finally, we speculate whether the illumination with strong laser light converts the LH2 complexes studied here into a quenched conformation that might be related to the development of the non-photochemical quenching mechanism that occurs in higher plants.

Reaction centers of the thermophilic microaerophile, Chloracidobacterium thermophilum (Acidobacteria) I: biochemical and biophysical characterization

Chloracidobacterium thermophilum is a microaerophilic, anoxygenic member of the green chlorophototrophic bacteria. This bacterium is the first characterized oxygen-requiring chlorophototroph with chlorosomes, the FMO protein, and homodimeric type-1 reaction centers (RCs). The RCs of C. thermophilum are also unique because they contain three types of chlorophylls, bacteriochlorophyll a_P esterified with phytol, Chl a_PD esterified with Δ2,6-phytadienol, and Zn-BChl a_P' esterified with phytol, in the approximate molar ratio 32:24:4. The light-induced difference spectrum of these RCs had a bleaching maximum at 839 nm and also revealed an electrochromic bandshift that is probably derived from a BChl a molecule near P840⁺. The F_X [4Fe-4S] cluster had a midpoint potential of ca. - 581 mV, and the spectroscopic properties of the P⁺ F _X ^- spin-polarized radical pair were very similar to those of reaction centers of heliobacteria and green sulfur bacteria. The data further indicate that electron transfer occurs directly from A₀^- to F_X, as occurs in other homodimeric type-1 RCs. Washing experiments with isolated membranes suggested that the PscB subunit of these reaction centers is more tightly bound than PshB in heliobacteria. Thus, the reaction centers of C. thermophilum have some properties that resemble other homodimeric reaction centers but also have specific properties that are more similar to those of Photosystem I. These differences probably contribute to protection of the electron transfer chain from oxygen, contributing to the oxygen tolerance of this microaerophile.

Electronic and vibrational properties of carotenoids: from in vitro to in vivo

Carotenoids are among the most important organic compounds present in Nature and play several essential roles in biology. Their configuration is responsible for their specific photophysical properties, which can be tailored by changes in their molecular structure and in the surrounding environment. In this review, we give a general description of the main electronic and vibrational properties of carotenoids. In the first part, we describe how the electronic and vibrational properties are related to the molecular configuration of carotenoids. We show how modifications to their configuration, as well as the addition of functional groups, can affect the length of the conjugated chain. We describe the concept of effective conjugation length, and its relationship to the S₀ → S₂ electronic transition, the decay rate of the S₁ energetic level and the frequency of the ν₁ Raman band. We then consider the dependence of these properties on extrinsic parameters such as the polarizability of their environment, and how this information (S₀ → S₂ electronic transition, ν₁ band position, effective conjugation length and polarizability of the environment) can be represented on a single graph. In the second part of the review, we use a number of specific examples to show that the relationships can be used to disentangle the different mechanisms tuning the functional properties of protein-bound carotenoids.

Photoprotection in intact cells of photosynthetic bacteria: quenching of bacteriochlorophyll fluorescence by carotenoid triplets

Upon high light excitation in photosynthetic bacteria, various triplet states of pigments can accumulate leading to harmful effects. Here, the generation and lifetime of flash-induced carotenoid triplets (³Car) have been studied by observation of the quenching of bacteriochlorophyll (BChl) fluorescence in different strains of photosynthetic bacteria including Rvx. gelatinosus (anaerobic and semianaerobic), Rsp. rubrum, Thio. roseopersicina, Rba. sphaeroides 2.4.1 and carotenoid- and cytochrome-deficient mutants Rba. sphaeroides Ga, R-26, and cycA, respectively. The following results were obtained: (1) ³Car quenching is observed during and not exclusively after the photochemical rise of the fluorescence yield of BChl indicating that the charge separation in the reaction center (RC) and the carotenoid triplet formation are not consecutive but parallel processes. (2) The photoprotective function of ³Car is not limited to the RC only and can be described by a model in which the carotenoids are distributed in the lake of the BChl pigments. (3) The observed lifetime of ³Car in intact cells is the weighted average of the lifetimes of the carotenoids with various numbers of conjugated double bonds in the bacterial strain. (4) The lifetime of ³Car measured in the light is significantly shorter (1-2 μs) than that measured in the dark (2-10 μs). The difference reveals the importance of the dynamics of ³Car before relaxation. The results will be discussed not only in terms of energy levels of the ³Car but also in terms of the kinetics of transitions among different sublevels in the excited triplet state of the carotenoid.

Singlet and triplet energy transfer dynamics in self-assembled axial porphyrin–anthracene complexes: towards supra-molecular structures for photon upconversion

Singlet and triplet energy transfer dynamics in anthracene–ruthenium porphyrin complexes, and their application to photon upconversion.

Triplet-triplet energy transfer in artificial and natural photosynthetic antennas

In photosynthetic organisms, protection against photooxidative stress due to singlet oxygen is provided by carotenoid molecules, which quench chlorophyll triplet species before they can sensitize singlet oxygen formation. In anoxygenic photosynthetic organisms, in which exposure to oxygen is low, chlorophyll-to-carotenoid triplet-triplet energy transfer (T-TET) is slow, in the tens of nanoseconds range, whereas it is ultrafast in the oxygen-rich chloroplasts of oxygen-evolving photosynthetic organisms. To better understand the structural features and resulting electronic coupling that leads to T-TET dynamics adapted to ambient oxygen activity, we have carried out experimental and theoretical studies of two isomeric carotenoporphyrin molecular dyads having different conformations and therefore different interchromophore electronic interactions. This pair of dyads reproduces the characteristics of fast and slow T-TET, including a resonance Raman-based spectroscopic marker of strong electronic coupling and fast T-TET that has been observed in photosynthesis. As identified by density functional theory (DFT) calculations, the spectroscopic marker associated with fast T-TET is due primarily to a geometrical perturbation of the carotenoid backbone in the triplet state induced by the interchromophore interaction. This is also the case for the natural systems, as demonstrated by the hybrid quantum mechanics/molecular mechanics (QM/MM) simulations of light-harvesting proteins from oxygenic (LHCII) and anoxygenic organisms (LH2). Both DFT and electron paramagnetic resonance (EPR) analyses further indicate that, upon T-TET, the triplet wave function is localized on the carotenoid in both dyads.

Light harvesting in phototrophic bacteria: structure and function

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

Bacteriopheophytin triplet state in Rhodobacter sphaeroides reaction centers

It is well established that photoexcitation of Rhodobacter sphaeroides reaction centers (RC) with reduced quinone acceptors results in the formation of a triplet state localized on the primary electron donor P with a significant yield. The energy of this long-lived and therefore potentially damaging excited state is then efficiently quenched by energy transfer to the RC spheroidenone carotenoid, with its subsequent decay to the ground state by intersystem crossing. In this contribution, we present a detailed transient absorption study of triplet states in a set of mutated RCs characterized by different efficiencies of triplet formation that correlate with lifetimes of the initial charge-separated state P(+)H A (-) . On a microsecond time scale, two types of triplet state were detected: in addition to the well-known spheroidenone triplet state with a lifetime of ~4 μs, in some RCs we discovered a bacteriopheophytin triplet state with a lifetime of ~40 μs. As expected, the yield of the carotenoid triplet increased approximately linearly with the lifetime of P(+)H A (-) , reaching the value of 42 % for one of the mutants. However, surprisingly, the yield of the bacteriopheophytin triplet was the highest in RCs with the shortest P(+)H A (-) lifetime and the smallest yield of carotenoid triplet. For these the estimated yield of bacteriopheophytin triplet was comparable with the yield of the carotenoid triplet, reaching a value of ~7 %. Possible mechanisms of formation of the bacteriopheophytin triplet state are discussed.

Ultrafast Electron Transfer Kinetics in the LM Dimer of Bacterial Photosynthetic Reaction Center from Rhodobacter sphaeroides

It has become increasingly clear that dynamics plays a major role in the function of many protein systems. One system that has proven particularly facile for studying the effects of dynamics on protein-mediated chemistry is the bacterial photosynthetic reaction center from Rhodobacter sphaeroides. Previous experimental and computational analysis have suggested that the dynamics of the protein matrix surrounding the primary quinone acceptor, QA, may be particularly important in electron transfer involving this cofactor. One can substantially increase the flexibility of this region by removing one of the reaction center subunits, the H-subunit. Even with this large change in structure, photoinduced electron transfer to the quinone still takes place. To evaluate the effect of H-subunit removal on electron transfer to QA, we have compared the kinetics of electron transfer and associated spectral evolution for the LM dimer with that of the intact reaction center complex on picosecond to millisecond time scales. The transient absorption spectra associated with all measured electron transfer reactions are similar, with the exception of a broadening in the QX transition and a blue-shift in the QY transition bands of the special pair of bacteriochlorophylls (P) in the LM dimer. The kinetics of the electron transfer reactions not involving quinones are unaffected. There is, however, a 4-fold decrease in the electron transfer rate from the reduced bacteriopheophytin to QA in the LM dimer compared to the intact reaction center and a similar decrease in the recombination rate of the resulting charge-separated state (P(+)QA(-)). These results are consistent with the concept that the removal of the H-subunit results in increased flexibility in the region around the quinone and an associated shift in the reorganization energy associated with charge separation and recombination.

Photoprotection Mechanism of Light-Harvesting Antenna Complex from Purple Bacteria

Photosynthetic light-harvesting apparatus efficiently capture sunlight and transfer the energy to reaction centers, while they safely dissipate excess energy to surrounding environments for a protection of their organisms. In this study, we performed pump-probe spectroscopic measurements with a temporal window ranging from femtosecond to submillisecond on the purple bacterial antenna complex LH2 from Rhodobacter sphaeroides 2.4.1 to clarify its photoprotection functions. The observed excited state dynamics in the time range from subnanosecond to microsecond exhibits that the triplet-triplet excitation energy transfer from bacteriochlorophyll a to carotenoid takes place with a time constant of 16.7 ns. Furthermore, ultrafast spectroscopic data suggests that a molecular assembly of bacteriochlorophyll a in LH2 efficiently suppresses a generation of triple bacteriochlorophyll a.

Spectral heterogeneity and carotenoid-to-bacteriochlorophyll energy transfer in LH2 light-harvesting complexes from Allochromatium vinosum

Photosynthetic organisms produce a vast array of spectral forms of antenna pigment-protein complexes to harvest solar energy and also to adapt to growth under the variable environmental conditions of light intensity, temperature, and nutrient availability. This behavior is exemplified by Allochromatium (Alc.) vinosum, a photosynthetic purple sulfur bacterium that produces different types of LH2 light-harvesting complexes in response to variations in growth conditions. In the present work, three different spectral forms of LH2 from Alc. vinosum, B800-820, B800-840, and B800-850, were isolated, purified, and examined using steady-state absorption and fluorescence spectroscopy, and ultrafast time-resolved absorption spectroscopy. The pigment composition of the LH2 complexes was analyzed by high-performance liquid chromatography, and all were found to contain five carotenoids: lycopene, anhydrorhodovibrin, spirilloxanthin, rhodopin, and rhodovibrin. Spectral reconstructions of the absorption and fluorescence excitation spectra based on the pigment composition revealed significantly more spectral heterogeneity in these systems compared to LH2 complexes isolated from other species of purple bacteria. The data also revealed the individual carotenoid-to-bacteriochlorophyll energy transfer efficiencies which were correlated with the kinetic data from the ultrafast transient absorption spectroscopic experiments. This series of LH2 complexes allows a systematic exploration of the factors that determine the spectral properties of the bound pigments and control the rate and efficiency of carotenoid-to-bacteriochlorophyll energy transfer.

Up-converted fluorescence from photosynthetic light-harvesting complexes linearly dependent on excitation intensity

Weak up-converted fluorescence related to bacteriochlorophyll a was recorded from various detergent-isolated and membrane-embedded light-harvesting pigment-protein complexes as well as from the functional membranes of photosynthetic purple bacteria under continuous-wave infrared laser excitation at 1064 nm, far outside the optically allowed singlet absorption bands of the chromophore. The fluorescence increases linearly with the excitation power, distinguishing it from the previously observed two-photon excited fluorescence upon femtosecond pulse excitation. Possible mechanisms of this excitation are discussed.

Multi-Level, Multi Time-Scale Fluorescence Intermittency of Photosynthetic LH2 Complexes: A Precursor of Non-Photochemical Quenching?

The light harvesting complex LH2 is a chromoprotein that is an ideal system for studying protein dynamics via the spectral fluctuations of the emission of its intrinsic chromophores. We have immobilized these complexes in a polymer film and studied the fluctuations of the fluorescence intensity from individual complexes over 9 orders of magnitude in time. Combining time-tagged detection of single photons with a change-point analysis has allowed the unambigeous identification of the various intensity levels due to the huge statistical basis of the data set. We propose that the observed intensity level fluctuations reflect conformational changes of the protein backbone that might be a precursor of the mechanism from which nonphotochemical quenching of higher plants has evolved.

The open, the closed, and the empty: time-resolved fluorescence spectroscopy and computational analysis of RC-LH1 complexes from Rhodopseudomonas palustris

We studied the time-resolved fluorescence of isolated RC-LH1 complexes from Rhodopseudomonas palustris as a function of the photon fluence and the repetition rate of the excitation laser. Both parameters were varied systematically over 3 orders of magnitude. On the basis of a microstate description we developed a quantitative model for RC-LH1 and obtained very good agreement between experiments and elaborate simulations based on a global master equation approach. The model allows us to predict the relative population of RC-LH1 complexes with the special pair in the neutral state or in the oxidized state P(+) and those complexes that lack a reaction center.

Membrane development in purple photosynthetic bacteria in response to alterations in light intensity and oxygen tension

Studies on membrane development in purple bacteria during adaptation to alterations in light intensity and oxygen tension are reviewed. Anoxygenic phototrophic such as the purple α-proteobacterium Rhodobacter sphaeroides have served as simple, dynamic, and experimentally accessible model organisms for studies of the photosynthetic apparatus. A major landmark in photosynthesis research, which dramatically illustrates this point, was provided by the determination of the X-ray structure of the reaction center (RC) in Blastochloris viridis (Deisenhofer and Michel, EMBO J 8:2149-2170, 1989), once it was realized that this represented the general structure for the photosystem II RC present in all oxygenic phototrophs. This seminal advance, together with a considerable body of subsequent research on the light-harvesting (LH) and electron transfer components of the photosynthetic apparatus has provided a firm basis for the current understanding of how phototrophs acclimate to alterations in light intensity and quality. Oxygenic phototrophs adapt to these changes by extensive thylakoid membrane remodeling, which results in a dramatic supramolecular reordering to assure that an appropriate flow of quinone redox species occurs within the membrane bilayer for efficient and rapid electron transfer. Despite the high level of photosynthetic unit organization in Rba. sphaeroides as observed by atomic force microscopy (AFM), fluorescence induction/relaxation measurements have demonstrated that the addition of the peripheral LH2 antenna complex in cells adapting to low-intensity illumination results in a slowing of the rate of electron transfer turnover by the RC of up to an order of magnitude. This is ascribed to constraints in quinone redox species diffusion between the RC and cytochrome bc1 complexes arising from the increased packing density as the intracytoplasmic membrane (ICM) bilayer becomes crowded with LH2 rings. In addition to downshifts in light intensity as a paradigm for membrane development studies in Rba. sphaeroides, the lowering of oxygen tension in chemoheterotropically growing cells results in a gratuitous formation of the ICM by an extensive membrane biogenesis process. These membrane alterations in response to lowered illumination and oxygen levels in purple bacteria are under the control of a number of interrelated two-component regulatory circuits reviewed here, which act at the transcriptional level to regulate the formation of both the pigment and apoprotein components of the LH, RC, and respiratory complexes. We have performed a proteomic examination of the ICM development process in which membrane proteins have been identified that are temporally expressed both during adaptation to low light intensity and ICM formation at low aeration and are spatially localized in both growing and mature ICM regions. For these proteomic analyses, membrane growth initiation sites and mature ICM vesicles were isolated as respective upper-pigmented band (UPB) and chromatophore fractions and subjected to clear native electrophoresis for isolation of bands containing the LH2 and RC-LH1 core complexes. In chromatophores, increasing levels of LH2 polypeptides relative to those of the RC-LH1 complex were observed as ICM membrane development proceeded during light-intensity downshifts, along with a large array of other associated proteins including high spectral counts for the F1FO-ATP synthase subunits and the cytochrome bc1 complex, as well as RSP6124, a protein of unknown function, that was correlated with increasing LH2 spectral counts. In contrast, the UPB was enriched in cytoplasmic membrane (CM) markers, including electron transfer and transport proteins, as well as general membrane protein assembly factors confirming the origin of the UPB from both peripheral respiratory membrane and sites of active CM invagination that give rise to the ICM. The changes in ICM vesicles were correlated to AFM mapping results (Adams and Hunter, Biochim Biophys Acta 1817:1616-1627, 2012), in which the increasing LH2 levels were shown to form densely packed LH2-only domains, representing the light-responsive antenna complement formed under low illumination. The advances described here could never have been envisioned when the author was first introduced in the mid-1960s to the intricacies of the photosynthetic apparatus during a lecture delivered in a graduate Biochemistry course at the University of Illinois by Govindjee, to whom this volume is dedicated on the occasion of his 80th birthday.

Molecular adaptation of photoprotection: triplet states in light-harvesting proteins

The photosynthetic light-harvesting systems of purple bacteria and plants both utilize specific carotenoids as quenchers of the harmful (bacterio)chlorophyll triplet states via triplet-triplet energy transfer. Here, we explore how the binding of carotenoids to the different types of light-harvesting proteins found in plants and purple bacteria provides adaptation in this vital photoprotective function. We show that the creation of the carotenoid triplet states in the light-harvesting complexes may occur without detectable conformational changes, in contrast to that found for carotenoids in solution. However, in plant light-harvesting complexes, the triplet wavefunction is shared between the carotenoids and their adjacent chlorophylls. This is not observed for the antenna proteins of purple bacteria, where the triplet is virtually fully located on the carotenoid molecule. These results explain the faster triplet-triplet transfer times in plant light-harvesting complexes. We show that this molecular mechanism, which spreads the location of the triplet wavefunction through the pigments of plant light-harvesting complexes, results in the absence of any detectable chlorophyll triplet in these complexes upon excitation, and we propose that it emerged as a photoprotective adaptation during the evolution of oxygenic photosynthesis.

Singlet molecular oxygen and primary mechanisms of photo-oxidative damage of chloroplasts. Studies based on detection of oxygen and pigment phosphorescence

Photogeneration of singlet oxygen molecules (1O2), their vibrationally excited stateand dimols (1O2)2has been shown by measuring photosensitised delayed luminescence in pigment-containing media. All singlet oxygen species are formed as a result of energy transfer to O2from triplet pigment molecules. Monomeric pigment molecules are the most efficient singlet oxygen generators. The1O2quantum yields are 40–80% in aerobic solutions of monomeric chlorophylls and pheophytins. Pigment aggregation causes a strong decrease in singlet oxygen production. The1O2quantum yield in chloroplasts has been estimated using literature and experimental data on formation of the chlorophyll triplet states in the photosynthetic apparatus. The most probable value is 0.1%. One of the major sources of singlet oxygen is likely to be the triplet states of newly formed pigment molecules which are not quenched by carotenoids and can be detected by measuring low-temperature pigment phosphorescence. Quenching of singlet oxygen by the thylakoid components has been analysed and the1O2lifetime estimated. The data suggest that carotenoids and chlorophylls are the most efficient physical1O2quenchers and the1O2lifetime is about 70 ns in thylakoids. The quantum yield of1O2-induced pigment photodestruction was estimated to be about 10−6–10−5. This value is close to the quantum yield of chlorophyll photobleaching experimentally observed in aerobic suspensions of isolated chloroplasts. The intensity of pigment phosphorescence at 77 K correlates with the rate of chlorophyll photobleaching in plant materials. The data suggest that1O2generation by the pigment triplet states is the most likely reason for chloroplast photodamage. The intensity of pigment phosphorescence can be used as an index of the degree of plant photo-oxidative stress.

Ab inito study on triplet excitation energy transfer in photosynthetic light-harvesting complexes

We have studied the triplet energy transfer (TET) for photosynthetic light-harvesting complexes, the bacterial light-harvesting complex II (LH2) of Rhodospirillum molischianum and Rhodopseudomonas acidophila, and the peridinin-chlorophyll a protein (PCP) from Amphidinium carterae. The electronic coupling factor was calculated with the recently developed fragment spin difference scheme (You and Hsu, J. Chem. Phys. 2010, 133, 074105), which is a general computational scheme that yields the overall coupling under the Hamiltonian employed. The TET rates were estimated based on the couplings obtained. For all light-harvesting complexes studied, there exist nanosecond triplet energy transfer from the chlorophylls to the carotenoids. This result supports a direct triplet quenching mechanism for the photoprotection function of carotenoids. The TET rates are similar for a broad range of carotenoid triplet state energy, which implies a general and robust TET quenching role for carotenoids in photosynthesis. This result is also consistent with the weak dependence of TET kinetics on the type or the number of π conjugation lengths in the carotenoids and their analogues reported in the literature. We have also explored the possibility of forming triplet excitons in these complexes. In B850 of LH2 or the peridinin cluster in PCP, it is unlikely to have triplet exciton since the energy differences of any two neighboring molecules are likely to be much larger than their TET couplings. Our results provide theoretical limits to the possible photophysics in the light-harvesting complexes.

Light collection and harvesting processes in bacterial photosynthesis investigated on a picosecond time scale

Fluorescence lifetimes have been determined for four strains of Rhodopseudomonas sphaeroides. Chromatophore samples were excited with a single picosecond flash, and the fluorescence was detected with a streak camera. The decay times are 100 psec in strains 2.4.1 and Ga, and 300 psec in the carotenoidless strain R-26. These times are related to the transfer of energy from the light-harvesting antenna pigment molecules to the photochemical reaction center. In strain PM-8 dpl, which lacks reaction centers, the lifetime is 1.1 nsec. In addition, we have obtained curves relating the quantum yield of fluorescence to the photon density of the excitation pulse. These curves can be fit with a simple model that relates excitonic processes to properties of the photosynthetic unit and that qualitatively describes differences between the mutant strains.

Excitation energy pathways in the photosynthetic units of reaction center LM- and H-subunit deletion mutants of Rhodospirillum rubrum

Light-induced reaction dynamics of isolated photosynthetic membranes obtained from wild-type (WT) and reaction center (RC)-subunit deletion strains SPUHK1 (an H-subunit deletion mutant) and SK Delta LM (an (L+M) deletion mutant) of the purple non-sulphur bacterium Rhodospirillum rubrum have been investigated by femtosecond transient absorption spectroscopy. Upon excitation of the spirilloxanthin (Spx) S(2) state at 546 nm, of the bacteriochlorophyll Soret band at 388 nm and probing spectral regions, which are characteristic for carotenoids, similar dynamics in the SPUHK1, SK Delta LM and WT strains could be observed. The excitation of Spx S(2) is followed by the simultaneous population of the lower singlet excited states S(1) and S* which decay with lifetimes of 1.4 and 5 ps, respectively for the mutants, and 1.4 and 4 ps, respectively, for the wild-type. The excitation of the BChl Soret band is followed by relaxation into BChl lower excited states which compete with excitation energy transfer BChl-to-Spx. The deexcitation pathway BChl(Soret) --> Spx(S(2)) --> Spx(S(1)) occurs with the same transition rate for all investigated samples (WT, SPUHK1 and SK Delta LM). The kinetic traces measured for the Spx S(1) --> S(N) transition display similar behaviour for all samples showing a positive signal which increases within the first 400 fs (i.e. the time needed for the excitation energy to reach the Spx S(1) excited state) and decays with a lifetime of about 1.5 ps. This suggests that the Spx excited state dynamics in the investigated complexes do not differ significantly. Moreover, a longer excited state lifetime of BChl for SPUHK1 in comparison to WT was observed, consistent with a photochemical quenching channel present in the presence of RC. For long delay times, photobleaching of the RC special pair and an electrochromic blue shift of the monomeric BChl a can be observed only for the WT but not for the mutants. The close similarity of the excited state decay processes of all strains indicates that the pigment geometry of the LH1 complex in native membranes is unaffected by the presence of an RC and allows us to draw a model representation of the WT, SK Delta LM and SPUHK1 PSU complexes.

Low-temperature pulsed EPR study at 34 GHz of the triplet states of the primary electron Donor P865 and the carotenoid in native and mutant bacterial reaction centers of Rhodobacter sphaeroides

The photosynthetic charge separation in bacterial reaction centers occurs predominantly along one of two nearly symmetric branches of cofactors. Low-temperature EPR spectra of the triplet states of the chlorophyll and carotenoid pigments in the reaction center of Rhodobacter sphaeroides R-26.1, 2.4.1 and two double-mutants GD(M203)/AW(M260) and LH(M214)/AW(M260) have been recorded at 34 GHz to investigate the relative activities of the "A" and "B" branches. The triplet states are found to derive from radical pair and intersystem crossing mechanisms, and the rates of formation are anisotropic. The former mechanism is operative for Rb. sphaeroides R-26.1, 2.4.1, and mutant GD(M203)/AW(M260) and indicates that A-branch charge separation proceeds at temperatures down to 10 K. The latter mechanism, derived from the spin polarization and operative for mutant LH(M214)/AW(M260), indicates that no long-lived radical pairs are formed upon direct excitation of the primary donor and that virtually no charge separation at the B-branch occurs at low temperatures. When the temperature is raised above 30 K, B-branch charge separation is observed, which is at most 1% of A-branch charge separation. B-branch radical pair formation can be induced at 10 K with low yield by direct excitation of the bacteriopheophytin of the B-branch at 590 nm. The formation of a carotenoid triplet state is observed. The rate of formation depends on the orientation of the reaction center in the magnetic field and is caused by a magnetic field dependence of the oscillation frequency by which the singlet and triplet radical pair precursor states interchange. Combination of these findings with literature data provides strong evidence that the thermally activated transfer step on the B-branch occurs between the primary donor, P865, and the accessory bacteriochlorophyll, whereas this step is barrierless down to 10 K along the A-branch.

In vivo carotenoid triplet formation in response to excess light: a supramolecular photoprotection mechanism revisited

Carotenoids have been known for their photoprotective role for about 50 years. However, despite many advances in laser flash photolysis, no photodynamic studies have been so far performed on whole cells to determine the harmful threshold of light. In the present work, we investigate the coupling between energy conversion and energy deactivation, in isolated complexes of RC-LH1 and LH2 increasingly integrated systems up to intact cells of the purple anaerobic photosynthetic bacterium Rubrivivax gelatinosus. A continuous light similar to the mean daily sun irradiance on the surface of the earth is found to saturate the in vivo electron transfer turnover and to give rise to carotenoid triplet formation. This accounts for the widespread use of carotenoids among phototrophic prokaryotes and emphasizes their essential protective role in the natural environment.

Fluorescence spectroscopy of conformational changes of single LH2 complexes

We have investigated the energy landscape of the bacterial photosynthetic peripheral light-harvesting complex LH2 of purple bacterium Rhodopseudomonas acidophila by monitoring sequences of fluorescence spectra of single LH2 assemblies, at room temperature, with different excitation intensities as well as at elevated temperatures, utilizing a confocal microscope. The fluorescence peak wavelength of individual LH2 complexes was found to abruptly move between long-lived quasi-stable levels differing by up to 30 nm. The frequency and size of these fluorescence peak movements were found to increase linearly with the excitation intensity. These spectral shifts either to the blue or to the red were accompanied by a broadening and decrease of the intensity of the fluorescence spectrum. The probability for a particle to undergo significant spectral shift in either direction was found to be roughly the same. Using the modified Redfield theory, the observed changes in spectral shape and intensity were accounted for by changes in the realization of the static disorder. Long lifetimes of the quasi-stable states suggest large energetic barriers between the states characterized by different emission spectra.

An unusual pathway of excitation energy deactivation in carotenoids: singlet-to-triplet conversion on an ultrafast timescale in a photosynthetic antenna

Carotenoids are important biomolecules that are ubiquitous in nature and find widespread application in medicine. In photosynthesis, they have a large role in light harvesting (LH) and photoprotection. They exert their LH function by donating their excited singlet state to nearby (bacterio)chlorophyll molecules. In photosynthetic bacteria, the efficiency of this energy transfer process can be as low as 30%. Here, we present evidence that an unusual pathway of excited state relaxation in carotenoids underlies this poor LH function, by which carotenoid triplet states are generated directly from carotenoid singlet states. This pathway, operative on a femtosecond and picosecond timescale, involves an intermediate state, which we identify as a new, hitherto uncharacterized carotenoid singlet excited state. In LH complex-bound carotenoids, this state is the precursor on the reaction pathway to the triplet state, whereas in extracted carotenoids in solution, this state returns to the singlet ground state without forming any triplets. We discuss the possible identity of this excited state and argue that fission of the singlet state into a pair of triplet states on individual carotenoid molecules constitutes the mechanism by which the triplets are generated. This is, to our knowledge, the first ever direct observation of a singlet-to-triplet conversion process on an ultrafast timescale in a photosynthetic antenna.

How carotenoids protect bacterial photosynthesis

The essential function of carotenoids in photosynthesis is to act as photoprotective agents, preventing chlorophylls and bacteriochlorophylls from sensitizing harmful photodestructive reactions in the presence of oxygen. Based upon recent structural studies on reaction centres and antenna complexes from purple photosynthetic bacteria, the detailed organization of the carotenoids is described. Then with specific reference to bacterial antenna complexes the details of the photoprotective role, triplet triplet energy transfer, are presented.

Escape probability and trapping mechanism in purple bacteria: revisited

Despite intensive research for decades, the trapping mechanism in the core complex of purple bacteria is still under discussion. In this article, it is attempted to derive a conceptionally simple model that is consistent with all basic experimental observations and that allows definite conclusions on the trapping mechanism. Some experimental data reported in the literature are conflicting or incomplete. Therefore we repeated two already published experiments like the time-resolved fluorescence decay in LH1-only purple bacteria Rhodospirillum rubrum and Rhodopseudomonas viridis chromatophores with open and closed (Q(A)(-)) reaction centers. Furthermore, we measured fluorescence excitation spectra for both species under the two redox-conditions. These data, all measured at room temperature, were analyzed by a target analysis based on a three-state model (antenna, primary donor, and radical pair). All states were allowed to react reversibly and their decay channels were taken into consideration. This leads to seven rate constants to be determined. It turns out that a unique set of numerical values of these rate constants can be found, when further experimental constraints are met simultaneously, i.e. the ratio of the fluorescence yields in the open and closed (Q(A)(-)) states F(m)/F(o) approximately 2 and the P(+)H(-)-recombination kinetics of 3-6 ns. The model allows to define and to quantify escape probabilities and the transfer equilibrium. We conclude that trapping in LH1-only purple bacteria is largely transfer-to-the-trap-limited. Furthermore, the model predicts properties of the reaction center (RC) in its native LH1-environment. Within the framework of our model, the predicted P(+)H(-)-recombination kinetics are nearly indistinguishable for a hypothetically isolated RC and an antenna-RC complex, which is in contrast to published experimental data for physically isolated RCs. Therefore RC preparations may display modified kinetic properties.