RESUMO
Typical purple bacterial photosynthetic units consist of light harvesting one/reaction centre 'core' complexes surrounded by light harvesting two complexes. Factors such as the number and size of photosynthetic units per cell, as well as the type of light harvesting two complex that is produced, are controlled by environmental factors. In this paper, the change in the type of LH2 present in the Rhodopsuedomonas acidophila strain 7050 is described when cells are grown at a range of different light intensities. This species contains multiple pucBA genes that encode the apoproteins that form light-harvesting complex two, and a more complex mixture of spectroscopic forms of this complex has been found than was previously thought to be the case. Femto-second time resolved absorption has been used to investigate how the energy transfer properties in the membranes of high-light and low-light adapted cells change as the composition of the LH2 complexes varies.
Assuntos
Complexos de Proteínas Captadores de Luz/metabolismo , Complexos de Proteínas Captadores de Luz/efeitos da radiação , Luz , Rodopseudomonas/crescimento & desenvolvimento , Rodopseudomonas/efeitos da radiação , Rodopseudomonas/classificação , Rodopseudomonas/metabolismoRESUMO
We demonstrate angle-resolved coherent (ARC) wave mixing using 4 fs light pulses derived from a laser source that spans 550-1000 nm. We believe this to be the shortest pulse duration used to date in coherent multi-dimensional spectroscopy. The marriage of this ultra-broad band, few-cycle coherent source with the ARC technique will permit new investigations of the interplay between energy transfers and quantum superposition states spanning 8200 cm-1. We applied this configuration to measurements on the photosynthetic low light (LL) complex from Rhodopseudomonas palustris in solution at ambient temperature. We observe bi-exponential population dynamics for energy transfer across 5500 cm-1 (0.65 eV), which we attribute to energy transfer from the Qx transition of bacteriochlorophylls to the B850 pigment of the complex. We believe for the first time, to the best of our knowledge, we demonstrate that ARC maps can be recorded using a single laser pulse.
RESUMO
Photosynthetic antenna complexes harvest sunlight and efficiently transport energy to the reaction center where charge separation powers biochemical energy storage. The discovery of existence of long lived quantum coherence during energy transfer has sparked the discussion on the role of quantum coherence on the energy transfer efficiency. Early works assigned observed coherences to electronic states, and theoretical studies showed that electronic coherences could affect energy transfer efficiency--by either enhancing or suppressing transfer. However, the nature of coherences has been fiercely debated as coherences only report the energy gap between the states that generate coherence signals. Recent works have suggested that either the coherences observed in photosynthetic antenna complexes arise from vibrational wave packets on the ground state or, alternatively, coherences arise from mixed electronic and vibrational states. Understanding origin of coherences is important for designing molecules for efficient light harvesting. Here, we give a direct experimental observation from a mutant of LH2, which does not have B800 chromophores, to distinguish between electronic, vibrational, and vibronic coherence. We also present a minimal theoretical model to characterize the coherences both in the two limiting cases of purely vibrational and purely electronic coherence as well as in the intermediate, vibronic regime.
Assuntos
Complexos de Proteínas Captadores de Luz/química , Complexos de Proteínas Captadores de Luz/metabolismo , Fotossíntese , Rhodobacter sphaeroides/química , Vibração , Transferência de Energia , Teoria Quântica , Rhodobacter sphaeroides/metabolismoRESUMO
Fluorescence imaging of hybrid nanostructures composed of a bacterial light-harvesting complex LH2 and Au nanorods with controlled coupling strength is employed to study the spectral dependence of the plasmon-induced fluorescence enhancement. Perfect matching of the plasmon resonances in the nanorods with the absorption bands of the LH2 complexes facilitates a direct comparison of the enhancement factors for longitudinal and transverse plasmon frequencies of the nanorods. We find that the fluorescence enhancement due to excitation of longitudinal resonance can be up to five-fold stronger than for the transverse one. We attribute this result, which is important for designing plasmonic functional systems, to a very different distribution of the enhancement of the electric field due to the excitation of the two characteristic plasmon modes in nanorods.
Assuntos
Fluorescência , Ouro/química , Complexos de Proteínas Captadores de Luz/química , Nanotubos/química , Ouro/metabolismo , Complexos de Proteínas Captadores de Luz/metabolismoRESUMO
We investigate the nature of the S* excited state in carotenoids by performing a series of pump-probe experiments with sub-20 fs time resolution on spirilloxanthin in a polymethyl-methacrylate matrix varying the sample temperature. Following photoexcitation, we observe sub-200 fs internal conversion of the bright S2 state into the lower-lying S1 and S* states, which in turn relax to the ground state on a picosecond time scale. Upon cooling down the sample to 77 K, we observe a systematic decrease of the S*/S1 ratio. This result can be explained by assuming two thermally populated ground state isomers. The higher lying one generates the S* state, which can then be effectively frozen out by cooling. These findings are supported by quantum chemical modeling and provide strong evidence for the existence and importance of ground state isomers in the photophysics of carotenoids.
Assuntos
Modelos Teóricos , Temperatura , Isomerismo , Luz , Nitrogênio/química , Polimetil Metacrilato/química , Teoria Quântica , Análise Espectral , Xantofilas/químicaRESUMO
We have investigated the spectral diffusion and the electron-phonon coupling of B800 bacteriochlorophyll a molecules in the peripheral light-harvesting complex LH2 for three different species of purple bacteria, Rhodobacter sphaeroides, Rhodospirillum molischianum, and Rhodopseudomonas acidophila. We come to the conclusion that B800 binding pockets for Rhodobacter sphaeroides and Rhodopseudomonas acidophila are rather similar with respect to the polarity of the protein environment but that the packaging of the alphabeta-polypeptides seems to be less tight in Rb. sphaeroides with respect to the other two species.
Assuntos
Proteínas de Bactérias/química , Bacterioclorofila A/metabolismo , Biofísica/métodos , Complexos de Proteínas Captadores de Luz/química , Proteobactérias/metabolismo , Rhodobacter/metabolismo , Rodopseudomonas/metabolismo , Rhodospirillaceae/metabolismo , Cristalografia por Raios X/métodos , Difusão , Elétrons , Modelos Moleculares , Conformação Molecular , Peptídeos/química , Ligação Proteica , Proteobactérias/fisiologiaRESUMO
Low-temperature (1.4 K), single-molecule fluorescence-excitation spectra have been recorded for individual reaction center-light-harvesting 1 complexes from Rhodopseudomonas palustris and the PufX(-) strain of Rhodobacter sphaeroides. More than 80% of the complexes from Rb. sphaeroides show only broad absorption bands, whereas nearly all of the complexes from Rps. palustris also have a narrow line at the low-energy end of their spectrum. We describe how the presence of this narrow feature indicates the presence of a gap in the electronic structure of the light-harvesting 1 complex from Rps. palustris, which provides strong support for the physical gap that was previously modeled in its x-ray crystal structure.
Assuntos
Proteínas de Bactérias/química , Complexos de Proteínas Captadores de Luz/química , Rhodobacter sphaeroides/química , Rodopseudomonas/química , Espectrometria de FluorescênciaRESUMO
We report on a sub-20-fs transient absorption study of the S2(1(1)B(+)(u))-->S1(2(1)A(-)(g)) internal conversion in a series of carotenoids with a number of conjugated double bonds (N) ranging from 5 to 15. For the longer carotenoids (N>or=9), the measurements reveal the existence of an additional intermediate excited state lying between the optically allowed S2 state and the lower-lying forbidden S1 state. This state enables us to explain the nonmonotonic dependence of the S2-->S1 conversion rate on N and is expected to play an important role in photosynthetic light harvesting.
Assuntos
Carotenoides/química , Espectrofotometria/métodos , Espectroscopia de Luz Próxima ao Infravermelho/métodos , Relação Estrutura-AtividadeRESUMO
The conductivity of two photosynthetic protein-pigment complexes, a light harvesting 2 complex and a reaction center, was measured with an atomic force microscope capable of performing electrical measurements. Current-voltage measurements were performed on complexes embedded in their natural environment. Embedding the complexes in lipid bilayers made it possible to discuss the different conduction behaviors of the two complexes in light of their atomic structure.
Assuntos
Proteínas de Bactérias/química , Elétrons , Bicamadas Lipídicas/química , Fotossíntese , Complexo de Proteína do Fotossistema II/química , Complexo de Proteína do Fotossistema II/efeitos da radiação , Proteínas de Bactérias/ultraestrutura , Condutividade Elétrica , Transferência de Energia , Microscopia de Força Atômica , Complexo de Proteína do Fotossistema II/ultraestrutura , Rhodobacter sphaeroides/química , Rodopseudomonas/química , Especificidade da EspécieRESUMO
Integral membrane proteins are solubilized by their incorporation into a detergent micelle. The detergent micelle has a critical influence on the formation of a three-dimensional crystal lattice. The bulk detergent phase is not seen in X-ray crystal structures of integral membrane proteins, due to its disordered character. Here, we describe the detergent structure present in crystals of the peripheral light-harvesting complex of the purple bacteria Rhodopseudomonas acidophila strain 10050 at a maximal resolution of 12A as determined by neutron crystallography. The LH2 molecule has a toroidal shape and spans the membrane completely in vivo. A volume of 16% of the unit cell could be ascribed to detergent tails, localized on both the inner and outer hydrophobic surfaces of the molecule. The detergent tail volumes were found to be associated with individual LH2 molecules and had no direct role in the formation of the crystalline lattice.
Assuntos
Detergentes/química , Membranas Intracelulares/química , Proteínas de Membrana/química , Difração de Nêutrons , Complexo de Proteínas do Centro de Reação Fotossintética/química , Rodopseudomonas/química , Rodopseudomonas/classificação , Cristalização , Detergentes/análise , Interações Hidrofóbicas e Hidrofílicas , Micelas , Rodopseudomonas/citologia , SolubilidadeRESUMO
We present the first direct evidence of the presence of an intermediate singlet excited state (Sx) mediating the internal conversion from S2 to S1 in carotenoids. The S2 to Sx transition is extremely fast and is completed within approximately 50 femtoseconds. These results require a reassessment of the energy transfer pathways from carotenoids to chlorophylls in the primary step of photosynthesis.
Assuntos
Carotenoides/química , Carotenoides/metabolismo , Luz , beta Caroteno/química , beta Caroteno/metabolismo , Fenômenos Químicos , Físico-Química , Clorofila/química , Clorofila/metabolismo , Cicloexanos , Transferência de Energia , Licopeno , Fotossíntese , Análise Espectral , TemperaturaRESUMO
This paper presents a concise review of the structural factors which control the energy of the Q(y) absorption band of bacteriochlorophyll a in purple bacterial antenna complexes. The energy of these Q(y) absorption bands is important for excitation energy transfer within the bacterial photosynthetic unit.
RESUMO
By means of steady-state fluorescence spectroscopy we explore the photophysics of two lowest lying singlet excited states in two natural 15-cis-carotenoids, namely phytoene and phytofluene, possessing three and five conjugated double bonds (N), respectively. The results are interpreted in relation to the photophysics of all-transcarotenoids with varying N. The fluorescence of phytofluene is more Stokes-shifted relative to that of phytoene, and is ascribed to the forbidden S1-->S0 transition, with its first excited singlet state (S1) lying 3340 cm-1 below the dipole allowed second excited singlet state (S2), at 77 K. For phytoene the S2 and S1 potential surfaces are closer in energy, probably giving rise to the mixed S2 and S1 fluorescence characteristics. The origin of phytoene fluorescence is discussed and is suggested to be due to the S1-->S0 transition; with the S1 state located 1100 cm-1 below S2 at 77 K. The dependence of the fluorescence quantum yield on temperature and viscosity shows that large amplitude molecular motions are involved in the radiationless relaxation process of phytoene. The transition dipole moment of absorption and emission are parallel in phytoene and nonparallel in phytofluene.
Assuntos
Carotenoides , Algoritmos , Carotenoides/química , Carotenoides/isolamento & purificação , Estrutura Molecular , Fotoquímica , Rhodospirillum rubrum/metabolismo , Espectrometria de Fluorescência , Espectrofotometria Atômica , EstereoisomerismoRESUMO
The B800-820, or LH3, complex is a spectroscopic variant of the B800-850 LH2 peripheral light-harvesting complex. LH3 is synthesized by some species and strains of purple bacteria when growing under what are generally classed as "stressed" conditions, such as low intensity illumination and/or low temperature (<30 degrees C). The apoproteins in these complexes modify the absorption properties of the chromophores to ensure that the photosynthetic process is highly efficient. The crystal structure of the B800-820 light-harvesting complex, an integral membrane pigment-protein complex, from the purple bacteria Rhodopseudomonas (Rps.) acidophila strain 7050 has been determined to a resolution of 3.0 A by molecular replacement. The overall structure of the LH3 complex is analogous to that of the LH2 complex from Rps. acidophila strain 10050. LH3 has a nonameric quaternary structure where two concentric cylinders of alpha-helices enclose the pigment molecules bacteriochlorophyll a and carotenoid. The observed spectroscopic differences between LH2 and LH3 can be attributed to differences in the primary structure of the apoproteins. There are changes in hydrogen bonding patterns between the coupled Bchla molecules and the protein that have an effect on the conformation of the C3-acetyl groups of the B820 molecules. The structure of LH3 shows the important role that the protein plays in modulating the characteristics of the light-harvesting system and indicates the mechanisms by which the absorption properties of the complex are altered to produce a more efficient light-harvesting component.
Assuntos
Proteínas de Bactérias , Complexos de Proteínas Captadores de Luz , Complexo de Proteínas do Centro de Reação Fotossintética/química , Rodopseudomonas/química , Bacterioclorofilas/química , Carotenoides/química , Cristalização , Cristalografia por Raios X , Ligação de Hidrogênio , Modelos Moleculares , Porfirinas/química , Estrutura Secundária de ProteínaRESUMO
Biological membranes are composed of a complex mixture of lipids and proteins, and the membrane lipids support several key biophysical functions, in addition to their obvious structural role. Recent results from X-ray crystallography are shedding new light on the precise molecular details of the protein-lipid interface.
Assuntos
Membrana Celular/química , Cristalografia por Raios X/métodos , Lipídeos/química , Bacteriorodopsinas/química , Cardiolipinas/química , Membrana Celular/metabolismo , Complexo IV da Cadeia de Transporte de Elétrons/química , Metabolismo dos Lipídeos , Modelos Moleculares , Complexo de Proteínas do Centro de Reação Fotossintética/químicaRESUMO
Previously, the spatial arrangement of the carotenoid and bacteriochlorophyll molecules in the peripheral light-harvesting (LH2) complex from Rhodopseudomonas acidophila strain 10050 has been determined at high resolution. Here, we have time resolved the energy transfer steps that occur between the carotenoid's initial excited state and the lowest energy group of bacteriochlorophyll molecules in LH2. These kinetic data, together with the existing structural information, lay the foundation for understanding the detailed mechanisms of energy transfer involved in this fundamental, early reaction in photosynthesis. Remarkably, energy transfer from the rhodopin glucoside S(2) state, which has an intrinsic lifetime of approximately 120 fs, is by far the dominant pathway, with only a minor contribution from the longer-lived S(1) state.
Assuntos
Carotenoides/química , Complexo de Proteínas do Centro de Reação Fotossintética/química , Bacterioclorofilas/química , Fenômenos Biofísicos , Biofísica , Transferência de Energia , Cinética , Complexos de Proteínas Captadores de Luz , Fotoquímica , Rodopseudomonas/química , EspectrofotometriaRESUMO
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.
Assuntos
Carotenoides/análogos & derivados , Carotenoides/metabolismo , Fotossíntese , Complexo de Proteínas do Centro de Reação Fotossintética/metabolismo , Xantofilas/análogos & derivados , Cinética , Rhodospirillum rubrum/metabolismo , Análise Espectral/métodosRESUMO
In this work we have selectively released the 800 nm absorbing bacteriochlorophyll a molecules of the LH2 protein from the photosynthetic bacterium Rhodopseudomonas acidophila, strain 10050, and replaced them with chlorophyll a (Chla). A combination of low-temperature electronic absorption, resonance Raman and site-selection fluorescence spectroscopies revealed that the Chla pigments are indeed bound in the B800 binding site; this is the first work that formally proves that such non-native chlorins can be inserted correctly into LH2.
Assuntos
Proteínas de Bactérias , Clorofila/química , Complexos de Proteínas Captadores de Luz , Complexo de Proteínas do Centro de Reação Fotossintética/química , Rodopseudomonas/química , Sítios de Ligação , Clorofila A , Temperatura Baixa , Ligação Proteica , Espectrometria de Fluorescência , Análise Espectral RamanRESUMO
This concise review describes the current status of research into how carotenoids function in bacterial photosynthesis. This is illustrated by reference to very recent studies on both the photoprotective and antenna functions of carotenoids. The major remaining open questions on the detailed molecular mechanisms involved in these reactions are highlighted.
RESUMO
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.