Fast Energy Transfer and Exciton Dynamics in Chlorosomes of the Green Sulfur Bacterium Chlorobium tepidum (original) (raw)
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Energy transfer and trapping of excitations in membranes of Heliobacterium chlorum at 15 K
Biochimica Et Biophysica Acta - Bioenergetics, 1994
The transfer of excitation energy in membrane fragments of Heliobacterium chlorum was studied at 15 K by picosecond transient absorption spectroscopy. Due to the high repetition rate of the laser pulses, the primary electron donor was in the oxidized state in essentially all reaction centers. The absorbance difference spectrum showed that excitations in the antenna rapidly accumulate on the long wavelength absorbing component bacteriochlorophyll (BChl) g 808. Analysis of the kinetics of excited antenna BChl g showed a lifetime of less than 1 ps for excitations on BChl g 778, whereas excited BChl g 793 showed a somewhat slower decay of about 2 ps. Time resolved anisotropy measurements showed a very rapid randomization of excitations amongst pigments with similar absorption spectrum within less than 1 ps. Kinetics as well as the absorbance difference spectrum showed that the decay of excitations accumulated on BChl g 808 was strongly inhomogeneous. About 50% of the excitations disappeared with a time constant of 4 ± 2 ps, while the others showed decays with time constants of 20 + 5 ps and 100 + 20 ps. The amplitude of the 20 ps component decreased with the energy density of the excitation pulses and this decay component is attributed, at least in part, to singlet-triplet quenching. The main relaxation process of excitations with a time constant of about 4 ps is most probably due to energy transfer to the oxidized reaction center. The 100 ps decay component is associated with BChls g 808 that are relatively isolated, either spatially or energetically.
Journal of Physical Chemistry B, 1997
The technique of accumulated photon echoes was used to study optical dephasing of the lowest Q Y transitions of the bacteriochlorophyll a complex of the green sulfur bacteria Prosthecochloris aestuarii. Variations in the photon echo decay as a function of wavelength are interpreted in terms of (phonon-assisted) downward relaxation within the Q Y manifold as a mechanism of energy transfer at the lowest temperatures. A temperature dependence study revealed contributions to the total dephasing both from transitions within so-called two level systems (TLS) of the host as well as activation of transitions to higher electronic levels within the Q Y manifold at temperatures above 5 K. Conclusions are drawn about the nature of energy transfer at low temperatures.
The Journal of Physical Chemistry B, 1997
The technique of accumulated photon echoes was used to study optical dephasing of the lowest Q Y transitions of the bacteriochlorophyll a complex of the green sulfur bacteria Prosthecochloris aestuarii. Variations in the photon echo decay as a function of wavelength are interpreted in terms of (phonon-assisted) downward relaxation within the Q Y manifold as a mechanism of energy transfer at the lowest temperatures. A temperature dependence study revealed contributions to the total dephasing both from transitions within so-called two level systems (TLS) of the host as well as activation of transitions to higher electronic levels within the Q Y manifold at temperatures above 5 K. Conclusions are drawn about the nature of energy transfer at low temperatures.
Biophysical Journal, 2003
The excited-state relaxation within bacteriochlorophyll (BChl) e and a in chlorosomes of Chlorobium phaeobacteroides has been studied by femtosecond transient absorption spectroscopy at room temperature. Singlet-singlet annihilation was observed to strongly influence both the isotropic and anisotropic decays. Pump intensities in the order of 10 11 photons 3 pulse ÿ1 3 cm ÿ2 were required to obtain annihilation-free conditions. The most important consequence of applied very low excitation doses is an observation of a subpicosecond process within the BChl e manifold (;200-500 fs), manifesting itself as a rise in the red part of the Q y absorption band of the BChl e aggregates. The subsequent decay of the kinetics measured in the BChl e region and the corresponding rise in the baseplate BChl a is not single-exponential, and at least two components are necessary to fit the data, corresponding to several BChl e!BChl a transfer steps. Under annihilation-free conditions, the anisotropic kinetics show a generally slow decay within the BChl e band (10-20 ps) whereas it decays more rapidly in the BChl a region (;1 ps). Analysis of the experimental data gives a detailed picture of the overall time evolution of the energy relaxation and energy transfer processes within the chlorosome. The results are interpreted within an exciton model based on the proposed structure.
Spectral hole burning study of intact cells of green bacterium Chlorobium limicola
FEBS Letters, 1993
Spectral hole burmng studies of intact cells of the green bacterium, Chlorobium lrmicolu. have proven that the Q,-absorption system of antenna bacteriochlorophyll c (BChl c) should be interpreted in terms of the delocalized exciton level structure of an oligomer. For the first time the O-0 band of the lowest exciton state of BChl c oligomers has been directly detected as the lowest energy inhomogeneously broadened band (FWHM -100 cm-': position of maximum, at -774 nm) of the near-infrared BChl c band of 1.8K excitation spectrum (FWHM = 830 cm-'; position of maximum, at 751 nm). Light-harvesting antenna; Pigment oligomerization: Energy transfer; Chlorobrum hnucola cell Published by Elsevier Science Publishers B. V.
Biochimica Et Biophysica Acta-bioenergetics, 2000
We have studied energy transfer in chlorosomes of Chlorobium limicola UdG6040 containing a mixture of about 50% bacteriochlorophyll (BChl) c and BChl d each. BChl d-depleted chlorosomes were obtained by acid treatment. The energy transfer between the different pigment pools was studied using both steady-state and time-resolved fluorescence spectroscopy at room temperature and low temperature. The steady-state emission of the
Insights into the Excitonic States of Individual Chlorosomes from Chlorobaculum tepidum
Biophysical Journal, 2014
Green-sulfur bacteria have evolved a unique light-harvesting apparatus, the chlorosome, by which it is perfectly adapted to thrive photosynthetically under extremely low light conditions. We have used single-particle, optical spectroscopy to study the structure-function relationship of chlorosomes each of which incorporates hundreds of thousands of self-assembled bacteriochlorophyll (BChl) molecules. The electronically excited states of these molecular assemblies are described as Frenkel excitons whose photophysical properties depend crucially on the mutual arrangement of the pigments. The signature of these Frenkel excitons and its relation to the supramolecular organization of the chlorosome becomes accessible by optical spectroscopy. Because subtle spectral features get obscured by ensemble averaging, we have studied individual chlorosomes from wildtype Chlorobaculum tepidum by polarization-resolved fluorescence-excitation spectroscopy. This approach minimizes the inherent sample heterogeneity and allows us to reveal properties of the exciton states without ensemble averaging. The results are compared with predictions from computer simulations of various models of the supramolecular organization of the BChl monomers. We find that the photophysical properties of individual chlorosomes from wild-type Chlorobaculum tepidum are consistent with a (multiwall) helical arrangement of syn-anti stacked BChl molecules in cylinders and/or spirals of different size.
Photosynthesis Research, 2002
The role of carotenoids in chlorosomes of the green sulfur bacterium Chlorobium phaeobacteroides, containing bacteriochlorophyll (BChl) e and the carotenoid (Car) isorenieratene as main pigments, was studied by steady-state fluorescence excitation, picosecond single-photon timing and femtosecond transient absorption (TA) spectroscopy. In order to obtain information about energy transfer from Cars in this photosynthetic lightharvesting antenna with high spectral overlap between Cars and BChls, Car-depleted chlorosomes, obtained by inhibition of Car biosynthesis by 2-hydroxybiphenyl, were employed in a comparative study with control chlorosomes. Excitation spectra measured at room temperature give an efficiency of 60-70% for the excitation energy transfer from Cars to BChls in control chlorosomes. Femtosecond TA measurements enabled an identification of the excited state absorption band of Cars and the lifetime of their S1 state was determined to be ~10 ps. Based on this lifetime, we concluded that the involvement of this state in energy transfer is unlikely. Furthermore, evidence was attained for the presence of an ultrafast (<100 fs) energy transfer process from the S2 state of Cars to BChls in control chlorosomes.
Excitation energy transfer in chlorosomes of green bacteria: theoretical and experimental studies
Biophysical Journal, 1996
A theory of excitation energy transfer within the chlorosomal antennae of green bacteria has been developed for an exciton model of aggregation of bacteriochlorophyll (BChl) c (d or e). This model of six exciton-coupled BChl chains with low packing density, approximating that in vivo, and interchain distances of -2 nm was generated to yield the key spectral features found in natural antennae, i.e., the exciton level structure revealed by spectral hole burning experiments and polarization of all the levels parallel to the long axis of the chlorosome. With picosecond fluorescence spectroscopy it was demonstrated that the theory explains the antenna-size-dependent kinetics of fluorescence decay in chlorosomal antenna, measured for intact cells of different cultures of the green bacterium C. aurantiacus, with different chlorosomal antenna size determined by electron microscopic examination of the ultrathin sections of the cells. The data suggest a possible mechanism of excitation energy transfer within the chlorosome that implies the formation of a cylindrical exciton, delocalized over a tubular aggregate of BChl c chains, and Forster-type transfer of such a cylindrical exciton between the nearest tubular BChl c aggregates as well as to BChl a of the baseplate.
Biophysical Journal, 2000
The energy transfer processes in isolated chlorosomes from green bacteria Chlorobium tepidum and Chloroflexus aurantiacus have been studied at low temperatures (1.27 K) by two-pulse photon echo and one-color transient absorption techniques with ϳ100 fs resolution. The decay of the coherence in both types of chlorosomes is characterized by four different dephasing times stretching from ϳ100 fs up to 300 ps. The fastest component reflects dephasing that is due to interaction of bacteriochlorophylls with the phonon bath, whereas the other components correspond to dephasing due to different energy transfer processes such as distribution of excitation along the rod-like aggregates, energy exchange between different rods in the chlorosome, and energy transfer to the base plate. As a basis for the interpretation of the excitation dephasing and energy transfer pathways, a superlattice-like structural model is proposed based on recent experimental data and computer modeling of the Bchl c aggregates (1994. Photosynth. Res. 41:225-233.) This model predicts a fine structure of the Q y absorption band that is fully supported by the present photon echo data.