Zoya Fetisova

Excitation energy transfer in chlorosomes of green bacteria: theoretical and experimental studies.

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 approximately 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.

Experimental evidence of oligomeric organization of antenna bacteriochlorophyll c in green bacterium Chloroflexus aurantiacus by spectral hole burning

Spectral hole burning has been used to prove experimentally the existence in natural antenna of one of the predicted structural optimizing factors — antenna pigment oligomerization [J. Theor. Biol. 140 (1989) 167]—ensuring high efficiency of excitation energy transfer from antenna to reaction center, This point has been examined for the chlorosomal antenna of green bacterium Chloroflexus auranticus by hole burning in fluorescence excitation and emission spectra of intact cells at 1.8 K. The persistent hole spectra have been found to be consistent with a strongly exciton‐coupled bacteriochloraphyll c (BChl c) chromophore system. The lowest exciton state of BChl c oligomers has been direclly detected and separated as the lowest energy inhomogeneously broadened band (FWHM ∼90 cm−1, position of maximum, at ∼752 nm) from the near‐infrared BChl c band (FWHM ∼350 cm−1, position or maximum, at ∼74 nm) of 1.8 K excitation spectrum.

Unit building block of the oligomeric chlorosomal antenna of the green photosynthetic bacterium Chloroflexus aurantiacus: modeling of nonlinear optical spectra

Abstract The size of a unit BChl c aggregate of the chlorosomal antenna of the green photosynthetic bacterium Chloroflexus aurantiacus was estimated by using the relative difference absorption measurements of the oligomeric BChl c and monomeric BChl a bands [S. Savikhin et al., FEBS Lett. 430 (1998) 323]. A quantitative fit of the data was obtained within the framework of the exciton model proposed before [Z.G. Fetisova et al., Biophys. J. 71 (1996) 995]. The model assumes that a unit building block of the antenna has a form of a tubular aggregate of L=6 linear chains, containing N BChl c molecules each. The simultaneous fit of the linear and nonlinear (pump-probe) absorption gave the intrachain interaction energy of 400 cm −1 , the site inhomogeneity value of 280 cm −1 , and the number of pigments in each individual linear chain N=4. The size of the antenna unit was found to be L×N=24 exciton-coupled BChl c molecules.

Excitation delocalization in the bacteriochlorophyll c antenna of the green bacterium Chloroflexus aurantiacus as revealed by ultrafast pump-probe spectroscopy.

Room temperature absorption difference spectra were measured on the femtosecond through picosecond time scales for chlorosomes isolated from the green bacterium Chloroflexus aurantiacus. Anomalously high values of photoinduced absorption changes were revealed in the BChl c Qy transition band. Photoinduced absorption changes at the bleaching peak in the BChl c band were found to be 7–8 times greater than those at the bleaching peak in the BChl a band of the chlorosome. This appears to be the first direct experimental proof of excitation delocalization over many BChl c antenna molecules in the chlorosome.

Strongly exciton-coupled BChle chromophore system in the chlorosomal antenna of intact cells of the green bacteriumChlorobium phaeovibrioides: A spectral hole burning study

Spectral hole burning studies of intact cells of the green bacteriumChlorobium phaeovibrioides have proven that the Qy-absorption system of antenna bacteriochlorophylle (BChle) should be interpreted in terms of the delocalized exciton level structure of an aggregate. For the first time the 0-0 band of the lowest exciton state of BChle aggregates has been directly detected as the lowest energy inhomogeneously broadened band (FWHM ∼ 100 cm−1; position of maximum, at ∼ 739 nm) of the near-infrared BChle band in the 1.8 K excitation spectrum (FWHM=750 cm−1; position of maximum, at 715 nm). The comparative analysis of the hole spectra, measured for the three species of BChlc- ande-containing green bacteria, has shown that the 0-0 transition bands of the lowest exciton state of BChlc ande aggregates display fundamentally similar spectral features: (1) the magnitude of inhomogeneous broadening of these bands is about 100 cm−1; (2) at the wavelength of the maximum of each band, the amplitude of the preburnt excitation spectrum makes up 20% of the maximum amplitude of the spectrum; (3) the spectral position of each band coincides with the spectral position of the longest wavelength band of the circular dichroism spectrum; (4) the width of these bands is ∼ 2.3-times less than that of monomeric BChl in vitro.

Light control over the size of an antenna unit building block as an efficient strategy for light harvesting in photosynthesis

It was shown that an increase in the bacteriochlorophyll (BChl) c antenna size observed upon lowering growth light intensities led to enhancement of the hyperchromism of the BChl c Qy absorption band of the green photosynthetic bacterium Chloroflexus aurantiacus. With femtosecond difference absorption spectroscopy, it was shown that the amplitude of bleaching of the oligomeric BChl c Qy band (as compared to that for monomeric BChl a) increased with increasing BChl c content in chlorosomes. This BChl c bleaching amplitude was about doubled as the chlorosomal antenna size was about trebled. Both sets of findings clearly show that a unit BChl c aggregate in the chlorosomal antenna is variable in size and governed by the grow light intensity, thus ensuring the high efficiency of energy transfer within the BChl c antenna regardless of its size.

Spectral hole burning study of intact cells of green bacterium Chlorobium limicola

Spectral hole burning studies of intact cells of the green bacterium, Chlorobium limicola, have proven that the Qy‐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 0‐0 band of the lowest exciton state of BChl c oligomers has been directly detected as the lowest energy inhomogeneously broadened band (FWHM −100 cm−1; position of maximum, at ~774 nm) of the near‐infrared BChl c band of 1.8K excitation spectrum (FWHM = 830 cm−1; position of maximum, at 751 nm).

Study of the chlorosomal antenna of the green mesophilic filamentous bacterium Oscillochloris trichoides.

Whole cells, chlorosome-membrane complexes and isolated chlorosomes of the green mesophilic filamentous bacterium Oscillochloris trichoides, representing a new family of the green bacteria Oscillochloridaceae, were studied by optical spectroscopy and electron microscopy. It was shown that the main light-harvesting pigment in the chlorosome is BChl c. The presence of BChl a in chlorosomes was visualized only by pigment extraction and fluorescence spectroscopy at 77 K. The molar ratio BChl c: BChl a in chlorosomes was found to vary from 70:1 to 110:1 depending on light intensity used for cell growth. Micrographs of negatively and positively stained chlorosomes as well as of ultrathin sections of the cells were obtained and used for morphometric measurements of chlorosomes. Our results indicated that Osc. trichoides chlorosomes resemble, in part, those from Chlorobiaceae species, namely, in some spectral features of their absorption, fluorescence, CD spectra, pigment content as well as the morphometric characteristics. Additionally, it was shown that similar to Chlorobiaceae species, the light-harvesting chlorosome antenna of Osc. trichoides exhibited a highly redox-dependent BChl c fluorescence. At the same time, the membrane B805–860 BChl a antenna of Osc. trichoides is close to the membrane B808–866 BChl a antenna of Chloroflexaceae species.

Energy Transfers in the B808–866 Antenna from the Green Bacterium Chloroflexus aurantiacus

Energy transfers within the B808-866 BChl a antenna in chlorosome-membrane complexes from the green photosynthetic bacterium Chloroflexus aurantiacus were studied in two-color pump-probe experiments at room temperature. The steady-state spectroscopy and protein sequence of the B808-866 complex are reminiscent of well-studied LH2 antennas from purple bacteria. B808-->B866 energy transfers occur with approximately 2 ps kinetics; this is slower by a factor of approximately 2 than B800-->B850 energy transfers in LH2 complexes from Rhodopseudomonas acidophila or Rhodobacter sphaeroides. Anisotropy studies show no evidence for intra-B808 energy transfers before the B808-->B866 step; intra-B866 processes are reflected in 350-550 fs anisotropy decays. Two-color anisotropies under 808 nm excitation suggest the presence of a B808-->B866 channel arising either from direct laser excitation of upper B866 exciton components that overlap the B808 absorption band or from excitation of B866 vibronic bands in nontotally symmetric modes.

Energy transfer in photoactive complexes obtained from green bacterium Chlorobium limicola

Abstract The pigment · protein complexes enriched with bacteriochlorophyll a were derived from green bacterium Chlorobium limicola . The light dependences of bacteriochlorophyll fluorescence yield and lifetime as well as the portion of photooxidized reaction centers P -840 were recorded. These combined data revealed that in addition to the known fluorescence with a lifetime exceeding 2 ns, the short-living emission exists in a picosecond time scale. The latter belongs to the main portion of bacteriochlorophyll a and its lifetime and yield are governed by the P -840 state. According to the time-lever method developed for phase fluorimetry, its physiological lifetime amounts to no more than 40–100 ps and the limiting value to 10–25 ps. The fluorescence yield and the portion of molecules belonging to different funds are also determined. The above data prove the energy migration in bacteriochlorophyll a to be of the excitonic type at a moderate rate of molecular interaction. The quantum yield of triplet formation in antenna bacteriochlorophyll is negligible in a state of active photosynthesis, as in purple bacteria and Photosystem I of plants.