[Model of aggregation of pigments in the chlorosomal antenna of the green bacteria Chloroflexus aurantiacus]. / Model' agregatsii pigmentov v khlorosomnoi antenne zelenykh bakterii Chloroflexus aurantiacus.

Independent experimental and theoretical evaluation was performed for the adequacy of our previously proposed general molecular model of structural organization of light-harvesting pigments in chlorosomal bacteriochlorophyll (BChl) c/d/e-containing superantenna of different green bacteria. Simultaneous measurement of hole burning in the optical spectra of chlorosomal BChl c and temperature dependence of steady-state fluorescence spectra of BChl c was accomplished in intact cells of photosynthetic green bacterium Chloroflexus aurantiacus; this allows unambiguous determination of the structure of exciton levels of BChl c oligomers in this natural antenna, which is a fundamental criterion for adequacy of any molecular model for in vivo aggregation of antenna pigments. Experimental data were shown to confirm our model of organization of oligometric pigments in chlorosomal BChl c antenna of green bacterium Chloroflexus aurantiacus. This model, which is based on experimental data and our theory of spectroscopy of oligomeric pigments, implies that the unit building block of BChl c antenna is a cylindrical assembly containing six excitonically coupled linear pigment chains whose exciton structure with intense upper levels provides for the optimal spectral properties of the light-harvesting antenna.

Model of detergent-induced spectral changes of the B800-850 complex from Chromatium minutissimum.

The absorption and circular dichroism spectra of the B800-850 complex from Chromatium minutissimum before and after the Triton X-100 treatment were simulated by means of standard exciton theory, taking into account inhomogeneous broadening. To explain the spectral changes of the B800-850 complex treated with Triton X-100, we have assumed that all bacteriochlorophyll pigments absorbing at 850 nm exhibit the same additional rotation of approximately 20 degrees around the axis perpendicular to the membrane plane. This has been sufficient to fit the transformation in absorption and circular dichroism spectra induced by detergent treatment of the B800-850 complex.

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.

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.

Functioning of oligomeric-type light-harvesting antenna.

In our previous work, we developed, for the first time, a theory of excitation energy transfer within an oligomeric-type light-harvesting antenna and, in particular, within the chlorosome of green bacteria (Biophys.J., 1996, vol.71, pp.995-1010). The theory was recently developed for a new original exciton model of aggregation of chlorosomal pigments, bacteriochlorophylls (BCh1) c/d/e (Biochem, Mol.Biol.Int., 1996, vol.40, No.2, pp. 243-252). In this paper, it was demonstrated with picosecond fluorescence spectroscopy that this theory explains the antenna-size-dependent kinetics of fluorescence decay in chlorosomal antenna, measured for intact cells of different cultures of the green bacterium Chlorobium limicola with different chlorosomal antenna size determined by electron microscopic examination of the ultrathin sections of the cells. According to our model, the energy transfer dynamics within the chlorosome imply the formation of a cylindrical exciton, delocalized over a tubular aggregate of BCh1 c chains, and inductive-resonance-type transfer of such a cylindrical exciton between the nearest tubular BCh1 c aggregates and to BCh1 a of the chlorosome.

Structure of bacteriochlorophyll aggregates in chlorosomes of green bacteria: a spectral hole burning study.

Exciton level structure, homogenous absorption and hole-burning spectra were calculated for different models of bacteriochlorophyll aggregation in chlorosomal antennae of green bacteria. It was demonstrated that none of the earlier proposed models of noninteracting linear bacteriochlorophyll aggregates and linear bacteriochlorophyll chains assembling in tubular aggregates with high density of packing, exhibits the in vivo exciton level structure revealed by hole-burning experiments on intact cells of green bacteria. The models of linear exciton-coupled bacteriochlorophyll chains with a low packing density, approximating that in vivo, were proposed as alternative, to obtain the main spectral features found in natural antennae.

Exciton delocalization in the antenna of purple bacteria: exciton spectrum calculations using Z-ray data and experimental site inhomogeneity.

Electron absorption and circular dichroism spectra of the peripheral light-harvesting complex (LH2) of photosynthetic purple bacteria were calculated taking into account the real-life spatial arrangement and experimental inhomogeneous broadening of bacteriochlorophyll molecules. It was shown that strong excitonic interactions between 18 bacteriochlorophyll molecules (BCh1850) within the circular aggregate of the LH2 complex result in an exciton delocalization over all these pigment molecules. The site inhomogeneity (spectral disorder) practically has no influence on exciton delocalization. The splitting between two lowest exciton levels corresponds to experimentally revealed splitting by hole-burning studies of the LH2 complex.

Site inhomogeneity and exciton delocalization in the photosynthetic antenna.

The influence of energy disorder on exciton states of molecular aggregates (the dimer and the circular aggregate) was analyzed. The dipole strength and inhomogeneous line shapes of exciton states were calculated by means of numerical diagonalization of Hamiltonian with diagonal energy disorder without intersite correlation. The disorder degree corresponding to destruction of coherent exciton states was estimated. The circular aggregates were treated as a model of light-harvesting antenna structures of photosynthetic bacteria. It was concluded that the site inhomogeneity typical for LH1 and LH2 complexes of purple bacteria cannot significantly influence the exciton delocalization over the whole antenna.

Excitation delocalization over the whole core antenna of photosynthetic purple bacteria evidenced by non-linear pump-probe spectroscopy.

Anomalously high values of photoinduced absorption changes were revealed in the antenna of photosynthetic purple bacteria. They were found to be 4-16 times greater at the bleaching peak of the antenna than at the bleaching peak of the BChl dimer of the reaction center. This is direct proof of excitation delocalization over many pigment molecules. Calculations according to the model of exciton delocalization over all core antenna BChls allow one to explain the observed phenomenon.

Exciton dynamics in circular aggregates: application to antenna of photosynthetic purple bacteria.

A theoretical model of exciton dynamics in circular molecular aggregates of light-harvesting bacteriochlorophyll of photosynthetic bacteria is proposed. The spectra and anisotropy of photoinduced absorption changes in the femto- and picosecond time domain are under its scope. The excited state of aggregate was treated due to the standard exciton theory, taking into account a pigment inhomogeneity. Dephasing processes via the exciton-phonon interactions were described by means of the Haken-Strobl equation. It was shown that only two exciton levels are dipole-allowed in the case of homogeneous circular aggregate. The pigment inhomogeneity results in the appearance of several weak transitions to higher exciton levels. It was proposed that the minor band (B896) in an absorption spectrum of the B875 complex as well as the similar minor band in spectra of B800-850 complex correspond to electron transition from the ground to the lowest exciton level, whereas the major band corresponds to transition to the higher exciton level. The proposed model shows the subpicosecond decay of anisotropy at the short-wavelength side of absorption band and a high degree of anisotropy at the long-wavelength side, even at high temperatures.

Exciton states of the antenna and energy trapping by the reaction center.

Forward and back energy transfer between antenna and RC in the photosynthetic apparatus of purple bacteria was studied taking into account the exciton states of the antenna. The exciton states were calculated for core antenna configuration in the form of a circular aggregate of N identical BChl molecules with the CN-symmetry. The influence of pigment inhomogeneity on the proposed exciton description of the antenna and its interaction with RC was investigated. The ratio between the rate constants of forward and back energy transfer between the exciton levels of the antenna and RC was obtained as a function of the temperature, the number of antenna BChls and the antenna exciton level position with respect to BChl special pair level of RC. A versatile analytical expression for this ratio which is independent of the BChl special pair level position and its dipole orientation was derived. The proposed model results in an irreversible excitation trapping by RC even at room temperature.

Excitonic interactions in the light-harvesting antenna of photosynthetic purple bacteria and their influence on picosecond absorbance difference spectra.

A new model of the light-harvesting antenna (core complex) of purple photosynthetic bacteria is proposed based on excitonic interactions in circular aggregates of bacteriochlorophyll molecules. The calculated absorbance difference spectra of circular aggregates demonstrate all special features observed in the experimental spectra of purple bacteria. In particular, the absorption changes with high amplitude of bleaching at the long-wavelength side of the absorption band at different excitation energy are predicted.