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.

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Spectral hole burning has been used to prove cxpcrimcnially the cxislcnce in natural antenna of one of the predicted structural optimizing factors -antenna pigment oligomcrization [J, Theor. Biol. I40 (1989) lG7]-ensuring high cllicicncy ofexcitation energy transfer from antenna toreaction center, This point has been examined for ~hc chlorosomal antenna of green bacterium Cllkurqf/~srrs aurunfiucus by hole burning in fluorescence excitation and emission spectra of intact cells at I .X K. The prrsistcnt hole spectra have been found tc be consistent with a strongly cxciton-couplcd bactcriochlorophyll c (BChl c) chromophorc sys~cm. The lowcat cxcilon state of BChl c oligomers has been directly detected and separated as the lowest energy inhomogcncously broadened bond (FWHM -90 cm-', position of maximum, at -752 nm) from the near-infrared BChl c band (FWHM -350 cm-'. poshion of maximum, at -742 nm) of I.8 K excitation spectrum.

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 Q y 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.z 1998 Federation of European Biochemical Societies.

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.