Pigment organization and energy transfer in the green photosynthetic bacterium Chloroflexus aurantiacus

Abstract

The transfer of excitation energy and the pigment arrangement in isolated chlorosomes of the thermophilic green bacterium Chloroflexus aurantiacus were studied by means of absorption, fluorescence and linear dichroism spectroscopy, both at room temperature and at 4 K. The low temperature absorption spectrum shows bands of the main antenna pigments BChl c and carotenoid, in addition to which bands of BChl a are present at 798 and 613 nm. Fluorescence measurements showed that excitation energy from BChl c and carotenoid is transferred to BChl a, which presumably functions as an intermediate in energy transfer from the chlorosome to the cytoplasmic membrane. Measurements of fluorescence polarization and the use of two different orientation techniques for linear dichroism experiments enabled us to determine the orientation of several transition dipole moments with respect to each other and to the three principal axes of the chlorosome. The Qy transition of BChl a is oriented almost perfectly perpendicular to the long axis of the chlorosome. The Qy transition of BChl c and the γ-carotene transition dipole are almost parallel to each other. They make an angle of about 40° with the long axis and of about 70° with the short axis of the chlorosome; the angle between these transitions and the BChl a Qy transition is close to the magic angle (55°).

Circular dichroism of green bacterial chlorosomes

“…chlorosome CD spectra (Betti et al 1982, van Dorssen et al 1986, Blankenship et al 1988b are generally similar to that in Fig. 1.…”

“…If the 750-nm absorption peak shifts to even slightly shorter wavelengths during isolation, the longest-wavelength negative CD band is lost or greatly attentuated, while the band at 720 nm is strengthened. A similar phenomenon may also occur with Cf-the negative band at ---720 nm is somewhat larger in previously published spectra (van Dorssen et al 1986, Blankenship et al 1988b)-but chlorosomes from that thermophilic organism appear to be considerably more stable at room temperature.…”

“…) and Chloroflexus (Cf.) chlorosomes were quite different (Betti et al 1982, Olson et al 1985, van Dorssen et al 1986, Blankenship et al 1988b. While both exhibited a large, derivative-shaped near-infrared (NIR) feature, the longest wavelength band was found to be positive in Cb., while in Cf.…”

Presence and significance of minor antenna components in the energy transfer sequence of the green photosynthetic bacterium Chloroflexus aurantiacus

“…The presence of the long wavelength antenna (F910) was suggested by the fluorescence polarization spectrum [33] and similarly F781 was observed in the fluorescence spectrum of chlorosomes [34] and whole cells [3] measured at 4 K, even though its presence was not inferred. A long-lived baseplate fluorescence (F819) suggests a difference in function between two baseplate Bchl a's.…”

Excitons in tubular molecular aggregates

“…The strong variation of the CD with the cylinder length offers an explanation to a well-known problem in the literature on chlorosomes, namely the strong variation of this spectrum as observed in different experiments [15][16][17][18][19][20]. Two types of spectra are typically reported in the experimental literature.…”