Linearly polarized light absorption spectra of chlorosomes, light-harvesting antennas of photosynthetic green sulfur bacteria

Absorption spectra of chlorosomes isolated from several species of photosynthetic green sulfur bacteria using linearly polarized light were measured in a compressed polyacrylamide gel at three-dimensional geometric orientations. Chlorosomes containing self-aggregates of bacteriochlorophyll(BChl)-e gave two distinct Soret absorption peaks, which were assigned to By and Bx bands at longer and shorter wavelengths, respectively. The transition dipole of By band was determined to be parallel to the long axis of ellipsoid-shaped chlorosomes, while that of Bx parallel to the short axis. Analysis of linear dichroism and polarization degrees supported the above orientation, while the alignment of Bx transition dipoles was limited. The features are also observed in chlorosomes having BChls-c and d. The absorption bands in the Soret region by linearly polarized light are shown for the first time to be useful for the investigation of supramolecular structures of chlorosomal self-aggregates, as in the redmost Qy bands reported earlier.