PARENT SESSION
Posters P3A Bacteriochlorophyll based antenna systems. Abstracts (219-238)

Microscopic investigation of single chlorosomes from green photosynthetic bacteria at cryogenic temperature. Yoshitaka Saga^*,1, Yutaka Shibata², Daisuke Yamamoto², Tsutomu Kouyama², Shigeru Itoh², Hitoshi Tamiaki¹, ¹ Department of Bioscience and Biotechnology, Ritsumeikan University, Shiga, Japan² Division of Materials Science, Graduate School of Science, Nagoya University, Nagoya, Japan

ABSTRACT- Chlorosomes are extramembranous antenna complexes of green photosynthetic bacteria. In chlorosomes, bacteriochlorophyll(BChl)s-c, d, or e self-aggregate to form light-harvesting apparatus. Light-energy harvested by BChl aggregates is transferred to BChl-a/protein complexes called baseplates in a chlorosomal membrane. To unravel the spectral properties of individual chlorosomes at cryogenic temperature, fluorescence emission spectra of single chlorosomes containing BChl-c, which were isolated from green filamentous bacterium Chloroflexus (Cfl.) aurantiacus and green sulfur bacterium Chlorobium (Chl.) tepidum, were measured on a quartz plate and in a frozen buffer at liquid-helium temperature by a confocal fluorescence laser microscope. These chlorosomes on a substrate were also observed with a contact mode by a home-made atomic force microscope (AFM). Single chlorosomes from Cfl. aurantiacus exhibited emission bands from BChl-c aggregates around 760 nm. Fluorescence bands from BChl-c self-aggregates in single chlorosomes from Chl. tepidum were positioned around 780 nm, and their spectral shapes were rather heterogeneous relative to those from Cfl. aurantiacus. BChl-a emission in both types of single chlorosomes adsorbed on a quartz plate were greatly weak or not detected. AFM observation proved that shape and size of both chlorosome types at low temperature were similar to the images observed by a transmission electron microscope at room temperature. In single non-adsorbed chlorosomes, the spectral properties of BChl-c aggregates were similar to those in single adsorbed chlorosomes. In contrast, BChl-a emission due to excitation energy transfer from BChl-c self-aggregates could be clearly observed in almost non-adsorbed single chlorosomes. This suggests that baseplates of most adsorbed chlorosomes were faced to a quartz plate, and quenching of BChl-a fluorescence and/or denaturation of baseplates might occur at the interface.

KEY WORDS: chlorosome, single-molecule spectroscopy, bacteriochlorophyll-c, green photosynthetic bacteria