title: Anisotropic distribution of emitting transition dipoles in chlorosome from Chlorobium tepidum: fluorescence polarization anisotropy study of single chlorosomes.
abstract: The polarization anisotropy of fluorescence spectra from single chlorosomes isolated from a green sulfur bacterium, Chlorobium (Cb.) tepidum, was observed at 13 K. As the polarizer was rotated, the intensities of the fluorescence bands of both bacteriochlorophyll (BChl)-c self-aggregates and BChl-a in baseplate proteins showed clear oscillations. From the oscillation, the values of the degree of polarization (DP) and the phase shift (PS) between the BChl-c and BChl-a bands were determined for each single chlorosome. The DP versus PS plot for Cb. tepidum chlorosomes showed linear correlations between the PS and the DP values for both BChl-c and BChl-a fluorescence bands. This tendency could be explained from a simulation assuming a random orientation of chlorosomes and a triaxial orientation distribution of emitting transition dipoles within a single chlorosome. The intensity ratios among the X-/Y-/Z-principal transition dipoles were estimated to be 0.3/0.5/1 and 1/0.6/0.1 for the BChl-c and BChl-a fluorescence bands, respectively. Here, the X-, Y-, and Z-axes are perpendicular, parallel to the cytoplasmic membrane, and parallel to the chlorosome long axis, respectively. A theoretical calculation based on the exciton theory was conducted to reproduce the observed triaxial orientation distribution of emitting transition dipoles. The simulation revealed that a deformation introduced to the circular cross section of the rod-shaped BChl-c self-aggregates could qualitatively reproduce results of this study.
authors.affiliation: Department of Physics, Graduate School of Science, Nagoya University, Nagoya, 464-8602, Japan. yshibata@bio.phys.nagoya-u.ac.jp
authors.forename: Yutaka
authors.initials: Y
authors.lastname: Shibata
authors.forename: Yoshitaka
authors.initials: Y
authors.lastname: Saga
authors.forename: Hitoshi
authors.initials: H
authors.lastname: Tamiaki
authors.forename: Shigeru
authors.initials: S
authors.lastname: Itoh
mesh_terms: D016880:Anisotropy; D041883:Chlorobium / Q000166:cytology* / Q000378:metabolism*; D003198:Computer Simulation; D005454:Fluorescence Polarization; D013050:Spectrometry, Fluorescence