Anisotropic distribution of emitting transition dipoles in chlorosome from <Emphasis Type="Italic">Chlorobium tepidum</Emphasis>: fluorescence polarization anisotropy study of single chlorosomes |
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Authors: | Yutaka Shibata Yoshitaka Saga Hitoshi Tamiaki Shigeru Itoh |
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Institution: | (1) Department of Physics, Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan;(2) Department of Chemistry, Faculty of Science and Engineering, Kinki University, Higashi-Osaka Osaka, 577-8502, Japan;(3) Department of Bioscience and Biotechnology, Faculty of Science and Engineering, Ritsumeikan University, Kusatsu Shiga, 525-8577, Japan |
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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. |
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Keywords: | Fluorescence polarization anisotropy Confocal microscope Single molecule spectroscopy Photosynthetic antenna complex Bacteriochlorophyll-c Green photosynthetic bacteria |
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