Probing binding site of bacteriochlorophyll a and carotenoid in the reconstituted LH1 complex from Rhodospirillum rubrum S1 by Stark spectroscopy |
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Authors: | Katsunori Nakagawa Satoru Suzuki Ritsuko Fujii Alastair T. Gardiner Richard J. Cogdell Mamoru Nango Hideki Hashimoto |
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Affiliation: | (1) Department of Applied Chemistry, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso-cho, Nagoya 466-8555, Japan;(2) Department of Physics, Graduate School of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan;(3) Glasgow Biomedical Research Centre, Institute of Biomedical and Life Sciences, University of Glasgow, 120 University Place, Glasgow, Scotland, 8QQ, UK |
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Abstract: | Stark spectroscopy is a powerful technique to investigate the electrostatic interactions between pigments as well as between the pigments and the proteins in photosynthetic pigment–protein complexes. In this study, Stark spectroscopy has been used to determine two nonlinear optical parameters (polarizability change Tr(Δα) and static dipole-moment change |Δμ| upon photoexcitation) of isolated and of reconstituted LH1 complexes from the purple photosynthetic bacterium, Rhodospirillum (Rs.) rubrum. The integral LH1 complex was prepared from Rs. rubrum S1, while the reconstituted complex was assembled by addition of purified carotenoid (all-trans-spirilloxanthin) to the monomeric subunit of LH1 from Rs. rubrum S1. The reconstituted LH1 complex has its Qy absorption maximum at 878 nm. This is shifted to the blue by 3 nm in comparison to the isolated LH1 complex. The energy transfer efficiency from carotenoid to bacteriochlorophyll a (BChl a), which was determined by fluorescence excitation spectroscopy of the reconstituted LH1 complex, is increased to 40%, while the efficiency in the isolated LH1 complex is only 28%. Based on the differences in the values of Tr(Δα) and |Δμ|, between these two preparations, we can calculate the change in the electric field around the BChl a molecules in the two situations to be E Δ ≈ 3.4 × 105 [V/cm]. This change can explain the 3 nm wavelength shift of the Qy absorption band in the reconstituted LH1 complex. |
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Keywords: | Bacteriochlorophyll a Binding-site Carotenoid LH1 complex Purple photosynthetic bacterium Reconstitution Rhodospirillum rubrum Stark spectroscopy |
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