Probing binding site of bacteriochlorophyll a and carotenoid in the reconstituted LH1 complex from Rhodospirillum rubrum S1 by Stark spectroscopy
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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.
KeywordsBacteriochlorophyll a Binding-site Carotenoid LH1 complex Purple photosynthetic bacterium Reconstitution Rhodospirillum rubrum Stark spectroscopy
- BChl a
N,N-lauryl dimethyl aminooxide
Light-harvesting 1 complex
- Rs. rubrum S1
Rhodospirillum rubrum strain S1
This work was supported by the Grant-in-aid from the Japanese Ministry of Education, Culture, Sports, Science and Technology (Grants No. 17204026, No. 17654083). HH, RF, MN, ATG and RJC thank for the financial support from SICP/JST. HH also acknowledges for the support from PRESTO/JST. RJC and ATG thank for the financial support from BBSRC.
- Kakitani Y, Akahane J, Ishii H, Sogabe H, Nagae H, Koyama Y (2007) Conjugation-length dependence of the T1 lifetimes of carotenoids free in solution and incorporated into the LH2, LH1, RC, and RC-LH1 complexes: possible mechanisms of triplet-energy dissipation. Biochemistry 46:2181–2197PubMedCrossRefGoogle Scholar
- Liptay W (1974) Dipole moments and polarizabilities of molecules in excited electronic states. In: Lin EC (ed) Excited states, vol 1. Academic Press, New York, USA, pp 129–229Google Scholar
- Nakagawa K, Suzuki S, Fujii R, Gardiner AT, Cogdell RJ, Nango M, Hashimoto H (2007) Electrostatic effect of surfactant molecules on bacteriochlorophyll a and carotenoid binding site in the LH1 complex isolated from Rhodospirillum rubrum S1 probed by Stark spectroscopy. Photosynth Res (in press)Google Scholar