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Spectroscopic evidence for triplet excitation energy transfer among carotenoids in the LH2 complex from photosynthetic bacterium Rhodopseudomonas palustris

  • Juan Feng
  • Qian Wang
  • Xujia Zhang
  • Youguo Huang
  • Xicheng Ai
  • Xingkang Zhang
  • Jianping Zhang
Article
  • 52 Downloads

Abstract

The LH2 complex from Rhodopsudomonas (Rps.) palustris is unique in the heterogeneous carotenoid compositions. The dynamics of triplet excited state Carotenoids (3Car* has been investigated by means of sub-microsecond time-resolved absorption spectroscopy both at physiological temperature (295 K) and at cryogenic temperature (77K). Broad and asymmetric T n ←T 1 transient absorption was observed at room temperature following the photo-excitation of Car at 532 nm, which suggests the contribution from various carotenoid compositions having different numbers of conjugated C=C double bonds (Nc=c). The triplet absorption bands of different carotenoids, which superimposed at room temperature, could be clearly distinguished upon decreasing the temperature down to 77 K. At room temperature the shorter-wavelength side of the main Tn04T1 absorption band decayed rapidly to reach a spectral equilibration with a characteristic time constant of ∽1 μs, the same spectral dynamics, however, was not observed at 77 K. The aforementioned spectral dynamics can be explained in terms of the triplet-excitation transfer among heterogeneous carotenoid compositions. Global spectral analysis was applied to the time-resolved spectra at room temperature, which revealed two spectral components peaked at 545 and 565 nm, and assignable to the Tn04 T1 absorption of Cars with Nc=c=11 and Nc=c=13, respectively. Surprisingly, the decay time constant of a shorter-conjugated Car, i.e. 0.72 ώs (aerobic) and 1.36 ώs (anaerobic), is smaller than that of a longer-conjugated Car, i.e. 2.12 us (aerobic) and 3.75 ώs (anaerobic), which is contradictory to the general rule of carotenoids and relative polyenes. The results are explained in terms of triplet-excitation transfer among different types of Cars. It is postulated that two Cars with different conjugation lengths coexist in an α, β-subunit in the LH2 complex.

Keywords

purple photosynthetic bacteria carotenoid excited-state time-resolved spectroscopy 

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Copyright information

© Science in China Press 2004

Authors and Affiliations

  • Juan Feng
    • 1
  • Qian Wang
    • 1
  • Xujia Zhang
    • 2
  • Youguo Huang
    • 2
  • Xicheng Ai
    • 1
  • Xingkang Zhang
    • 1
  • Jianping Zhang
    • 1
  1. 1.State Key Laboratory for Structural Chemistry of Unstable and Stable Species Institute of ChemistryChinese Academy of SciencesBeijingChina
  2. 2.National Laboratory of Biomacromolecules Institute of BiophysicsChinese Academy of SciencesBeijingChina

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