PARENT SESSION
Posters P3A Bacteriochlorophyll based antenna systems. Abstracts (219-238)

Structural analysis and excitation transfer dynamics of B808-866 light-harvesting complexes from the green bacterium Chloroflexus aurantiacus. Yueyong Xin¹, Su Lin¹, Gabriel Montaņo¹, Robert Blankenship^*,1, ¹ Department of Chemistry and Biochemistry, Tempe, Arizona, USA

ABSTRACT- Light-harvesting complex B808-866 from green bacterium Chloroflexus aurantiacus has been isolated using a new protocol. Reverse phase HPLC analysis demonstrated that the number of BChls in the B808-866 antenna is 36 +/- 2, the main carotenoid type is -carotene, and the molar ratio of BChl to carotenoid is 2:1. The steady-state absorption and fluorescence spectra and protein sequence of the B808-866 complex are reminiscent of the well-studied LH2 peripheral antenna from purple bacteria, while the amino acid sequence and circular dichroism spectrum shows that the complex is more similar to the LH1 inner antenna. The complex has been reconstituted into 2D crystals, which have been characterized by electron microscopy, dynamic light scattering and atomic force microscopy. The results suggest that the B808-866 antenna form a ring like LH1 but consists of 12 building blocks and the ring diameter is close to 20 nm, which may surround several reaction centers. Energy transfer dynamics in the B808-866 antenna have been investigated in detail at room temperature and 77 K using ultrafast transient absorption spectroscopy with selective excitation. The data show that energy transfer from the B808 to the B866 band takes 1.6 ps at room temperature and slows down to 2.4 ps at 77K. Based on a previous linear dichroism study, the distance between BChl 808 and BChl 866 of about 24 A was calculated according to the Forster mechanism. Within the B866 band, there is a fast excitation equilibrium process occurring among the individual subpopulations with a time constant of 300 fs. The excitation transfer from carotenoids to BChl 866 has a low efficiency of 15% at room temperature. By comparing with in vitro electronic state dynamics of -carotene, which has a life time of 5.1 ps in methanol, the S1 state accounts for the carotenoid to Bchl energy transfer, which takes 4 ps. The rate of energy transfer remains essentially the same at 77 K. A model is proposed that can explain our results from parallel studies of the structural and functional aspects of this unique antenna complex.

KEY WORDS: energy transfer, antenna, Chloroflexus aurantiacus, carotenoid