Pigment organization and exciton dynamics in the B808-866 antenna of the green bacterium Chloroflexus aurantiacus.

The model for the B808-866 antenna of the green photosynthetic bacterium Chloroflexus aurantiacus based on femtosecond pump-probe studies is proposed. The three-dimensional structure of the B808-866 antenna is assumed to be similar to the structure of the B800-850 antenna of purple bacteria, i.e. it has the form of two concentric rings of N strongly coupled BChl866 pigments and of N/2 weakly coupled BChl808 monomers. The Qy transition dipoles of BChl808 and BChl866 molecules form the angles 43 degrees +/- 3 degrees and 8 degrees +/- 4 degrees, respectively, with the plane of the corresponding rings. The lowest limit of the BChl866 aggregate size is N = 18. The anomalously high bleaching value of the BChl866 band with respect to the monomeric BChl808 band provides evidence for a high degree of exciton delocalization. To account for this phenomenon, the exciton model describing exciton dynamics in the spectrally disordered circular BChl866 aggregate is developed. According to this model, the effective exciton size in this antenna (Neff) is 6-8.