Unit building block of the oligomeric chlorosomal antenna of the green photosynthetic bacterium Chloroflexus aurantiacus: modeling of nonlinear optical spectra

The size of a unit BChl c aggregate of the chlorosomal antenna of the green photosynthetic bacterium Chloroflexus aurantiacus was estimated by using the relative difference absorption measurements of the oligomeric BChl c and monomeric BChl a bands [S. Savikhin et al., FEBS Lett. 430 (1998) 323]. A quantitative fit of the data was obtained within the framework of the exciton model proposed before [Z.G. Fetisova et al., Biophys. J. 71 (1996) 995]. The model assumes that a unit building block of the antenna has a form of a tubular aggregate of L=6 linear chains, containing N BChl c molecules each. The simultaneous fit of the linear and nonlinear (pump-probe) absorption gave the intrachain interaction energy of 400 cm−1, the site inhomogeneity value of 280 cm−1, and the number of pigments in each individual linear chain N=4. The size of the antenna unit was found to be L×N=24 exciton-coupled BChl c molecules.