Exciton simulations of optical spectra of the FMO complex from the green sulfur bacterium Chlorobium tepidum at 6 K

Optical spectra (absorption, circular dichroism, linear dichroism: and triplet-minus-singlet), measured at 6 K, are presented for the Fenna-Matthews-Olson (FMO) complex from the green sulfur bacterium Chlorobium (C.) tepidum. Significant differences are observed in comparison with the corresponding spectra of Prosthecochloris (P.) aestuarii. The spectra of C. tepidum FMO were simulated using exciton calculations, based on the recently resolved structure of this complex [Li et al. J. Mel. Biol. 1997, 271 456]. These calculations apply the same basic assumptions as were earlier used for the FMO complex from P. aestuarii [Louwe et al. J. Phys. Chem. B 1997, 101, 11280], except that the site energies of all but one of the bacteriochlorophylls had to be changed in order to optimize the simulations. The good agreement that was obtained supports the assumptions in these model calculations, and the assignment of the site energies makes it possible to consider the spectra in relation to the structural differences between the two types of FMO complex.