The Fenna-Matthews-Olson (FMO) trimer protein from the green sulfur bacterium Chlorobaculum tepidum is an important model system of exciton dynamics and excitation energy transfer (EET) in photosynthetic complexes. In addition, FMO is a popular model for excitonic calculations, with many theoretical parameter sets reported describing linear and nonlinear optical spectra; often taking only a single monomer into account. However, resonant spectral holes burned into the low-energy (825 nm) band cannot be accounted for without a model of uncorrelated EET between the trap states of different monomers in the FMO trimer. It is proposed that to properly account for various optical spectra of FMO exciton delocalization is restricted to a single monomer, which is described by Redfield theory, while intermonomer EET rates are then described by Generalized Förster theory. Due to fast exciton relaxation, intermonomer EET results predominately from the lowest energy exciton states (contributed to by BChls a 3 and 4). Using experimentally determined electron-phonon coupling strength (S), phonon spectral density, and reorganization energy (Eλ), simulated optical spectra were obtained for the entire FMO trimer. The latter was accomplished with Redfield simulations using Monte-Carlo disorder averaging and Nelder-Mead Simplex algorithm for parameter optimization. Simultaneous fits of low-temperature absorption, emission and HB spectra placed constraints on the determined pigment site energies. Chlorosome–Baseplate–FMO EET is discussed in light of the resulting simulations of isolated FMO samples and experimental spectra for chlorosomes of various complexities (i.e., chlorosome–baseplate, baseplate-depleted chlorosomes, and chlorosome–baseplate–FMO systems).

New insight into the FMO antenna protein: Hole burning and modeling study

249th National Meeting (2015)

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