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The FennaMatthewsOlson (FMO) antenna complex from the green sulfur bacterium Chlorobaculum (C.) tepidum is an important model protein to study exciton dynamics and excitation energy transfer in photosynthetic complexes. Single point mutations provide an excellent opportunity for the study of exciton structure and interexciton level relaxation. In this bacterium, the energy harvested by chlorosomes is funnelled via the baseplate pigments and FMO trimers to the reaction center (RC), where photochemistry takes place. The structure of the FMO protein from C. tepidum was solved and revealed a homotrimeric complex containing eight bacteriochlorophylls (BChl) a per monomer [1]. In this work we focus on two mutants: 1) V152N (here mutation occurs near BChl 1, i.e. valine is replaced by asparagine); and 2) W184F (where tryptophan 184 near BChl 6 is replaced with phenylalanine). In order to obtain information on the excited state lifetimes (T1) and interexciton level relaxation in the mutants we measured low temperature absorption/emission spectra and performed high-resolution resonant HB experiments. It is well known that the excited state lifetimes, that are related to the so-called homogeneously broadened lines (hom), are easily obtained via hole-burning spectroscopy as 1/T2 = (chom) 1 = (T1) –1 + (T2) , where T2 is the total dephasing time and T2 is the pure dephasing time; c is the velocity of light. At T = 4 K hom, obtained for excitonically coupled systems are strongly dominated by the exciton level decay times (T1). T1 = (2chom), where hom = 1⁄2 ZPH. In the modeling studies we apply a recently developed algorithm [2] to simultaneously fit multiple optical spectra. Intra-monomer interactions are modeled using non-Markovian Redfield theory. Calculations are performed for the FMO trimer. Fig. 1 compares the extracted resonant holes (four black narrow spikes) obtained for the typically studied wild type (WT) FMO with holes burned in the V152N mutant (Frame A; ten red holes) and W16F mutant (Frame C; eleven red holes). Spectra a (black) and b (red) are the absorption spectra obtained at 4 K for typical WT FMO, V152N (Frame A) and W16F (Frame C) mutant samples respectively. All resonant holes in Fig. 1 are obtained with a constant fluence f = 6 J/cm. The absence of resonant bleach in WT FMO sample at B  12450 cm, as illustrated in Fig. 1A/B, implies very fast (femtosecond) energy relaxation in this spectral range in agreement with ultrafast pump-probe and 2DES data [3]. That is, the lifetimes of the states excited in this region are too short to allow for an observable resonant bleach (broad ZPH). Likewise, the presence of ZPHs at higher energies in V152N (Fig. 1A) and W16F (Fig. 1C) indicates to slower (ps) energy relaxation for higher energy excitons compared to WT FMO. The slower experimentally estimated T1 values for V152N (~1 ps) and W184F (1.2-1.9 ps)

Impact of single point mutations on the excitonic structure and dynamics in FMO complex