Energy Transfer Kinetics and Low Energy Vibrational Structure of the Three Lowest Energy Qy-States of the Fenna−Matthews−Olson Antenna Complex

Burn wavelength (λ B )-dependent nonphotochemical hole spectra are reported for the lowest energy Q y -absorption band of the Fenna−Matthews−Olson (FMO) trimer complex from Prosthecochloris aestuarii . This band at 825 nm is contributed to by three states that stem from the lowest energy state of the subunit of the trimer. The spectra reveal unusually rich and quite sharp low energy satellite structure that consists of holes at 18, 24, 36, 48, 72, 120, and 165 cm -1 as measured relative to the resonant hole at λ B . The possibility that some of these holes are due to correlated downward energy transfer from the two higher energy states that contribute to the 825 nm band could be rejected. Thus, the FMO complex is yet another example of a photosynthetic complex for which structural heterogeneity results in distributions for the values of the energy gaps between Q y -states. The results of theoretical simulations of the hole spectra are consistent with the above holes being due to intermolecular phonons and low energy intramolecular vibrations of the bacteriochlorophyll a (BChl a ) molecule. The 36 cm -1 and higher energy modes are most likely due to the intramolecular BChl a modes. The simulations lead to the determination of the Huang−Rhys (S) factor for all modes. They range between 0.05 and 0.25 in value. The temperature dependencies of the spectral dynamics for the three contributing states are similar to those reported for the FMO complex from Chlorobium tepidum (Rätsep et al., J . Phys . Chem . B 2000, 103, 5736). The contribution to the dynamics from pure dephasing/spectral diffusion due to the glasslike two-level systems of the protein is identical for all three states. The lifetimes of the highest and intermediate energy Q y -states due to downward energy transfer are 26 and 99 ps, respectively, at liquid helium temperatures.